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be considered suitable as backfill is inorganic clay (CH). Use of CH soils should be avoided in
confined areas if a high degree of compaction is needed to minimize backfill settlement or to
provide a high compression modulus.
The swelling (and shrinking) characteristics of expansive clay vary with the type
of clay mineral present in the soil, the percentage of that clay mineral, and the change in water
content. The active clay minerals include montmorillonite, mixed-layer combinations of
montmorillonite and other clay minerals, and under some conditions chlorites and vermiculites.
Problems may occur from the rise of groundwater, seepage, leakage, or elimination of surface
evaporation that may increase or decrease the water content of compacted soil and lead to the
tendency to expand or shrink. If the swelling pressure developed is greater than the
restraining pressure, heave will occur and may cause structural distress. Compaction on the
wet side of optimum moisture content will produce lower magnitudes of swelling and swell
pressure. Expansive clays that exhibit significant volume increases should not be used as
backfill where the potential for structural damage might exist. Suitability should be based upon
laboratory swell tests.
Additives, such as hydrated lime, quicklime, and fly ash, can be mixed with some
highly plastic clays to improve their engineering characteristics and permit the use of some
materials that would otherwise be unacceptable. Hydrated lime can also be mixed with some
expansive clays to reduce their swelling characteristics. Laboratory tests should be performed
to determine the amount of the additive that should be used and the characteristics of the
backfill material as a result of using the additive. Because of the complexity of soil additive
systems and the almost completely empirical nature of the current state of the art, trial mixes
must be verified in the field by test fills.
Commercial By-Products. The use of commercial by-products, such as
furnace slag or fly ash as backfill material, may be advantageous where such products are
locally available and where suitable natural materials cannot be found. Fly ash has been used
as a lightweight backfill behind a 7.6 m (25 ft) high wall and as an additive to highly plastic
clay. The suitability of these materials will depend upon the desirable characteristics of the
backfill and the engineering characteristics of the products.
Processing of Backfill Materials. The construction of subsurface structures
often requires the construction of elements of the structure within or upon large masses of
backfill. The proper functioning of these elements is often critically affected by adverse
behavioral characteristics of the backfill. Behavioral characteristics are related to material
type, water content during compaction, gradation, and compaction effort. While compaction
effort may be easily controlled during compaction, it is difficult to control material type, water
content, and gradation of the material as it is being placed in the backfill; control criteria must
be established prior to placement.
Material Type. Backfill material should consist of a homogeneous material of
consistent and desirable characteristics. The field engineer must ensure that only the
approved backfill material is used and that the material is uniform in nature and free of any