CHP 9.2 (RMR)

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Rock mass classification systemRock mass classification system::

The most comprehensive & objective classification systems of The most comprehensive & objective classification systems of rock mass are: rock mass are:

-- RMR (Rock Mass Rating) RMR (Rock Mass Rating) -- QQ-- System. System.

However, they are more complex than RQD. However, they are more complex than RQD.

The system includes several essential properties of a rock The system includes several essential properties of a rock body that evaluate its gross properties; i.e. body that evaluate its gross properties; i.e. material propertiesmaterial properties+ + mass propertiesmass properties

Rock Mass Rating, RMRRock Mass Rating, RMR::

Also called Also called geomechanicsgeomechanics classification, based on the work of classification, based on the work of BieniawskiBieniawski (1973)(1973)

The system includes The system includes ‘‘standstand--up timeup time’’ of an unsupported of an unsupported excavationexcavation’’ & & 6 other parameters6 other parameters::-- uniaxialuniaxial compressive strength of intact rock compressive strength of intact rock

material, UCSmaterial, UCS-- Rock quality designation, RQDRock quality designation, RQD-- Joint or discontinuity spacingJoint or discontinuity spacing-- Conditions of discontinuity (infilling, persistence)Conditions of discontinuity (infilling, persistence)-- Conditions of groundwater Conditions of groundwater -- Orientation of discontinuity (strike & dip angle).Orientation of discontinuity (strike & dip angle).

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Collection of field data for RMRCollection of field data for RMR::

The rating of 6 parameters of the RMR are given in Table 1 to 6.The rating of 6 parameters of the RMR are given in Table 1 to 6.

For eliminating doubts due to subjective For eliminating doubts due to subjective judgmentsjudgments, the rating , the rating for different parameters should be given a range in preference for different parameters should be given a range in preference to a single value. to a single value.

(1) (1) UniaxialUniaxial compressive strength of intact rock compressive strength of intact rock material (qmaterial (qcc))

The strength of the intact rock material should be obtained The strength of the intact rock material should be obtained from rock cores, the ratings based on from rock cores, the ratings based on uniaxialuniaxial compressive compressive strength (preferred) & pointstrength (preferred) & point--load strength as shown in Table 1. load strength as shown in Table 1.

Table 1: Strength of intact rock material Table 1: Strength of intact rock material ((BieniawskiBieniawski, 1979), 1979)

QQuuaalliittaattiivvee ddeessccrriippttiioonn

CCoommpprreessssiivvee SSttrreennggtthh ((MMPPaa))

PPooiinntt--llooaadd ssttrreennggtthh ((MMPPaa))

RRaattiinngg

Exceptionally strong

> 250 8 15

Very strong 100 – 250 4 – 8 12 Strong 50 – 100 2 – 4 7 Average 25 – 50 1 – 2 4 Weak 10 – 25 Use of uniaxial

compressive strength is preferred

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Very weak 2 – 10 - do - 1 Extremely weak 1 – 2 - do - 0 Note: At compressive strength less than 0.6 MPa, many rock material would be regarded as soil

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(2) (2) Rock Quality Designation (RQD)Rock Quality Designation (RQD)RQD should be determined as previously discussed and rating RQD should be determined as previously discussed and rating are given in Table 2.are given in Table 2.

Table 2: Rock Quality Designation RQD Table 2: Rock Quality Designation RQD ((BieniawskiBieniawski, 1979), 1979)

Qualitative description RQD (%) Rating Excellent 90 - 100 20 Good 75 - 90 17 Fair 50 - 75 13 Poor 25 - 50 8 Very poor 0 - 25 3

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(3) (3) Spacing of discontinuities Spacing of discontinuities The term discontinuity includes joints, beddings or foliations, The term discontinuity includes joints, beddings or foliations, shear zones, minor faults, or other weakness planes. The linear shear zones, minor faults, or other weakness planes. The linear distance between two adjacent discontinuities should be distance between two adjacent discontinuities should be measured for all sets of discontinuities & the rating should be measured for all sets of discontinuities & the rating should be obtained from Table 3 for the most critical discontinuity.obtained from Table 3 for the most critical discontinuity.

