Bft1©E?

Ii

ii1^i

KS

ssSSssnimi

iif1

^i&&fe•fe

^ HcHml ^ ra mlals ^ ^ ^

Information Circular 9020

Design of Bulkheads for ControllingWater in Underground Mines

By Gregory J. Chekan

UNITED STATES DEPARTMENT OF THE INTERIORDonald Paul Hodel. Secretary

BUREAU OF MINESRobert C. Norton, Director

AR303620

Chekan, G. J. (Gregory J.)Design of bulkheads for controlling water in underground mines.

(Information circular : 9020)

Bibliography: p. 23-24.

Supt. of Docs, no.: I 28.27:9020.1. Mine drainage. 2. Mine water. 1. Title. II. Scries: Informa-

tion circular (United States. Bureau of Mines) : 9020.

TN294.U4 [TN3211 622s [622'.5] S4-600374

SR30362I

J

•JLibrary of Congress Cataloging m Publication Data:

J

CONTENTS

Abstract....................................................................... 1Introduction...,.................................................;............. 2Bulkhead design methods........................................................ 3Types of bulkheads........................................................... '3Factors to consider in bulkhead design....................................... 3Thin and thick plate designs................................................. 4Thin plate design.......................................................... 4Thick plate design......................................................... 4Trench depth............................................................... 5

South African plug design.................................................... 6Plug design formulas....................................................... 6The relation of water leakage to plug length............................... 10

Single and double bulkhead seals............................................. 11Single bulkhead seals...................................................... 11Double bulkhead seals...................................................... 12

Concrete specifications and placement methods.................................. 12Specifications............................................................... 12Placement.................................................................... 14

Pressure grouting.............................................................. 15Grouting materials........................................................... 16Portland cement grout...................................................... 16Chemical grouts............................................................ 17

~Grouting methods............................................................. 17\ier pillars................................................................ 18

*v— illsr considerations........................................................ 18Pillar width formulas........................................................ 18

Monitoring water pressure...................................................... 20Obtaining MSHA approval........................................................ 22Discussion..................................................................... 22References..................................................................... 23Bibliography................................................................... 24Appendix A.—Hydrostatic testing of a single bulkhead seal..................... 26Appendix B.—Flexural strength analysis for concrete block bulkhead............ 36

ILLUSTRATIONS

1. Methods of designing plugs to retain high water pressures................ 72. Dimensions of experimental plug.......................................... 93. Plug constructed on the 1,200-ft level of the Friendensville Zinc Mine... 104. Length of plugs based on ultimate pressure gradient values............... 115. Double bulkhead seal..................................................... 126. Concrete being mixed underground and placed in forms by concrete pump.... 157. Pumping concrete from the surface through a single vertical borehole to a

central underground site................................................ 158. Pumping concrete from the surface through a vertical borehole directly to

bulkhead site........................................................... 15A-l. Diagram of single bulkhead seal.......................................... 26A-2. Location of bulkhead in Safety Research Coal Mine........................ 27* 3. Bulkhead under construction.............................................. 28

Cross section of 1-in pipe grouted into concrete block................... 29Diagram of test apparatus................................................ 29

AR303622

ii

ILLUSTRATIONS—Continued

A-6. Drill plan for injecting polyurethane grout in strata surrounding thebulkhead............................................................... 30

A-7. Packer-mixer assembly installed in borehole............................. 31A-8. Polyurethane grout emerging from strata................................. 31A-9. Diagram of standpipe, pressure gauge, and porous tube arrangement....... 34A-10. Polyethylene porous tube installed on the inby side of bulkhead......... 35B-l. Correction factor for bulkhead width-to-height ratios................... 36

TABLES

1. Summary of test results on experimental plug............................ 82. Parallel plugs of the Witwatersrand and Orange Free State gold fields

for which there are records of loads.applied of over 1,000 psi......... 93. Ultimate compressive, tensile, and shear strengths for 1:2:4 concrete

mix.................................................................... 13A-l. Test procedure for increraentally pressurizing bulkhead.................. 29

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

ft foot psi pound per square inch

gal/h gallon per hour psig pound per square inch, gauge

in inch V dc volt, direct current

raA milliampere wt pet weight percent

rain minute yr year

mm millimeter

DESIGN OF BULKHEADS FOR CONTROLLING WATER IN UNDERGROUND MINES

By Gregory J. Chekan '

ABSTRACT

This Bureau of Mines report presents three methods for designingbulkheads to impound water underground: (1) thin and thick plate de-sign; (2) South African plug design; and (3) single and double bulkheadseal design. Related areas critical to the long-term effectiveness ofunderground water impoundments are also addressed. These include bulk-head anchorage, concrete specifications and placement, the grouting ofpermeable strata, and the sizing of barrier pillars. A case studyinvolving hydrostatic tests conducted on a single bulkhead seal con-structed in the Safety Research Coal Mine of the Bureau's PittsburghResearch Center is presented in an appendix.

i,!Mining engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA,

AR30362if

INTRODUCTIONv_y

Bulkheads are commonly used to seal loading, which could be present for theabandoned workings and protect adjacent entire life of the bulkhead. In extremeactive mines from explosion; however, cases, this pressure may reach 500 psibulkheads can also be used to control (approximately 1,150 ft of waterhead) andunwanted inflows of water. Ground water last for several days, until pumping orseepage from poorly sealed shafts, water- draining operations can be initiated.bearing strata, and abandoned mine areas Permeation of acid water is another majorused for impoundment are the major structural concern, for it deterioratessources of water inflow. In some in- the bulkhead and its anchorage, as wellstances, water levels in these abandoned as the ground around the bulkhead.areas can rise rapidly or go completely When designing and constructing a bulk-undetected while the areas are accumulat- head for the purpose of impounding water,ing excessive hydrostatic pressure. This several general criteria should be met:poses a potential inundation hazard tothe active mine, especially if the bulk- 1. The bulkhead should be designed tohead is not suitably designed to retain withstand the static forces of hydro-water, static pressure rather than the dynamicThe Federal Coal Mine Safety and Health forces of an explosion.

Act of 1977 requires that bulkheads whichseal abandoned areas be "explosionproof" 2. The bulkhead should be constructedbut makes no requirements on their abil- from a material, such as concrete, whichity to perform as water seals.2 The Bu- will resist deterioration by water.reau has conducted extensive researchinto the design and construction of ex- 3. The bulkhead should be constructedplosionproof bulkheads and the forces ex- sufficiently thick and properly anchored, \. Jerted upon them from coal dust and meth- and the surrounding strata should beane ignitions. Although these designs pressure grouted to minimize watermay have application for impounding wa- seepage.ter, there are differences between ex-plosion pressures and hydrostatic pres- The ability to safely impound water un-sures and the forces that they exert on a derground will become increasingly impor-structure. tant in future years. Inundation bulk-In the case of an explosionproof bulk- heads will be needed to protect active

head, the structure may never experience workings in areas where mining is ina significant loading until an explosion close proximity to surface water bodiesoccurs. A methane or coal dust explosion or water-bearing strata. Mining compa-exerts a dynamic loading on the bulkhead nies are also beginning to examine thethat rarely exceeds 50 psig. As a gen- possibility of impounding water under-eral rule, pressure at 200 ft or more ground as a means of eliminating thefrom the origin of an explosion will not costly treatment of acid mine water be-exceed 20 psig unless coal dust accumula- fore discharge. Presently, there is notions are abnormal and the incombustible commonly accepted design method for con-content of the dust is far less than re- structing bulkheads for this purpose.quired by law (_U.3 In contrast, inunda- Prior to constructing a bulkhead for im-tion bulkheads are usually subject to a pounding water, a mine operator mustconstant hydrostatic pressure, a static first notify the Mine Safety and Health_______________________________ Administration (MSHA) and then submit de-

Regulations governing the sealing of tailed design and construction plans forabandoned areas are covered in the Code _________________________________of Federal Regulations, Title 30, Chapter 3Underlined numbers in parentheses re- .1, Part 75, Subchapter D, Subparts 329-1, fer to items in the list of references ^—'329-2, 330 and 330-1. preceding the appendixes.

*R303625

pproval. This usually requires the as- marginal, and more research needs to beV_/istance of a professional engineering conducted to add to our present knowledge

consultant who has had prior experience of plug and bulkhead design. This reportin this subject. seeks to increase the mine operators' un-

it is strongly emphasized that this re- derstanding of inundation bulkhead designport is not intended to serve as a com- and other problems associated with under-plete guide to the design of bulkheads ground water impoundments. Such informa-for impounding water underground. Pub- tion is essential to safe mining in areaslished literature on this subject is where inundation is possible.

BULKHEAD DESIGN METHODS

TYPES OF BULKHEADS '(2,) the entire entry and simply limit theheight of water to the height of the dam;

Bulkheads constructed for impounding but large volumes of water can be im-water can be classified into five types. pounded in this way.

Control.—Bulkheads that are planned in FACTORS TO CONSIDER IN BULKHEAD DESIGNadvance and constructed to seal abandonedmines and prevent the inflow of water in- Several factors should be consideredto an adjacent active mine. They are in- before designing and constructing a bulk-stalled between barrier or chain pillars ; head to impound water:with no means of access to the sealed-offarea. Pipes with valves are usually cast 1. The bulkhead should be located ininto the bulkhead to measure and control competent ground that is not excessivelyvater levels. They are designed to with- fractured or broken, preferably in areas

v :and the maximum water pressure that can of stable ground (1_). However, in most^-develop. This is usually equal to the coal mines ground movements such as roof

depth of the bulkhead below the surface. convergence and floor heave are inevit-Emergency.—Bulkheads constructed under able, and supplemental roof supports

emergency conditions to seal off unex- (timbers and cribs) should be installedpected inrushes of water. They are de- at the site.signed to withstand the maximum waterpressure that can develop, with no means 2. The bulkhead, in most cases, shouldof access to the sealed-off area. be designed to withstand the maximum hy-Precautionary.—Bulkheads planned in , drostatic pressure that can develop.

advance and constructed in main entries Practical limits of potential inundationand haulage roads to control flooding can be determined by plotting on a coalshould an inundation occur. Watertight contour map the expected mine pool ele-doors are cast into the bulkheads to pro- vations and corresponding ground surfacevide a travelway for workers and equip- elevations. Areas where excessive water-ment. These bulkheads are designed to heads may accumulate can then be pro-withstand the maximum water pressure that jected. To convert waterhead, which iscan develop. . expressed in feet, to hydrostatic pres-Consolidation.—Bulkheads constructed sure, which is expressed in pounds per

to control water inflow during high-pres- square inch, multiply the waterhead bysure grouting and ground . consolidation 0.434.operations. They are temporary struc-tures which are removed after ground 3. The concrete for constructing thesealing is completed. bulkhead must be properly mixed andOpen Dam Walls.—Open dam walls are placed to achieve acceptable strengthssed to impound water for treatment or upon curing. (See section on "Concrete

and reuse. They do not seal Specifications and Placement Methods.")

