Rock Tunneling

7/21/2019 Rock Tunneling

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ROCK TUNNELING

(By: KU Internship team, Mid-Bhotekoshi HPP, 2015)

1. DRILL AND BLAST METHODThis tunneling method involves the use of explosives. Drilling rigs are used to drill blast holes on

the proposed tunnel surface to a designated depth for blasting. Explosives and timed detonators

(Delay detonators) are then placed in the blast holes. Once blasting is carried out, waste rocs

and soils are transported out of the tunnel before further blasting.

Drilling and blasting method is suitable for tunneling in medium to high strength rocs. !t can be

applied to a wide range of roc conditions. "ome of its features include versatile e#uipment, fast

start$up and relatively low capital cost tied to the e#uipment.

1.1 Procedure

%s the name suggests, drilling and blasting wors as follows&

% number of holes are drilled into the roc face.

The holes are then filled with explosives.

Detonating the explosive causes the roc to collapse.

The smoe and dust produced after the explosion is removed.

The rubble is removed and the new tunnel surface is reinforced.

'epeating these steps will eventually create a tunnel.

Figure 1 Drill and Blast Cycle


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1.2 Drillin

The tunnel face can be roughly divided into four sections for drilling. The main goal is to ensure

the optimum number of correctly placed and accurately drilled holes. This helps to ensure

successful charging and blasting, as well as produce accurate and smooth tunnel walls, roof and

floor.

Figure 2 Types of holes in tunnel face

1.! Drillin P"##ern

The drilling pattern ensures the distribution of the explosive in the roc and desired blasting

result. "everal factors must be taen into account when designing the drilling pattern& roc

drillability and blastability, the type of explosives, blast vibration restrictions and accuracy

re#uirements of the blasted wall etc. hen designing a drilling pattern in tunneling, the main

goal is to ensure the optimum number of correctly placed and accurately drilled holes. This helps

to ensure successful charging and blasting, as well as produce accurate and smooth tunnel walls,

roof and floor. % drilling pattern optimied in this way is also the most economical and efficientfor the given conditions.

1.!.1 Hole Si$e

*ole sies under +mm in diameter are often considered small, holes between -mm $ /-mm

intermediate, and those over /-mm large. 0ost tunneling operations today are based on hole

sies between + $ 1mm in diameter. Only cut holes are larger than 1mm. 'oc drills and

mechanied drilling e#uipment used in tunneling are designed to give optimum performance in

this hole range.

1.!.2 Cu# Hole

The blasting se#uence in a tunnel always starts from the 2cut3, a pattern of holes at or close to

the center of the face, designed to provide the ideal line of deformation. The placement,

arrangement and drilling accuracy of the cut is crucial for successful blasting in tunneling. %

wide variety of cut types have been used in mining and construction, but basically they fall into

two categories& cuts based on parallel holes, and cuts that use holes drilled at certain angles. The

most common types of cut today is the parallel and 4 cut.


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Figure 3 V Cut

The 4 cut is the older of the two and is still widely used in construction. !t is an effective type of

cut for tunnels with a fairly large cross$section and re#uires fewer holes than a parallel cut. The

parallel cut was introduced when the first mechanied drilling machines came on the maret

maing accurate parallel drilling possible.

1.!.! S#o%e&ole

The holes surrounding the cut are called stopeholes (blast holes). The diameter of a stopehole is

typically between - $ 1mm. *oles smaller than -mm may re#uire drilling an excessive

number of holes to ensure successful blasting. *oles bigger than 1mm can result in excessive

charging and an uncontrolled blast.

*oles are placed around the cut section in an evenly distributed pattern using a space5burden

ratio of &.. 6urden is the minimum distance from the axis of the blast hole to the free face,

and spacing is the distance between blast holes in the same row. !f hole sie is between -1 $

1mm, typical spacing and burden are both between .7m $ .+m. %ctual roc conditions and

ability to drill in the re#uired positions are factors that can reduce or add to the number of holesneeded. The design of the drilling pattern can now be carried out and the cut located in the cross

section in a suitable way.

1.!.' Con#our Hole( "nd )loor Hole(

8loor holes have approximately the same spacing as stope holes, but the burden is somewhat

smaller9 from 7.:m to .m. !naccurate or incorrect drilling and charging of the floor holes can

leave unblasted bumps, which are difficult to remove later. The contour holes lie in the perimeter

of the drilling pattern. !n smooth blasting, contour holes are drilled closer to each other and are

specially charged for smooth blasting purposes. "pacing is typically from 7.1m to 7$:m and

burden varies between and .;1 times the space. This type of layout maes it possible to usespecial smooth blasting explosives, which limits the width and depth of the fracture one in the

walls and roof caused by blasting. !n special circumstances, two or more smooth blasting rows

can be used.