Table 3: Spacing of discontinuitiesTable 3: Spacing of discontinuities ((BieniawskiBieniawski, 1979), 1979)

Description Spacing (m) Rating Very wide > 2 20 Wide 0.6 – 2 15 Moderate 0.2 – 0.6 10 Close 0.06 – 0.2 8 Very close < 0.06 5 Note: If more than one discontinuities sets are present and the spacing of discontinuities of each set varies, consider the set with the lowest rating

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(4) (4) Condition of discontinuityCondition of discontinuityThis parameter includes roughness of discontinuity surfaces, This parameter includes roughness of discontinuity surfaces, their separation (aperture or opening), length or continuity, their separation (aperture or opening), length or continuity, weathering of the discontinuity surfaces, and infilling (gouge) weathering of the discontinuity surfaces, and infilling (gouge) material. The details of rating are given in Table 4.material. The details of rating are given in Table 4.

Table 4: Condition of discontinuitiesTable 4: Condition of discontinuities ((BieniawskiBieniawski, 1979), 1979)

Description Rating Very rough and weathered, wall rock tight and discontinuous, no separation

30

Rough and slightly weathered, wall rock surface separation < 1 mm

25

Slightly rough and moderately to highly weathered, wall rock surface separation < 1 mm

20

Slickensided wall rock surface or 1 – 5 mm thick gouge or 1 – 5 mm wide continuous discontinuity

10

5 mm thick soft gouge, 5 mm wide continuous discontinuity

0

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(5) (5) Ground water conditionGround water conditionIn the case of tunnel, the rate of inflow of ground water in In the case of tunnel, the rate of inflow of ground water in litreslitresper minute per 10 m length of the tunnel should be determined, per minute per 10 m length of the tunnel should be determined, or general condition can be described as or general condition can be described as completely drycompletely dry, , dampdamp, , wet drippingwet dripping & & flowingflowing. .

If actual water pressure data is available, these should be If actual water pressure data is available, these should be stated & expressed in terms of the ratio of the seepage stated & expressed in terms of the ratio of the seepage pressure to the major principal stress. pressure to the major principal stress.

The ratings as per the water condition are given in Table 5.The ratings as per the water condition are given in Table 5.

Table 5: Ground water conditionTable 5: Ground water condition ((BieniawskiBieniawski, 1979), 1979)

Inflow per 10m tunnel length (litre/min)

None < 10 10 – 25 25 – 125 > 125

Joint water pressures / major principal stress

0 0 – 0.1 0.1 – 0.2 0.2 – 0.5 > 0.5

General description Completely dry

damp wet dripping flowing

Rating

15 10 7 4 0

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Rating of the above 5 parameters (Table 1 to Table 5) Rating of the above 5 parameters (Table 1 to Table 5) are added to obtain what is called the are added to obtain what is called the basic rock mass basic rock mass ratingrating -- RMRRMRbasicbasic


(6) (6) Orientation of discontinuitiesOrientation of discontinuitiesOrientation of discontinuities means the DIP and STRIKE of Orientation of discontinuities means the DIP and STRIKE of discontinuities (weakness planes). discontinuities (weakness planes).

The dip angle is the angle between the horizontal and the The dip angle is the angle between the horizontal and the discontinuity plane taken in a direction in which the plane dipsdiscontinuity plane taken in a direction in which the plane dips. .

The value of the dip and strike should be recorded as shown in The value of the dip and strike should be recorded as shown in Table 6, the orientation of tunnel axis or slope face or Table 6, the orientation of tunnel axis or slope face or foundation alignment should also be recorded.foundation alignment should also be recorded.

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Table 6: Orientation of discontinuitiesTable 6: Orientation of discontinuities

A. Orientation of tunnel/slope/foundation axis: ……….. B. Orientation of discontinuities Set-1 Average strike: ….. (from ….. to ….. ) Dip ….. Set-2 Average strike: ….. (from ….. to ….. ) Dip ….. Set-3 Average strike: ….. (from ….. to ….. ) Dip …..


