Environmental Engineering Reference
In-Depth Information
Figure 9.33.
Filter zone following construction of other zones which is generally undesirable.
Care must also be taken to avoid contamination where haul roads cross the filter zones.
This can be facilitated by covering the filter with a geotextile or geomembrane before
building the haul road. If practicable, it is wise to shape the surface of the embankment
fill, so that surface water flows away from the filter.
9.6.3
Compaction of filters
The general objectives for the compaction of filter zones are:
(a) Filters upstream and downstream of the dam core;
- Sufficient compaction to ensure the filter shear strength does not influence the sta-
bility of the embankment;
- For saturated filters, compaction should be to a relative density (density index) such
that flow liquefaction under earthquake is not possible, i.e. a relative density
70%
is required;
- The deformation modulus of the filter should be compatible with the adjacent core
material and rockfill;
-Avoid excessive breakdown of the filter materials by the compaction equipment.
(b) Horizontal or near horizontal filters;
- The compaction should be to a relative density (density index) such that flow lique-
faction under earthquake is not possible, i.e. a relative density of
70% is required;
-Avoid excessive breakdown of the filter materials by the compaction equipment.
Filters should be compacted in layers using a vibratory smooth steel drum roller. Filters
are usually well graded granular materials and are readily compacted to a dense condi-
tion. Hence for the majority of dams, a “methods” type specification for compaction is
the most practicable, i.e. a maximum layer thickness, coupled with a number of passes of
a vibratory roller of a specified static weight and centrifugal force.
Table 9.11
gives some
examples of “standards” type specifications, where a density index (relative density) is
specified. The latter is not the preferred basis for the routine (daily) control of placement
on the dam because of the cost involved in carrying out the tests, the delay in the results
and the difficulty of carrying out the testing required, particularly in Zone 2B filters. Zone
2B filters will commonly have gravel up to 50 mm or 75 mm size, requiring a density in
place test hole at least 200 mm or 300 mm diameter and laboratory compactions in non
standard 300 mm or 500 mm diameter cylinders.
