geotech - rockmass class

7/30/2019 Geotech - Rockmass Class

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ROCK MASS CLASSIFICATIONS

Marek Caa Dept. of Geomechanics, Civil Engineering & Geotechnics


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Rock Mass ClassificationRock Mass Classification

Why?

How does this help us in tunnel design?



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Rock Mass ClassificationRock Mass Classification

WHY?WHY?



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Ground interactionGround interaction



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Summary of rock mass characteristics, testingSummary of rock mass characteristics, testing

methods and theoretical considerationsmethods and theoretical considerations



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Engineering Rock Mass Classification SchemesEngineering Rock Mass Classification Schemes

Developed for estimation of tunnel support

Used at project feasibility and preliminary design stages

Simple check lists or detailed schemes Used to develop a picture of the rock mass and its

variability

Used to provide initial empirical estimates of tunnel

support requirements

Are practical engineering tools which force the user toexamine the properties of the rock mass

Do Not replace detailed design methods

Project specific



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TerzaghisTerzaghis Rock Mass Classification (1946)Rock Mass Classification (1946)

Rock Mass Descriptions

Intact

Stratified

Moderately jointed

Blocky and Seamy

Crushed

Squeezing

Swelling



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Intact rock contains neither joints nor hair cracks. Hence, ifit breaks, it breaks across sound rock. On account of theinjury to the rock due to blasting, spalls may drop off theroof several hours or days after blasting. This is known as

a spalling condition. Hard, intact rock may also beencountered in the popping condition involving thespontaneous and violent detachment of rock slabs fromthe sides or roof.

Stratified rock consists of individual strata with little or noresistance against separation along the boundariesbetween the strata. The strata may or may not be

weakened by transverse joints. In such rock the spallingcondition is quite common.

Moderately jointed rock contains joints and hair cracks, butthe blocks between joints are locally grown together or so

intimately interlocked that vertical walls do not requirelateral support. In rocks of this type, both spalling and



T hiT hi R k M Cl ifi i (1946)R k M Cl ifi ti (1946)


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Blocky and seamy rock consists of chemically intact or almost

intact rock fragments which are entirely separated from eachother and imperfectly interlocked. In such rock, vertical wallsmay require lateral support.

Crushed but chemically intact rock has the character of crusher

run. If most or all of the fragments are as small as fine sandgrains and no recementation has taken place, crushed rockbelow the water table exhibits the properties of a water-bearing

sand. Squeezing rock slowly advances into the tunnel without

perceptible volume increase. A prerequisite for squeeze is ahigh percentage of microscopic and sub-microscopic particles of

micaceous minerals or clay minerals with a low swellingcapacity.

Swelling rock advances into the tunnel chiefly on account of

expansion. The capacity to swell seems to be limited to thoserocks that contain clay minerals such as montmorillonite, with ahigh swelling capacity.




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Rock Quality Designation Index (RQD)Rock Quality Designation Index (RQD)

(Deere et al. 1967)(Deere et al. 1967)

Aim : to provide a quantitative estimate of rock mass

quality from drill logs

Equal to the percentage of intact core pieces longer than

100mm in the total length of core



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RQDRQD

Directionally dependant parameter

Intended to indicate rock mass quality in-situ

Adapted for surface exposures as Jv number of

discontinuities per unit volume

Used as a component in the RMR and Q systems

Palmstrom (1982)

Priesta i Hudsona (1976)

- number of joints per unit length

vJRQD 3.3115 =

( ) 1.01.01100 += eRQD



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Procedure for Measurement and Calculation of RQDProcedure for Measurement and Calculation of RQD



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Weathering of Basalt with depthWeathering of Basalt with depth



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Multi parameter Rock Mass ClassificationMulti parameter Rock Mass Classification

SchemesSchemes

Rock Mass Structure Rating (RSR)

Rock Mass Rating (RMR)

Rock Tunnelling Quality Index (Q)

Geological Strength Index (GSI)



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Rock Mass Structure Rating (RSR) (1972)Rock Mass Structure Rating (RSR) (1972)