(6) (6) Orientation of discontinuitiesOrientation of discontinuitiesThe influence of the strike & dip of the discontinuities is The influence of the strike & dip of the discontinuities is considered with respect to the direction of tunnel considered with respect to the direction of tunnel drivagedrivage or or slope face orientation or foundation alignment. slope face orientation or foundation alignment.

To facilitate a decision whether the strike & dip are To facilitate a decision whether the strike & dip are favourablefavourableor not, reference should be made to or not, reference should be made to Table 7Table 7 & & Table 8Table 8 which which provide a quantitative assessment of critical joint orientation provide a quantitative assessment of critical joint orientation effect with respect to tunnels & dams foundation respectively. effect with respect to tunnels & dams foundation respectively.

Once the ratings for the effect of the critical discontinuity arOnce the ratings for the effect of the critical discontinuity are e known, as shown in known, as shown in Table 9Table 9 an arithmetic sum of the joint an arithmetic sum of the joint adjustment rating in and the adjustment rating in and the RMRRMRbasicbasic is obtained. This number is obtained. This number is called the final rock mass rating RMR.is called the final rock mass rating RMR.

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Table 7: Assessment of joint orientation effect on Table 7: Assessment of joint orientation effect on tunnels (dips are apparent dips along tunnel axis) tunnels (dips are apparent dips along tunnel axis)

((BieniawskiBieniawski, 1989), 1989)

Strike perpendicular to tunnel Drive with dip Drive against dip

Strike parallel to tunnel axis

Irrespective of strike

Dip 450 - 900 Dip 200 - 450 Dip 450 - 900 Dip 200 - 450 Dip 200 - 450 Dip 450 - 900 Dip 00 - 200 Very favourable

Favourable Fair Unfavoura-ble

Fair Very unfavourable

Fair

Table 8: Assessment of joint orientation effect on Table 8: Assessment of joint orientation effect on stability of dam foundationstability of dam foundation ((BieniawskiBieniawski, 1989), 1989)

Dip 00 - 100 Dip direction

Dip 00 - 100

Upstream Downstream

Dip 300 - 600

Dip 600 - 900

Very favourable

Unfavourable Fair Favourable Very Unfavourable

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Table 9: Adjustment for joint orientation Table 9: Adjustment for joint orientation ((BieniawskiBieniawski, 1979), 1979)

Joint orientation assessment for:

Very favourable

Favourable Fair Unfavourable Very Unfavourable

Tunnels

0 -2 -5 -10 -12

Raft foundation

0 -2 -7 -15 -25

Slopes

0 -5 -25 -50 -60

Estimation of RMREstimation of RMR::

The rock mass rating is an algebraic sum of ratings for all the The rock mass rating is an algebraic sum of ratings for all the parameters in parameters in Table 1 to 5Table 1 to 5 & & Table 9Table 9, after the adjustments for , after the adjustments for orientation of discontinuities given in orientation of discontinuities given in Table 7 and 8Table 7 and 8. .

The sum of ratings for 4 parameters in The sum of ratings for 4 parameters in Table 2 to Table 5Table 2 to Table 5 is is called called Rock Condition RatingRock Condition Rating (RCR) which discounts the effect (RCR) which discounts the effect of strength (of strength (σσcc) of intact rock material & orientation of joints. ) of intact rock material & orientation of joints.

Heavy blastingHeavy blasting creates new fractures, hence it is suggested creates new fractures, hence it is suggested that 10 points should be added to get RMR for that 10 points should be added to get RMR for undisturbedundisturbed rock rock masses (e.g. excavation by TBM & road headers), and 3 to 5 masses (e.g. excavation by TBM & road headers), and 3 to 5 points may be added depending upon the quality of the points may be added depending upon the quality of the controlled blasting.controlled blasting.

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Estimation of RMREstimation of RMR::On the basis of RMR values for a given engineering structure, On the basis of RMR values for a given engineering structure, the rock mass is classified in 5 classes (see Table 10) as: the rock mass is classified in 5 classes (see Table 10) as:

Group I:Group I: Very goodVery good RMR 100 RMR 100 –– 81 81 Group II:Group II: GoodGood RMR 80 RMR 80 –– 6161Group III:Group III: FairFair RMR 60 RMR 60 –– 4141Group IV:Group IV: PoorPoor RMR 40 RMR 40 –– 2121Group V:Group V: Very poor Very poor RMR < 20. RMR < 20.