0R303626

4. Anchorage of the bulkhead to mine a » maximum entry dimension, ft;roof, ribs, and floor is important, anddepends on design as well as on strata p - hydrostatic pressure, psi;type and condition. Some design methodsrely on the strength of the concrete and ft « allowable tensile strength ofbearing against the irregularities in the construction material, psi.rock surface to provide anchorage. Oth-ers require the excavation of trenches. If a • 18 ft, p - 100 psi (230 ft of wa-Anchorage requirements for each design terhead), and ft (for concrete) » 150 psimethod are discussed in their respective (O, a bulkhead approximately 12.7 ftsections. thick and unanchored4 would be needed to

impound 230 ft of water.5. Adequate pressure grouting of the In practice, this design formula may be

immediate strata surrounding the bulkhead conservative. Research indicates thatis probably the most significant factor bulkheads designed accordingly have re-in the bulkhead's long-term performance. sisted much higher dynamic loads (explo-Deterioration of the anchoring strata by sion pressure) with a considerable marginacid water permeation is a major struc- of safety (_!_)• This observation suggeststural concern, especially if large pres- not only that bulkheads designed by thissures are anticipated over the life of method can resist a much higher dynamicthe bulkhead. A brief review of grouting load, but also that the design methodmaterials and methods frequently used to could be modified to more realisticallyseal coal mine strata is given in the represent the response of the structuresection on "Pressure Grouting." under static load. Such is the case in

thick plate design.THIN AND THICK PLATE DESIGNS (3)

Thick Plate DesignThin and thick plate formulas for de-

signing bulkheads are derived through The Bureau conducted a series of modelstatic analysis techniques, but the ef- test in the early 1930's (-.S) and foundfectiveness of these designs for impound- that restraining the edges of a bulkheading water has not been thoroughly evalu- caused a dramatic increase in strength,ated through full-scale prototype tests. well beyond what was expected from plateThese designs apply only to bulkheads theory. Full-scale explosion tests alsoconstructed from homogeneous and isotrop- showed that bulkheads that were recessedic materials. into the roof, ribs, and floor, and that

had thickness-to-width ratios of at leastThin Plate Design 1 to 10, resisted much higher pressures

than the design pressure. It was con-This design assumes that the bulkhead eluded that recessing the ends of the

is to act as a simply supported thin bulkhead into the surrounding strata al-plate, spanning the width of the entry; lows the structure to act as a flat arch.its structural behavior under static load Under load, this arching behavior trans-is characterized by bending at mldspan. mits a lateral thrust to the strata*Under these conditions, bending failure which then act as a buttress.is governed by the tensile strength of Attempts have been made to explain thisthe construction material. Using this arching behavior through static designanalysis, the required bulkhead thickness models. Whitney (j>) developed an archis predicted to be model that assumed the bulkhead to fall

as two rigid walls, fractured at bothT * 0.865 a /p/ft (1) _______________________________

^Anchorage is supplied by bearing re-where T » bulkhead thickness, ft; sistance between rock and concrete.

^303627

sides and along the midspan. Using this to recess the bulkhead into the roof,V-^assumption, the design formula is pre- ribs, and floor contributes to this in-

dicted to be creased strength. The trenches assure- that the applied load develops through

T - pa /p/fc (2) the bulkhead and is then transferred tothe load-bearing capacity of the coal

where T • bulkhead thickness, ft; roof, ribs, and floor.The thick plate design approach has two

a - maximum entry dimension, ft; principal drawbacks. First, the archingbehavior described earlier does not occur

p - hydrostatic pressure, psi; until there is considerable cracking orfracturing of the bulkhead. The failure

fc • allowable compressive of a bulkhead under these circumstancesstrength of construction can be catastrophic, especially if thematerial, psi; hydrostatic pressure exceeds the design

1 pressure. Second, the strength of theS^ /I + Ay2 bulkhead depends directly on the bearingE /I + Ay2 - 1 j strength of the coal, roof, ribs, and

floor strata. The Bureau has conductedwhere E * Modulus of elasticity of research along these lines to determine

construction material, psi; the compressibility and bearing strengthof in-place coal (5_, 7). Future design

Sc = ultimate compressive criteria should include the bearingstrength of construction strengths of the coal, the roof, and thematerial, psi; floor to assure adequate design.

T/a, thickness-to-width Trench Depth (_1 , j4)ratio.

The required trench depth to properlyFor average concrete mixtures; Sc = 3,000 anchor bulkheads has not yet been deter-psi, E 2 3 million psi and with thick- mined through either model or full-scaleness-to-width ratios (y) of at least 1 to tests. However, acceptable requirements10, p is ~0.670. Therefore, the thick- for minimum trench depths for bulkheadsness formula becomes less than 3 ft thick can be presumed

from research conducted on explosionproofT - 0.670 a /p/fc (3) bulkheads.

For concrete bulkheads, Rice (4_-J>),If a - 18 ft, p - 100 psi (230 ft of recommended trench depths, in the coal

waterhead), and fc (for concrete) « 1,000 ribs, of at least one-tenth the width ofpsi, a bulkhead approximately 3.8 ft the entry (0.1 W) after all loose coal onthick and firmly anchored (recessed into either rib had been scaled away. Howev-the roof, ribs, and floor) would be er, if the coal is distinctively soft orneeded to impound 230 ft of water. broken, a trench depth of one-fifth theDesign equation 3 is very similar to width of the entry (0.2 W) was advised.

design equation 1, the major difference In accordance, Mitchell (O recommendedbeing that the allowable tensile strength that rib trench depths be at least 2 ft(ft) is replaced by the allowable com- or the thickness of the bulkhead, which-pressive strength (fc). For most materi- ever is greater.als, fc is 5 to 10 times ft. This allows Floor trenches should be a minimum ofa reduction in required design thickness 12 in deep, provided the immediate floorof 50 to 70 pet, provided that there is strata have not been softened by water.dequate anchorage. Excavating trenches If this is the case, trenching should

AR30362;

proceed until a competent stratum is SOUTH AFRICAN PLUG DESIGN (_2, 8-9) V>reached. After excavation, holes shouldbe drilled along the centerline of the South African plug research was con-trench to accommodate steel reinforcing ducted during the late 1950's and earlyrods of at least 7/8-in diam and 38 in 1960's to resolve inundation problemslong. The steel rods should be firmly encountered in the mines of the Witwa-grouted no less than 18 in deep, and tersrand and Orange Free State goldspaced at no more than 18-in intervals. fields. U. S. Garrett and L. T. Campbell

If feasible, trenching of the roof is Pitt (8_-9_) designed, constructed, andrecommended. Owing to roof sag, the roof tested an experimental concrete plug thatis usually where most water seepage will withstood a hydrostatic pressure in ex-occur. Trenching the roof may be a dif- cess of 6,000 psi. Their research led toficult task because of the unpredictable a better understanding of the criterianature of most roof rock. The immediate that influence plug design. Althougharea should be stabilized with supplemen- their tests were conducted under condi-tal supports, and care should be exer- tions unique to deep gold mines of Southcised during the trenching operation. Africa, the assumptions and theory whichTrenches should be cut at least 8 in formed the basis for their design formuladeep. Once the trenches are complete, can also be applied to water impoundmentsadditional anchorage should be provided in underground coal mines.with steel reinforcing rods of at least7/8-in diam and 30 in long. The steel Plug Design Formulasrods should be firmly grouted into theroof along the centerline of the bulkhead Garrett and Campbell Pitt consideredat a depth of no less than 18 in, with no three possible methods for designingmore than 18-in spacing between rods. plugs to retain high water pressuresWhen excavating the trenches the fol- (fig. 1):

lowing should be observed:1. The plug would be constructed as a

1. Select a site where the ribs, roof, slab with all four sides recessed intoand floor are competent and not affected the rock, with or without steel rod rein-by long-term weathering or excessive forcement (plate design).ground movement and stress. As a routine -measure, supplemental supports should be 2. The walls of the drive wouldinstalled at the selected site. tapered so the load could be transmitted

from the plug to the rock wall by2. Trim all loose coal from the ribs, compression.

making them as straight as possible. The :_*;same applies for loose rock on the floor 3. The plug would be parallel to tha,;and roof. walls of the drive, and there would be

need for recessing or tapering. Anchor-i3. Cut the trenches with hydraulic or age would be provided by the concrete

pneumatic tools, taking care to avoid un- bearing, against the irregularitiesnecessarily fracturing the strata. Ex- the rock surface, after all loose mater$~plosives should not be used to excavate al had been scaled away. 'the trenches unless very hard, competentstrata are encountered. In many instances, these plugs had ^

be constructed under emergency condj|4. Keep the width of the trenches tions. Time was the most important fa'c

the same as that of the bulkhead. All tor, and site preparation had to be mtrenches should be cut as square as pos- mal. For this reason, they choosesible, especially at the inner and outer third method (parallel plug design)jcorners where the floor and rib trench rather than the first two methods, botlmeet.

Government Mining Engineer as adequatecriteria for parallel plug design. Thelength of the plug was derived from

pab - 2(a+b)lfs (5)

Therefore: JV 2(a+b)fsSlab keyed into walls

plug

where 1 * length of the plug, ft,

a » width of the entry, ft,

b - height of the entry, ft,

p - hydrostatic pressure, psi,

and fs - allowable shear stress forrock or concrete, whicheveris the lesser,^ psi.

To learn more about this design, Gar-rett and Campbell Pltt constructed asmaller experimental plug, with doors,and tested it hydrostatically (8). Thedimensions of this experimental plug areshown in figure 2. With each test, thehydrostatic pressure was increased andthe strata surrounding the plug werepressure-grouted more extensively to sealleaks. A summary of the test results isshown in table 1. Upon completion of the

_ test Garrett and Campbell Pitt made sev-Kt-Y eral observations, on which basis they

Hi TAP firm nf revised the parallel plug design underUireiMUII Ul the a8SUmptlon that when a plug is in-

stalled in an entry, half of the rocksurfaces will be at an angle (45°) that

.. „ . would resist movement of the plug by com-iMOl tO SCale pression. The other half of the rock

FIGURE 1.. Methods of designing plugs to re- surfaces would resist movement by ten-. . ,. , *j if ui c sion, provided the contact between rocktain high-water pressures. Adapted from W. S. and plug was ensured through adequateGarrett and L. T. Compbell Pitt (8). . pressure grouting. The length of the

parallel plug was represented asof which required ground excavation be-fore the plug could be installed. _______________________________At that time, the accepted theory for 5The Government Mining Engineer of•

parallel plug design assumed that the South Africa recommends 85 psi as the al-load, induced by hydrostatic pressure, lowable shear stress (fs) for concretewas transmitted from the concrete plug to placed in the normal manner and 120 psithe rock as punching shear around the for plugs where positive contact betweenperimeter of the plug and along its rock and plug is ensured by subsequentfull length. This was accepted by the pressure grouting (_2).

AR3U3630

pab - 2(a+b)l/2 1 tan 45°fc (5) of a very complicated stress condition.They felt that movement of the rock, from

- pab stress induced by the hydrostatic load,Therefore: i - (a+b)fc might likely be the governing factor in

assessing the stress condition in thewhere 1 - length of the plug, ft, concrete of the plug itself. Therefore,

the values that could be set on eithera - width of the entry, ft, allowable shear stress or allowable com-

press ive strength depend on the effee-fa » height of the entry, ft, tiveness with which the concrete of the

plug is confined by the surrounding rock.p - hydrostatic pressure, psi, Using this analysis, it is impossible to

recommend a particular formula for as-and fc - allowable compressive sessing the stress condition in the con-

strength of the rock or crete until some satisfactory quantita-concrete, whichever is the tive results can be produced on whatlesser,6 psi. actually takes place in the rock sur-

rounding a plug under a hydrostatic loadGarrett and Campbell Pitt realized that (9).

the two formulas were oversimplifications Table 2 gives a list of parallel plugs,_____________________________ constructed in the Witwatersrand and6Garrett and Campbell Pitt used 600 psi Orange Free State gold fields. Garrett

as the allowable compressive strength of and Campbell Pitt calculated the respec-concrete. tive values of fs and fc from the two

TABLE 1. - Summary of test results on experimental plugTest

X • « * •

*»••••

•J • • • •

*l • • • •

J • • • •

Date

7/31/57

8/15/57to9/ 5/579/12/57

10/ 3/57

10/ 8/57

10/15/57

10/17/57

Cementation

No cementation.

