Environmental Engineering Reference
In-Depth Information
14.2
SPECIFICATION OF EARTHFILL
It is common to specify the following for earthfill for dam construction.
(a)
The source and Unified Soil Classification of the earthfill
For example, “clay and sandy clay, medium plasticity, from Borrow area A”.
(b)
Maximum size and particle size distribution
The maximum particle size (of gravel or rock fragments) in the earthfill is limited to
ensure compaction is not affected. It will usually be specified as not greater than a size
in the range 75-125 mm.
The particle size distribution is further defined to:
(i) Ensure there is sufficient silt and clay fines passing 0.075 mm to give the required
low permeability for the core. It is normal to require at least 15% passing
0.075 mm. For most clays, sandy clay and clayey sands this will be readily met.
(ii) Be compatible with the grading requirements assumed for the design of the filters.
This may be achieved by specifying the particle size grading envelope for the soil
(based on laboratory tests from the borrow area, as shown in Figure 9.26).
Alternatively for soils with no gravel content, the minimum D
85B
i.e. the size for
which 85% is finer used for the filter design could be specified. For gravelly soils,
it will be necessary to express this in terms of the grading of the soil passing the
4.75 mm sieve, as is required for filter design.
It is important to measure the particle size in the embankment after compaction to
allow for breakdown of weak rock particles and mixing of fine and coarse soils from
the borrow area. During site investigations, the effect of such breakdowns can be
assessed by carrying out particle size distributions on samples which have been sub-
jected to compaction.
(c)
The Atterberg limits
Most authorities specify a minimum plasticity index and some also place a maximum on
liquid limit. The latter may be due to the presence in the borrow area of particularly high
plasticity clays, which may be difficult to compact, but in general there should be no need
to place an upper bound on the liquid limit. In fact there is evidence that more plastic
clays are likely to be less erodible than other soils so are advantageous to have in the core.
The authors' preference is to specify an allowable range of liquid limit and plasticity
index by relating to the “A” line on the plasticity chart. Figure 14.1 gives an example.
The critical issue really is to specify limits which can be satisfied by the material you
wish to use from the borrow area.
Figure 14.1.
An example of specification of Atterberg limits for earthfill.