Separate RMR should be obtained for tunnels of different Separate RMR should be obtained for tunnels of different orientations after taking into account the orientation of tunnelorientations after taking into account the orientation of tunnelaxis with respect to the critical joint set (Table 6). axis with respect to the critical joint set (Table 6).

In terms of quality & mass strength, group I rock is more In terms of quality & mass strength, group I rock is more suitable for excavation of tunnel compared to groups with suitable for excavation of tunnel compared to groups with lower RMR, the tunnel also requires less support.lower RMR, the tunnel also requires less support.

Table 10: Design parameters & engineering Table 10: Design parameters & engineering properties of rock mass (properties of rock mass (BieniawskiBieniawski, 1979), 1979)

Rock Mass Rating (Rock Class)

Parameters & properties of rock mass

100-81 (I) 80-61 (II) 60-41(III) 40-21 (IV) < 20 (V) Classification of rock mass

Very good Good Fair Poor Very poor

Average stand-up time

10 years for 15 m span

6 months for 8 m span

1 week for 5 m span

10 hours for 2.5 m span

30 min.for 1 m span

Cohesion of rock mass (MPa)*

> 0.4

0.3 – 0.4

0.2 – 0.3

0.1 – 0.2

< 0.1

Angle of internal friction of rock mass

> 450

350 – 450

250 – 350

150 – 250

150

Note * These values are applicable to slopes only in saturated and weathered rock mass

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Estimation of RMREstimation of RMR::

Separate RMR should be obtained for tunnels of different Separate RMR should be obtained for tunnels of different orientations after taking into account the orientation of tunnelorientations after taking into account the orientation of tunnelaxis with respect to the critical joint sets (Table 6). axis with respect to the critical joint sets (Table 6).

RMR can be used for estimating many useful parameters such RMR can be used for estimating many useful parameters such as the as the unsupported spanunsupported span, the , the standstand--up timeup time (bridging action (bridging action period) & the period) & the support pressuresupport pressure for an underground opening. for an underground opening.

It can also be used for selecting a It can also be used for selecting a method of excavationmethod of excavation & & permanent supportpermanent support system for underground excavation in rock system for underground excavation in rock ((BieniawskiBieniawski, 1976)., 1976).

Deformation modulusDeformation modulus & & allowable bearing pressureallowable bearing pressure may also may also be estimated. be estimated.

Table 6: Orientation of discontinuitiesTable 6: Orientation of discontinuities

A. Orientation of tunnel/slope/foundation axis: ……….. B. Orientation of discontinuities Set-1 Average strike: ….. (from ….. to ….. ) Dip ….. Set-2 Average strike: ….. (from ….. to ….. ) Dip ….. Set-3 Average strike: ….. (from ….. to ….. ) Dip …..

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Application of RMRApplication of RMR::

(1) (1) Average standAverage stand--up time for arched roofup time for arched roof::

The standThe stand--up time depends upon effective span of the opening up time depends upon effective span of the opening which is defined as the width of the opening or the distance which is defined as the width of the opening or the distance between the tunnel face and the last support, whichever is smallbetween the tunnel face and the last support, whichever is smaller. er. For arched openings the standFor arched openings the stand--up time would be significantly up time would be significantly higher than that for a flat roof. higher than that for a flat roof.

Controlled blasting will further increase the standControlled blasting will further increase the stand--up time as up time as damage to the rock mass is decreased. damage to the rock mass is decreased.

It is important not It is important not to delay supporting of the roofto delay supporting of the roof in the case of in the case of rock with high standrock with high stand--up time, as this may lead to deterioration in up time, as this may lead to deterioration in the rock which ultimately reduces the standthe rock which ultimately reduces the stand--up time. up time.