!n tunneling, however, contour holes are blasted with stope holes, but timed to detonate last. The

result in smooth contour excavation mostly depends on drilling accuracy. The re#uired amount of

shotcreting and concrete casting can be significantly reduced by using smooth blasting,


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particularly in poor roc conditions. "mooth blasting increases the number of holes needed for

the drilling pattern by roughly 7 $ 1


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The two$duct system is practical in long tunnels (? - m). The system removes explosion gases

fast and effectively. %fter the explosion gases are removed, both ducts can be used for blowing

ventilation to get even more fresh air into the tunnel during loading and transportation.

Figure 5 Twoway ventilation using two ducts

The one$duct system is practical in tunnels up to -$1 m in length. ith this system there is onlyone ventilation duct in the tunnel. %t the tunnel face end, there is a two$fan system which

controls the ventilation according to the stage in the drill > blast cycle. During drilling, charging,

loading > hauling, the system is used for conventional blowing ventilation. %fter blasting, a

transverse fan is used to remove explosion gases through the duct while the other fan blows fresh

air towards the face to ensure that all explosive gases are mixed and removed. The one$duct

system removes explosion gases fast and effectively, and is more cost$effective than the two$duct

system.

Figure ! Twoway ventilation "y single duct

1.0 Sc"lin

The purpose of scaling is to clear loose roc from walls and surfaces after blasting. 0anuallydone it is hard wor involving many safe haards such as falling roc and dust, and re#uiring

awward woring positions. "caling is often very time consuming when done manually. Today,

modern mechanied scaling e#uipment is used whenever possible.


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1. Mucin

0ucing is done after the fumes have been removed. 'elevant loading and hauling devices can

be used for removing the muc. Two basic types of transportation system available for moving

materials in the tunnel are railroad tracs and various rubber$tired vehicles.

1..1 R"il(3 8rom the standpoint of energy consumption, rail haulage provides by far the mostefficient handling of materials in the tunnel. 'ail$mounted vehicles are moved, singly or in

trains, by locomotives powered by either internal combustion engines or electric motors.

1..2 Ru44er#ired +e&icle(3 Transportation with rubber$tired vehicles offers great flexibility

because its operation is not restricted to locations having fixed facilities, as is the case with

railroad trac. % wide range of vehicle sies and configurations, all typically diesel$powered, are

available. "tandard front end loaders can be used for transport. %lthough primarily intended for

loading, such units may be economically used for short haul distances. Dump trucs can be used

to haul materials at greater distances.

1.5 Liin

!n the past, an evenly spaced string of light bulbs was the usual type of general lighting. On some

pro@ects, no general lighting is provided, and personnel are supplied with flashlights or cap lamps

for emergencies and general use. "afety regulations may re#uire specific lighting standards in

tunnels and should be consulted when planning a particular @ob. 8loodlights are used for lighting

wor areas, and are mounted in strategic locations on @umbos and other woring structures.

1.6 Roc Su%%or#

%s the tunnel is incrementally excavated the roof and sides of the tunnel need to be supported to

stop the roc falling into the excavation. The philosophy and methods for roc support vary

widely but typical roc support systems can include roc bolts or roc dowels, shotcrete and ribs.

1.6.1 Roc 4ol#( or roc do,el(

% roc bolt is a long anchor bolt, for stabiliing roc excavations, which may be used in tunnels

or roc cuts. !t transfers load from the unstable exterior, to the confined (and much stronger)

interior of the roc mass.

'oc bolts are almost always installed in a pattern, the design of which depends on the roc

#uality designation and the type of excavation. 'oc bolts wor by AnittingA the roc mass

together sufficiently before it can move enough to loosen and fail by unravelling (piece by

piece). 'oc bolts may also be used to support wire mesh, but this is usually a small part of their

function.


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Figure # Typical roc$ "olting pattern for a tunnel

1.5.2 S&o#cre#e

"hotcrete is concrete conveyed through a hose and pneumatically pro@ected at high velocity onto

a surface, as a construction techni#ue. !t is reinforced by conventional steel mesh, and5or fibers.

The need for ductility, toughness, and a residual strength is generally met by incorporating short,

thin pieces of wire or sheet steel into the mix (fiber shotcrete). elded wire fabric (8) was

introduced into shotcrete usage to provide ductility9 however, steel fiber now provides this

characteristic more effectively. elded wire fabric is not recommended now a days due topractical reasons. Even properly spaced fabric (- in. B - in. or / in. B / in.) is #uite stiff, maing

installation time consuming, difficult, and therefore costly. hen used in drill$and$blast tunnels,

considerable excess shotcrete may be re#uired to fill overbrea to which 8 cannot be

properly formed. "ometimes, woven wire mesh is used in con@unction with roc bolts for safety

when roc is reinforced.

