TM 5-818-4/AFM 88-5, Chap. 5
CHAPTER 2
PLANNING AND DESIGN OF STRUCTURES AND
EXCAVATIONS TO ACCOMMODATE BACKFILL OPERATIONS
2-1. General. Many earthwork construction prob-
face structures have occurred in confined zones where
lems can be eliminated or minimized through proper
required densities are difficult to achieve because of
design, thorough planning, and recognition of problem
restricted working room and relatively low compaction
areas effecting backfill operations. Recognition and
effort of equipment that is too lightweight. The use of
consideration must be given in planning to design fea-
small equipment to achieve required compaction is
tures that will make backfilling operations less diffi-
also more expensive than heavy equipment since thin-
cult to accomplish. Examples of problem areas and
ner lifts are required. However, because small compac-
how forethought in design and planning can help to
tion equipment can operate in spaces as narrow as 2
eliminate backfill deficiencies are presented in the fol-
feet in width, such equipment is necessary to achieve
lowing paragraphs.
the required densities in some areas of most backfill
projects. Therefore, the designer should plan structure
2-2. Effect of excavation and structural
and excavation areas to minimize the use of small com-
configuration on backfill operations.
paction equipment.
Some of the problems encountered in earthwork con-
c. Structure configuration. The designer familiar
struction are related to the excavation and the configu-
with backfilling operations can avoid many problems
ration of the structures around which backfill is to be
associated with difficult to reach confined zones,
placed. It is the designer's responsibility to recognize
which are created by structural shapes obstructing the
these problems and to take the necessary measures to
placement and compaction of backfill, by considering
minimize their impact on the backfill operations.
the impact of structural shape on backfill operations.
In most cases, structural shapes and configurations
area of sufficient dimensions to permit the operation
that facilitate backfill operations can be used without
of heavy compaction equipment without endangering
significantly affecting the intended use of the struc-
the integrity of adjacent structures around which com-
ture.
pacted backfill operations are conducted. Figure 2-1
(1) Curved bottom and wall structures. Areas be-
shows examples of open zones. In these zones where
low the spring line of circular, elliptical, and similar
large compaction equipment, can operate, it is general-
shaped structures are difficult to compact backfill
ly not too difficult to obtain the desired density if ap-
against because compaction equipment cannot get un-
propriate materials and proper backfill procedures are
der the spring line. If possible, structures should be de-
used. For areas that can be economically compacted by
signed with continuously curved walls and flat floors
heavy equipment, the designer can avoid problems by
such as in an igloo-shaped structure. For structures
including in the design provisions sufficient working
where a curved bottom is required to satisfy the in-
space between structures or between excavation slopes
tended function, it may be advisable for the designer
and structures to permit access by the heavy compac-
to specify that a template shaped like the bottom of
tion equipment. Generally, a working space of at least
the structure be used to guide the excavation below the
12 feet between structure walls and excavation slopes
spring line so that uniform foundation support will be
and at least 15 feet between structures is necessary for
provided.
heavy equipment to maneuver. In addition to maneu-
(2) Complex structures. Complex structures have
vering room, the designer must also consider any ad-
variable shaped walls and complex configurations in
verse loading caused by the operation of heavy equip-
plan and number of levels. These structures can also be
ment too close to structure walls, as discussed in para-
simple structures interconnected by access shafts, tun-
graph 2-3d.
nels, and utility conduits. Because of their irregular
b. Confined zones. Confined zones are defined as
shapes and configurations the different types of struc-
areas where backfill operations are restricted to the
tures significantly increase excavation and backfill
use of small mechanical compaction equipment (fig.
problems.
2-2) either because the working room is limited or be-
(a) Typical examples of complex structures are
cause heavy equipment (fig. 2-1) would impose exces-
stepped multilevel structures and multichambered
sive soil pressures that could damage the structure.
structures with interconnecting corridors (fig. 2-3).
Most deficiencies in compacted backfill around subsur-
Complex structures are generally more difficult to
2-1
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