UFC 3-260-02
30 June 2001
remolded, become "quick" when remolded, or have expansive characteristics) result in lower permanent
deformations, less susceptibility to pumping, and improved overall performance.
7. TREATMENT OF PROBLEM SOILS. Although compaction increases the strength of most soils,
some soils decrease in stability when scarified, worked, and rolled. There are also some soils that
shrink excessively during dry periods and expand excessively when allowed to absorb moisture. When
these soils are encountered, special treatment is required. General descriptions of the soils in which
these conditions may occur and suggested methods of treatment are outlined as follows:
a. Clays that Lose Strength When Remolded. These types of clays 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, bearing values
should be obtained for both the undisturbed soil and the soil remolded and compacted to the design
density at the design moisture content and adjusted to the future moisture content conditions. If the
undisturbed value is the higher, no compaction should be attempted, and construction operations should
be conducted 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 requirements.
b. Soils that Become "Quick" When Molded. 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 sand 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, normal compaction
requirements should be applied. 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. There are cases where
drainage is not feasible and a high water table cannot be lowered, or cases where such soils become
saturated from sources other than high water table and cannot be dried out (as in necessary
construction during wet seasons). In such cases, the subgrade should not be disturbed, and additional
thickness of base and pavement should be used to ensure that the subgrade will not be overstressed or
compacted during subsequent traffic by aircraft.
c. Soils with Expansive Characteristics. Soils with expansive characteristics, if highly compacted,
will swell and produce uplift pressures of considerable intensity if the moisture content of the soil
increases after compaction. This action may result in intolerable differential heaving of flexible
pavements. Where the amount of swell is less than about 3 percent (as determined from soaked CBR
test), special consideration will not normally be needed. However, where an airfield subgrade includes
interspersed patches of soil with different swell characteristics, even amounts of swell less than
3 percent may require special consideration.
(1) Proper moisture content and density. A common method of treating a subgrade with
expansive characteristics is to compact it at a moisture content and to a unit weight that will minimize
expansion. The proper moisture content and unit weight for compaction control of a soil with marked
expansion characteristics are seldom the optimum moisture content and unit weight determined by the
compaction test. These factors may be determined from a study of the relations between moisture
content, unit weight, percentage of swell, and CBR for a given soil. A combination of moisture, density,
6-12
Previous Page
