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factors, on the effective weight of the compaction equipment and the weight, earth pressure
coefficient, and Poisson's ratio of the backfill material. Compaction-induced earth pressures
against walls are also described in Chapter 6.
The designer must evaluate the economics of the extra cost of structures
designed to withstand very close-in operation of heavy compaction equipment versus the extra
cost associated with obtaining required compaction of backfill in thin lifts with smaller
compaction equipment. A more economical alternative might be to specify how close to the
walls different weights of compaction equipment can be operated.
One method of reducing lateral earth pressures behind walls has been to use
about 1.2 m (4 ft) of uncompacted granular (sand or gravel) backfill above the base of the wall.
Soil backfill can then be compacted in layers above the granular backfill. Compression of the
granular material prevents the buildup of excessive lateral pressures against the wall.
Evaluation, Design, and Processing of Backfill Materials. The evaluation,
design, and proper processing of backfill materials are extremely important phases of the pre-
construction operations. The purpose of the evaluation phase is to determine the engineering
characteristics of potential backfill materials. The design phase must take into account the
engineering characteristics required of the backfill and specify materials that, when compacted
properly, will have these characteristics. Proper processing of the backfill material will ensure
that desirable engineering characteristics will be obtained as the material is placed.
Evaluation of Backfill Materials. Evaluation of backfill materials consists of
exploration, sampling, and laboratory testing to determine the engineering characteristics of
potential backfill materials. Detailed instructions for exploration, sampling, laboratory testing,
and foundation design are presented in Chapters 2 and 3. However, to emphasize the need
for an adequate investigation, some aspects of planning and investigation that should be
considered are discussed in the following paragraphs.
Field Exploration and Sampling. Field exploration and sampling are extremely
important to the design of foundations, selection of backfill, and planning for construction. A
great amount of material will be available from required excavations, and the investigation for
foundation conditions should include the sampling and evaluation of these materials for
possible use as backfill. Where an adequate volume of suitable backfill cannot be obtained
from the construction excavation, the exploration and sampling program must be expanded to
find other sources of suitable material whether from nearby borrow areas or commercial
The purpose of the investigation is to delineate critical conditions and provide
detailed information on the subsurface deposits so that proper design and construction,
including backfilling operations, can be accomplished with minimum difficulty. Thus careful
planning is required prior to the field exploration and sampling phase of the investigation.
Available geologic and soil data should be studied, and if possible, preliminary borings should
be made. Once a site has been tentatively selected, orientation of the structure to the site