Lesson 1/Learning Event 2
Remolded Clay. The types of clays that show a decrease in strength when remolded are generally in
the CH and OH groups. They are clays that have been consolidated to a very high degree, either under
an overburden load or by alternate cycles of wetting-and-drying, or that have by other means developed
a definite structure. They have a high strength in the undisturbed state. Scarifying, reworking, and
rolling these soils in cut areas may produce a lower bearing value than that of the undisturbed soils.
When such clay soils are encountered, obtain the bearing values for both the undisturbed soil and the
soil removed; compact them to the design density at the design moisture content and adjust them to the
future moisture content conditions. If the undisturbed value is the higher, do not attempt any
compaction and conduct construction operations to produce the least possible disturbance of the soil.
Since compaction cannot be effected in these cases, the total thickness design above the subgrade may
be governed by the required depth of compaction rather than the CBR method.
Remolded Silts. Experience has shown that some deposits of silt, very fine sand, and rock flour
(predominantly in classifications ML and SC) when compacted in the presence of a high water table will
pump water to the surface and become "quaky" or "spongy" with a loss of practically all bearing value.
The condition can also develop in most silts and poorly draining, very fine sands if these materials are
compacted at a high moisture content, because the compaction reduces the air voids so that the available
water fills practically all the void space. Therefore, it is difficult to obtain the desired densities in these
silts and very fine sands at moisture contents greater than optimum. Also during compaction of the
base, the water from a wet, spongy silt subgrade will often enter the subbase and base with detrimental
effects. The bearing value of these silts and very fine sands is reasonably good if they can be compacted
at the proper moisture content. Drying is not difficult if the source of water can be removed, since the
soils are usually friable and can be scarified readily. If the soils can be dried, apply normal compaction
requirements. However, removing the source of water is often very difficult and in some cases
impossible in the allotted construction period. In cases of high water table, drying is usually not
satisfactory until the water table is lowered, as recompacting operations will again cause water to be
pumped to the surface. Local areas of this nature are usually treated satisfactorily by replacing the soil
with subbase and base materials or with a dry soil that is not critical to water.
In cases where drainage is not feasible and a high water table cannot be lowered, or in cases where such
soils become saturated from other sources than high water table and cannot be dried out (as in necessary
construction during wet seasons), the subgrade should not be disturbed. Additional thicknesses of base
and pavement should be used to insure that the subgrade will not be overstressed or compacted during
subsequent traffic. Anticipate pumping and detrimental actions previously described whenever silts or
very fine sand subgrade are accompanied by a high water table. This pumping action limits the ability
to obtain compaction in the immediately overlying material which must be considered in the design.