15 AUGUST 2005
Special Problems. In open zones, compaction of backfill will not
generally present any particular problems if proper compaction procedures normally
associated with the compaction of soils are exercised and the materials available for
use, such as backfill, are not unusually difficult to compact. The majority of the
problems associated with backfill will occur in confined zones where only small
compaction equipment producing a low compaction effort can be used or where
because of the confined nature of the backfill zone even small compaction equipment
cannot be operated effectively.
Considerable latitude exists in the various types of small compaction
equipment available. Unfortunately, very little reliable information is available on the
capabilities of the various pieces of equipment. Depending upon the soil type and
working room, it may be necessary to establish lift thickness and compaction effort
based essentially on trial and error in the field. For this reason, close control must be
maintained particularly during the initial stages of the backfill until adequate compaction
procedures are established.
Difficult Structures. Circular, elliptical and arched walled structures are
particularly difficult to adequately compact backfill beneath the under side of haunches
because of limited working space. Generally, the smaller the structure the more difficult
it is to achieve required densities. Rock, where encountered, must be removed to a
depth of at least 0.15 m (6 in) below the bottom of the structure and the overdepth
backfilled with suitable material before foundation bedding for the structure is placed.
Some alternate bedding and backfill placement methods are discussed below.
One method is to bring the backfill to the planned elevation of the spring
line using conventional heavy compaction equipment and methods. A
template in the shape of the structure to be bedded is then used to
reexcavate to conform to the bottom contours of the structure. If the
structure is made of corrugated metal, allowance should be made in the
grade for penetration of the corrugation crests into the backfill upon
application of load. Success of this method of bedding is highly
dependent on rigid control of grade during reexcavation using the
template. This procedure is probably the most applicable where it is
necessary to use a cohesive backfill.
Another method of bedding placement is to sluice a clean granular backfill
material into the bed after the structure is in place. This method is
particularly adapted to areas containing a maze of pipes or conduits.
Adequate downward drainage, generally essential to the success of this
method, can be provided by sump pumps or, if necessary, by pumping
from well points. Sluicing should be accompanied by vibrating to ensure
adequate soil density. Concrete vibrators have been used successfully for
this purpose. This method should be restricted to areas where conduits or
pipes have been placed by trenching or in an excavation that provides
confining sides. Also, this method should not be used below the
groundwater table in seismic zones, since achieving densities high
enough to assure stability in a seismic zone is difficult.