Civil Engineering Reference
In-Depth Information
Fig. 2.11
Examples where piping can occur.
2.11.3 Alleviation of piping
The risk of piping can occur in several circumstances, such as a cofferdam (Fig.
2.11a
) or the downstream
end of a dam (Fig.
2.11b
).
In order to increase the factor of safety against piping in these cases, two methods can be adopted.
The first procedure involves increasing the depth of pile penetration in Fig.
2.11a
and inserting a sheet
pile at the heel of the dam in Fig.
2.11b
; in either case there is an increase in the length of the flow path
for the water with a resulting drop in the excess pressure at the critical section. A similar effect is achieved
by laying down a blanket of impermeable material for some length along the upstream ground surface
(Fig.
2.11b
).
The second procedure is to place a surcharge on the ground surface of the downstream side, the weight
of which increases the downward forces, or to install a drainage system (e.g. perforated pipe protected
by a filter) at the downstream side.
2.12 Design of soil filters
As seen above, water seeping out of the soil can lead to piping and therefore drainage should be provided
in such situations to ensure ground stability. To prevent soil particles being washed into the drainage
system, soil filters can be provided as the interface between base material and drain. The design procedure
for a filter is largely empirical, but it must comprise granular material fine enough to prevent soil particles
being washed through it and yet coarse enough to allow the passage of water.
The following formulae are used in the specification of the filter material, based initially on the work of
Terzaghi and developed through the experimental research of Sherard
et al.
(1984a, 1984b):
D
15
ilter
>
5
×
D
15
of base material
D
15
ilter
<
5
×
D
85
of base material
The first equation ensures that the filter layer has a permeability several times higher than that of the
soil it is designed to protect. The requirement of the second equation is to prevent piping within the filter.
The ratio D
15
(filter)/D85
85
(base) is known as the
piping ratio
.
The required thickness of a filter layer depends upon the flow conditions and can be estimated with
the use of Darcy's law of flow. The filter material should be well graded, with a grading curve more or
less parallel to the soil. All material should pass the 75 mm size sieve and not more than 5% should pass
