Page 321 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

278 Engineering Rock Mass Classification

Figure 23.2 shows which criterion to ascertain when grouting is needed in the dam
foundation. The foundation of a concrete dam should go deeper than debris into a good
grouted-rock mass. Its depth in rock mass should be more than twice the scour depth
(Figure 20.4). The foundation should be undulating for seismic stability by increasing
the joint wall roughness coefficient (JRC) of the dam foundation surface.

Singh (2009) proved that the sliding angle of friction between concrete and rock
masses is higher at low normal stresses than previously believed. Table 20.8 provides
the shear strength parameters from in situ direct shear tests at 20 different hydroelectric
project sites in the Himalayas in India. The residual strength parameters are likely to re-
duce with more sliding of the concrete blocks. Shear strength of rock mass is anisotropic
and the least in the direction of tectonic movement, so the dam axis should be inclined to
the direction of tectonic motion for better strength of rock mass.

Example 20.1

A clear water reservoir is to be built by cutting the top of the hill of a highly weathered
rock mass of about 35 m height. It is igneous boulder rock mass with no boulder bigger
than 60 to 75 cm on average in size. Suggest the allowable bearing pressure. It is not
practicable to do plate load tests at the site. The rock mass is classified as poor according
to RMR after rating adjustment of the slope all around the site.

Table 20.2 suggests a least bearing pressure of 30 t/m2 (0.3 MPa). In view of the steep
slopes, the recommended allowable bearing pressure is 15 t/m2 (0.15 MPa), which is
sufficient to take the pressure due to water and the tank. The minimum distance of foun-
dation from the edge of the natural slope is 2 m (beyond the filled up soil). The depth of
foundation is 0.5 m below the plane of excavation of the hilltop. The raft foundation is
provided to prevent its cracking due to possible differential settlement as well as pene-
tration of water toward the slope, which may cause distress to these slopes. Suitable
drainage measures for the surface water should also be implemented.

Example 20.2

A 270 m high chimney is to be built for a thermal powerhouse. The rock mass is granite
beneath a soil cover of 25 m. The average UCS of rock material is 85 MPa. A core loss of
40% was observed during drilling, but all pieces of rock core were longer than 20 cm.
The site is located in a no earthquake zone. Design the foundation.

A raft foundation is suggested with 26 m long cast in situ concrete piles of 60 cm
diameter in because the structure is very tall. The piles should be socketed into the rock
mass up to 1 m in depth (1D–2D in strong rocks). The minimum spacing of piles should
be 1.8 m c/c (3D).

The safe bearing pressure of the rock mass according to Eq. (20.1) is

qa ¼ qc Nj Nd

Nj ¼ p3ffiffiffiþffiffiffiffiffiðffiffisffiffi=ffiffiBffiffiffiÞffiffiffiffiffiffiffiffiffiffiffi, s ffi 0:2 m, B ¼ 0:6 m, d=s ffi core loss=100 and ffi 0:4,
10 1 þ ð300 d=sÞ

Nj ¼ p3ffiffiþffiffiffiffiðffiffi0ffiffiffi:ffi2ffiffiffi=ffiffi0ffiffiffi:ffi6ffiffiffiÞffiffiffiffiffiffiffiffiffi ¼ 0:03, Nd ¼ 0:8 þ 0:2 h=D (between 1 and 2), Nd ¼ 0:8 þ
10 1 þ ð300 Â 0:4Þ

0:2 Â 1=0:6 ¼ 1:1, and qa ¼ 85 Â 0:03 Â 1:1 ¼ 2:8 MPa ðt=m2Þ and is < safe compres-
sive stress in concrete.