Relationship between RMR rating, standRelationship between RMR rating, stand--up time & up time & UnsuppotedUnsuppoted span span ((BieniawskiBieniawski, 1989), 1989)

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(2) (2) Estimation of support pressureEstimation of support pressure::

The estimation of support pressure for openings with flat roof The estimation of support pressure for openings with flat roof is given as (is given as (UnalUnal, 1983): , 1983):

PPvv = {(100 = {(100 −− RMR) / 100} RMR) / 100} ×× γγ ×× BB

PPvv : support pressure: support pressureγγ : rock density: rock densityBB : tunnel width: tunnel width



For rock tunnel with arched roof the estimation of shortFor rock tunnel with arched roof the estimation of short--term term support pressure is given as (support pressure is given as (GoelGoel & & JethwaJethwa, 1991): , 1991):

PPvv = {(0.75 = {(0.75 ×× BB0.10.1 ×× HH0.50.5 −− RMR) / (2 RMR) / (2 ×× RMR)} RMR)} MPaMPa

PPvv : short: short--term roof support pressure (term roof support pressure (MPaMPa))H : depth of tunnel in m (> 50 m)H : depth of tunnel in m (> 50 m)BB : span of opening in m: span of opening in m((Method of excavation by conventional blasting methodMethod of excavation by conventional blasting method))Comprehensive guidelines Comprehensive guidelines

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BieniawskiBieniawski (1989) provides a comprehensive guidelines for (1989) provides a comprehensive guidelines for selection of tunnel selection of tunnel stabilisationstabilisation methods. This is applicable to methods. This is applicable to tunnels excavated with conventional drill & blast method. tunnels excavated with conventional drill & blast method.

These guidelines depend upon factors like depth below These guidelines depend upon factors like depth below surface (in situ overburden stress) tunnel size & shape & surface (in situ overburden stress) tunnel size & shape & method of excavation. The method of excavation. The stabilisationstabilisation measures are the measures are the permanent and not temporary (or primary) support. permanent and not temporary (or primary) support.

Table 11: RMR guide for excavation & Table 11: RMR guide for excavation & stabilisationstabilisation methods in methods in rock tunnelrock tunnel

Excavation shape: horseshoe; width: 10m; vertical stress: 25MPa; construction method drill and blast Rock Mass Excavation Support

Rating Rock bolts (20mm dia. fully bonded)

Shotcrete Steel sets

Very good rock (I)

RMR: 81-100

Full face 3m advance

Generally, no support required except for occasional spot bolting

Good rock (II)

RMR: 61-80

Full face 1.0-1.5m advance. Complete support 20m from

face.

Local bolts in crown 3m long, spaced 2.5m with occasional

wire mesh.

50mm in crown where required

None

Fair rock (III)

RMR: 41-60

Top heading and bench 1.5 - 3m in advance top heading.

Commence support after each blast. Complete support 10m

from face.

Systematic bolts 4m long, spaced 1 - 2m in crown and

walls with wire mesh in crown.

50 - 100mm in crown and

100mm in sides

None

Poor rock (IV)

RMR 21-40

Top heading and bench 1.0 - 1.5m advance in top heading.

Install support concurrently with excavation 10m from face.

Systematic bolts 4 - 5m long, spaced 1 - 1.5m in crown and

walls with wire mesh.

100 - 150mm in crown and

100mm in sides

Light to medium ribs spaced 1.5m where required

Very poor rock (V)

RMR < 20

Multiple drifts. 0.5 - 1.5m advance in top heading. Install

support concurrently with excavation. Shotcrete as soon

as possible after blasting.

Systematic bolts 5 - 6m long, spaced 1 - 1.5m in crown and

walls with wire mesh. Bolt invert

150 - 200mm in crown, 150mm in

sides and 50mm on face

Medium to heavy ribs spaced 0.75m with steel lagging and forepoling if

required

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(3) (3) Modulus of deformationModulus of deformation::Nicholson & Nicholson & BieniawskiBieniawski (1990) developed an empirical (1990) developed an empirical expression for modulus reduction factor (MRF). This factor is expression for modulus reduction factor (MRF). This factor is calculated in order to derive modulus of deformation for a rock calculated in order to derive modulus of deformation for a rock mass using its RMR and Youngmass using its RMR and Young’’s modulus of its core sample: s modulus of its core sample:

