TM 5-818-4/AFM 88-5, Chap. 5
(c) Additives, such as hydrated lime, quicklime,
controlled during compaction, it is difficult to control
and fly ash, can be mixed with some highly plastic
material type, water content, and gradation of the
clays to improve their engineering characteristics and
material as it is being placed in the backfill; control
permit the use of some materials that would otherwise
criteria must be established prior to placement.
be unacceptable. Hydrated lime can also be mixed with
a. Material type. Backfill material should consist of
some expansive clays to reduce their swelling char-
a homogeneous material of consistent and desirable
acteristics (TM 5-818-1/AFM 88-3, Chapter 7). Lab
characteristics. The field engineer must ensure that
oratory tests should be performed to determine the
only the approved backfill material is used and that
amount of the additive that should be used and the
the material is uniform in nature and free of any
characteristics of the backfill material as a result of
anomalous material such as organic matter or clay
using the additive. Because of the complexity of soil-
pockets. Stratified material should be mixed prior to
additive systems and the almost complete empirical
placing to obtain a uniform blend. Excavated material
nature of the current state of the art, trial mixes must
to be used as backfill should be stockpiled according to
be varified in the field by test fills.
class or type of material.
(6) Commercial by-products. The use of commer-
b. Water content. While water content can be ad-
cial by-products, such as furnace slag or fly ash as
justed to some extent after placing (but before com-
backfill material, may be advantageous where such
pacting), it is generally more advantageous to adjust
products are locally available and where suitable nat-
the water content to optimum compaction conditions
ural materials cannot be found. Fly ash has been used
before placing. Adjustment of water content can be ac-
as a lightweight backfill behind a 25-foot-high wall
complished by aeriation (disking or turning) or sprin-
and as an additive to highly plastic clay. The suitabil-
kling the material in 12- to 18- inch layers prior to
ity of these materials will depend upon the desirable
placing or stockpiling. If the material is stockpiled,
characteristics of the backfill and the engineering
provisions should be made to maintain a constant
characteristics of the products.
moisture content during wet or dry seasons.
3-4. Processing of backfill materials. The
construction of subsurface structures often requires
the construction of elements of the structure within or
sisting of crushed rock, gravel, or sand require limita-
upon large masses of backfill. The proper functioning
tions on maximum and minimum particle-size or
of these elements are often critically affected by ad-
gradation distributions. Where materials cannot be lo-
verse behavioral characteristics of the backfill. Be-
cated that meet gradation criteria, it may be advanta-
havioral characteristics are related to material type,
geous to require processing of available material by
water content during compaction, gradation, and com-
sieving to obtain the desired gradation.
paction effort. While compaction effort may be easily
3-7
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