Engineering Rock Mechanics

500

Stress Reduction Factor (SRF)

Appendix C: Rock mass classification fables - RMR and Q

10.0

5.0

2.5

7.5

5.0

2.5

5.0

2.5

1.0

(i) Weakness zones intersecting excavation, which may cause loosening of

rock mass when tunnel is excavated

(a) Multiple occurrences of weakness zones containing clay or

chemically disintegrated rock, very loose surrounding rock

(any depth)

(b) Single weakness zones containing clay or chemically

disintegrated rock (excavation depth t50 m)

(c) Single weakness zones containing clay or chemically

disintegrated rock (excavation depth >50 m)

(d) Multiple shear zones in competent rock (clay-free), loose

surrounding rock (any depth) a

(e) Single shear zones in competent rock (clay-free) (depth of

excavation t50 m) a

(f) Single shear zones in competent rock (clay-free) (depth of

excavation >50 m) a

(g) Loose open joints, heavily jointed or 'sugar cube', etc. (any

depth) a

(ii) Competent rock, rock stress problems Oc/OI at101

(h) Low stress, near surface > 200 > 13

(j) Medium stress 200-10 13-0.66

0.5-2.0 (k) High-stress, very tight structure (usually 10-5 0.6M.33

favourable to stability, may be

unfavourable for wall stability)

5-10 (1) Mild rock burst (massive rock) 5-2.5 0.33-0.16

10-20 (m) Heavy rock burst (massive rock) <2.5 t0.16

(iii) Squeezing rock; plasticflow of incompetent rock under the influence of

high rock pressures

(n) Mild squeezing rock pressure

(p) Heavy squeezing rock pressure

(9) Mild swelling rock pressure

5-10
10-20
(iv) Swelling rock; chemical swelling activity depending on presence of water
5-10
10-15 (r) Heavy swelling rock pressure
a Reduce these SRF values by 2550% if the relevant shear zones only influence,
but do not intersect, the excavation.
bFew case records available where depth of crown below surface is less than
span width; suggest SRF increase from 2.5 to 5 for such cases. For strongly
anisotropic stress field (if measured): 5 5 al/a3 I 10: reduce a, and to 0.80,
and 0.80~; a,/q > 10: reduce O, and a, to 0.60, and 0.60, (o, = unconfined
compressive strength, a, = tensile strength (point load), o1 and a3 are major and
minor principal stresses).