Civil Engineering Reference
In-Depth Information
Protective filters are usually constructed in layers, each of which is coarser than the one below it, and
for this reason they are often referred to as reversed filters. Even when there is no risk of piping, filters
are often used to prevent erosion of foundation materials and they are extremely important in earth dams.
Example 2.6:
Buoyant uplift
An 8 m thick layer of silty clay is overlying a gravel stratum containing water under arte-
sian pressure. A stand pipe was inserted into the gravel and water rose up the pipe to
reach a level of 2 m above the top of the clay (Fig.
2.12)
.
The clay has a particle specific gravity of 2.7 and a natural water content of 30%. The
permeability of the silty clay is 3.0
×
10
−
8
m/s.
It is proposed to excavate 2 m into the soil in order to insert a wide foundation which,
when constructed, will exert a uniform pressure of 100 kPa on to its supporting soil.
Determine: (a) the unit rate of flow of water through the silty clay in m
3
per year before
the work commences; (b) how safe the foundation will be against heaving (i) at end of
excavation (ii) after construction of the foundation.
Solution:
(a) Assume that GWL occurs at top of clay.
Head of water in clay
=
8 m
Head of water in gravel
=
10 m
⇒
Head of water lost in clay
=
2 m
q Aki
=
Consider a unit area of 1 m
2
then:
2
8
q
= × ×
1 3 10
−
8
×
−
9
3
=
7 5 10
7 5 10
.
×
m s
/
=
.
.
×
−
9
× × × ×
60 60 24 365
=
0 237
m
3
/year per m of surface area
2
(b) (i) e
=
wG
s
=
0.3
×
2.7
=
0.81
G e
e
kN m
+
+
9 81
3 51
1 81
.
.
γ
=
γ
=
.
sat
w
1
3
=
19 0
.
/
Fig. 2.12
Example
2.6
.
