Page 106 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

78 Engineering Rock Mass Classification

FIGURE 7.4 Prediction of ground condition. (From Kumar, 2002)

occurred only where Jr/Ja exceeds 0.5. This observation confirmed the study of Singh and
Goel (2002). If Jr/Ja was significantly less than 0.5, then a squeezing phenomenon was
encountered in many tunnels under high overburden in the Himalayas. Thus, a semi-

empirical criterion for mild rock burst in the tunnels is suggested as follows:

sy ¼ 0:60 À 1:0 ð7:12Þ
q0 cmass

and

Jr=Ja > 0:50 ð7:13Þ

where q0cmass ¼ biaxial strength of rock mass (Eq. 7.14) and sy ¼ maximum tangential
stress at tunnel periphery. Predictions should be made on the basis of Figures 7.3 and 7.4.

Rock Burst

The upper right corner zone in Figure 7.3 is marked by dotted lines. Spalling and mild to
moderate rock burst cases in tunnels from Indian hydroelectric and mining projects are in
this region, which indicates a probable zone of rock burst condition. The inter-block
shear strength parameter (Jr/Ja) of Barton et al. (1974) is found to be more than 0.5
for all tunneling cases encountering the mild to moderate rock burst condition.

Criterion of Bhasin and Grimstad

Using the results of Eq. (7.1), Bhasin and Grimstad (1996) developed a monogram
(Figure 7.5) between rock mass strength, in situ stress, and rock behavior in tunnels with
rock mass quality (Q) for estimating the ground conditions.

THEORETICAL/ANALYTICAL APPROACH

Theoretically, the squeezing conditions around a tunnel opening are encountered if
(Eq. 13.20)