Civil Engineering Reference
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to a close approximation can be described as isotropic (Section 8.6), represented by the
elastic constants E and
(Figs. 8.10 and 8.11, upper left). The unconfined compressive
strength of the intact rock is relatively high. Thus, the intact rock is assumed to remain
elastic during tunnel excavation at low to medium overburden. Thus, the rock mass
strength is governed by the strength of the discontinuities. The shear strengths on the
horizontal, bedding-parallel discontinuities B and the vertical joints J are described by
the parameters c
B
,
ν
φ
B
(Figs. 8.10 and 8.11, upper left). The tensile strength
normal to the discontinuities B and J can be assumed to be zero.
φ
B
, c
J
and
Figure 8.10
Phenomena related to tunneling in swelling rock, idealized representation
Seepage flow in the coupled model is also described according to the homogeneous model
(Section 6.4.2). It is assumed that seepage flow only takes place along the discontinuities.
The permeability of the rock mass is then significantly governed by the mean apertures
2a
iB
and 2a
iJ
and, to a minor extent, by the mean spacing s
B
and s
J
of the discontinuities
(Fig. 8.10, upper left). The specific storage coefficient S
0
of the rock mass, which controls
transient seepage flow, for an unconfined phreatic surface can be approximately calculat-
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