Introduced the concept of rating components to arrive at

a numerical value

Demonstrates the logic in a quasi-quantitative rock mass

classification

Has limitations as based on small tunnels supported by

steel sets only

RSR = A + B + C



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Rock Structure RatingRock Structure RatingParameter A: General area geologyParameter A: General area geology

Considers (a) rock type origin

(b) rock hardness

(c) geotechnical structure



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Considers (a) joint spacing

(b) joint orientation (strike and dip)

(c) direction of tunnel drive

Rock Structure RatingRock Structure RatingParameter B: Geometry : Effect of discontinuity patternParameter B: Geometry : Effect of discontinuity pattern



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Considers (a) overall rock mass quality (on the basis of A + B)

(b) joint condition

(c) water inflow

Rock Structure RatingRock Structure RatingParameter C: Groundwater, joint conditionParameter C: Groundwater, joint condition



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RSR support estimates for a 7.3m diameterRSR support estimates for a 7.3m diameter

circular tunnelcircular tunnel

(After Wickham et al. 1972)

Examples

RSR = 62

2 shotcrete

1 rockbolts @

5ft centres

RSR = 30

5 shotcrete

1 rockbolts @

2.5ft centres

OR 8WF31 steel

sets @ 3ft centres



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Geomechanics Classification orGeomechanics Classification or

Rock Mass Rating System (RMR) (Rock Mass Rating System (RMR) (BieniawskiBieniawski 1976)1976)

Based upon

uniaxial compressive strength of rock material rock quality designation (RQD)

spacing of discontinuities

condition of discontinuities

groundwater conditions

orientation of discontinuities



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Rock Mass Rating SystemRock Mass Rating System

Rock mass divided into structural regions

Each region is classified separately Boundaries can be rock type or structural, eg: fault

Can be sub divided based on significant changes, eg:

discontinuity spacing



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BUT: 1976 to 1989 Bieniawski

System refined by greater data Ratings for parameters changed

Adapted by other workers for different situations

PROJECT SPECIFIC SYSTEMS



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Development of Rock Mass Rating SystemDevelopment of Rock Mass Rating System



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(After Bieniawski 1989)



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Rating Class Description

81-100 I Very Good Rock 61-80 II Good Rock

41-60 III Fair Rock

12-40 IV Poor Rock

Less than 20 V Very Poor Rock



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Guidelines for excavation and support of 10mGuidelines for excavation and support of 10m

span rock tunnels in accordance with the RMR systemspan rock tunnels in accordance with the RMR system

(After Bieniawski 1989)



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Prediction of inPrediction of in--situ deformation modulus Esitu deformation modulus Emm

from rock mass classificationsfrom rock mass classifications




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Nicholson & Bieniawski (1990)

Bieniawski (1978) and Serafim & Pereira (1983)

Hoek i Brown (1997)

Verman (1993

H depth, = 0.16-0.3 (decreases with rock strength)

)82.22/(2

9.00028.0RMR

s

rm

eRMRE

E

+=

)(501002 GPaRMRforRMREm >=

)(5010 40/)10( GPaRMRforE RMRm


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Estimates of support capacity for tunnelsEstimates of support capacity for tunnels

of different sizesof different sizes




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Support pressure - Unal (1983)

s - tunnel width

sRMR

pv

= 100

100

Hoek (1994): m m ei

RMR

=100

28

s e

RMR

=100

9

mi - constant from 4 (weak shales) to 32 (granite).

R sRcrm c= ( )RR

m m srrmc

= +2 42

Aydan & Kawamoto (2000)5.20016.0 RMRRcrm =

Kalamaras & Bieniawski (1995)85

15

2

=

RMRRR ccrm




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Aydan & Kawamoto (2000)

( )RMRRMRRMR

RR ccrm +=

1006

Lets assume: 60=RMR MPaRc 80=

Hoek:

Aydan:

Kalamaras & Bieniawski:

MPaRc 67.8=

MPaRc 62.44=

MPaRc 18.21=

Aydan & Kawamoto (2000) RMRrm 05.022 +=

rm

rmcrm

rm

R

c

cos

sin1

2

=Marek Caa Dept. of Geomechanics, Civil Engineering & Geotechnics

k lli Q li d Q i d


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Rock Tunnelling Quality Index QRock Tunnelling Quality Index QBartonBarton,, LienLien,, LundeLunde

Based on case histories in Scandinavia

Numerical values on a log scale

Range 0.001 to 1000


Q Cl ifi i SQ Cl ifi i S


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Q Classification SystemQ Classification System

(After Barton et al. 1974)


Q Cl ifi ti S tQ Cl ifi ti S t


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represents the structure of the rockmass crude measure of block or particle size


Q Cl ifi ti S tQ Cl ifi ti S t


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represents roughness and frictionalcharacteristics of joint walls or infill material


Q Cl ifi ti S tQ Classification System


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consists of two stress parameters SRF can be regarded as a total stress parameter

measure of

loosening load as excavated through shear zones

rock stress in competent rock

squeezing loads in plastic incompetent rock

JW is a measure of water pressure


Classification of individual parameters used inClassification of individual parameters used in


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the Tunnelling Quality Index Qthe Tunnelling Quality Index Q




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the Tunnelling Quality Index Qthe Tunnelling Quality Index Q (contd)




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the Tunnelling Quality Index Qthe Tunnelling Quality Index Q (contd)


Q Classification SystemQ Classification SystemSRFSRF updateupdate


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Q Classification SchemeQ Classification Scheme


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Resolves to three parameters

Block size ( RQD / Jn

)

Interblock shear strength ( Jr/ Ja )

Active stress ( Jw / SRF )




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Resolves to three parameters

Block size ( RQD / Jn

)

Interblock shear strength ( Jr/ Ja )

Active stress ( Jw / SRF )

Does NOT include joint orientation


Equivalent Dimension DEquivalent Dimension Dee


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Estimated support categories based on theEstimated support categories based on the

t lli lit i d Qtunnelling quality index Q


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tunnelling quality index Qtunnelling quality index Q




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Roof pressure:31

= Q

JJp

r

nroof

Length of the bolts: (roof) (walls)ESR

sL

15.02 +=

31

32.0

= QJ

Jpr

n

roof

Bhasin & Grimstad (1996): 3140

= Q

J

sp

r

roof

Youngs modulus:

Seismic wave velocity: ]/[100

log5.3 skmR

QV cp +=


LH

ESR=+2 0 15.

[ ]MPaR

QE c33

310=

RMRRMRQQ -- CorrelationsCorrelations


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Rock Mass Classification SystemRock Mass Classification System


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RMR and Q system or variants are the most widely used

both incorporate geological, geometric and

design/engineering parameters to obtain a value of

rock mass quality

empirical and require subjective assessment


Rock Mass Classification SystemRock Mass Classification System


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Approach:

accurately characterise the rockmass ie: full and

complete description of the rockmass

assign parameters for classification later

always use two systems for comparison


Geological Strength Index (GSI)Geological Strength Index (GSI)


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Method to link the constants m and s of Hoek-Brown

failure criterion to observations in the field

ie: a possible solution to the problem of estimating

strength of jointed rockmass

A system for estimating the reduction in rockmassstrength for different geological conditions

Overcomes deficiencies of RMR for poor quality rock


Estimate of Geological StrengthEstimate of Geological Strength

Index GSIIndex GSI

b d l i l d i i

Estimation of constants based upon rockEstimation of constants based upon rock

mass structure and discontinuity surfacemass structure and discontinuity surface

di i


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based on geological descriptionsbased on geological descriptions conditionsconditions


Geological Strength Index (GSI)Geological Strength Index (GSI)


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Geological Strength IndexGeological Strength Index

(GSI)(GSI)


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(GSI)(GSI)

Estimate of Geological

Strength Index GSI

based on geologicaldescriptions.


Plots of cohesive strength and friction anglesPlots of cohesive strength and friction angles

f diff t GSI df diff t GSI d ll


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for different GSI andfor different GSI andmmii valuesvalues


Klasyfikacja KF


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