Geoscience Reference
In-Depth Information
In case of non-cohesive soils (
0) the tangential stress can become much
larger and maintain a cavity under relatively quite low pressures. This effect is
referred to as arching (see Fig 15.7).
x
y
x
x
A
B
A
y
B
4
z
2
2
z
10 m
50 kPa
Figure 15.7 3D simulation of arching effects: vertical stress in A is smaller than in B.
Vesi developed formulae for expanding cavities in cohesive or frictional soils,
both for spherical and cylindrical situations and valid for Mohr-Coulomb
behaviour, including initial isotropic stress, soil weight and volume change
in the
plastic zone (dilatancy). He expressed his results in the following form
q = cF
c
+
0
F
(15.19)
0
is the initial stress and
F
c
and
F
are cylindrical cavity expansion
factors, each of them a function of
I
r
,
Here,
. For these functions tables and
graphs have been prepared for practical use. They are applicable for small cavities
or relatively large displacements, i.e.
|w|
and
r
0
. The previous analysis explains that
Vesi's approach may be applied for contracting cavities as well. For tunnels,
which can be conceived as large horizontal cavities, the situation may be more
complex, but fundamentals discussed here do apply to some extent.
D
A HORIZONTAL CYLINDRICAL CAVITY IN A UNIFORM SEMI
-
INFINITE SOIL
A typical example of a horizontal cylindrical cavity in soil is a pipeline or a
tunnel. Such a structure will induce a change of local soil stresses, because the
weight is different from the soil weight. Also the boring process will induce
changes in local stresses particularly at the boring front due to slurry pressures and
over-cutting. In cohesive soils or in frictional soils the response will be different
(e.g. see Fig 15.4) and also groundwater may play an important role. Furthermore,
over time, induced stresses will relax due to consolidation and creep. During all
these processes the soil deforms and at the surface settlements may occur, which
could affect nearby buildings and infrastructure. Insight in the stress distribution is
essential and a simple approximation after Verruijt is discussed here.
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