Civil Engineering Reference
In-Depth Information
water pressure within the ground, u (below the groundwater level, GWL,
this is equal to the water pressure in the ground). The total primary stress
is then the summation of
′
and u (equation 3.3).
=
′
+ u
(3.3)
It is often the effective stresses which dictate the behaviour of the ground
in terms of shear strength as the pore water is assumed to have no shear
strength. However, it should not be forgotten that the water pressure must
be included when determining the loads acting on the tunnel lining, i.e. the
effective stress generates bending and the total stress generates normal
forces in the tunnel lining. The procedure to calculate the vertical and
horizontal primary stresses (both total and effective) is as follows:
1
Calculate the vertical total stress using equation 3.1 (if the ground is
layered then this is the summation of all the layers above the tunnel
depth, i.e.
2
z
2
etc., where the subscripts 1 and 2 refer to different
strata above the tunnel).
1
z
1
+
2
Calculate the pore water pressure at the tunnel depth. For example, if
the tunnel is below the groundwater level and the water pressure can
be assumed hydrostatic, then u =
w
is the unit weight of
water and z
w
is the depth below the level of the groundwater level (if
the groundwater is flowing then u will be different).
w
z
w
, where
v
- u,
i.e. the effective vertical stress is the average stress acting between the
particle to particle contacts within the ground material.
v
′
v
′
3
The effective vertical stress,
, is then calculated from
=
4
Multiplying the effective vertical stress by K
0
gives the effective
horizontal stress,
h
′
= K
0
v
′
.
5
To determine the total horizontal stress,
h
, the pore pressure, u,
determined previously (water pressure acts equally in all directions, i.e.
K
0
is 1.0 for water) is added to
h
′
, i.e.
h
=
h
′
+ u.
When a tunnel is excavated, it disturbs the primary stress conditions.
Assuming that the tunnel construction is stable, this requires a redistribution
of the stresses around the void. This is known as arching. The stresses form
a new equilibrium and this is called the secondary stress condition. It can
also happen temporarily for partial or separate construction phases, for
example if there is a partial heading far in advance of the remaining heading
construction.
3.3 Stability of soft ground
One of the key parameters that influences the choice of tunnelling technique
is the stability of the ground as the tunnel is constructed. This is particularly
