Question and answers: design of underground excavations 377fracture problems, the choice of a west-east orientation should definitely
be made based on this information alone.
The stress components perpendicular to the tunnel axis would then
be equal. This means that a circular tunnel cross-section would be
preferable because it will minimize stress concentrations.
Such design thinking, based on the rock mechanics principles alone,
can often be all that is required to ensure the success of a rock engin-
eering project. Wang and Fan (2000) give an example of the strategic
optimization of new coal mine development in soft rock in China based
on such principles. The optimization procedure has enabled coal mines
to be operated much more effectively than before because the roadway
maintenance has been significantly reduced.
This example illustrates the significance of the concept of stress as
a tensor. Although clear from the principles discussed, we may not be
intuitively expecting that horizontal tunnelling in different directions
through a homogeneous rock mass will produce different effects: in one
direction the tunnel is stable and in another direction it is unstable. The
fundamental reason for this phenomenon is that rock can sustain a shear
stress.
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
about 2000 m and use these for injecting water at high pressure
into the naturally fractured rock mass. The water will take heat from
the surrounding rock, and will then be extracted from production
boreholes and used for heating. A key question is the stability of the
fractured rock mass during this process.
Extensive site investigations have revealed that the rock mass
contains three pervasive sets of fractures, oriented at OOO/OO,
165/90 and 245/90, all of which have a friction angle of 44O. Also,
a stress determination programme has established that the three
principal stresses have orientations 000/90, 040/00 and 130/00.
Their magnitudes (in MPa) are given by 262, 6 + 122 and 15 + 282
respectively, where z is the depth below ground surface. Although
the intact rock is essentially impermeable, the rock mass is so highly
fractured that it is naturally saturated with groundwater and hence
effective stress conditions exist within the fractures.
The water injection holes have to be located relative to the pro-
duction hole such that the pressurized water will flow along the
natural fractures between them. For the target depth, examine the
effective normal stress induced on a fracture from each of the three
sets, and identify which set will be jacked open by the injected water.
Hence, propose a layout for the injection boreholes. Bear in mind
that the boreholes can be deviated during drilling such that they
run horizontally, so that a vertical flow regime can be established if
necessary.
' Wang Tongliang and Fan Qiuyan (2000) Optimization of Soft Rock Engineering with
Particular Reference to Coal Mining. Int. J. Rock Mech. Min. Sci. 37.