Page 161 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 9 Rock Mass Number 129

COEFFICIENT OF VOLUMETRIC EXPANSION
OF FAILED ROCK MASS

The ground response (reaction) curve depends upon the strength parameters of rock mass
and also the coefficient of volumetric expansion of rock mass (k) in the broken zone.
Jethwa (1981) estimated values of k as listed in Table 9.7. A higher degree of squeezing
was associated with higher k values.

Example 9.3

A rock mass has three joint sets, each spaced at 15 cm. The joints are rough and the joint
profile is almost planar. The joint surface of critical joints is altered with a sandy clay
coating. The rock mass is moist only. The plan is to design a road tunnel of 9 m diameter
at a depth of 350 m. Find out the ground condition likely to be encountered. If it is a
squeezing condition, what would be the safe depth to avoid the squeezing condition?
Also estimate the support pressure and the supports to be used.

Each joint set of the three joint sets has a joint spacing of 15 cm (joint frequencies ¼ 6
joints per meter). Therefore, volumetric joint count (Jv) ¼ 6 Â 3 ¼ 18 (Eq. 4.3). Using
volumetric joint, RQD ¼ 115 À 3.3 Jv ¼ 55%. Jn ¼ 9 (three joint sets); Jr ¼ 1.5 (rough
planar); Ja¼ 3.0 (sandy clay coating); Jw ¼ 1.0 (moist only); SRF ¼ 1.0 (competent rock,
medium stress); and for a road tunnel, ESR ¼ 1.0. Using Eq. (8.1b), Q ¼ 3.05 (approx-
imately) 3.00. Using Eq. (8.2), vertical support pressure ¼ 0.092 MPa; since SRF ¼ 1,
therefore, rock mass number N ¼ Q ¼ 3.0.

For N ¼ 3.0 and tunnel diameter 9 m, the safe tunnel depth to avoid squeezing ground
condition is 320 m (Eq. 7.4). Any tunnel having rock cover more than 320 m may face
squeezing ground condition in rock masses with N ¼ 3.0. In this example, the tunnel
depth is 350 m, hence a squeezing condition is expected. To avoid squeezing, either
design the tunnel with a cover of less than 320 m or reduce its diameter. A tunnel diameter
of 3.5 m would encounter a non-squeezing ground condition (Eq. 7.4). This is cor-
roborated by an unsupported span of 3.0 m obtained from Barton’s approach (Eq. 8.12).

The support pressure (Eq. 9.10) using rock mass number N and considering f(N) value
as 1.5 (allowing 1–2% normalized tunnel closure/deformation, Table 9.3) is 0.17 MPa.

TABLE 9.7 Coefficient of Volumetric Expansion of Failed Rock Mass
(k) within the Broken Zone

S. No. Rock type k
1 Phyllites 0.003
2 Claystones/siltstones 0.01
3 Black clays 0.01
4 Crushed sandstones 0.004
5 Crushed shales 0.005
6 Metabasics (Goel, 1994) 0.006

Source: Jethwa, 1981.