3 1 8 Design of surface excavations
In order to determine the modulus at greater depths, it is usually
necessary to perform plate-loading tests at the bottom of large diameter
boreholes. Although large diameter plate-loading tests would, in theory,
provide the correct values, the enormous loads required for such tests
mean that the engineering is prohibitively costly. This also shows that,
for a small foundation, the plate loading test will give an appropriate
modulus, although for a larger foundation the composite modulus may
be more appropriate.
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.A78.2 The diagram below, from Jimeno et al. (1995)*, shows qualit-
atively how the profile of a pre-split plane is affected by the relative
orientations of the plane and the fracture set.Two mechanisms are acting during pre-split blasting: stress waves and
high gas pressures. Each of these contributes to the generation of the
pre-split plane, although the effect of the former is attenuated through
the use of decoupled charges.
Although the gas pressure effect will allow the formation of the
pre-split plane, the reflection and refraction of the stress waves will
locally deviate the plane according to the rock structure - causing the
irregularities illustrated in the diagram.478.3 A quarry is to be opened in a rock mass which contains four
fracture sets with dip directions and dip angles as follows:Jimeno C. L., Jimeno E. L. and Carcedo E J. A. (1995) Drilling and Blasting of Rocks,
Balkema, Rotterdam, 391pp.