Page 218 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

180 Engineering Rock Mass Classification

df ¼ C1 sy À C2 ð13:20Þ
a qc

where C1 ¼ 1.37 (1 þ 0.4v); C2 ¼ 0.57; v ¼ peak particle velocity due to remote seismic
event in m/sec; and a ¼ radius of tunnel.

Thus the length of (resin) bolt is equal to df þ FAL (fixed anchor length). It is advised

to use yielding bolts that can deform up to 80 mm. It is better to excavate in many steps to

reduce the strain energy released as it causes rock burst. Moreover, highly stressed tunnel

faces should be shaped convexly to remove potentially unstable rock before it can cause

serious safety hazards to workers near the tunnel face (Kaiser, 2006).

TENSILE STRENGTH ACROSS DISCONTINUOUS JOINTS

The length of joints is generally less than 5 m in tunnels in young rock masses except
for bedding planes. Discontinuous joints thus have tensile strength. Mehrotra (1996)
conducted 44 shear block tests on both nearly dry and saturated rock masses. He also
obtained non-linear strength envelopes for various rock conditions. These strength
envelopes were extrapolated carefully in tensile stress regions so that they were tangen-
tial to Mohr’s circle for uniaxial tensile strength as shown in Figure 13.4. It was noted that
the non-linear strength envelopes for both nearly dry and saturated rock masses
converged to nearly the same uniaxial tensile strength across discontinuous joints (qtj)
within the blocks of rock masses. It is related to Barton’s rock mass quality
(Figure 13.5) as follows:

qtj ¼ 0:029 g Q0:31, MPa ð13:21Þ

where g is the unit weight of the rock mass in g/cc (T/m3). In case of tensile stresses, the

criterion of failure is as shown in Eq. (13.22).

FIGURE 13.4 Estimation of tensile strength of rock mass from Mohr’s envelope. (From Mehrotra, 1992)