Environmental Engineering Reference
In-Depth Information
ICOLD (1990) recommend compaction at a water content above optimum water con-
tent so as to avoid a flocculated soil structure, and to avoid brittleness which will promote
formation of cracks. They suggest that a permeability of lower than 10
7
m/sec is
required.
The authors' opinion is that a water content between optimum and optimum plus 2%,
and a density ratio of greater than 98% (standard compaction) is desirable, and that the
water content should not be below optimum
1%. If the soil was at optimum
2% or
even say optimum
3%, one might relax the density ratio requirement to 97%, in would
probably be of low permeability and less likely to crack than if compacted at opti-
mum
1%.
This will necessitate use of thin layers, particularly adjacent outlet pipes, where rollers
may not be used in some cases. Supervision and testing need to be very thorough. Care
must be taken to avoid drying of the surface of layers of earthfill, which could result in
cracking, and a preferred path for piping failure to initiate.
7.7.2.3
Careful detailing of pipes or conduits through the embankment
Pipes through embankments should be avoided if possible since it is very difficult to
ensure good compaction around the pipe, and differential settlement around the pipe can
also lead to cracking of the soil. If pipes must be placed through an embankment then sup-
port the pipe on a concrete footing and use filters to surround the downstream end of the
pipe as shown in Figures 13.16, 13.17 and 13.18.
It is considered that reliance only on good compaction with or without lime modifica-
tion is a relatively high risk option and is not recommended.
7.7.2.4
Lime or gypsum modification of the soil
Most dispersive soils can be rendered non dispersive by addition of a small quantity of
lime (Ca(OH)
2
or CaO) or gypsum (CaSO
4
or CaSO
4
(2H
2
O)).
This process is one of cation exchange, with the Ca
ions exchangeable for Na
ions. Laboratory tests can be carried out to determine the required amount of lime or gyp-
sum, e.g. Sherard pinhole tests using soil with different amounts of lime or gypsum.
Commonly one would require 2% to 3% lime, and would allow a margin of 1% or 2%
above that indicated by the laboratory tests to allow for difficulty in mixing the lime with
the soil. The lime should be mixed with the soil using a pulveriser. This breaks up the soil
so that 80% to 90% of the particles are less than 25 mm diameter and facilitates good
mixing of the lime.
7.7.2.5
Sealing of cracks in the abutment and cutoff trench
If the soil used for construction is dispersive, particular care must be taken to seal cracks
in the cutoff foundation, and the sides of the cutoff trench so the soil will not erode
into the cracks. Extensive use of slush concrete or shotcrete is likely. If the cutoff trench
is in soil, it may be necessary to provide a filter on the downstream side as shown in
Figure 10.8.
7.7.3
Turbidity of reservoir water
The presence of dispersive soil in the reservoir area, or in the catchment, can lead to tur-
bidity of the water in the reservoirs. Grant et al. (1977) describe how turbidity in the water
for Cardinia Creek Dam, Melbourne, was prevented by adding 6500 tonnes of gypsum to
the water on first filling, at a concentration of 40 mg/litre of water. Again the process is one
of Ca
displacing the Na
in the soil and makes it less dispersive. Gypsum was added to
water as it entered the storage. Cardinia Creek Dam is an off river storage reservoir.
