ROCK MASS CLASSIFICATIONS

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ROCK MASS CLASSIFICATIONS

Marek Cała – Dept. of Geomechanics, Civil Engineering & Geotechnics

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Rock Mass Classification Rock Mass Classification

• Why?

• How does this help us in tunnel design?

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Rock Mass Classification Rock Mass Classification

WHY?WHY?

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Ground interaction Ground interaction

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

methods and theoretical considerations methods and theoretical considerations

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Types of failure which occur in rock masses Types of failure which occur in rock masses

under low and high in-situ stress levels under low and high in-situ stress levels

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Engineering Rock Mass Classification Schemes Engineering 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 to examine the properties of the rock mass

• Do Not replace detailed design methods

• Project specific

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Terzaghi’s Rock Mass Classification (1946) Terzaghi’s 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, if it breaks, it breaks across sound rock. On account of the injury to the rock due to blasting, spalls may drop off the roof several hours or days after blasting. This is

known as a spalling condition. Hard, intact rock may also be encountered in the popping condition involving the spontaneous and violent detachment of rock slabs from the sides or roof.

• Stratified rock consists of individual strata with little or no resistance against separation along the boundaries

between the strata. The strata may or may not be

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

• Moderately jointed rock contains joints and hair cracks, but the blocks between joints are locally grown together or so intimately interlocked that vertical walls do not

require lateral support. In rocks of this type, both spalling and popping conditions may be encountered.

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• Blocky and seamy rock consists of chemically intact or

almost intact rock fragments which are entirely separated from each other and imperfectly interlocked. In such rock, vertical walls may 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 sand grains and no recementation has taken place,

crushed rock below 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 a high percentage of microscopic and sub-microscopic particles of micaceous minerals or clay minerals with a low swelling capacity.

• Swelling rock advances into the tunnel chiefly on account of expansion. The capacity to swell seems to be limited to those rocks that contain clay minerals such as montmorillonite,

with a high 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 ‘J_v’ 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

Jv

RQD 115  3.3



¹ ⁰^.¹

^ 

⁰^.¹^

100  ^

 e

RQD

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

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Weathering of Basalt with depth Weathering of Basalt with depth

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Multi parameter Rock Mass Classification Multi parameter Rock Mass Classification

Schemes Schemes

• 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 Rating Rock Structure Rating

Parameter A: General area geology Parameter 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

Parameter B: Geometry : Effect of discontinuity pattern Parameter 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

Parameter C: Groundwater, joint condition Parameter C: Groundwater, joint condition

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

circular tunnel circular 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 or Geomechanics Classification or

Rock Mass Rating System (RMR) (Bieniawski 1976) Rock Mass Rating System (RMR) (Bieniawski 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 System Rock 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 System Development 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 10m Guidelines for excavation and support of 10m

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

(After Bieniawski 1989)

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Prediction of in-situ deformation modulus E Prediction of in-situ deformation modulus E_m_m

from rock mass classifications from 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 0.9 0028

.

0 ^RMR

s

rm RMR e

E

E  

) (

50 100

2 RMR for RMR GPa

E

_m

  

) (

50

10

⁽ ¹⁰⁾^/⁴⁰

for RMR GPa E

_m



^RMR^



40 / ) 10

10(

10

 

 ^c ^RMR

m

E R

) (

10 3

.

0 H ⁽ ²⁰⁾^/³⁸ GPa E_m  ^  ^RMR^

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Prediction of in-situ deformation modulus E

Prediction of in-situ deformation modulus E_m_mfrom rock massfrom rock mass classificationsclassifications

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Estimates of support capacity for tunnels Estimates of support capacity for tunnels

of different sizes of different sizes

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Rock Mass Rating System Rock Mass Rating System Support pressure - Unal (1983)

s - tunnel width

RMR s

p_v     100

100

Hoek (1994):

m m e

_i

RMR



100

28

s e

RMR



100 9

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

R

_crm

 sR

_c ^R^rrm ^ ^R₂^c



^m^ ^m² ^ ⁴^s



Aydan & Kawamoto (2000) R_crm  0.0016RMR²^.⁵

Kalamaras & Bieniawski (1995)

85

15 2

 R RMR  R_crm ^c

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Rock Mass Rating System Rock Mass Rating System Aydan & Kawamoto (2000)

 ^RMR 

RMR R RMR R

_crm _c



 

100 6

Let’s assume:

RMR  60

R_c  80 MPa

Hoek:

Aydan:

Kalamaras & Bieniawski:

MPa R_c  8.67

MPa R_c  44.62

MPa R_c  21.18

Aydan & Kawamoto (2000)



_rm

 22  0 . 05 RMR

rm rm rm crm

c R



 cos

sin 1

2  

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Rock Tunnelling Quality Index Q

Rock Tunnelling Quality Index Q – Barton, Lien, Lunde – Barton, Lien, Lunde

• Based on case histories in Scandinavia

• Numerical values on a log scale

• Range 0.001 to 1000

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‘‘Q’ Classification SystemQ’ Classification System

(After Barton et al. 1974)

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• represents the structure of the rockmass

• crude measure of block or particle size

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• represents roughness and frictional

characteristics of joint walls or infill material

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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

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Classification of individual parameters used in Classification of individual parameters used in

the Tunnelling Quality Index Q the Tunnelling Quality Index Q

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the Tunnelling Quality Index Q

the Tunnelling Quality Index Q (cont’d)

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the Tunnelling Quality Index Q

the Tunnelling Quality Index Q (cont’d)

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‘‘Q’ Classification SystemQ’ Classification System – SRF update – SRF update

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Q Classification Scheme Q Classification Scheme

Resolves to three parameters

• Block size ( RQD / J_n)

• Interblock shear strength ( J_r / J_a )

• Active stress ( J_w / SRF )

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Resolves to three parameters

• Block size ( RQD / J_n)

• Interblock shear strength ( J_r / J_a )

• Active stress ( J_w / SRF )

• Does NOT include joint orientation

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Equivalent Dimension D Equivalent Dimension D_e_e

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Estimated support categories based on the Estimated support categories based on the

tunnelling quality index Q tunnelling quality index Q

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Roof pressure: _^ ^³¹









  Q

J p J

r n roof

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

L  2  0.15s

3 1

3 2 .

0 















  Q

J p J

r n roof

Bhasin & Grimstad (1996): ⁴⁰ _ ^³¹



 



  Q

J p s

r roof

Young’s modulus:

Seismic wave velocity: [ / ]

log 100 5

.

3 R km s

Q V_p   ^c

L H

 2 0 15ESR.



^MPa



Q R E ³³ ^c

10 3



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RMR –

RMR – QQ - - CorrelationsCorrelations

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Rock Mass Classification System Rock Mass Classification System

• 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

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Rock Mass Classification System Rock Mass Classification System

Approach:

• accurately characterise the rockmass ie: full and complete description of the rockmass

• assign parameters for classification later

• always use two systems for comparison

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

• 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 rockmass strength for different geological conditions

• Overcomes deficiencies of RMR for poor quality rock

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Estimate of Geological Strength Estimate of Geological Strength

Index GSI Index GSI

based on geological descriptions based on geological descriptions

Estimation of constants based upon rock Estimation of constants based upon rock mass structure and discontinuity surface mass structure and discontinuity surface

conditions conditions

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

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Geological Strength Index Geological Strength Index

(GSI) (GSI)

Estimate of Geological Strength Index GSI

based on geological descriptions.

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

for different GSI and

for different GSI and mm_i_i values values

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ROCK MASS CLASSIFICATIONS