"hotcrete can be dry mix$shotcrete or wet mix$shotcrete. Dr/7i8 (&o#cre#econsists of a

mixture of damp aggregate and cement fed into a placing machine, fed at a uniform rate into an

airstream to travel through a hose to the nole. The water of hydration is added at the nole

before discharge to the surface. ater is manually controlled, permitting ad@usting to changing

surface wetness. Cowdered accelerators are added to the dry mix as it is fed into the placer. !fli#uid, the accelerator is mixed with the feed water before it goes to the nole.

The ,e#7i8 %roce((consists of mixing measured #uantities of aggregate, cement, and water,

and introducing the resulting mix into a vessel for discharge pneumatically or mechanically

through a hose to final delivery from a nole. !t has the advantage of rigidly controlling the

water5cement (5=) ratio of the product. "uccessful methods have been devised to introduce

#uic$acting accelerators to the delivery hose. Cumping low$slump concrete is commonly a


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problem, and so a slightly higher than desirable water content is used. 6y use of accelerators,

such concrete can be made to adhere overhead, but ultimate strength usually suffers. *owever,

the method has been found convenient for use with less$silled operators.

The strength of shotcrete should be tested and approved for used in pro@ects. "hotcrete testing is

a three$part process. The first stage, compatibility checing, is re#uired before the proposedmaterials and their sources are approved. =ement$accelerator compatibility is of prime

importance. "imilarly, compatibility of the entire mix and proportions must be established by

meeting the various re#uirements with proposed mixes prepared, cured, and tested in the

laboratory. The second stage, field trials, begins upon completion of the first part. 0aterial from

the approved sources should be combined per approved mixes by the production e#uipment to be

used and then shot by a certified noleman into appropriate sie boxes. %fter curing in the

manner proposed for the production wor, samples should be taen and tested.

The third stage, production testing, has three parts. 8irst, the field trial process should be

repeated at the heading during production shotcreting upon demand by the engineer. "econd,

cores should be taen from the in$place shotcrete, at specified intervals. The primary purpose ofthese is to chec thicness and adhesion9 however, compressive strength should also be tested.

The third part is the overall checing of the in$place concrete. !n addition to a visual chec for

defects, the shotcrete should be sounded at fre#uent intervals (locations) by striing with a

geologistAs or similar hammer. "ound, adhering concrete will give a distinct ringing sound.

aminated shotcrete or voids behind the shotcrete will result in a drummy or hollow sound. !f

drummy, the area should be recheced thoroughly and the approximate boundaries determined.

=ores should then be taen and examined. Defective shotcrete should be removed and replaced

with sound shotcrete.

1.5.! Ri4(

"teel ribs set close to the tunnel surface and bloced from it are normally used as the initial

support system for roc tunnels, especially those constructed by conventional drill$and$ blast

methods. ood, concrete, or steel lagging may be placed between the ribs to secure blocy or

raveling ground, or welded wire fabric can also be used.


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ROCK SUPPORT )OR +ARIOUS ROCK CONDITIONSDsfa

'oc 0ass =lass

"upport

'oc bolts (ength

5+ to Tunnelidth)

"hotcrete "teel "ets

4ery Food 'oc

!

'0'& $77

Fenerally no support re#uired except for occasional spot bolting.

Food 'oc

!!

'0'& /$7

ocally bolts in roof

7 ft. long, spaced

ft. with occasional

wire mesh.

; in. in roof where

re#uired.

Gone.

8air 'oc

!!!

'0'& -$/7

"ystematic bolts ;

ft. long, spaced 1$/ ft.

in roof and walls with

wire mesh in crown.

; to - in. in roof and

in. on walls.

Gone.

Coor 'oc

!4

'0'& ;$-7

"ystematic bolts ;$

1 ft. long, spaced +$

1 ft. in roof and walls

with wire mesh.

- to / in. in roof and

- in. on walls.

ight to medium ribs

spaced 1 ft. where

re#uired.

4ery Coor 'oc

4

'0'&7$;7

"ystematic bolts 1$

;7 ft. long, spaced +$

1 ft. in roof and walls

with wire mesh.

/ to in. in roof. / in.

on walls and ; in. on

face.

0edium to heavy ribs

spaced ; ft / in. with

steel lagging and

forepoling if

re#uired. =loseinvert.

'oc support for various 'oc 0ass 'atings can be chosen using the following chart as a

guideline&


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Rock Tunneling

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