Page 314 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 20 Allowable Bearing Pressure for Shallow Foundations 273

TABLE 20.4 Value of Nj Nj
Spacing of discontinuities (cm) 0.4
300 0.25
100–300 0.1
30–100

depth (to follow IS code). The depth of subsurface exploration in a rocky area should be
more than two times the width of concerned footing.

Faults and shear zones are seen often in the Himalayas, so the sites of tall structures
are changed repeatedly until a safe site is discovered. The natural frequency of vibrations
of tall structures, high silos, and large bridges is so low that seismic forces are insignif-
icant. Instead, wind forces govern the design of tall structures. Foundations should be
robust structures, embedded all along into the rock mass. They are restrained from all
sides to prevent excessive displacements during vibrations. A safe edge distance from
the slope (e.g., 10 m in high hills) should be planned due to possible surveying errors
in steep hilly terrain. (An error of 1 mm in contour lines means an error of 50 m horizon-
tally on a map on a scale of 1:50,000.) Stability of slopes, together with heavily loaded
foundations, is of critical importance. A minimum factor of safety of 1.2 is recommended
in the static case and 1.0 in the dynamic case. Block shear tests and uniaxial jacking tests
should be conducted carefully on the undisturbed rock mass inside the drifts or pits up
to the foundation level to get realistic strength parameters and moduli of deformations for
the detailed design of tall and very costly structures on rocks.

COEFFICIENT OF ELASTIC UNIFORM COMPRESSION
FOR MACHINE FOUNDATIONS

The coefficient of uniform compression (Cu) is defined as the ratio between pressure and
corresponding settlement of block foundation. Typical values of coefficient of elastic
uniform compression (Cu) for machine foundations on a rock mass are listed in
Table 20.5 (Ranjan et al., 1982). The coefficient of uniform shear is generally Cu/2.
It may be noted that Cu is less than 10 kg/cm3 in very poor rocks.

The elastic modulus of rock mass Ee (Eq. 8.19) may be used for calculating Cu. Cyclic
plate load tests are more reliable for this purpose.

SCOUR DEPTH AROUND BRIDGE PIERS

Approximately 400 bridge sites were surveyed by Hopkins and Beckham (1999). They
observed insignificant rock scour around exposed bridge piers and abutments. Only a few
sites experienced rock scour holes. Figure 20.4 shows the trend of correlation between
depth of scour holes and RQD. It is observed that the depth of scour is less than
about 30 cm below bed level where RQD is more than 10%. Scour depth was maximum
up to 1 m at zero RQD, and the minimum depth of socketing of well foundations or
bridge piers is 50 cm (Peck et al., 1974). The design scour depth is twice the actual scour