Environmental Engineering Reference
In-Depth Information
17.3
CUTOFF FOUNDATION FOR EMBANKMENT DAMS
17.3.1
The overall objectives
The requirements for excavation below general foundation level to achieve a suitable cut-
off foundation are described below. It should be noted that each case will be determined
on its merits and often there will be trade-offs between the following:
-The desire to achieve a low permeability foundation;
- The depth and hence volume and cost of excavation required to achieve a cutoff;
- The extent of grouting planned in the foundation below the cutoff;
- The protection downstream to control foundation erosion, e.g. filters over the surface
of the foundation.
High groundwater levels, e.g. in alluvial soils, may determine the practical depth to
which a cutoff may be taken, since dewatering is usually expensive. The guidelines given
below, therefore, are stating the “desirable” requirements rather than what may be prac-
ticable in some cases.
It should also be noted that many of the features described below cannot be readily
identified in the base of the cutoff excavation, but are apparent in the sides of the excava-
tion. Hence it is normal to require progressive excavation and cleanup, with a minimum
excavation depth of about 0.5 m below general excavation level to confirm that the
requirements have been achieved.
Having defined the cutoff foundation, the further objective when placing the earthfill is
to ensure a low permeability contact between the earthfill and the foundation. This is
done by:
- Modifying the shape of the foundation surface to provide a surface suitable for earth-
fill compaction (Section 17.6);
- Using compaction equipment which facilitates compaction but does not damage the
foundation (Section 14.2);
- Using more deformable, less erodible soils adjacent to the contact - i.e. using higher
plasticity soils placed wet of optimum water content (Section 14.2);
- Filling erodible seams or open joints in the rock prior to placing the core material
(Section 17.6).
17.3.2
Cutoff in rock
- Remove rock with open joints and other fractures which would otherwise lead to a highly
permeable structure. In many cases this will result in a foundation which has a perme-
ability generally less than say 15 Lugeons. For large, high hazard dams it would be nor-
mal to aim for a foundation with a permeability generally less than 7 Lugeons. However
it is emphasised that this may not be practicable in many situations and higher perme-
ability rock will be left in the foundations.
- Remove rock with clay infilled joints, roots etc., which may erode under seepage flows
to yield a high permeability rock. This is particularly important where the clay has been
transported into the joints and/or is dispersive, as this indicates its likely erodibility.
- Carry out slope modification and treatment as described in Section 17.6.
- Where the exposed rock is susceptible to slaking by wetting and drying (e.g. many
shales) or breakdown under trafficking, it should be covered with a cement-sand grout
(thickness usually
10 mm to 25 mm), pneumatically applied mortar or concrete (min-
imum thickness 50 mm and preferably not less than 150 mm). Generally this should be
