Page 369 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

324 Engineering Rock Mass Classification

where mr ¼ Hoek-Brown rock material constant to be found from triaxial tests on
rock cores.

In Eqs. (26.8) and (26.9), s and n are Hoek-Brown constants for the rock mass given

by the following relationships:

s ¼ À ð26:8Þ
GSI 100
exp 9 À 3D

n ¼ 1 þ 1 À ð26:9Þ
eÀGSI=15 eÀ20=3
26

D is a disturbance factor that depends upon the degree of disturbance to which the rock
mass has been subjected by blast damage and stress relaxation. It varies from 0 for
undisturbed in situ rock masses to 1 for very disturbed rock masses (Table 26.4).
Cheng and Liu (1990) found that a zone of blast damage extended for a distance of
approximately 2.0 m with D ¼ 0.7 around all large excavations (caverns). While using
the disturbance factor D, its values given in Table 26.4 are selected judiciously. The
actual value of D is a function of rock mass quality and blasting practices.

Experience in the design of slopes in very large open pit mines has shown that the
Hoek-Brown criterion for undisturbed in situ rock masses (D ¼ 0) results in shear
strength parameters that are too optimistic. The effects of heavy blast damage as well

TABLE 26.4 Guidelines for Estimating Disturbance Factor D

Appearance of rock mass Description of rock mass Suggested
value of D
Excellent quality controlled blasting or
excavation by tunnel boring machine D¼0
results in minimal disturbance to the
confined rock mass surrounding a tunnel.

Mechanical or hand excavation in poor D¼0
quality rock masses (no blasting) results in D ¼ 0.5
minimal disturbance to the surrounding No invert
rock mass.
Where squeezing problems result in
significant floor heave, disturbance can be
severe unless a temporary invert, as shown
in the photograph, is placed.