Rock and concretecontacts ceraentatedat 3,000 psi.Rock surrounding theplug ceraentated at6,000 psi.Leaks sealed bycementation.

Further cementationto seal leaks.

Footwall leakcementated.

Pressure,psi75;200;310.

650-1,750

1,800;2,500.

4,300

5,700

6,200

6,800

Remarks

Heavy leakage on rock and concretecontacts particularly at hanging;tappings on hanging contact closedoff to build up pressure >75 psi.Leakage on the rock and concretecontacts reduced; leakage pastplug 50 gal/h at 1,750 psi.Total leakage past plug 156 gal/hat 1,800 psi; 300 gal/h at 2,500psi.

An old diamond drill hole began toleak at 3,000 psi; leakage 128gal/h at 4,300 psi; leakagestopped when pressure was reducedto 2,000 psi.Leakage not measured; pipe sleevecorrugated with crests of corru-gations 15 in, 27 in, and 37 infrom the door face.Leakage in footwall of the drive~400 gal/h; no further distortionof the pipe sleeve was apparent.Leakages in footwall and hangingof main drive; pressure could notbe raised further.

Source: W. S. Garrett and L. T. Campbell Pitt (8).

2 /JAWW

/ Waterchamber

PLAN, sectional2" 12" 12" l2Vl2l/2»

,»- Mild steel tubeAT with welded flanges

ELEVATION, general arrangement SECTION A-AFIGURE 2. • Dimensions of experimental, plug. Adapted from W. S. Garrett and L. T. Campbell Pitt

(8).• ' ' • • •

TABLE 2. - Parallel plugs of the Witwatersrand and Orange Free State goldfields for which there are records of loads applied of over 1,000 psi

Mine and location

Free State Geduld: No. 2 Shaft..West Driefontein:

8 and 12 levels. ...............

Virginia-Merrie:

Dimensions,1 ftH

47

41313.5

12.2512.25

W

11

41010

1111

L

100

7.641.663

3612

Pressure,psi

2,250

6,8001,8271,650

1,3401,340

*s.psi

100

88512475

108324

fe.psi

200

1,790248150

216648

Pressuregradient,psi/ft

22.5

887.043.526.2

39.2111.7

'H - height; W » width; L - length.Source: W. S. Garrett and L. T. Campbell Pitt (9).

design formulas. Note the high values of located near Alientown, PA. This precau-fs and fc for the West Driefontein exper- tionary plug separates the main shaftimental plug, the plug that formed the from the stopes. In the event of an in-basis for their design assumptions. undation, the watertight doors are closedA plug similar to those constructed in to prevent the main shaft from flooding.7

South Africa is shown in figure 3. Thisparticular plug is situated on the 1,200- _____________________________________

level of the Friendensville Zinc Mine 'For more information on the Friendens-(owned by Gulf and Western Industries), ville Mine, see Cox (10).

AR303632

10

FIGURE 3. - Plug constructed on the 1,200-ft level of the Friendensville Zinc Mine.

The Relation of Water Leakage p » hydrostatic pressure, psi,to Plug Length

1 = length of plug, ft.The effectiveness of a plug to impound

water depends on the ability to minimize According to the tests on the experi-water leakage. Water can leak past a mental plug in table 1, the limiting val-plug in several ways: along the plug- ue of the pressure gradient was achievedrock interface; through the cracks, frac- on four occasions when leakage became ex-tures, and fissures in the rock surround- tensive and obvious:ing the plug; or through the concrete ofthe plug itself. These three modes of 1. Before grouting of the plug-rockwater leakage are dependent upon the interface when hydrostatic pressurelength of the plug and the resistance of reached 75 psi, the pressure gradient wasthe rock to the permeation of water. 75 psi/7.67 ft • 9.8 psi/ft.Garrett and Campbell Pitt felt that theleakage aspects of the rock could be used 2. After grouting the plug-rock inter-as criteria for plug design. They ex- face but before grouting the rock, v/henpressed this leakage as a pressure gradi- the hydrostatic pressure reached 1,750ent in the rock: psi the pressure gradient was 1,750 psi/

7.67 = 228 psi/ft.p.g. = P/l (6)

3. After grouting the rock to 6,000where p.g. = pressure gradient, psi/ft, psi with cement, an approximate

11hydrostatic pressure of 3,000 psi was ex- QQrted. The pressure gradient was ±3,000

"psi/7.67 ft « ±400 psi/ft. 70

4. After extensive chemical grout in- cjections in the rock, the hydrostatic _pressure rose to 6,800 psi and the pres- "* _sure gradient was 6,800 psi/7.67 ft - 887 P-

_ A

L L I

The pressure gradients calculated oabove are unique to the particular rock 3 ^(quartzite) in which the experimental £.plug was constructed, but the theory of 20using a safe pressure gradient as designcriterion offers a valuable means of tak- 10ing into account the important factor ofthe ground surrounding the plug. Garrett wand Campbell Pitt believed that values 2 4 6 8 10 12 14 16for minimum and maximum pressure gradi- WATERHEAD (H), I03 ftents could be established experimentally CI/-IIOCT . , , , . ,for various rocks. Also, they recora- F|GURE 4. - Length of plugs based on ultimate pres-mended that plugs designed accordingly sure, «;adient V°!UV, A' MiniT length ,of pluf thatshould have a leakage factor of safety would be required if the contact between plug and rockof at least 4 and as much as 10 in some ''« pouted. No factor of safety; B minimum lengthcases, depending on conditions such as when the contact is grouted but before the rock is grouted.-ractures in the rock after mining and ^ factor of safety; C, minimum lengthwhennormalgrout.>e subsequent redistribution of stress, ing of rock was at 6,000 psi. No factor of safety. (4C,porosity of the rock, and its acceptance '-9., means 4 x C.); D, similarto C, but w.th the additionof grout of chemicals to seal rock fissures. C isthenapplicableFigure'4 shows the minimum length of to a normally grouted plug but with no factor of safety.

plugs based on ultimate pressure gradient Ad°Pted *'™ W- s- Garrett and L. T. Campbell P,tt (8).values obtained from the tests on the ex-perimental plug. Also given are curvesfor plugs with safety factors of 4, 6, 8, . was performed in the East, as part ofand 10 with the provision that the sur- Federal and State acid mine drainage re-rounding rock is grouted to at least the search and abatement programs (11, 13)«same pressure which the plug is designed . . " , . .to resist. Single Bulkhead Seals

SINGLE AND DOUBLE BULKHEAD SEALS :<_1_1-_13) Single bulkhead seals are usually con-structed from concrete, grouted aggre-

Single and double bulkheads seals, also gate, or concrete block. They are corn-known as hydraulic seals, are. commonly monly used to seal off abandoned minesused to permanently seal abandoned drift from active workings, and such seals haveand slope mines and protect the environ- been documented to withstand water pres-ment from the undesirable effects of acid sure as high as 70 psi (161.5 ft of•nine drainage. Historically, thickness waterhead) (H). In many instances,and anchorage requirements for these plate theory is used in their design,6seals have been derived from experience or minimum thicknesses and anchorageand are based on the immediate ground _________________________________________conditions and the amount of water to be 8Thick plate design, discussed.earliermpounded. Various types of hydraulic in this report, is applicable for deter-

v_>eals have been demonstrated in the mining thickness and anchorage require-United States. Most of this sealing work ments for single bulkhead seals.

6R30363I*

12

requirements are derived from research on Not to scaleexplosionproof bulkheads. The Bureaurecommends that a bulkhead must withstanda dynamic pressure of at least 50 psi forit to be explosionproof O_).To learn more about the application of

explosionproof designs for impoundingwater, the Bureau hydrostatically testeda concrete block bulkhead commonly usedin coal mines to resist explosion. Thebulkhead is 16 in thick with the blocklaid in a transverse pattern and a pi-laster at center span for additional sup-port. It withstood 50 psi of water pres-sure before tests were stopped owing towater leakage through the bulkhead struc-ture. Details of this research are pre-sented in appendix A.There are numerous problems associated

with the impoundment of water by singlebulkhead seals. The long-term effective-ness of these seals is questionable evenunder low pressure, because of their rel-atively short thicknesses. When large cirnoc c n LI L n L j i L •hydrostatic pressures are anticipated, FIGURE 5. - Double bulkhead seal show.ng re-double bulkhead seals are considered more taming bulkheads and concrete center.effective.

thickness, but the South African plugDouble Bulkhead Seals design may be applicable in this situa-

tion. Since no trenching is required forThe double bulkhead seal is constructed the center seal, the anchorage depends

by placing two retaining bulkheads in a on the strength of the concrete bearingmine entry and then placing an imperme- against the irregularities in the rockable seal in the space between them (fig. surface. However, steel rods can be5). The front and rear bulkheads pro- grouted into the roof, ribs, and floorvide a form for the center seal which is surrounding the center seal to provideformed by placing concrete or injecting additional shear strength.grout into a preplaced aggregate. The The double bulkhead method of sealingretaining bulkheads are constructed from mine entries has been successfully demon-concrete, concrete block, or brick and strated at Moraine State Park, Butlershould be sufficiently thick and well an- County, PA, under the State's "Operationchored to hold the center seal in place Scarlift" reclamation program. Thesewhile it is poured and as it cures. seals were placed in inaccessible mineThe required thickness for double bulk- entries through vertical boreholes

head seals range from 10 to 45 ft de- drilled from the surface. A total of 69pending on the ground conditions, the seals were installed ranging in thicknessstrength of the concrete, and the amount from 17 to 40 ft. The seals were con-of water to be impounded. There are no structed from a fly ash, sand, gravel,recommended designs for calculating this and cement mixture (12).

CONCRETE SPECIFICATIONS AND PLACEMENT METHODS

SPECIFICATIONS upon the method of construction, thethickness of the bulkhead, the contents

Concrete for constructing bulkheads can and proportions of the mix and curingachieve different strengths depending time.

AR303635

13

For explosionproof bulkheads the Bureau allowable shear or corapressive strengthsrecommends (JU j*_~5) a mix, by volume, ,of of the concrete or rock be used. In most1 part type 1 Portland9 cement, 2 parts cases, the allowable shear and compres-clean sand, and 4 parts clean gravel. sive strengths of coal and other strati-Only enough water should be added to make fled rock will be less than those forthe mix homogeneous and give a stiffness concrete and should be used in these de-consistency that will enable it to be sign equations to assure adequate marginsproperly placed in the form. Overwa- of safety.tering must be avoided, for it reduces During the curing of a large mass ofthe strength of the concrete. The Bu- confined concrete, such as a plug, crack-reau contracted a professional engineer- ing and shrinkage can occur. For thising laboratory to conduct strength tests reason, prolonged, thorough .. curing is aon the above mix. Test cylinders were significant factor in attaining imperme-prepared, and after curing for 28 days able watertight .concrete. Cracking isthe samples were tested for ultimate com- usually caused by high heat of hydrationpressive and tensile strengths. Shear generated during curing. This weakensstrength was calculated by using an equa- the concrete and may affect its abilitytion developed from Mohr's Circle which to resist design pressure. Shrinkage canrelates compressive, tensile, and shear affect anchorage and is a result of ex-strength of the sample. The strength cessive water content or inadequate ag-values are given in table 3. gregate composition. Some shrinkage is

inevitable in concrete, and pressureTABLE 3. - Ultimate compressive, grouting is necessary to improve contacttensile, and shear strengths between the bulkhead and surroundingfor 1:2:4 concrete mix rock. The addition of pozzolans, such

as fly ash, to concrete can improve work-

Strength, psi:Compressive...Tensile.......Shear.........

