Engineering Rock Mechanics

Post-peak sfrengfh behaviour 147

Table 8.1 Approximate relations between rock mass quality and the material

constants in the Hoek-Brown failure criterion (from Hoek and Brown (1988)).

Undisturbed values are in italics

0.128

0.00009

0.947

0.00198

EMPIRICAL FAILURE CRITERION

01 = ma + \/~o.os + W!

01 = major principal atfeu

OS = minor principal atra

0. 
   
   
   
   • uniuial complask strenfi
     of intact mck. and
     m and 8 are rmpiiil constinu.
   
   
   
   

0.183 0.275 0.311

o.Ow0) 0.00009 0.00009

1.353 2.030 2.301

0.00198 0.00198 0.00196

INTACT ROCK SAMPLES

lataratwy size spdmenr fiec m 1.00

from dbconfi'nuit*r s 1.00

CSlR ratinc RMR = 100 m 7.00

NGI ratinc: Q E SW s 1.00

VERY COOD QUALITY ROCK MASS

lirhtb intcrkckhg undkfurkd rock m 2.40

w'th unweJtlkndjo*u at 1 to In. I 0.082

CSlR wing: RMR = 85 m 4.10

NCI filing: Q ic 100 s 0.189

COOD QUALITY ROCK MASS

~ Fresh to slightly mathered rock. srifFy m 0.575

disturbed dth PhtS Jf I lo 3m. s 0.00293

CSlR raring: RMR = 65 m 2.006

1 NGI rating: Q = io s 0.0205

0.029

0.000003

0.447

0.06019

10.00

1.00

10.00

1.00

3.43

0.082

5.85

0.189

0.821

0.00293

2.865

0.0205

0.041 0.061 0.069

O.OooOo3 0,000003 0.000003

0.639 0.959 1.087

0.00019 0.00019 0.00019

8.78 9.95

0.007

0.00a0001

0.219

0.00002

FAIR QUALITY ROCK MASS

%IJ/ YU Ormod.Pat#b WJfhend m

Mu spaad a~ 0.3 to lm. S

CSlR nling: RMR P 44 m

NCI rating: Q = 1 I

0.010 0.015 0.017

0.0000001 o.oow1 0.000000:

0.313 0.469 0.532

0.00002 0.00002 0.00001

c I

VERY POOR QUALITY ROCK MASS

Numerous heavily urafhudjoinu spaced rn

<SOmmdth~. Warfemc&m?h1Fna. s

CSlR rating: RMR = 3 m

NCI ratinc: Q = 0.01 I

25.00

1.00

25.00

1.00

8.56

0.082

14.63

0.189

2.052

0.00293

7.163

0.0205

0.458

0.00000)

3.381

0.00196

0.102

0.000w3

1.598

O.WO19

0.025

0.0000w

0.782

0.00002

8.3 Post-peak strength behaviour

In Chapter 6, on intact rock, and in Chapter 7, on discontinuities, wc have
demonstrated that it is possible to describe the complete mechanical
behaviour from initial deformation, via the peak strength, to the failure
process. In the case of intact rock, the post-peak strength behaviour can be
characterized using the shape of the complete stress-strain curve.
Similarly, in the case of discontinuities, we can discuss the residual frictional
value that is reached after the discontinuity has been fully sheared.
However, it is much more difficult to provide any simple characterization