15 AUGUST 2005
Complex Structures. Complex structures have variable shaped walls and
complex configurations in plan and number of levels. These structures can also be simple
structures interconnected by access shafts, tunnels, and utility conduits. Because of their
irregular shapes and configurations the different types of structures significantly increase
excavation and backfill problems.
Typical examples of complex structures are stepped multilevel structures and
multichambered structures with interconnecting corridors (Figure 8-3.3). Complex structures
are generally more difficult to compact backfill around and are more likely to have settlement
problems (paragraph 8-220.127.116.11). Although the multilevel step structure (Figure 8-3.3(a)) is not
particularly difficult to compact backfill around, at least for the first level, the compaction of
backfill over the offset structure will generally require the use small equipment. Small
equipment will also be required for compaction of backfill around and over the access corridor
and between the two chambers (Figure 8-3.3 (b)). Where possible, the design should
accommodate intended functions into structures with uniformly shaped walls and a simple
Where structures of complex configurations are necessary, construction of a
three-dimensional model during the design and planning phases will be extremely beneficial.
From the model, designers can more easily foresee and eliminate areas in which it would be
difficult to place and compact backfill.
Service Conduits. Since compaction of backfill is difficult around pipes and
conduits, group utility lines together or place in a single large conduit where feasible, rather
than allow to form a haphazard maze of pipes and conduits in the backfill. Run utility lines
either horizontally or vertically wherever possible. Coordinate plans for horizontally run
appurtenances, such as utility lines, access tunnels, and blast-delay tubing, with the
excavation plans so that wherever feasible these appurtenances can be supported by
undisturbed soils rather than by compacted backfill.