Slump, in.......

Test 1

2,933233NApNAp

Test 2

3,074255NApNAp

Average ability, reduce heat of hydration andshrinkage, and increase resistance to

3,004 sulfates contained in water. However,244 caution must be exercised in the selec-766 tion of pozzolans, because their proper-1.5 ties vary widely and excessive amounts

NAp Not applicable. may have adverse effects on the concrete,such as increased shrinkage and reduced

Equations 1-5 require using the allow- strength and durability (14), Before se-able flexural, shear, and compressive lecting a mix, trial mixes should bestrength values of the construction made, especially when using admixturesmaterial so as to provide adequate mar- and pozzolans.gins of safety. As a rule of thumb, al- To attain concrete with specific prop-lowable strength values range from 20 to erties, other types of Portland cement30 pet of the ultimate strength. Accord- can be used. Type II is for general use,ing to Garrett and Campbell Pitt (9), more specifically when moderate sulfateconcrete of great strength is not impor- resistance and heat of hydration are de-tant. If 2,500 psi ultimate compressive sired. Type IV gives low heat of hydra-strength is obtained after 28 days, the tion, and Type V is used when high sul-safety factor is >4. They used 600 psi fate resistance is desired. Standardas the allowable compressive strength specifications for Portland cement are(fc) for concrete plugs designed accord- given in ASTM Designation C150, Part 14.ing to equation 5. In addition, plug Standard specifications for fly ash anddesign equations 4 and 5 require that the raw or calcined natural pozzolans for use

as mineral admixtures to cement concreteReference to specific equipment (or are given in ASTM Designation C618, Part

trade names or manufacturers) does not 14,imply endorsement by the Bureau of Mines.

AR303636

14

PLACEMENT side from which the concrete is placed), ,the form boards should be about half s>-•'

There are several commonly used methods height with the remainder of the boardsfor placing concrete for bulkheads in ac- cut and readily available for insertioncessible mine entries. When constructing as needed.only one or two bulkheads, concrete can Before placing the concrete, any waterbe mixed underground by hand or machine in the floor trench must be removed. Theand placed in the forms either manual- concrete is placed in successive horizon-ly or with concrete pumps (fig. 6). If tal layers, with care taken to fill theseveral larger bulkheads are to be con- rib recesses completely. There must bestructed, concrete is usually placed no delay greater than 30 rain between mix-from the surface through a vertical bore- ing and placing the concrete for the en-hole to a central underground site where tire bulkhead, because greater delaysa slurry-distributor is located. The cause cold joints that could weaken theslurry-distributor remixes the wet con- structure. However, if unavoidable de-crete and pumps it to the individual lays are foreseen, steel reinforcingbulkhead sites (fig. 7). Another varia- rods, of at least 7/8-in diam and 16 intion of this method requires the drilling long, should be installed vertically inof one or several vertical boreholes di- the last layer about 18 in apart and pro-rectly to the entry where the bulkhead(s) jecting upward 8 in or more. The surfaceare to be placed. This method is used of the cold joint should be cleaned be-when several large plugs or double bulk- fore placing fresh concrete. As the roofhead seals are needed. Retaining bulk- is approached, placing the concrete be-heads or rigid wood forms are then built comes more difficult but cannot be ne-between the boreholes, and concrete is glected. Concrete should be worked wellthen introduced directly from the surface into the roof cavity and also around thethrough the boreholes to the space be- reinforcing rods extending from the roof vVtween the forms (fig. 8). and into recesses in both coal ribs.When placing large volumes of concrete, Forms should not be removed for at

such as a plug, the coarse aggregate may least 4 to 7 days after the concrete isseparate from the mix and settle, causing placed. Noticeable voids on the out-the concrete to lose its strength charac- by side of the bulkhead can be filledteristics upon curing. To avoid this and with stiff concrete or a cement gun, ifalso to improve concrete workability, the available.coarse aggregate can be preplaced between For bulkheads over 3 ft thick, build-the retaining bulkheads or forms and con- ing retaining bulkheads for forms is thecrete grout injected into the aggregate. best approach. Retaining bulkheads canThe aggregate can be preplaced by hand or be constructed from concrete, concretestowed pneumatically.10 block, or brick, but the latter two areFor bulkheads less than 3 ft thick (\), preferred to forego the need of con-

concrete can be mixed underground by hand structing wood forms for the concrete.or machine and forms constructed from Mortar for block and brick must be prop-lumber. Plywood forms should be at least erly mixed to ensure good bonding.3/4 in thick and support boards 2 in by 4 Concrete for the center plug can bein. Boards must be properly spaced so mixed and placed underground by machinethe forms can resist the hydraulic head or from the surface through verticalof the concrete as it is placed and borehole(s). Hand mixing and placingcures. The ends of the forms should be should not be attempted, owing to theflush with the edges of the trenches in large volumes involved. As mentionedthe roof, ribs, and floor and should not earlier, the coarse aggregate can be pre-extend Into them. On the outby side (the placed between the retaining bulkheads_______________________________ and concrete grout injected through the

10Details for building seals and plac- outby side of the bulkhead or from the v^Xing aggregate by pneumatic stowing are surface through vertical boreholes.provided by Maksimovic (15-16).

AR303637

15

Plywood, -in minimumthickness

Not to scale J ,Steel reinforcing rods-*FIGURE 6. • Concrete being mixed underground and placed in forms by concrete pump.

Bulkheocin place

__ __ _ Not to scaleNot to scale

FIGURE 7.• Pumping concrete from the surface • FIGURES.* Pumping concrete from the surfacethrough a single vertical borehole to a central on- •• . through a vertical borehole directly to bulkheadderground site. , site.

PRESSURE GROUTING (IT)

Pressure grouting involves the injec- surrounding inundation bulkheads is ation of fluid materials, under pres- most significant factor in their long-sure, into rock or soil to fill pore terra performance, because it reducesspaces, consolidate material, and prevent strata permeability and increaseswater migration. Grouting the strata strength and durability with respect to

AR303638

16

aqueous solutions. There are many fac- III is used when early strength gainstors to consider in planning a grouting required within 10 days or less. It alsoprogram, such as the drilling, spacing, has a finer grind, which improves its in-and depth of holes; the proper selection jectability. Type IV generates less heatof grouting materials and equipment; the than type II and develops strength at acontrol of grout volumes and injection very slow rate. It is rarely used inpressures; and a knowledge of the strata grouting. Type V has a high resistanceto be grouted. Pressure grouting is a to sulfates and is used when groundwaterhighly specialized technique requiring with extremely high sulfate content isexperience and sound engineering judg- encountered.ment, so that procuring the services of Mineral admixtures are finely dividedqualified personnel is essential. materials that are added to neat cementIn some respect, pressure grouting is grout to improve or achieve a specific

an art, for which the establishment of characteristic. Calcium chloride, sodiumrigid rules and procedures is not fea- silicate, and gypsum, when used in smallsible. However, a knowledge of basic amounts (2 to 4 wt pet), act as accelera-grouting materials and methods is tors and decrease the setting time of therecommended. grout. Accelerators are used when there

is little heat to aid in setting. TheyGROUTING MATERIALS may also be used to reduce grout migra-

tion, reduce erosion of new grout byAn important factor in the successful groundwater, and increase the rate of

grouting of permeable coals and other early strength gains. When high tempera-stratified rock is the selection of a tures are encountered, retarders such assuitable grouting material. There are sodium chloride and calcium lignosulfo-four basic types of grout: Portland ce- nate are used to increase setting time.ment, asphalt, clay, and chemical grouts. These admixtures allow the grout to mi-Technical literature and field experience grate properly into fine pore spaces be-show that Portland cement and chemical fore setting.grouts are the most applicable and effec- Fly ash and natural pozzolans such astive for grouting coal mine strata. diatomite and pumicite are admixtures

that when used in small amounts improvePortland Cement Grout the pumpability of the mix. They may al-

so be used as a filler and can compose upPortland cement is the most widely used to 30 wt pet of the mix. In this case,

grouting material because of cost, avail- they react chemically with the cement toability, and everyday knowledge of the produce cement itious properties and im-material. Neat cement grout consists of proved bonding. Other admixtures includePortland cement and water, but mineral bentonite, which is used to increaseadmixtures are often used with this base water requirements and reduce the unitto attain grouts with specific character- weight of the mix; latex additives, whichistics. There are five types of Portland improve bonding and increase grout re-cement (excluding air-entrailed cements) sistance to acids and other corrosivethat conform to ASTM Designation C150 and solutions; and aluminum powder, which in-can be used for cement grouts. Each type creases viscosity and causes the grout topossesses specific properties that may be expand slightly.needed to meet job requirements. Water-to-cement ratios for PortlandType I is a general-purpose cement cement grouts are indicated by either

suitable for most grouting jobs. It is weight or volume. The volume method isused when the special properties of the more convenient and most frequently usedother four types are not needed. Type II in the field. A sack of cement is con-has moderate resistance to the sulfates sidered to equal 1 ft3. The mixingin groundwater. With type II the heat ratios of water to cement used mostof hydration is less and develops at a frequently range from 1:1 to 4:1. Theslower rate than that of type I. Type choice of a .starting mix depends on such

AR303639

17

factors as the size and amount of pore investigation is done, the size and con-spaces in the strata, the amount of wa- tinuity of pore space in the strata willter the strata bears and experience with remain relatively unknown. The art ofgrouting similar strata. In general, successful grouting requires the abilitygrouting is started with a thin mix. to treat these unknowns through experl-Thicker mixes are used based on the abil- ence obtained from similar grouting work.ity of the strata to accept the grout. Three basic grouting methods are used ln-If the strata accepts the starting mix mine to seal and consolidate permeablereadily without pressure buildup, thicker strata surrounding a bulkhead: curtainmixes are considered in accordance with grouting, blanket grouting, and contactthe objectives of the grouting program. grouting.

Curtain grouting involves the construe-Chemical Grouts tlon of a curtain or barrier of grout by

drilling and grouting a linear sequenceIn recent years, the use of chemical of holes. Its primary purpose Is to re-

grouts to consolidate permeable rock and duce strata permeability. A grout cur-soil has gained increased popularity. tain can consist of a single row of holesThe primary advantages of chemical grouts or two or more parallel rows. "Primary"over Portland cement grouts are their irn- holes are initially drilled into theproved bonding characteristics, low vis- roof, ribs, or floor on rather widelycosity, better flowability, and good con- spaced centers ranging from 20 to AO ft.trol of setting time. Some chemical After the two primary holes have beengrouts are water reactive and expand grouted, a first intermediate hole Isslightly by contact with water, a feature drilled midway between them. After thisthat is advantageous in sealing fine pore hole is grouted, two secondary Interme-spaces in the rock. Some disadvantages diate holes are drilled midway betweeninclude possible toxicity, so that they the primary and first Intermediate hole.may not meet MSHA permissibility stan- This pattern of drilling and groutingdards for use underground. Also, they continues until grout consumption indi-are relatively higher in cost than Port1- cates the strata to be sufficientlyland cement grouts. > tight. Grout consumption should decreaseResearch and development is continuing as the spacing of intermediate holes be-

at a rapid pace, and currently a number come smaller.of commercial manufacturers produce chem- The hole depth for curtain grouting de-ical grouts and .injection equipment. pends on the flowability of the grout,Most grouts consist of two or more compo- the ability of the strata to acceptnents that must be mixed before injec- grout, and the distance the grout musttion. Because of the critical nature of migrate to create a satisfactory seal.proportioning, this mixing should only be Generally, the primary holes are thedone under the supervision of company ' deepest, with intermediate holes beingpersonnel. It is beyond the scope of drilled less deep with each successivethis report to review all commercial grouting.grouts currently available. If one is - Blanket grouting involves the injec-considering the use of a chemical grout, tlon of grout, under low pressures, intothe best approach is to consult directly shallow holes drilled on a grid pattern.with a company that has a proven grouting Its primary purpose is to Increase thetechnique. bearing strength of the strata. Blanket

grouting may be used to form a grout capGROUTING METHODS prior to curtain grouting and serve as a

