Environmental Engineering Reference
In-Depth Information
- The water content seldom becomes too high - most soils only take in water up to
about the optimum;
- Ripping to 0.6 m to 0.9 m will assist in allowing water to penetrate. Contour
ploughing can assist in hilly borrow areas;
-Water conditioning up to 4.5 m depth has been successfully achieved.
Earthfill which dries out, or gets wet, on the embankment must be tyned, its mois-
ture adjusted, then thoroughly reworked with a grader before recompaction. If the
soil is judged too wet or dry to be adjusted on the embankment, it should be removed
from the embankment.
The risk of wetting earthfill on the embankment if rain is about to fall is usually
reduced by requiring the surface to be “sealed” with a smooth drum roller, and con-
toured to allow runoff of surface water. The sealed surface must be tyned prior to
placing the next layer.
(h)
Fill adjacent to the foundation
It is normal to specify that the earthfill which is to be placed adjacent (within say
0.6 m) of the dam foundation is to be composed of finer, more plastic soil available
from the borrow area and is to be compacted at a higher water content (e.g.
OWC
3%) with rubber tyred construction equipment or rollers.
This is to faciliate squeezing the soil into the irregularities in the foundation.
Generally there is no “performance” specification (i.e. density ratio), because the lay-
ers are thin and testing is impracticable. On Thomson Dam, Snowy Mountains
Engineering Corporation (SMEC), designing for the Melbourne and Metropolitan
Board of Works (MMBW), required that the contact zone be compacted to a density
100% of that which would be achieved by compacting in the laboratory with stan-
dard compactive effort, at the field water content. This is a reasonable approach
which allows for the high compaction water content, but should not be specified except
for such large dams as Thomson (160 m high). For most dams, a methods specification
with good water content control is sufficient.
2% or OWC
(i)
Compaction of the edges of fill
Under normal operations, the outer 1 m to 1.5 m (measured horizontally) of an earthfill
embankment will not be adequately compacted by rollers. It is necessary to specify
either that the embankment is constructed oversize and trimmed back to the required
lines by removing this poorly compacted soil, or to require rolling of the surface up
and down the slope.
If left in place, the poorly compacted soil will often soften and lead to surficial slid-
ing, particularly on steeper slopes. While this will not in itself lead to failure of the
embankment, subsequent erosion or sliding may cause problems.
14.3
SPECIFICATION OF FILTERS
It is common to specify the following for filters:
(a)
Type of material and source
For example “washed, graded sand and gravel alluvium from Borrow Area F” or
“crushed, graded, slightly weathered to fresh granodiorite from Quarry G”.
(b)
Particle size grading
This is specified as outlined in Section 9.5.1.
It is important to make it clear in the specification for filters, that the particle size
specification will be strictly adhered to, including the limitations on fines (passing
0.075 mm) content, and that the contractor should expect to have to wash, screen into
