15 AUGUST 2005
responsibility to recognize these problems and to take the necessary measures to minimize
their impact on the backfill operations.
Open Zones. An open zone is defined as a backfill area of sufficient dimensions
to permit the operation of heavy compaction equipment without endangering the integrity of
adjacent structures around which compacted backfill operations are conducted. Figure 8-3.1
shows examples of open zones. In these zones where large compaction equipment, can
operate, it is generally not too difficult to obtain the desired density if appropriate materials and
proper backfill procedures are used. For areas that can be economically compacted by heavy
equipment, the designer can avoid problems by including in the design provisions sufficient
working space between structures or between excavation slopes and structures to permit
access by the heavy compaction equipment. Generally, a working space of at least 3.6 m (12
ft) between structure walls and excavation slope and at least 4.5 m (15 ft) between structures
is necessary for heavy equipment to maneuver. In addition to maneuvering room, the designer
must also consider any adverse loading caused by the operation of heavy equipment too close
to structure walls, as discussed in paragraph 2-3d.
Confined zones. Confined zones are defined as areas where backfill operations
are restricted to the use of small mechanical compaction equipment (Fig 8-3.2) either because
the working room is limited or because heavy equipment (Fig. 8-3.1) would impose excessive
soil pressures that could damage the structure. Most deficiencies in compacted backfill around
subsurface structures have occurred in confined zones where required densities are difficult to
achieve because of restricted working room and relatively low compaction effort of equipment
that is too lightweight. The use of small equipment to achieve required compaction is also
more expensive than heavy equipment since thinner lifts are required. However, because
small compaction equipment can operate in spaces as narrow as 0.6 m (2 ft) in width, such
equipment is necessary to achieve the required densities in some areas of most backfill
projects. Therefore, the designer should plan structure and excavation areas to minimize the
use of small compaction equipment.
Structure Configuration. The designer familiar with backfilling operations can
avoid many problems associated with difficult to reach confined zones, which are created by
structural shapes obstructing the placement and compaction of backfill, by considering the
impact of structural shape on backfill operations. In most cases, structural shapes and
configurations that facilitate backfill operations can be used without significantly affecting the
intended use of the structure.
Curved Bottom and Wall Structures. Areas below the spring line of circular,
elliptical, and similar shaped structures are difficult to compact backfill against because
compaction equipment cannot get under the spring line. If possible, structures should be
designed with continuously curved walls and flat floors such as in an igloo-shaped structure.
For structures where a curved bottom is required to satisfy the intended function, it may be
advisable for the designer to specify that a template shaped like the bottom of the structure be
used to guide the excavation below the spring line so that uniform foundation support will be