Environmental Engineering Reference
In-Depth Information
The Atterberg limit requirements should be seen only as a way of allowing rejection
of unsuitable material if a dispute arises with the contractor. Generally, it should be
possible to accept or reject material visually in the borrow area, at least after the job
has been under way for some time and the contractor and supervisors are familiar
with the materials.
(d)
Density ratio, water content and layer thickness
It is common practice to specify a density ratio
98% of standard maximum dry density,
with a water content between OWC
2%,
where OWC is standard compaction optimum water content. It should be noted that:
1% and OWC
1%, or OWC and OWC
- Standard, not modified compaction should be used. This is to ensure moist com-
paction which leads to low permeability, flexible fills. Compaction at around mod-
ified optimum water content leads to high densities, but the soil structure is likely
to be aggregated, leading to a higher permeability and more brittle fill. Recent stud-
ies (Wan and Fell, 2002, 2003) show dry compaction also is likely to result in clay
soils which are more erodible than if they are compacted wet of optimum.
- The requirement for density ratio
98% is reasonable and compatible with the
water content ranges shown. There is no advantage in specifying a higher density
ratio and it may be detrimental in that the contractor will be forced to compact dry
of optimum water content. For smaller dams, dams to be constructed in wet climates
and soils which are difficult to compact, it would not be unreasonable to relax the
compaction requirement to as low as 95% density ratio, provided that compaction
is carried out above optimum water content. However, compaction to only 95%
density ratio at say optimum
3% would lead to a permeable soil structure and
would not normally be acceptable. Figure 14.2 shows some of these effects.
Figure 14.2.
Compaction water content and the effect on properties.
