482 Questions 20.7 -20.7 0: design of underground excavations
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 44". Also, a stress determination pro-
gramme 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 sur-
face. 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 production
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.
Compute the water pressure required to open the critical fracture set
(1) 250 m above, (2) 250 m below and (3) at the target horizon.
Using these results, predict the flow regime and overall stability of the
rock mass when water is injected at the target horizon.
Does this affect the layout of the boreholes and operation of the
system, and if so, how?
420.4 A circular tunnel of radius 4 m is to be driven in a weak rock
mass subjected to an in situ hydrostatic stress field of 9 ma. The triaxial
compressive strengths of the rock mass in its initial and fractured states
are given by o1 = 8 + 403 (ma) and (rl = 303, respectively.
A preliminary analysis for design is to determine the relative instabil-
ities of the roof, sidewalls and floor.
Plot the ground response curves for the roof, sidewalls and floor,
taking G = 2.1 GPa, f = 1.4 and y = 25 kN/m3, and comment on the
appropriate support pressure.420.5 For the design of part of a large underground civil defence
facility in a rock mass, there are two competing excavation geometries,
as shown in the sketches of the vertical cross-sections given below. Both
geometries consist of excavated rooms separated by rock pillars.