; barrier to improve the migration of hlgh-Procedures for grouting permeable er pressure grout into deeper horizons,

strata vary, as dictated by the charac- but it is more commonly used to consol-teristics of the strata and the program idate fractured or severely weatheredobjectives. Regardless of how much ex- strata in a mine entry prior to bulk-ploratory drilling and other pregrouting head constftiptljojrl.Q OTUfj grouting method

18

strengthens the strata and provides water seepage along this concrete-stratabearing support when constructing a large interface. Over the long term, it mini-plug or excavating trenches to recess a mizes the premature failure of bulkheadbulkhead. Holes are drilled on 5- to 7- anchorage.ft centers and are shallow, 3 to 5 ft Holes for contact grouting are usuallydeep. Severely fractured strata may re- provided for by placing steel pipe orquire the holes to be drilled on tighter packers at predetermined locations alongspacings of 1 to 3 ft. the concrete-strata interface before theContact grouting involves the grouting concrete is poured. The pipes, which

of the voids between the roof, ribs, and protrude from the forms, act as a travel-floor of the entry and the bulkhead or way for the grout after the concreteplug. These voids result primarily from cures. At times, during the pouring ofimproper concrete placement and concrete the plug, the pipe may fill with concreteshrinkage while curing. This is consid- which must then be drilled out so thatered a most important grouting procedure, grout can migrate properly along thebecause it improves bonding and prevents interface.

BARRIER PILLARS

PILLAR CONSIDERATIONS head and the amount of water impounded?Third, what is the physical condition of

Bulkheads can be designed to withstand the barrier? Fourth, if the barrier werea considerable amount of hydrostatic to fail, is there sufficient time to warnpressure, but the seal is only a small and evacuate workers (18)?part of the water impoundment. The pe- In most cases, before an undergrourrimeter of the abandoned area, consisting impoundment is created by constructing_Jof chain or barrier pillars, forms a bulkheads, the width of the coal pillarslarge part of the impoundment and at is known. Determinations must then betimes may not be capable of withstanding made as to the limits of hydrostaticthe design pressure. Practical limits of pressure that the pillars can withstand.inundation can be determined by plottingon a coal contour map the expected mine PILLAR WIDTH FORMULAS (18-21)pool elevations and corresponding groundsurface elevations. Areas of excessive Determining whether an existing coalpressure are projected, and determina- pillar is sufficiently wide to resist ations are made as to the capability of specific waterhead is a complex problemthe coal pillars to withstand the antici- that cannot be solved with a high degreepated water pressure. of certainty. However, several formulas,There are no specific Federal regula- based on experience and empirical obser-

tions concerning the size of barrier pil- vation, have been developed for thislars that separate active mines from purpose.inundated abandoned areas. In general, The first is the Ashley, or Mine .In-there are two ways MSHA handles potential spector, Formula, established by a seven-inundations involving barrier pillars. member commission for the Commonwealth ofTheir first concern is whether the situa- Pennsylvania for incorporation into Statetion presents an imminent danger to the law. The primary objective of the corn-mine and the workers. Second, if no im- mission was to develop a method ofminent danger exists, is whether proper _______________________________procedures (such as drilling, etc.) are 11Section 107(a) of the 1977 Act coversbeing followed when mining toward or the imminent danger situation and Sec-adjacent to impounded water.11 A number tions 75.1200, 1200-1, 1200-2, 120of questions are considered to determine 1202, 1202-1, 1203, 1204, and 1701if an imminent danger exists. First, Title 30, Code of Federal Regulations,does the coal barrier have an adequate outline the criteria governing MSHA's en-width? Second, what is the hydrostatic forcement activiflias ..^ f- . .

Hn 0 Uob 4|

19

designing coal barriers to impound water T - average thickness of theand protect active mines from unexpected coal seam, ft,inundations. From the findings of thecommission, the minimum width of the bar- W2 - the estimated convergence onrier is expressed as the high stress side of the

pillar, mm,W » 20+4T+0.1D (7)

I - the base of the natural sys-where W - Width of the coal pillar, ft, tern of logarithm (2.72),

T - average thickness of the coal 5 - a constant which includes aseam, ft, factor to convert metric to

English units and a safetyand D - depth of overburden or the factor;

height of waterhead, ft.and 0.09 - a coefficient if caving fol-

Knowing W and T, the maximum waterhead lowing mining is permitted.that a barrier can withstand (D) can bedetermined. Holland suggests that the convergenceA second formula, developed in England factor, W2, be estimated as a function of

through observation and measurement, is overburden depth according tobased on the pressure arch concept ofstress distribution. Here the width of W2 - 10 *0.0012D QQ)the barrier is presumed to be

where W2 - Convergence, mm,W - 0.15D+60 (8)

A - 2.72,where W - Width of the coal pillar, ft, . • ...

and D » depth of overburden, ft.and D * depth of overburden, ft.

This relationship gives the convergenceThis formula does not take into account at various depths for a coal bed 7 ft

the thickness of the seam and consequent- thick and having a strength of 3,000 psily may be unsatisfactory for water im- in a 3-in cube.12 It should be notedpoundments. Data and field experience ' that under the assumed conditions, thepresented during the development of the Holland Formula will give wider barriersAshley Formula indicate that seam thick- than the Ashley Formula at all depths.ness is an important factor in the design The actual safety factor associatedof coal barriers. With all other factors with these formulas cannot be readily de-being equal, a thick seam requires a termined because of the many elementsthicker barrier than a thin seam. that have an unknown effect on the barri-The third formula, developed by C. T. er. These include stress redistribution

Holland (20), is the least used of the after mining, subsidence, geologic fea-three. It has been stated that this'for- tures such as slips and faults, the long-mula is not suitable for computing water term effects of water seepage, pore waterdams, although it has been compared with pressure, ground saturation, and dete-the two previous formulas (21). The rioration and the favorable aspects ofwidth of the barrier is given as the pressure grouting. Since no one formulagreater of is universally applicable, mine personnel

u T«;T f - *°& ^ W2 ^ /Q\ must exercise sound engineering judgmentw * 15T or 5 .o.09 log I ) "' ______________________________

12To estimate convergence for coalswhere W « Width of the coal pillar, having different seam thickness and

ft, strengths, see Holland (20).

AR3036l*2

20

in determining the practical limits impoundments should be based on the max-of potential inundation. The design iraum hydrostatic pressure that a coalof bulkheads for underground water barrier can withstand.

MONITORING WATER PRESSURE

The hydrostatic pressure in the inun- transmitting recorders. The floats comedated area should be monitored. There in various sizes, with a 3-in-diam floatare two basic methods to accomplish this: presently being the smallest. Because(1) from vertical boreholes drilled from the float must be counterweighted and thethe surface to the inundated area, and excess line must hang in the hole, the(2) in-mine, through piping cast into the smallest practical hole size is 4 in ID,bulkheads themselves. The first method although larger sizes are recommended toinvolves the drilling of a vertical bore- allow the float and counterweight to passhole from the surface to the inundated each other without interference. Thearea and measuring the height or pressure float-beaded line systems have been usedof a column of water with a water-level in holes as deep as 650 ft, with floatindicator or pressure transmitter. These depths up to 350 ft, with no difficulty.instruments are commercially available Still another method of monitoringfrom manufacturers of geophysical and hy- pressure from vertical holes is to in-drological instrumentation. stall strain-gauge type pressure trans-One type consists of a detection meter mitters. Pressure transmitters capable

and a water-sensitive electrode attached of operation in water at depths as greatto 300 to 500 ft of electrical cable num- as 5,000 ft are readily available. Al-bered in 1- to 5-ft intervals. The level though the difficulties of finding drift-of the water below the surface is mea- free transducers for long-term installa-sured by lowering the electrode down the tion and connecting them to a cable areborehole until a sharp needle deflection not trivial, a few manufacturers haveon the meter indicates that water is con- partial systems available. Pressuretacted. The approximate waterhead is de- transmitter systems are also availabletermined by subtracting this distance from companies serving oilfield needs.from the total depth to the coalbed where Most of these are strain-gauge type sys-the bulkhead(s) is(are) located. To con- terns, although one company makes a systemvert waterhead expressed in feet to hy- which uses a small-diameter tube and adrostatic pressure in pounds per square gas-filled chamber at the bottom of theinch, multiply by 0.434. hole with the pressure transducer at theAnother system uses a float to measure surface. Both the strain-gauge systems

the water column in the monitor wells. and the gas chamber system are capable ofThe float is connected to a steel wire operating under high pressures. However,upon which are crimped brass beads at these systems are very expensive and many6-in intervals, to prevent line slippage. are available for rental only, and on aThe line wraps around a measuring wheel short-term basis.and an idler pulley, and the wheel move- A second and more direct method of mon-ment drives a depth recorder. On the itoring hydrostatic pressure involvesolder mechanical recorders, the wheel casting a pipe into the bulkhead and in-drove a pen directly, and a spring or stalling a pressure gauge or pressureelectric motor ran a time drive. The transducer. Pipes can be either plasticspring drives can operate for up to 6 or metal, though metal pipes must be cor-months without winding, and the chart pa- rosion resistant. Care must also be tak-per comes in rolls good for up to 2 yr, en if plastic pipe is used to insure thatboth depending upon drive speed. Newer the pipe will be able to withstand therecorders have been especially designed maximum anticipated hydrostatic pressuresfor telephone transfer of the data, and (this of course applies to all valves and ;|an add-on device is available to con- fittings). Pipes should be installed 6vert the older mechanical recorders to to 12 in from the floor and 12 to 18 In'1

AR303643

21

from the rib having the lowest elevation. types) and titanium. They can be madeThe pipe should have a 1- to 2-in ID and with a number of thread types so that theextend 2 to 3 ft from either'side of the gauges can be screwed directly into abulkhead. Both ends of the pipe must be pipe cast into the bulkhead.threaded to permit the installation of a The pressure gauges and pressure trans-piezometer installed on the inby side of ducers must be chosen such that theirthe bulkhead (water side) and a pressure range is greater than the maximum hydro-gauge or pressure transducer and valve on static pressure that can develop behindthe outby side. The piezometer allows the bulkhead; they, should be calibratedwater to pass into the pipe, but traps before installation. A valve should alsosediments that can clog the pressure be included in the line between the bulk-gauge and affect readings. Piezometers head and any pressure transducer or gaugecan be handmade from a porous, fine- to allow their removal for replacement orgrained material such as sandstone, but a calibration. Usually the valve is keptmore preferable type is constructed from shut for direct reading gauges, exceptporous polyethylene. This type is avail- when the gauges are being read. Whenable commercially; it is lightweight and readings are made the valve should berelatively inexpensive.13 opened slowly to prevent shock damage to

It may often be desired to allow remote the gauge.reading of pressures from behind the Finally, it may be necessary in somebulkheads. Strain-gauge pressure trans- cases to have a warning system, should anducers for this use are readily avail- inundation occur by bulkhead or barrierable. Unlike the case of the vertical pillar failure. One method of accom-boreholes where the transducer (or trans- plishing this is to install remote read-mi tter) case and cable connector must ing water-level warning devices in thewithstand hydrostatic pressure and be areas of interest. A number of on-offleakproof, the in-mine transducers need type sensors are available for this use.only be intrinsically safe and be air- Most of these are float-level switchestight to prevent dust entry. Many pres- and indicators. These devices come in asure transducers are especially made for wide variety of configurations and poweruse in hazardous environments. ranges. Many of them are also designedTwo types of transducers are available. to actuate equipment, such as 'alarms,

Both may be obtained in the same sizes, pumps, or motors. The problem with theseand both require input voltages in the devices is that most have moving partsrange of 6 to 60 V dc. The first type that may be frozen by dust or corrosion.has a constant current output, usually in Another device, which has no movingthe range of 16 to 20 mA, and a fluctuat- parts, is the capacitive proximity sen-ing voltage in the range of 0.5 to 5.0 V. sor. These sensors are only suitable fordc. The second type maintains a constant sensing, in a yes-no fashion, the pres-voltage, and the output signal is a ence or absence, of water, but they re--changing current, usually varying between quire little power and have no movingA and 20 mA. The first type allows data parts. Typical capacitive proximity sen-transmission over a greater distance but sors operate on 10 to 12 V dc at currentsrequires higher power, and the second of 5 to 20 mA. They are not actuated bytype is more likely to be available in a moisture film on the sensor head andintrinsically safe models. are not affected by moderate quantitiesThe cases of the pressure transducers of dust.

come in a variety of materials, the most To insure the safety of the mine andcommon being stainless steel (several the workers, hydrostatic pressure must«—r-j.—___-- - _______ •'•-'•' not exceed the capacity of the bulk-

For more information regarding poly- head(s) or coal barrier(s). If dangerousej*ylene piezometers, see "Installation pressures are suspected, ways of reducing° piPin9" in appendix A. excessive pressure should be implemented.

