TM 5-818-4/AFM 88-5, Chap. 5
Figure 2- 1. Open backfill zones.
be grouped together or placed in a single large conduit
compact backfill around and are more likely to have
settlement problems (para 2-3a). Although the multi-
where feasible rather than allowed to form a haphaz-
ard maze of pipes and conduits in the backfill. Utility
level step structure (fig. 2-3a) is not particularly diffi-
cult to compact backfill around, at least for the first
lines should be run either horizontally or vertically
wherever possible. Plans for horizontally run appur-
level, the compaction of backfill over the offset struc-
tenances, such as utility lines, access tunnels, and
ture will generally require the use of small equipment.
Small equipment will also be required for compaction
blast-delay tubing, should be coordinated with the ex-
of backfill around and over the access corridor and be-
cavation plans so that wherever feasible these appur-
tween the two chambers (fig. 2-3b). Where possible,
tenances can be supported by undisturbed soils rather
the design should accommodate intended functions
than by compacted backfill.
into structures with uniformly shaped walls and a sim-
e. Excavation plans. The excavation plans should be
ple configuration.
developed with the backfill operations and the struc-
(b) Where structures of complex configurations
ture configurations in mind. The excavation and all
are necessary, construction of a three-dimensional
completed structures within the excavation should be
model during the design and planning phases will be
conducive to good backfill construction procedures,
extremely beneficial. From the model, designers can
and access should be provided to all areas so that com-
more easily foresee and eliminate areas in which it
paction equipment best suited to the size of the area
would be difficult to place and compact backfill.
can be used. The plans for excavation should also pro-
d. Service conduits. Since compaction of backfill is
vide for adequate haul roads and ramps. Positive exca-
difficult around pipes and conduits, utility lines should
vation slopes should be required in all types of soil de-
2-2
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