Questions 18,l-18,lO:
design of surface
excavations
418.1 A foundation is proposed on a rock mass consisting of a series
of horizontal strata. The results of a site investigation and associated
laboratory testing programme are shown in the table below.
Depth Description RQD Intact rock elas- Fracture defor-
tic modulus mation modulus
(m) VO) (GPa) (GPa/m)
3 Slightly weathered sandstone 98 5.0 20.010 Fresh, massive limestone 100 40 120(a) Calculate the effective elastic modulus for the uppermost 10 m of
the rock mass.
(b) A plate-loading test using a 0.5 m diameter rigid plate at the
ground surface has indicated an elastic modulus of 1.0 GPa. By consid-
ering the moduli of the various strata, explain why this might be. What
value of modulus should be used in the design?
(Aide memoire: For a rock mass consisting of i strata, each of which
contains a single set of planar, parallel fractures, oriented parallel to the
strata boundaries, the effective elastic modulus normal to the fractures,
E,, may be calculated using the expression
0 Moderately weathered sandstone 61 4.0 18.08 Slightly weathered limestone 80 39 115End of hole at 12 m
where, for each of the i strata, ti = thickness, Ei = Young’s modulus of
the intact rock, hi = fracture frequency, Edi = modulus of deformation of
the fractures.)
418.2 In a rock mass containing a single set of vertically dipping
fractures, consider how the fractures might adversely affect the profile of
a vertical slope excavated using pre-split blasting.