MRF = EMRF = Edd / / EErr = 0.0028 RMR= 0.0028 RMR22 + 0.9 + 0.9 ×× expexp(RMR/22.82(RMR/22.82))

Where:Where: EEdd is deformation modulus of rock mass is deformation modulus of rock mass EErr is deformation modulus of intact rock material is deformation modulus of intact rock material


(3) (3) Modulus of deformationModulus of deformation::MitriMitri et al. (1994) used the following equation to derive the et al. (1994) used the following equation to derive the modulus of deformation of rock masses: modulus of deformation of rock masses:

MRF = EMRF = Edd / / EErr = 0.5 { 1 = 0.5 { 1 −− coscos ( ( ππ ×× ((RMR / 100) }RMR / 100) }

Where:Where: EEdd is deformation modulus of rock mass is deformation modulus of rock mass EErr is deformation modulus of intact rock materialis deformation modulus of intact rock material

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(3) (3) Modulus of deformationModulus of deformation::

HoekHoek & Brown (1997) suggested the modulus of deformation of & Brown (1997) suggested the modulus of deformation of rock masses: rock masses:

EEdd = {= {√√ ((qqcc) / 10} ) / 10} ×× {10 {10 (RMR (RMR −− 10)/4010)/40 } } GPaGPa, q, qc c ≤≤ 100Mpa100Mpa

Where: qWhere: qcc is average is average uniaxialuniaxial crushing strength of intact rock crushing strength of intact rock material in material in MPaMPa..

The modulus of deformation of poor rock masses with water The modulus of deformation of poor rock masses with water sensitive minerals decrease significantly after saturation and sensitive minerals decrease significantly after saturation and with time after excavation. with time after excavation.


(4) (4) Allowable bearing pressureAllowable bearing pressure::

Foundation on weak, heterogeneous and highly undulating Foundation on weak, heterogeneous and highly undulating surfaces of subsurfaces of sub--surface rock mass may pose serious problem of surface rock mass may pose serious problem of differential settlement. The design of foundation depends on thedifferential settlement. The design of foundation depends on thesubsurface strata and its bearing capacity subsurface strata and its bearing capacity

Where the foundation rests directly on bedrocks (e.g. spun pile Where the foundation rests directly on bedrocks (e.g. spun pile & & endend--bearing pile), the bearing pressure can be obtained from bearing pile), the bearing pressure can be obtained from available classification tables.available classification tables.

Pressure acting on a rock bed due to building foundation should Pressure acting on a rock bed due to building foundation should not be more than the safe bearing capacity of rock foundation not be more than the safe bearing capacity of rock foundation system taking into account the effect of eccentricity. system taking into account the effect of eccentricity.

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It is often useful to estimate the safe bearing pressure (SBP) fIt is often useful to estimate the safe bearing pressure (SBP) for or preliminary design on the basis of the classification approach preliminary design on the basis of the classification approach (e.g. RMR). (e.g. RMR).

Orientation of joints plays a dominant role in stress distributiOrientation of joints plays a dominant role in stress distribution on below strip footing due to low shear modulus of bedrocks. below strip footing due to low shear modulus of bedrocks. Bearing capacity of rocks will be drastically low for near vertiBearing capacity of rocks will be drastically low for near vertical cal joints that strike parallel to the footing length as pressure bujoints that strike parallel to the footing length as pressure bulb lb extends deep into the strata extends deep into the strata –– see Figure.see Figure.

Shear zone and clay gouge, if present below foundation level, Shear zone and clay gouge, if present below foundation level, need to be treated to improve bearing capacity & reduce need to be treated to improve bearing capacity & reduce differential settlement.differential settlement.

Orientation of joints and stress distribution below strip footinOrientation of joints and stress distribution below strip footing due g due to low shear modulus (Singh, 1973)to low shear modulus (Singh, 1973)

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Similar to the effect of rocks displaying minerals arrangement Similar to the effect of rocks displaying minerals arrangement (lamination & foliation) (lamination & foliation) -- variations in UCS & failure strainvariations in UCS & failure strain


A rock mass classification for assessing net SBP is shown in A rock mass classification for assessing net SBP is shown in Table 12 (Peck et al., 1974). Table 12 (Peck et al., 1974).