22

This can be accomplished by pumping the the area behind the bulkhead into sumpswater out from the surface or draining in the active working areas.

OBTAINING MSUA APPROVAL

Creating an underground water impound- and mine operators should familiarizement by constructing bulkheads requires themselves with the State law. Initial-the approval of the MSHA District Mana- ly, the mine operator is required to pre-ger. Federal regulations governing min- pare detailed plans on bulkhead design,ing activities near water bodies, the construction, location, etc., and submitconstruction and inspection of bulkheads, them to the appropriate MS11A district of-the width of coal barriers needed, and fice for approval. If disapproved, areasMSHA's enforcement role are covered in of insufficiency are defined and the op-Title 30, CFR, Part 75. State regula- erator must make changes as recommended.tions may be more specific and stringent,

DISCUSSION

The practice of constructing bulkheads conservative strength values for rock andin underground coal mines for the purpose concrete in the design equations. It isof impounding water will become increas- not an uncommon practice to increase re-ing ly important in future years. While quired bulkhead thickness by a factor ofmost of the practical experience of de- 1.5 to 2, because a large safety factorsigning and constructing bulkheads has far outweighs most criteria in the theorybeen in hard-rock mining, it can be con- of design.eluded that no stringent guidelines or The success of any underground impound-theoretical design criteria have been ment depends on the ability of the entirewidely accepted. Bulkhead design can dam structure, consisting of the bulk-differ depending on the condition of the heads and barrier pillars, to withstandsurrounding rock and the anticipated hy- the anticipated hydrostatic pressure. Indrostatic pressure. Over-design is a some instances, the barrier pillars maycommon practice. Bulkheads are usually form the weakest link in the impoundmentconstructed sufficiently thick to resist and therefore, dictate the feasibilitythe force of hydrostatic pressure and the and practical limits of potential inunda-surrounding strata pressure grouted to tion. This is an important considerationminimize water seepage. because with time the barrier pillar canLittle reference has been made in this be just as prone to failure as the bulk- .

report to the safety factors associated head. The deterioration of coal barriers'with the designs discussed. Such safety and surrounding roof and floor strata bjfactors are difficult to assess because water seepage can be minimized througlof the many variables that can affect pressure grouting.bulkhead design. These include the maxi- Creating an underground water impound-mum waterhead the bulkhead is designed ment also creates the potential forto withstand; the type of anchorage; the inundation hazard. Mine personnel muststrength and condition of the anchoring exercise sound engineering judgment Itstrata; the existence of geologic anoma- design. To insure the safety of tllies in the immediate area; the favorable workers and the mine, water levels shouldaspects of pressure grouting; and the be monitored and controlled, groundmaximum pressure that the barrier pillars ments near the impoundment areacan safely withstand. However, accept- lized with supplemental supports, and theable margins of safety can be made in- bulkheads and barrier pillars inspect*herent to these designs if one selects regularly.

23

REFERENCES

1. Mitchell, D. W. Explosion-Proof 1961). S. Afr. Inst. Min. and Metall.,Bulkheads, Present Practices. BuMines RI 1961, v. 3, pp. 1283-1302.7581, 1971, 16 pp.

10. Cox, K. R. Hydrological Problems2. Lancaster, F. H. Research Into Un- and Their Resolution at the Friendens-

derground Plugs Transvaal and Orange Free ville Mine. Paper in Mine Drainage, ed.State Chamber of Mines, Res. Rep. 27/64, by G. 0. Argall and C. 0. Brawer (Proc.Aug. 1964, 130 pp. 1st Int. Mine Drainage Symp., Denver, CO,

May 15-18, 1979). Miller-Freeman, 1979,pp. 3-13.

3. Maser, K. R., D. S. Kingery, andD. Hoadley. Design of Reusable Explo- 11. Scott, R. L., and R. M. Hays. In-sion-proof Bulkhead for a Crosscut Refuge active and Abandoned Underground Mines -Chamber (contract H0133050, Foster-Miller Water Pollution Prevention and ControlAssociates, Inc.). BuMines OFR 48-76, (U.S. EPA contract 68-01-2907, MichaelNov. 1975, 197 pp.; NTIS PB 251714. Baker, Jr., Inc.). EPA-440/9-75-007,

June 1975, 322 pp.; NTIS PB 258263.4. Rice, G. S., H. P. Greenwald, and

H. C. Howarth. Tests of the Strength of 12. Penn Environmental Consultant Inc.Concrete Stoppings Designed To Resist the Processes, Procedures, and Methods ToPressure of Explosions in Coal Mines. Control Pollution From Mining Activities.BuMines RI 3036, 1930, 11 pp. EPA-430/9-73-011, Oct. 1973, 359 pp.;

NTIS PB 257 297.5. Rice, G. S., H. P. Greenwald, H. C.

Howarth, and S. Avins. Concrete Stop- 13. Bucek M. F., and J. L. Emel (HRB-pings in Coal Mines for Resisting Explo- Singer, Inc.). Long-Term EnvironmentalBions: Detailed Tests of Typical Stop- Effectiveness of Close Down Procedures -pings and Strength of Coal as a Buttress. Eastern Underground Coal Mines (con-BuMines B 345, 1931, 63 pp. tract 68-03-2216). EPA-600/7-77-083,

Aug. 1977, 155 pp.; NTIS PB 272 373.6. Whitney C. S., B. G. Anderson, and

E. Cohen. Design of Blast Resistant 14. U.S. Bureau of Reclamation (Dep.Structures for Atomic Explosions. J. Am. Interior). Concrete Manual. 8th ed.,Concrete Inst., v. 51, Mar. 1955, 94 pp. 1975, 627 pp.

7. Greenwald, H. P., S. Avins, and 15. Maksimovic, S. D., and J. C*G. S. Rice. Compressibility and Bearing Draper. Building Seals by PneumaticStrength of Coal in Place: Tests of Lat- Stowing in Mine Closure Operations. Bu-eral Compression of the Pittsburgh Coal Mines RI 8729, 1982, 23 pp.Bed. BuMines Tech. Paper 527, 1933,12 pp. 16. Maksimovic, S. D., and J. R. Lips-

comb. Sealing Openings in Abandoned8. Garrett, W. S., and L. T. Campbell Mines by Pneumatic Stowing. BuMines RI

Pitt. Tests on an Experimental Under- 8730, 1982, 26 pp.ground Bulkhead for High Pressures. J.S. African Inst. Min. and Metall., Oct. 17. U.S. Army and U.S. Air Force.1958, pp. 123-143. Grouting Methods and Equipment. U.S.

Army Tech. Manual 5-818-6 (Air Force Man-'• ___. Design and Construction of ual 88-32), Feb. 1970, 86 pp.

Underground Bulkheads and Water Barriers.*PCr in Transactior»s of the Seventh Com- 18. Wu, K. K., J. W. Fredland, J. M.

alth Mining and Metallurgical Con- Krese, and M. Lawless. Factors in thegtess (Johannesburg, South Africa, April Evaluation of Coal Barriers To Prevent

AR3036l*6

24

Inundation and the Effectiveness of Fed- 22. Kendorski, F. S. , I. Khosla, anderal Regulations. Pres. at Soc. Min. M. M. Singh. Criteria for DeterminingEng. AIME Annu. Meeting, Las Vegas, NV, When a Body of Surface Water ConstitutesFeb. 1980. Soc. Min. Eng. AIME preprint a Hazard to Mining (contract J0285011,80-88, 11 pp. Engineers International, Inc.). BuMines

OFR 45-81, 1979, 364pp.; NTIS PB Si-19. Ashley, G. H. Barrier Pillar Leg- 194938.

islation in Pennsylvania. Trans. AIME,Feb. 1930, pp. 76-97. 23. Roark, R. J. , W. C. Young. Formu-

las for Stress and Strain. McGraw-Hill,20. Holland, C. T. Factors in the De- 5th ed. , 1975, 624 pp.

sign of Barrier Pillars in Coal Mines.Proc. WV Coal Min. Inst. , 1964, pp. 109- 24. American Concrete Institute.126. Building Code Requirements for Concrete

Masonry Structures (ACI 531-79) and Corn-21. Cummings, A. B. , and I. A. Given. raentary - ACI 531R-79. Ch. 10. Allow-

SME Mining Engineering Handbook, ed. by able Stresses. 1978, pp. 55-58.I. A. Given. AIME, v. 1, 1973, pp. 13-105 to 13-107.

BIBLIOGRAPHY

Argall, G. 0., and C. 0. Brawer (eds.). Cummings, A. B., and I. A. Given.Mine Drainage. (Proc. 1st Int. Mine Ground Water and Ground Water Control.Drainage Symp., Denver, CO, May 15-18, Ch. in SME Mining Engineering Handbook,1979.) Miller-Freeman, 1979, 848 pp. ed. by I. A. Given. AIME, v. 2, 1973,

pp. 26-1 to 26-55.Ash, S. H. , and W. L. Eaton. Barrier

Pillars in the Anthracite Region of Penn- Davies, A. W. , and W. K. Baird. Watersylvania. Trans. AIME, New York Meeting, Dangers. Min. Eng. (London) , v. 136, No.Mar. 1947, v. 177, pp. 62-81. 186, Dec. 1976-Jan. 1977, pp. 175-184.

Badappanauar , M. G. Chemical Grouting Ducks tein, L. , I. Bogardi, and F. Szi-in Pervious and Rock Foundations. J. darovszky. Reliability of UndergroundInst. Eng. (India), Civil Eng. Div., v. Flood Control System. J. Hydraulics54, No. C13, 1974, pp. 110-115. Div,, Proc. ASCE, v. 107, No. HY5, 1981,

pp. 817-827.Baker, W. H. (ed.). Proceedings of the

Conference on Grouting in Geotechnical Dunn, R. B. Water Problems in Mines.Engineering (sponsored by The Geotech. Min. Eng., (London), v. 142, No. 251,Eng., Div. ASCE, New Orleans, LA, Feb. Aug. 1982, pp. 81-91.10-12, 1982). ASCE, 1982, 1018 pp.

