UFC 3-220-01N
15 AUGUST 2005
8-3.2.2.5
Lines and Grades. Exercise care in planning lines and grades for excavation to
ensure that uniform, adequate support is provided at the foundation level of important
structures. Generally, foundations consisting of part backfill and part undisturbed materials do
not provide uniform bearing and should be avoided wherever possible. The foundation should
be overexcavated where necessary and backfilled with compacted select material to provide
uniform support for the depth required for the particular structure. Where compacted backfill is
required beneath a structure, the minimum depth specified should be at least 450 mm (18
inches).
8-3.2.2.6
Thin-walled Metal Structures. Thin-walled, corrugated metal structures are
susceptible to deflections of structural walls when subjected to backfill loads. Minimize
adverse deflections by planning backfill operations so that compacted backfill is brought up
evenly on both sides of the structure to ensure uniform stress distribution. Temporary
surcharge loads applied to the structure crown may also be required to prevent vertical
distortions and inward deflection at the sides.
8-3.2.3
Backfill Problem Areas. Other features that have the potential to become
problem areas are discussed in the following paragraphs. These potential problem areas must
be considered during the planning and design phases to minimize deficiencies in structure
performance associated with backfill placement and to make backfilling operations less
difficult.
8-3.2.3.1
Settlement and Downdrag. In the construction of underground structures and
particularly missile launch site facilities, tolerances to movement are often considerably less
than those in normal construction. The design engineer must determine and specify allowable
tolerances in differential settlement and ensure that differential settlement is minimized and/or
accommodated. Settlement analysis procedures are outlined in Chapter 4.
8-3.2.3.2
Critical Zones. Critical backfill zones are those immediately beneath most
structures. Consolidation and swelling characteristics of backfill materials should be
thoroughly investigated so that materials having unfavorable characteristics will not be used in
those zones. Some settlement can be expected to take place, but it can be minimized by
requiring a higher than normal compacted density for the backfill. Cohesive backfill compacted
at water content as little as 3 to 4 percentage points below optimum may result in large
settlements caused by collapse of non-swelling soil material or heave of swelling materials
upon saturation after construction. Compacting cohesive backfill material at optimum water
content or slightly on the wet side of optimum generally will reduce the amount of settlement
and swelling that would occur. Confirm the reduction by consolidation and swell tests on
compacted specimens.
8-3.2.3.3
Service Conduits. Settlement within the backfill around structures will also
occur. A proper design will allow for the estimated settlement as determined from studies of
consolidation characteristics of the compacted backfill. Where service conduits, access
corridors, and similar facilities connect to the structure oversize sleeves, flexible connections
and other protective measures, as appropriate, may be used to prevent damage within the
8-45
Previous Page
