Environmental Engineering Reference
In-Depth Information
weak. An adequate foundation may be identified by blade refusal of a bulldozer but for
high dams light ripping may be necessary;
- Where there are unfavourably oriented weak seams in the rock, e.g. bedding surface
shears or landslide rupture (slide/surfaces), these will influence stability and may need
to be removed, or the design modified to accommodate them;
- It is unlikely that slope modification will be required under rockfill other than over-
hanging cliffs being treated by removing the overhanging rock, or filling the re-entrant
below the overhang with concrete;
- The surface should be cleaned of loose soil and rock with a bulldozer, grader, backhoe
or excavator sufficient to ensure the rockfill is supported on the rock foundation.
Intensive cleanup is not required and if there is loosened rock of good quality this may
be left in place.
If there are wide, erodible seams in the rock, oriented that they are likely to carry
seepage water from the storage, it may be necessary to cover them with concrete,
shotcrete under the upstream rockfill, or preferably by a filter layer under the downstream
rockfill. Wallace and Hilton (1972) describe backfilling compressible seams greater than
0.3 m wide with Zone 2B filter material to a depth of up to 0.9 m. In most cases this
will not be necessary unless the seams are wide enough to affect stability or pose an
erosion risk.
17.2.3
General foundation under horizontal filter drains
Horizontal filter drains will generally only be required if the foundation is soil or erodible
rock:
- Remove topsoil and weak compressible soil consistent with the assumptions made for
stability, settlement and particle size used for design of the filter;
- Where soil or rock in the foundation is fissured or has landslide rupture (slide)
surfaces or unfavourably oriented bedding surface shears are present, these will
influence stability and may need to be removed, or the design modified to accommo-
date them;
- Slope modification should not be required except to remove overhangs. However, if
earthfill is to be placed on top of the filter drain, slope modifications may be needed as
described in Section 17.6;
- The surface should not be rolled prior to placing the filter. Rolling will destroy the
soil structure and reduce the permeability, making it more difficult for seepage water
to flow into the filter drains. It is desirable for the foundation seepage to flow
into the filter drain so erosion is controlled, rather than being forced to emerge down-
stream of the toe of the embankment in an uncontrolled manner. This is an important
issue, often not well appreciated by construction personnel and even some dam
engineers;
- On low strength rock and on soil, trafficking with earthmoving equipment will contin-
uously break up the surface, necessitating final cleanup with an excavator or backhoe
working away from the cleaned up area. This is particularly a problem with weathered
schists, phyllites and similarly fissile rocks which break down very easily under equip-
ment. Once cleaned, the filter should be dumped on the cleaned up surface and spread
onto the surface without equipment trafficking directly onto the foundation;
-Immediately before placing the filter material, the surface should be cleaned of
loose dry and wet soil and rock. This may necessitate intensive work using light
equipment and hand methods. Final cleanup should involve an air or air-water jet
to “blow” away loose material. In many cases an air-water jet will be too severe and
cause erosion.
 
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