The terms net SBP and the The terms net SBP and the allowable bearing pressureallowable bearing pressure (ABP) are (ABP) are similar in terms of meaning. But, the net SBP here means the similar in terms of meaning. But, the net SBP here means the ultimate SBP, whereas the ABP means bearing pressure being ultimate SBP, whereas the ABP means bearing pressure being considered for the designs (considered for the designs (i.ei.e ABP after taking into account the ABP after taking into account the FOS).FOS).

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Table 12: Net safe bearing pressure Table 12: Net safe bearing pressure qqnsns for various for various rock types (Peck et al., 1974)rock types (Peck et al., 1974)

Rock types / material Safe Bearing Pressure qns (t/m2)

Massive crystalline bedrock including granite, diorite, gneiss, hard limestone & dolomite

1000

Foliated rocks such as slate and schist in sound condition

400

Bedded limestone in sound condition

400

Sedimentary rock including hard shale and sandstone

250

Soft or broken bed rock (excluding shale) & limestone

100

Soft shales 30

Estimation ofEstimation of allowable bearing pressure using RMRallowable bearing pressure using RMR::

The RMR system may also be used to obtain net ABP as proposed The RMR system may also be used to obtain net ABP as proposed by by MehrotraMehrotra (1992). (1992). The guidelines for the ABP (Table 13)The guidelines for the ABP (Table 13) are:are:

(1) The RMR should be obtained below the foundation at depth (1) The RMR should be obtained below the foundation at depth equal to the width of the foundation, provided RMR does not equal to the width of the foundation, provided RMR does not change with depth. If the upper part of the rock, within a depthchange with depth. If the upper part of the rock, within a depth of of about one fourth of the foundation width, is of lower quality thabout one fourth of the foundation width, is of lower quality the e value of this part should be used or the inferior rock should bevalue of this part should be used or the inferior rock should bereplaced with concrete. Values of replaced with concrete. Values of qqaa in the Table are to limit in the Table are to limit settlement & must not be increased if foundation is embedded settlement & must not be increased if foundation is embedded into rock. into rock.

(2) During earthquake loading, the values ABP may be increased (2) During earthquake loading, the values ABP may be increased by 50% in view of by 50% in view of rheologicalrheological behaviourbehaviour of rock masses.of rock masses.

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Table 13: Net allowable bearing pressure Table 13: Net allowable bearing pressure qqaa based on based on RMR (RMR (MehrotraMehrotra, 1992), 1992)

Class No. I II III IV V Description of rock

Very good

Good Fair Poor Very poor

RMR 100-81 80-61 60-41 40-21 20-0 qa (t/m2) 600-440 440-280 280-135 135-45 45-30

Classification for bearing pressureClassification for bearing pressure::

Another classification of rock masses for ABP is given in Another classification of rock masses for ABP is given in Table 14Table 14((KrahenbuhlKrahenbuhl & Wagner,1983). & Wagner,1983). [Note*: the 2[Note*: the 2ndnd column of Table 14 indicates sites with highly column of Table 14 indicates sites with highly weathered rock and weathered rock and unfavourableunfavourable geological structures, geological structures, subjected to instability]. subjected to instability].

There is a correlation between RQD and ABP but relatively There is a correlation between RQD and ABP but relatively conservative compared to conservative compared to Table 14Table 14. For . For socketedsocketed piles & shallow piles & shallow foundations, Gill (1980) gives the following formula: foundations, Gill (1980) gives the following formula:

Allowable Bearing Pressure, Allowable Bearing Pressure, qqaa = q= qcc ×× NNjj ×× NNdd

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Table 14: Allowable Pressure Table 14: Allowable Pressure qqaa of various rock types under of various rock types under different weathering conditions (different weathering conditions (KrahenbuhlKrahenbuhl & Wagner, 1983)& Wagner, 1983)