Erickson, H. B. Strengthening Rock byBeck, L. A. Sealing an Underground Injection of Chemical Grout. J. Soil

Mine in Pennsylvania. Min. Congr. J. , Mech. and Foundations Div., AIME, v. 94,July 1981, pp. 37-44. No. SM1, 1968, pp. 159-173, Paper 5744. v

Bogardi, I., L. Duckstein, A. Schmied- Flatau, A. S., R. W. Brockett,er, and F. Szidarovszky. Stochastic J. V. Brown. Grouts and Grouting:Forecasting of Mine Water Inrushes. Adv. vey of Materials and Practice. Civ. Eng. :in Water Re sour. , v. 8, 1980, pp. 3-8. and Pub. Works Rev., v. 68, No. 804, J

1973, pp. 591-601. 'Civil Engineering and Public Works Re-

view. Principles of Grouting Selection.V. 66, No. 783, 1971, pp. 1137-1142.

25

Frailey, D. Why, When and How to Seal Pugh, W. L. Water Problems at WestV_yAbandoned Workings Rather Than Ventilate. Cannock No. 5 Colliery. Min. Eng. (Lon-

Trans. Nat. Safety Congr., v. 7, 1968, don), Feb. 1980, pp. 669-679.pp. 21-22.

Skelly and Loy. Guidelines for MiningGarcia, J. A., and S. M. Cassidy. Near Water Bodies (contract H0252083).

Bulkheads for Coal Mines. AIME Tech. BuMines OFR 29-77, 1976, 107 pp.; NTIS PBPub. 789, 1937, 17 pp. 264-728/AS.

Graham, R. W. A Discussion of Pressure South African Council for ScientificGrouting. Q. CO Sch. Mines, v. 58, No. and Industrial Research, National Mechan-4, Oct. 1963, pp. 129-136. ical Engineering Research Institute. A

Survey of Factors Which Have a BearingMatthes, G. How Engineers Beat Shaft Upon the Design of Underground Plugs.

Flood in Karst Limestone. World Min., v. CSIR Rep. ME/MR/4267, 1963, 4 pp.2 8 , N o . 2 , 1975, p p . 48-52. , . - " . . - . .

____. A Survey of Underground PlugsMiller, J. T., and D. R. Thompson. Installed in South African Gold Fields.

Seepage and Mine Barrier Width. Paper in Contract Rep. C MEG/576, Nov. 1963; Cham-Fifth Symposium on Coal Mine Drainage Re- ber of Mines Res. Rep. 30/64, Aug. 1964,search (Proc. Coal and the Environment 42 pp.Tech. Conf., Louisville, KY, Oct. 22-24,1974). Nat. Coal Assoc., 1974, pp. 103- Tillson, B. F. Pump Stations and Water127. Control: Bulkheads, Dams Ejectors, Prim-

ing, Siphons, Sumps. Ch. in Mine Plant.Mitchell, R. J., J. D. Smith, and AIME, 1938, pp. 172-181.

W. J. Libby. Bulkhead Pressure Due toemented Hydraulic Mine Backfills. Can. Venburgh, L. C., D. Sokol, and M. V.Geotech. J., v. 12, No. 3, 1975, pp. 362- Damm. Investigative Programs for Design-371, ing and Modeling Mine Water Control Sys-

tems. Min. Eng. (London), Aug. 1982,Nishimatsa, Y., Y. Oka, and Y. Nishida. pp. 1217-1219.

The Failure and Displacement of ConcretePlug for Sealing in the Pit Mouth of Walker, J. Grouting Techniques. PlantClosed Mine. Rock Mechanics in Japan Eng., v. 11, No. 5, 1967, pp. 321-325.(Engl. Transl.), v. 3, 1979, pp. 114-H6. Whittaker, B. H., and R. N. Singh. De-

sign Aspects of Barrier Pillars AgainstOrchard, R. J. Working Under Bodies of Water-Logged Workings in Coal Mining Op-

Water. Min. Eng. (London), v. 134, No. erations. Paper in Symposium on Water in166, Mar. 1975, pp. 261-270. Mining Underground Works. Assoc. Nac. de

Ing. de Minas, Granada, Spain, 1978, v.Pearson, M. L., T. Preber, and P. J. 1, pp. 675-692.

Conroy. Outcrop Barrier Design Guide-lines for Appalachian Coal Mines (con- Wittke, W. Application of the Finitetract J0395069, Dames & Moore). BuMines Element Method to the Design of Under-OFR 134-81, Feb. 1981, 166 pp. ground Bulkheads. Erzmetall, v. 26, No.

_ 2, 1973, pp. 66-74.Folatty, J. M. Comments on U.S. Grout-

ing Practices. Paper in Foundation for World Mining. How West DriefonteinDams (Proc. Eng. Foundation Conf., Pacif- Gold Mine Fought and Won the Flood Bat-ic Grove, CA, Mar. 17-21, 1974). ASCE, tie. V. 23, Mar. 1969, pp. 38-43.

y > 9 7 * i PP. 47-55.

26

APPENDIX A.—HYDROSTATIC TESTING OF A SINGLE BULKHEAD SEAL

BULKHEAD CONSTRUCTION

The general diagram of the bulkhead is trench and a level concrete footer ap-shown in figure A-l. It was constructed proximately 4 in thick was poured. Afterfrom 6- by 8- by 16-in solid concrete allowing the footer to set, constructionblock and was located in a 6- by 18- by of the bulkhead began, as shown in figure20-ft dead-end room of the Bureau's Safe- A-3.ty Research Coal Mine, as shown in figure To facilitate the installation of anA-2. The first task was to trench the air release, water inlet, and pressureribs and floor to provide anchorage for gauge, piping had to be built intothe bulkhead. The roof was not trenched the bulkhead. Special blocks were madebecause this was considered too hazard- by drilling 1-1/4-in holes lengthwiseous. Using an air-driven jack hammer and through the block and grouting a 1-inchisel, trenches were dug 16 in into each steel pipe into place, as shown in figurerib and 22 in into the floor. Care was A-4. The blocks were then laid in theirtaken during this operation so that the respective courses as the bulkhead wasstrata were not unnecessarily cracked or constructed, as shown in figure A-3.fractured. When the trenching was com- As the bulkhead approached roof level,pleted, debris was removed from the floor gunite was sprayed in the cavity behind

Pilaster center

.Transverse patternof laying block

~ ~

(Not to scale)

FIGURE A-"I. - Diagram of single bulkhead seal.

AR3036l*9

27

Concrete block bulkhead

rF- Butt

D-Butt

] CDC-Bult

!_/—

50 100__•_____iScale, ft

FIGURE A-2. - Location of bulkhead in Safety Research Coal Mine.

the bulkhead, approximately 1/2 in to 1 pilaster on both the inby and outbyin thick, to seal cracks in the roof, sides. Eighteen-inch mechanical roofribs, and floor and provide a watertight bolts were used in securing the anglechamber. The bulkhead was then completed irons to the roof. The bulkhead curedto roof level and sealed tight against for a week before gunite was sprayed, 1/2the roof using 2- by 8- by 16-in solid in to 1 in thick, on the outby side. Af-block and mortar. Roof anchorage was ter spraying, the bulkhead was allowedprovided by securing 4- by 4-in angle to cure for another week before testingirons on either side of the bulkhead's began.

TEST APPARATUS

Figure A-5 shows a diagram of the test 3. Standpipe - a clear plexiglass tubearea with the following apparatus; that indicates water level behind the

bulkhead. "-1. Water pipe - inlet for water.

4. Pressure gauge - measures hydro-*• Air release - allows air to escape static pressure behind bulkhead.

/&-.. &s the chamber is filled with water.

AR303650

FIGURE A-3. • Bulkhead under construction in Safety Research Coal Mine.

5. Water meter - measures amount of 7. Water tank - water storage forwater used during testing. test.

6. Pump - air-driven pump with con-trols to regulate pressure behindbulkhead.

TEST PROCEDURE -,«. *

The test procedure consisted of incre- increment was maintained for approximate-*mentally building and then relieving hy- ly 10 min. Pressure was then dropped;drostatic pressure behind the bulkhead. back to zero and a new series of testsThis procedure simulated the actual in- started, as shown in table A-l.mine practice of allowing hydrostatic The bulkhead was inspected after each?pressure to build to a maximum level, and series of tests. The decision to proceed^then relieving it by pumping or draining to the next test depended on several fac-|>excess water behind the bulkhead. Ini- tors: (1) success of the previous test;*tial pressure was started at zero and (2) excessive water leakage or damageincreased at 5-psi intervals. Each roof, ribs, or floor strata; (3) visible

fifi30365

29

••V-

/4-in steel plate(welded to pipe)

-i_______16-

Concrete block

____ J38Bl-in

steel pipe

GroutScale, in

FIGURE A-4. - Cross section of 1-in pipe grouted into concrete block.

TABLE A-l. - Test procedure forIncrementally pressurizingbulkhead

Test

1. .......2........3........4........5........6........7........8........9........10.......TT —————— 1

Date

A /OO/QO

A/9R/R9A/OO /Q9

A/29/82V3/R9

VO/0031 Of.e/ O/QO5/ 4/825/ 4/825/ 5/82

Pressureinterval,1 psi

0- 50-100-150-200-250-300-350-AO0-450-50

Time,min10203040506070on90100

'5-psi intervals.

water leakage or damage to the bulkheadstructure.When testing commenced, water immedi-

ately began to leak from the roof, ribs,and floor strata surrounding the bulk- FIGURE A-5. • Diogram of test apparatus.

|> head. During the fourth series of testsat 20 psi pressure, water leakage became fear that infusing the strata with waterexcessive, but there was still no leaking would create larger cracks and generallythrough the bulkhead itself. Apparently, weaken ground conditions in the test* Eunite did not make a good sealant area. To seal and strengthen the sur-

tfc Pressure was forcing the water rounding strata, the application of a'•stl* the gunite and into the strata. pressure grouting technique was consid-

tlng was temporarily discontinued for ered the most practical approach.

AR303652

30

PRESSURE GROUTING

Pressure grouting was considered the the mixer-packer assembly. The grout wasmost practical method of sealing the pumped continuously, under pressure, asstrata in the test area. The strategy it migrated into the strata. Injectionwas to stop water permeation by forming pressures ranged from 500 to 700 psi.a grout curtain around the water chamber When the grout emerged from the strata,behind the bulkhead. In addition to as shown in figure A-8, the injection wasminimizing seepage, the grout would stopped, the hole abandoned, and a newstrengthen the ground by consolidating hole was drilled and injected. The in-the fractured strata. The Bureau sought jection started with hole Hj and pro-a company with expertise in a proven ceeded in numerical order to hole HI 4.pressure grouting technique to do the Some roof sag was experienced when thework. Representatives of Mobay Chemical grout was injected into the roof. ToCorp. were contacted; after consulta- provide additional support, four timberstion, the decision to inject their Roklok on 4-ft centers were installed acrossB-4 waterstop system into the fractured the entry, approximately 5 ft from thestrata was made. bulkhead. Extensometers, which detectThe B-4 system is a two-component poly- roof sag, were installed and monitored

urethane grout consisting of a polymetricisocyanate (component A) and a polyol TOP VIEWresin (component B), which are mixed andinjected into the strata. The mixturehas a low viscosity, which enables it toflow freely into fine cracks and fis-sures. As it migrates into the strata,it encounters water and expands drivingany remaining water out. The componentmixture solidifies in approximately 5min.The major concern was grout migration FRONT VIEW

to the blind face located behind the ——————bulkhead, because it was suspected thatmost of the water was seeping throughthis area. A drilling plan for injectingthe chemicals was developed. To reachthe blind face behind the bulkhead, twoholes over 50 ft long had to be drilledfrom entries to the right and left of thebulkhead. Other holes would be drilledaround the bulkhead to seal the ribs,roof, and floor, as shown in figure A-6.A total of 14 holes, varying in lengthfrom 4 ft to 50 ft, were drilled andinjected.Injecting the grout was relatively sim-

ple. A hole was first drilled into thestrata and the packer-mixer assembly,with an expansion shell, was insertedinto the borehole and anchored tightly,as shown in figure A-7. This assem-bly was then connected to the pumpingunit by high-pressure hoses. The twocomponents were pumped separately, then FIGURE A-6. - Drill plan for injecting polyure-mixed and injected into the strata via thane grout in strata surrounding the bulkhead.

