Questions and answers: underground excavation instability mechanisms^343
can be used for tunnel support but not for TBM excavation, because the
TBM cannot easily be changed.
There may be other factors involved depending on the purpose of the
tunnel. The performance requirements for a conveyor tunnel, road tun-
nel, rail tunnel, gas storage tunnel, pressurized water tunnel, radioactive
waste repository tunnel, explosive storage tunnel, defence headquarters
tunnel and public access museum tunnel will all be different. There may
also be environmental problems, such as the inflow of contaminated
water, as illustrated in Fig. 19.5.
Thus, we need to understand firstly that creating the excavation causes
the three primary effects of the rock moving in, the alteration of the rock
stress field, and increased flow of water, and secondly we should design
with a knowledge of the instability mechanisms that can occur, given
the project objective and hence the excavation requirements. The two
main factors causing instability are the movement of rock blocks and
the disturbance caused by the induced stresses. It is essential therefore
to understand the method for assessing the kinematic feasibility of rock
blocks, and how stress concentrations around underground excavations
can cause rock failure.
19.2 Questions and answers: underground
excavation instability mec hanisms
Questions 19.1 to 19.6 are a set of questions all concerning rock block
instability in the same underground rock cavern, and so we recommend
that they are approached in the correct order.
419.1 The diagram on the right shows 50m 5 Om
the cross-section of an underground ma-
chine hall. The hall is to be excavated in a
sequence of metamorphosed argillaceous
rocks, of unit weight 22 kN/m3, which con-
tains five fracture sets with the following
orientations:
Set Dip direction Dip angle
1 058O 54O
2 1 95O 70°
3 127O 81 O
4 1 60° 32O
5 335O 64O
Each set has a friction angle of approxim-
ately 30° and zero cohesion. It is proposed
that the axis of the machine hall will be
oriented on a heading 030° and will be horizontal.
formed by the intersection of the fracture sets and the roof.
Determine the kinematic feasibility of all the tetrahedral blocks