Rock Type Highly weathered structure un-favourable for stability*

Fairly weathered structure favourable for stability

Highly weathered structure favourable for stability

Fairly weathered structure favourable for stability

Un-weathered rock structure un-favourable for stability

Un-weathered rock structure favourable for stability

Marls, marls interbedded with sandstone

15

30

35

50

60

110

Calc-schist, calc-schist interbedded with quarzites

15

30

45

65

100

200

Slates, phyllites, schists interbedded with hard sandstones & quartzite or gneiss

20

35

60

75

90

130

Limestone, dolomite & marbles

50

80

90

130

150

200

Sandstone 40 to 60 (massive)

90 120 150 170 220

Calcareous conglomerates (massive)

60

100

120

200

200

330

Quartzite (massive)

50 to 70 150 120 180 200 330

Gneiss (massive) 30 to 60 150 120 180 200 330 Granite & plutonic rocks

20 250 > 330 − − −



Where:Where:qqcc: avg. laboratory : avg. laboratory uniaxialuniaxial compressive strengthcompressive strengthNNjj: empirical : empirical coeffcoeff. depending on the spacing of discontinuities . depending on the spacing of discontinuities ((see Table 15see Table 15), and calculated as:), and calculated as:

s: spacing of joint in cms: spacing of joint in cmB: footing width in cmB: footing width in cm

( ))s

Bs

jNδ×+×

+=300(110

3

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Table 15: Value of Joint Spacing & Table 15: Value of Joint Spacing & NNjj for estimation of for estimation of Allowable Bearing PressureAllowable Bearing Pressure

Spacing of discontinuities, cm Nj 300

0.4

100 – 300

0.25

30 – 100

0.1



δδ: opening of joints in cm: opening of joints in cmNNdd isis = 0.8 + 0.2 (h/D) < 2= 0.8 + 0.2 (h/D) < 2

≥≥ 1.01.0= = 1.01.0 for shallow foundations of buildingsfor shallow foundations of buildings

h: depth of socket in rockh: depth of socket in rockD: diameter of socketD: diameter of socket

( ))s

Bs

jNδ×+×

+=300(110

3

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Equation Equation qqaa = q= qcc ×× NNjj ×× NNdd may also be applied to shallow may also be applied to shallow foundation considering foundation considering NNdd = 1.0. It may be noted that the above = 1.0. It may be noted that the above correlation does not take into account for orientation of jointscorrelation does not take into account for orientation of joints. .

It is recommended that plate load test should be conducted on It is recommended that plate load test should be conducted on poor rocks where ABP is like to be less than 100 t/mpoor rocks where ABP is like to be less than 100 t/m22. .

Uncertainties on ABP may be improved by a larger number of Uncertainties on ABP may be improved by a larger number of observation pits, say at a rate of at least 3 pits per importantobservation pits, say at a rate of at least 3 pits per importantstructure. The load test should be conducted in the pit structure. The load test should be conducted in the pit representing the poorest rock qualities. representing the poorest rock qualities.

Coefficient of Elastic Uniform Compression for machine Coefficient of Elastic Uniform Compression for machine foundationfoundation::

The coefficient of uniform compression CThe coefficient of uniform compression Cuu is defined as the ratio is defined as the ratio between pressure and corresponding settlement of block between pressure and corresponding settlement of block foundationfoundation. .

Typical values of CTypical values of Cuu for machine foundation on rock mass are for machine foundation on rock mass are listed in Table 16. listed in Table 16.

The coefficient of uniform shear is generally taken as CThe coefficient of uniform shear is generally taken as Cuu/2. It may /2. It may be noted that Cbe noted that Cuu is less than 10 kg/cmis less than 10 kg/cm33 in very poor rocks. in very poor rocks.

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Table 16: Coefficient of Elastic Uniform Compression Table 16: Coefficient of Elastic Uniform Compression CCuu for rock masses (for rock masses (RanjanRanjan et al., 1982)et al., 1982)

Rock type Allowable Bearing Press (t/m2)

Cu (kg/cm2/cm)

Weathered granites − 17 Massive limestones 160 25 Flaky limestones 75 12 Shaly limestones 50 7 Soft shales 45 7 Saturated soft shales 33 1.5 Saturated non-plastic shales

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CHP 9.2 (RMR)

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