AR303653

31

FIGURE A-7. - Packer-mixer assembly installed in borehole.

FIGURE A-8. - Polyurefhane grout emerging from strata.

AR303651*

32

continuously. Approximately 0.3 in of migrating properly. These spot checksroof sag was measured during the roof showed that the technique was workinginjection, but no roof problems were well. The pressure rose rapidly as waterencountered. was pumped into the chamber behind thePeriodically during the injection, bulkhead and visible water seepage from

the bulkhead was pressurized at less the roof, ribs, and floor was signlfi-than 20 psi to determine if the grout was cantly less.

FINAL TESTS AND RESULTS

With the pressure grouting completed, initially showed signs of failure (40testing resumed and followed the origi- psi) was considered the maximum pressurenal plan of incrementally pressurizing the bulkhead could withstand.the bulkhead (see table A-l). Tests 1 It should be noted that the hydrostaticthrough 7 proceeded well; 35 psi (80 ft pressures were applied over a much short-of waterhead) was reached with no signs er time than under actual mine condi-of failure in the bulkhead or its anchor- tions. Infusing the strata with waterage. Water seepage from the surround- was considered dangerous to ground sta-ing strata was minimal, showing that the bility, limiting the time duration ofgrout adequately sealed the water chamber tests. The maximum pressure the bulkheadbehind the bulkhead. When 40 psi (92 ft withstood, 40 psi (92 ft of waterhead),of waterhead) was reached during test 8, includes no factor of safety, and actualthe bulkhead began to show signs of fail- pressures should be kept well below thisure. Water was leaking, although not limit because of this time factor. Aexcessively, at roof level and also flexural strength analysis of this bulk-through mortar joints located near the head, given in appendix B; Includes thebase of the bulkhead. Testing proceeded maximum allowable stress a bulkhead ofto tests 9 and 10. With each pressure this type could safely withstand.increase, water leakage through the roof Tests show that certain constructionand mortar joints became more severe. At and maintenance procedures should be fol-50 psi (115 ft of waterhead) the bulkhead lowed when building this particular bulk-and roof strata were leaking excessively. head, especially If it is to act as bothIt was at this point the bulkhead was an explosionproof and water seal. Theseconsidered to have failed and testing was procedures are detailed in the nextstopped. Though testing continued to 50 section.psi, the point at which the bulkhead

CONSTRUCTION AND MAINTENANCE PROCEDURES

The following procedures for bulkhead 2. Proper anchorage of the bulkheadconstruction and maintenance are based on to the mine roof, ribs, and floor i§test experiences and pertinent litera- important and depends on strata type aiture. If implemented properly, they will condition. In general, most coalsdecrease the possibility of an unexpected good anchorage characteristics,inundation and provide a greater degree trench depths In the ribs of 16 to 24 1of safety to the mine. should be adequate (4). Fireclays,

stones, and shales, which compose moilCONSTRUCTION floor strata, usually make good support

1. Construct the bulkhead in competent 1Underlined numbers in parenthese3ground that is not excessively broken or fer to items in the list of refererfractured, preferably where ground move- preceding the appendixes.ment has settled (I).1

33

material unless softened by water (_3). such as a waterproofing cement. ApplyFloor trenches should be at least 16 in several coats, and if possible seal bothdeep if the strata are in good condition. sides of the bulkhead.If the floor is in poor condition, trenchuntil competent strata are reached. 8. Pressure grouting is recommended

depending on the permeability and3. If feasible, trenching of the roof strength of the surrounding strata* Of-

is highly recommended. Due to roof sag, ten, the deterioration of the roof, rib,the roof is typically the place where and floor anchorage by water permeationmost seepage occurs. Also, during tests, creates a most significant structuralmost water seepage occurred at roof level concern. Tests showed that pressurebecause the bulkhead was not keyed into grouting will minimize water permeation,the roof. To trench the roof, first re- especially if large pressures are antici-move any headcoal or other deteriorate pated over the life of the bulkhead. Ifing roof rock so as to expose competent pressure grouting is necessary, the minestrata. Trenches should be made at least operator should carefully plan his strat-8 in deep, and care should be taken when egies and select a company with & proventrenching because of the unpredictable pressure grouting technique.nature of most roof rock.

MAINTENANCE4. Trenches can be cut with any hy-

draulic or air-driven tools, continuous 1. The bulkhead should be inspectedminers, or cutting machines. If extreme- according to the Code of Federal Regula-ly hard strata are encountered, trenching tlons, Title 30, Parts 75.303 and 75.305.may be carried 'out by drilling and shoot- Records of these inspections should being, but care should be taken to avoid kept and include significant factors suchunneccessarily fracturing the strata O). as present waterhead, visible water seep-

age through bulkhead or Immediate strata,5. Use only solid concrete block to or any visible deterioration in the bulk-

construct the bulkhead. Remove all de- head or ground conditions.bris from the floor trench, and pour alevel concrete footer on which to build. 2. Ground movements, such as floorMix cement properly to assure good bond- heave, roof convergence, and pillaring between blocks and make sure all spalllng, can damage the bulkhead and itscourses are laid level. anchorage, especially if it is con-

structed from rigid materials such as6. As the bulkhead is being built, concrete or concrete block (_1). Ground

fill in all gaps and cracks in the an- movements are difficult to predict, andchorage with concrete or cement. When in most mines they are Inevitable. Be-compieted, use the same material to seal cause of this, supplemental roof supportsthe bulkhead tightly against the roof, such as cribs and timbers should be in-ribs, and floor (3). stalled around the bulkhead as a routine

measure.7. The bulkhead will need a water

sealant to protect it from prematurely 3. With time, water may weaken thedeteriorating but allow the bulkhead to bulkhead and anchorage, but this can becure for several days before application. minimized by repatchlng and resealing.In constructing the bulkhead for tests, In extreme cases, where strata or bulk-gunite was used to seal the bulkhead. head deterioration may endanger worker*ut, because gunlte is sprayed on, air safety or the mine, more extensive mea-tccomes trapped in the material; this may sures such as pressure grouting or recon-«ake it porous. To avoid this, use seal- struction may be necessary.•nts that can be applied with a trowel

AR303656

34

4. Water pressure should not exceed wall) can be just as dangerous asthe design capacity of the bulkhead. excessive water pressures. If dangerousThis capacity can vary, depending on such water volumes are suspected, methods forfactors as construction practices, reducing excess water should be imple-strength and permeability of anchoring mented. This can include pumping thestrata, and ground stability. In addi- water out from the surface or drainingtion, large volumes of water stored the water from behind the bulkhead to un-behind a bulkhead (such as an open dam derground sumps.

INSTALLATION OF PIPING

Pipes for monitoring water levels be- facilitate the installation of thehind the bulkhead are recommended. The following: (1) A pressure gauge andpipes can be either plastic or metal. If stand pipe, installed on the outby sidemetal, they should be corrosion resist- of the bulkhead; (2) a porous tube whichant. Installation of pipes, by grout- traps sediment, but allows water to passing them into the concrete block, is dis- through, installed on the inby side.cussed earlier in this report. Figure A-9 shows this arrangement inPipes should be installed 6 to 12 detail.

in from the floor and 12 to 18 in from The pressure gauge measures hydrostaticthe rib that has the lower elevation. pressure on the bulkhead and should beThe pipe should be at least 7 to 10 ft able to register at least 50 psi. Thein length with a 1- to 2-in ID. Both standing pipe indicates the exact water-ends of the pipe must be threaded to head behind the bulkhead in a direct

height-to-height relationship. It can be'constructed from any clear plasticsuch as plexiglass. It should be in-*"f-stalled vertically to roof level withvalve, capable of withstanding a static^pressure greater than 50 psi, separating

^ the pressure gauge from the standitPorous ^ ' oopolyethylene tube

The porous tube used in tests wasof polyethylene and acquired from Piezoneter Research and Development Corp. Figure A-10 shows this tube installed oninby side of the bulkhead. The tubelows water to pass without affectl|pressure readings, and traps sedimetthat nay clog the stan,iing pipe and pi

•Stondpipe sure gauge.The valve that separates the star

pipe from the pressure gauge shouldmain closed until use. To determine

„ _ . , __ . waterhead behind the bulkhead, opetjNot to scale i rv- T '>m-J-1 valve and raeasure the height of

in the standing pipe. If the watelflows from the standing pipe, the;;behind the bulkhead has reached atroof level. If this is the case,j

-Pressure gouge the valve so that the waterheaddetermined from the pressure gauge«Jthe gauge, then divide the

FIGURE A-9. - Diagram of standpipe, pressure 0.434 to determine the waterhead jtgouge, and porous tube arrangement. tne bulkhead.

AR303658

36

APPENDIX B.—FLEXURAL STRENGTH ANALYSIS FOR CONCRETE BLOCK BULKHEAD

Properly constructed bulkheads that b » 72 in;are fixed on at least three sides (bothribs and floor) when subject to a hydro- and T - 16 in.static pressure are most likely to failin flexure (.22).1 The flexural stress on therefore Ft - 42.1 psi.the bulkhead is given by the followingequation (23): The maximum allowable flexural stress

. Ppb2 fA-n *or nonreinf°rced masonry unit con-t T2 structed of solid block is 40 psi as rec-

ommended by the ACI Code (24). There-where Ft - the flexural stress on the fore, the maximum allowable pressure that

bulkhead, psi; the bulkhead described in appendix A cansafely withstand, keeping within the llm-

(3 • correction factor, no units; its of the ACI Code, is approximately 2.6psi (6 ft of waterhead). This maximum

p - hydrostatic pressure, psi; allowable pressure may seem unreason-ably low when compared with the ultimate

b - bulkhead height, in; pressure of 40 psi that the bulkheadresisted. But, it must be realized that

and T - bulkhead thickness, in. the time duration of tests was very'short when compared with actual mine

p is a correction factor dependent upon conditions. This analysis neglects the;the width-to-height ratio of the bulkhead pilaster center, the gunite coating, and;;and the particular loading condition. the transverse pattern of laying tha"Figure B-l (22) gives the correction fac- block. These design features are diffi-tor for various waterheads, H. Using the cult to access but would provide addi-maximum condition given H » b, and a tional resistance to flexural failure. . /.width-to-height ratio of a/b - 3, p isapproximately 0.80. Substituting intoequation A-l and solving for Ft: '-2 —

. . H - bi.oh l - - "

where p - 0.80; .4

2.6 psi (6 ft of waterhead);.2

KEYH* Water heod

H«'/3b

0 i 2 3 4a/b

Underlined numbers in parentheses re-. FIGURE B-l. - Correction factor forfer to items in the list of references to-height ratios. Adopted from F.S. Kendorski,preceding the appendixes. la, and M. M. Singh (22). <•

-U.S. CPO 1985-505-019/20.041 INT.-BU.OF MINES.PGH..W

303659