Engineering Geology

E n g i n e e r i n g G e o l o g y
the opening. However, initial movements may be influenced by the natural stresses concentrated
around an opening and under certain boundary conditions may continue to act even
after large displacements have occurred.
The angle of friction for tight irregular joint surfaces commonly is greater than 45∞ and, as
a consequence, the included angle of any wedge opening into the roof of a cavern has to be
90∞ or more, if the wedge is to move into the cavern. A tight rough joint system therefore
only presents a problem when it intersects the surface of a cavern at relatively small angles
or is parallel to the surface of the cavern. However, if material occupying a thick shear zone
has been reduced to its residual strength, then the angle of friction could be as low as 15∞
and, in such an instance, the included angle of a wedge would be 30∞. Such a situation would
give rise to a very deep wedge that could move into a cavern. Displacement of wedges into
a cavern is enhanced if the ratio of the intact unconfined compressive strength to the natural
stresses concentrated around the cavern is low. Values of less than 5 are indicative of stress
conditions in which new extension fractures develop about a cavern during its excavation.
Wedge failures are facilitated by shearing and crushing of the asperities along discontinuities
as wedges are displaced.
The walls of a cavern may be influenced by the prevailing state of stress, especially if the
tangential stresses concentrated around the cavern approach the intact compressive
strength of the rock (Gercek and Genis, 1999). In such cases, extension fractures develop
near the surface of the cavern as it is excavated and cracks produced by blast damage
become more pronounced. The problem is accentuated if any lineation structures or discontinuities
run parallel with the walls of the cavern. Indeed, popping of slabs of rock may take
place from cavern walls.
Rock bursts have occurred in underground caverns at rather shallow depths, particularly where
they were excavated in the sides of valleys and on the inside of faults when the individual fault
passed through a cavern and dipped towards an adjacent valley. Bursting can take place at
depths of 200–300 m, when the tensile strength of the rocks varies between 3 and 4 MPa.
Three methods of blasting normally are used to excavate underground caverns (Fig. 9.18).
Firstly, in the overhead tunnel, the entire profile is drilled and blasted together or in parts
by horizontal holes. Secondly, benching with horizontal drilling may be used to excavate the
central parts of a large cavern. Thirdly, the bottom of a cavern may be excavated by benching,
the blastholes being drilled vertically. The central part of a cavern also can be excavated
by vertical benching, provided that the upper part has been excavated to a sufficient height
or that the walls of the cavern are inclined. Indeed, once the crown of a cavern has been
excavated, it may be more economical to excavate the walls in a series of large deep bench
cuts exposing substantial areas of wall in a single blast. Under these conditions, however,
498