TM 5-818-4/AFM 88-5, Chap. 5
to 4 percentage points below optimum may result in
posits to facilitate compaction of backfill against the
large settlements caused by collapse of nonswelling
slope and to ensure good bond between the backfill and
soil material or heave of swelling materials upon sat-
the excavation slopes. Loose material should be re-
uration after construction. Compacting cohesive back-
moved from the excavation slopes; in some cases,
fill material at optimum water content or slightly on
benches may be required to provide a firm surface to
the wet side of optimum generally will reduce the
compact backfill against.
amount of settlement and swelling that would occur.
f. Lines and grades. Care should be exercised in
The reduction should be confirmed by consolidation
planning lines and grades for excavation to ensure
and swell tests on compacted specimens (para 3-2b(4)).
that uniform, adequate support is provided at the
(2) Service conduits. Settlement within the back-
foundation level of important structures. Generally,
fill around structures will also occur. A proper design
foundations consisting of part backfill and part undis-
will allow for the estimated settlement as determined
turbed materials do not provide uniform bearing and
from studies of consolidation characteristics of the
should be avoided wherever possible. The foundation
compacted backfill. Where service conduits, access cor-
should be overexcavated where necessary, and back-
ridors, and similar facilities connect to the structure
filled with compacted select material to provide unif-
oversize sleeves, flexible connections and other protec-
orm support for the depth required for the particular
tive measures, as appropriate, may be used to prevent
structure. Where compacted backfill is required be-
damage within the structure.
neath a structure, the minimum depth specified should
(3) Differential settlement. Complex structures
be at least 18 inches.
are more susceptible to differential settlement because
g. Thin- walled metal structures. Thin-walled, corru-
of the potential for large variations in loads carried by
gated metal structures are susceptible to deflections of
each component foundation. In the multilevel stepped
structural walls when subjected to backfill loads. Ad-
structure (fig. 2-3a), the foundation supporting the
verse deflections can be minimized by planning back-
lower level offset component must also support the
fill operations so that compacted backfill is brought up
volume of backfill over that part of the structure.
evenly on both sides of the structure to ensure uniform
Measures must be taken to ensure that the proper
stress distribution. Temporary surcharge loads applied
functioning of all elements is not hampered by differ-
to the structure crown may also be required to prevent
ential settlement. The increased cost of proper design
vertical distortions and inward deflection at the sides.
and construction where unusual or difficult construc-
tion procedures are required is insignificant when
2-3. Backfill problem areas. Other features
compared with the cost of the structure. The cost of re-
that have the potential to become problem areas are
medial measures to correct deficiencies caused by im-
discussed in the following paragraphs. These potential
proper design and construction usually will be greater
problem areas have to be considered during the plan-
than the initial cost required to prevent the deficien-
ning and design phases to minimize deficiencies in
cies.
structure performance associated with backfill place-
(4) Downdrag. In addition to conventional service
ment and to make backfilling operations less difficult.
loads, cut and cover subsurface structures are suscepti-
a. Settlement and downdrag. In the construction of
ble to downdrag frictional forces between the struc-
underground structures and particularly missile-
ture and the backfill that are caused by settlement of
launch-site facilities, tolerances to movement are often
the backfill material adjacent to and around the struc-
considerably less than those in normal construction.
ture. Downdrag loads can be a significant proportion
The design engineer must determine and specify allow-
of the total vertical load acting on the structure and
able tolerances in differential settlement and ensure
must be considered in the structure settlement analy-
that differential settlement is minimized and/or ac-
commodated. Settlement analysis procedures are out-
significant shear forces along the outer surface of
lined in TM 5-818-l/AFM 88-3, Chapter 7. See ap-
structures with curve-shaped roofs and walls. The
pendix A, References.
of backfill, roughness of the structure's surface, and
immediately beneath most structures. Consolidation
magnitude of earth pressures acting against the struc-
and swelling characteristics of backfill materials
ture. Techniques for minimizing downdrag friction
should be thoroughly investigated so that materials
forces generally include methods that reduce the struc-
having unfavorable characteristics will not be used in
ture surface roughness such as coating the structure's
those zones. Some settlement can be expected to take
outer surface with asphalt or sandwiching a layer of
place, but it can be minimized by requiring a higher
polyethylene sheeting between the structure's outer
than normal compacted density for the backfill. Cohe-
surface and fiberboard (blackboard) panels. Backfill
sive backfill compacted at a water content as little as 3
settlement and associated downdrag can also be mini-
2-4
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