15 AUGUST 2005
anomalous material such as organic matter or clay pockets. Stratified material should be
mixed prior to placing to obtain a uniform blend. Stockpile excavated material to be used as
backfill according to class or type of material.
Water Content. While water content can be adjusted to some extent after
placing (but before compacting), it is generally more advantageous to adjust the water content
to optimum compaction conditions before placing. Adjustment of water content can be
accomplished by aeriation (disking or turning) or sprinkling the material in 305 mm to 450 mm
(12 to 18 in) layers prior to placing or stockpiling. If the material is stockpiled, provisions
should be made to maintain constant moisture content during wet or dry seasons.
Ensuring Gradation. Some backfill materials consisting
of crushed rock, gravel,
or sand require limitations on maximum and minimum particle-size or gradation distributions.
Where materials cannot be located that meet gradation criteria, it may be advantageous to
require processing of available material by sieving to obtain the desired gradation.
EARTHWORK: EXCAVATION AND PREPARATION FOR FOUNDATIONS
General. In general, excavation for subsurface structures will consist of open
excavation and shaft and tunnel excavation. Where excavation to great depths is required, a
variety of soils and rock may be encountered at a single site. Soils may range through a wide
spectrum of textures and water contents. Rock encountered may vary from soft rock, very
similar to a firm soil in its excavation requirements, to extremely hard rock requiring extensive
blasting operations for removal. Groundwater may or may not be present. The groundwater
conditions and the adequacy of groundwater control measures are important factors in
excavation, in maintaining a stable foundation, and in backfilling operations. The extent to
which groundwater can be controlled also influences the slopes to which the open excavation
can be cut, the bracing required to support shaft and tunnel excavation, and the handling of the
Good Construction Practices, and Problems. A majority of the problems
encountered during excavation are related to groundwater conditions, slope stability, and
adverse weather conditions. Many of the problems can be anticipated and avoided by
preconstruction planning and by following sound construction practices.
Groundwater. Probably the greatest source of problems in excavation
operations is groundwater. If the seepage of groundwater into an excavation is adequately
controlled, other problems will generally be minor and can be easily handled. Several points
should be recognized that, if kept in mind, will help to reduce problems attributable to
groundwater. In some instances, groundwater conditions can be more severe than indicated
by the original field exploration investigation since field explorations provide information only
for selected locations and may not provide a true picture of the overall conditions.