Civil Engineering Reference
In-Depth Information
Table 3.1
Typical values for K
0
Ground material
K
0
Sand
0.4-0.5
Clayey soil (between rock layers)
0.6-0.8
Slurry
1.0
Soft rock
0.4-0.6
Hard soil/rock
(0.2) 0.5-0.8 (1.2)
London Clay
0.6-1.5
normally consolidated soils, i.e. a soil that has not experienced greater
stresses acting on it in the past than are acting on it now, K
0
can be
estimated based on the internal friction angle,
, of the material, for
′
example K
0
= (1 - sin
). For overconsolidated clays, i.e. where the soil
has experienced larger stresses in the past than it is experiencing now, K
0
is likely to be greater than 1.0. Some examples of typical values of K
0
are
shown in Table 3.1.
′
3.5 Preliminary analytical methods
3.5.1 Introduction
It is impossible to take all the influences, parameters and boundary
conditions that are dependent on the geology and construction phases into
account in a calculation. Therefore, analytical models have been developed
which simplify reality to such an extent that the remaining parameters can
be dealt with in a calculation and at the same time lead to sensible results.
In the following discussion, three common analytical methods are
briefly described; the bedded-beam spring method, the continuum method
and the tunnel support resistance method. The assessment of which method
to use, depends on the tunnel depth. In soft soil, two conditions can be
defined as:
•
shallow, C
2D, i.e. where the ground above the tunnel crown in
assumed to have no bearing capacity;
•
deep, C
3D, i.e. where the ground above the tunnel crown is acting
as a support;
where C is the tunnel crown depth and D is the tunnel diameter.
C
2D: The excavation process creates a softening zone in the crown
area, which for shallow tunnels in soft ground reaches the ground surface.
As a result, no arching can develop over the crown. The ground in this
area has no bearing capacity and acts only as a load on the tunnel lining.
For the unsupported area, on average, an angle of 90 degrees is assumed
at the tunnel crown (Figure 3.6a). This is a very conservative approach.
