ELSEVIER GEO-ENGINEERING BOOK SERIES VOLUME 5 Part 2 pps

Terzaghi’s rock load theory 33

Table 3.5 Recommendations of Singh et al. (1995) on support pressure for rock tunnels and caverns.

Terzaghi’s classification Classification of Singh et al. (1995)

Recommended support

pressure (MPa)

Category Rock condition Rock load factor Hp Category Rock condition pv ph Remarks

I. Hard and intact 0 I. Hard and intact 0 0 –

II. Hard stratified or

schistose

0 to 0.5B II. Hard stratified or schistose 0.04–0.07 0 –

III. Massive, moderately

jointed

0 to 0.25B III. Massive, moderately

jointed

0.0–0.04 0 –

IV. Moderately blocky

seamy and jointed

0.25B to 0.35

(B + Ht)

IV. Moderately blocky seamy

very jointed

0.04–0.1 0–0.2 pv Inverts may be

required

V. Very blocky and

seamy, shattered

arched

0.35 to 1.1 (B+ Ht) V. Very blocky and seamy,

shattered highly jointed,

thin shear zone or fault

0.1–0.2 0–0.5 pv Inverts may be

required, arched

roof preferred

VI. Completely crushed

but chemically intact

1.1 (B+ Ht) VI. Completely crushed but

chemically unaltered,

thick shear and fault zone

0.2–0.3 0.3–1.0 pv Inverts essential,

arched roof

essential

continued

34 Tunnelling in weak rocks

Table 3.5—Continued

Terzaghi’s classification Classification of Singh et al. (1995)

Recommended support

pressure (MPa)

Category Rock condition Rock load factor Hp Category Rock condition pv ph Remarks

VII. Squeezing rock at

moderate depth

1.1 to 2.1 (B+ Ht) VII. Squeezing rock

condition

VIIA. Mild squeezing

(ua/a upto 3%)

0.3–0.4 Depends on primary

stress values, ph may

exceed pv

Inverts essential. In

excavation flexible support

preferred. Circular section

with struts recommended.

VIIB. Moderate squeezing

(ua/a = 3 to 5%)

0.4–0.6 As above As above

VIII. Squeezing rock at

great depth

2.1 to 4.5 (B+ Ht) VIIC. High squeezing

(ua/a >5%)

6.0–1.4 As above As above

IX. Swelling rock upto 80 m VIII. Swelling rock

VIIIA. Mild swelling 0.3–0.8 Depends on type and

content of swelling

clays, ph may exceed

pv

Inverts essential in

excavation, arched roof

essential.

VIIIB. Moderate swelling 0.8–1.4 As above As above

VIIIC. High swelling 1.4–2.0 As above As above

Notations: pv = Vertical support pressure; ph = horizontal support pressure; B = width or span of opening; Ht = height of opening; ua = radial tunnel closure; a = B/2;

thin shear zone = upto 2 m thick.

Terzaghi’s rock load theory 35

by 5.5 m. The estimated roof support pressures from Table 3.5 were found comparable

with the measured values irrespective of the opening size and the rock conditions (Singh

et al., 1995). They have further cautioned that the support pressure is likely to increase

directly with the excavation width for tunnel sections through slickensided shear zones,

thick clay-filled fault gouges, weak clay shales and running or flowing ground conditions

where interlocking of blocks is likely to be missing or where joint strength is lost and

rock wedges are allowed to fall due to excessive roof convergence on account of delayed

supports beyond stand-up time. It may be noted that wider tunnels require reduced spacing

of bolts or steel-arches and thicker linings since rock loads increase directly with the

excavation width even if the support pressure does not increase with the tunnel size.

REFERENCES

Barton, N., Lien, R. and Lunde, J. (1974). Engineering classificaion of Rock Masses for the Design

of Tunnel Support. NGI Publication No.106, Oslo, 48.

Brekke, T. L. (1968). Blocky and seamy rock in tunnelling. Bull. Assoc. Eng. Geol., 5(1), 1-12.

Cecil, O. S. (1970). Correlation of Rock Bolts – Shotcrete Support and Rock Quality Parameters in

Scandinavian Tunnels. PhD thesis, University of Illinois, Urbana, 414.

Deere, D. U., Peck, R. B., Parker, H., Monsees, J. E. and Schmidt, B. (1970). Design of tunnel

support systems. High Res. Rec., 339, 26-33.

Goel, R. K., Jethwa, J. L. and Dhar, B. B. (1996). Effect of tunnel size on support pressure. Technical

Note. Int. J. Rock Mech. and Min. Sci. & Geomech. Abstr., Pergamon, 33(7), 749-755.

Rose, D. (1982). Revising Terzaghi’s tunnel rock load coefficients. Proc. 23rd U.S.Sym. Rock

Mech., AIME, New York, 953-960.

Singh, Bhawani, Jethwa, J. L. and Dube, A. K. (1995). A classification system for support pressure

in tunnels and caverns. J. Rock Mech. & Tunnelling Technology, India, 1(1), January,

13-24.

Sinha, R. S. (1989). Underground Structures – Design and Instrumentation. Elsevier Science,

U.K., 480.

Terzaghi, K. (1946). Rock defects and loads on tunnel support. Introduction to Rock Tunnelling

with Steel Supports. Eds: R. V. Proctor and T. L. White, Commercial Sheering & Stamping

Co., Youngstown, Ohio, U.S.A., 271.

Verman, M. K. (1993). Rock Mass – Tunnel Support Interaction Analysis. PhD thesis, IIT Roorkee,

Roorkee, India, 258.

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4

Rock mass rating (RMR)

“Effectiveness of knowledge through research (E) is E = mc2

; where m is mass of

knowledge and c is communication of knowledge by publications.”

Z. T. Bieniawski

4.1 INTRODUCTION

The geomechanics classification or the rock mass rating (RMR) system was initially

developed at the South African Council of Scientific and Industrial Research (CSIR) by

Bieniawski (1973) on the basis of his experiences in shallow tunnels in sedimentary rocks

(Kaiser et al., 1986). Since then the classification has undergone several significant evo￾lutions: in 1974 – reduction of classification parameters from 8 to 6; in 1975 – adjustment

of ratings and reduction of recommended support requirements; in 1976 – modification of

class boundaries to even multiples of 20; in 1979 – adoption of ISRM (1978) rock mass

description, etc. It is, therefore, important to state which version is used when RMR-values

are quoted. The geomechanics classification reported by Bieniawski (1984) is referred in

this book.

To apply the geomechanics classification system, a given site should be divided into

a number of geological structural units in such a way that each type of rock mass is repre￾sented by a separate geological structural unit. The following six parameters (representing

causative factors) are determined for each of the structural unit:

(i) Uniaxial compressive strength of intact rock material,

(ii) Rock quality designation RQD,

(iii) Joint or discontinuity spacing,

(iv) Joint condition,

(v) Ground water condition and

(vi) Joint orientation.

Tunnelling in Weak Rocks

B. Singh and R. K. Goel

© 2006. Elsevier Ltd