Geology Reference
In-Depth Information
Water table
64
4 mbGL
Total vertical stress
(bulk weight of
soil)
140
8 mbGL
10 mbGL
60
182
effective stress
σ′
60, i.e.
122 kN/m 2
182
=
Water
pressure
50
100
150
200
250
Figure B6-1.2 Total and effective stresses(vertical)
At depth
Total vertical stress
(
Water pressure (u)
kN/m 2
Effective vertical stress
(
σ v )kN/m 2
)kN/m 2
σ
4m
4×16 = 64
0
64
8m
64 + (4×19) = 140
4×10 = 40
140
-
40 = 100
-
10m, at tunnel crown
140 + (2×21) = 182
6×10 = 60
182
60 = 122
Therefore, before tunnel construction, the estimated vertical effective stress at the tunnel crown is 122
kN/m 2 . During construction, due to seepage into the tunnel the water table would be lowered or this
might be done deliberately to excavate
'
'
flowing or ravelling of the soil into the
tunnel. If the water pressure dropped, so the effective stress would increase. If the water table was
lowered so that water pressure was zero at tunnel crown level, then the effective stress would equal the
total stress (= 182 kN/m 2 ) . 1
in the dry
to avoid
At some locations, however, tectonic or topographic stresses
can be dominant even very close to the Earth
s surface, with
horizontal stresses sometimes locked in from a previous geologi-
cal event and far in excess of that due to gravity and the Poisson
'
1 Note: the actual stress conditions near a tunnel would be more complex than this
calculation. The tunnel would distort the stress
eld
-
refer to Muir Wood (2000) or
Hoek et al . (1995).
 
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