Environmental Engineering Reference
In-Depth Information
9
Design, specification and construction of filters
9.1
GENERAL REQUIREMENTS FOR DESIGN AND THE FUNCTION OF
FILTERS
9.1.1
Functional requirements
Filters in embankment dams and their foundations are required to perform two basic
functions:
(a) Prevent erosion of soil particles from the soil they are protecting;
(b) Allow drainage of seepage water.
Filters are usually specified in terms of their particle size distribution. They are required
to be sufficiently fine, relative to the particle size of the soil they are protecting (the “base
soil”), to achieve function (a), while being sufficiently coarse to achieve function (b).
To achieve these functions the ideal filter or filter zone will (ICOLD 1994):
- Not segregate during processing, handling, placing, spreading or compaction;
- Not change in gradation (by degradation or break down) during processing, handling,
placing and/or compaction, or degrade with time e.g. by freeze-thaw or wetting and
drying by seepage flow;
- Not have any apparent or real cohesion, or ability to cement as a result of chemical,
physical or biological action, so the filter will not allow a crack in the soil it is protect-
ing to persist through the filter;
- Be internally stable, that is the fines particles in the filter should not erode from the fil-
ter under seepage flows;
- Have sufficient permeability (and, if a drain, thickness) to discharge the seepage flows
without excessive build-up of head;
- Have the ability to control and seal the erosion which may have initiated by a concen-
trated leak, backward erosion, or suffusion (internal instability) in the base soil.
9.1.2
Flow conditions acting on filters
Figure 9.1
illustrates the basic flow conditions that can occur between a filter and base
soil. These are:
N
1
Flow normal to the base soil - filter interface, with potentially high gradient condi-
tions; e.g. at the downstream face of the earthfill core, the contact between the hori-
zontal drain and the foundations, and within the foundation where seepage is across
bedding.
N
2
Flow normal to the base soil - filter interface, with low gradient conditions; e.g. at the
upstream face of the earthfill core under reservoir drawdown conditions, or into the
upper Zone 2A filter for the horizontal drain.
P
Flow parallel to the interface, e.g. at the base of rip-rap layers, or in the foundation
where seepage is along bedding.
