Questions 20,1120.1 0:
design of underground
excavations
420.1 A vertical shaft is to be sunk through a rock mass in which the
magnitudes of the two horizontal principal stresses have a ratio of 3 : 1.
A clear space of 5 m x 5 m must be maintained within the shaft for
equipment installation. The three excavation cross-sections shown in the
sketch are under consideration for the shaft design.
Discuss the relative design merits of each cross-section in terms of
rock mechanics principles, and hence provide recommendations for the
optimal shape (and in the case of profile (b) and (c), optimal orientation)
for the excavation.
420.2 You have been asked for an initial decision on the orientation of
an underground structure. The structure comprises ten parallel tunnels,
each 5 km long, to be built in a rock mass containing definite fracture
sets. You have also been asked about the best shape for the cross-section
of the tunnels.
The dip direction and dip of the fracture sets are 270/45 and 90/45;
the direction of the maximum principal stress is horizontal, west-east,
and its magnitude is twice the value of both the vertical and the other
horizontal stress component. The ratio of the maximum principal stress
component to the intact rock uniaxial compressive strength is 0.5.
What would be your initial design based on this information alone?
420.3 A site is under investigation for use in a geothermal energy
project. It is planned to drill a number of boreholes to a depth of