UFC 3-220-01N
15 AUGUST 2005
jobsite. Filter material must be stockpiled according to gradation. For graded filter systems,
the materials must be placed with care to minimize mixing of individual components.
8-3.2.3.7.2 Filter Cloths. Both woven and non-woven filter cloths, which have been found
satisfactory for use as a filter media for subsurface drains, are available. When granular filter
materials are not economically available, a single wrap of filter cloth around a pipe may be
used in lieu of a coarser backfill. When available granular filter material is too coarse to satisfy
filter criteria for the protected soil, a single layer of filter cloth may be used adjacent to the
protected soil. To reduce the chance of clogging, no filter cloth should be specified with an
open area less than 4 percent and or equivalent opening size (EOS) of less than the No. 100
sieve (0.15 mm (0.0059 inch)). A cloth with openings as large as allowable should be
specified to permit drainage and prevent clogging. Additional information on air- field drainage
is contained in TM 5-820-2/AFM 88-5.
Filter cloth can also provide protection for excavated slopes and serve as a filter
to prevent piping of fine-grained soils. In one project, sand was not available for backfill behind
a wall and coarse gravel had to be used to collect seepage. The filter cloth used to protect the
excavated slope served as a filter against piping of the natural silty clay under seepage
gradients out of the excavated slope after the coarse gavel backfill was placed.
8-3.2.3.8
Earth Pressures. The rationale design of any structure requires the designer to
consider all loads acting on the structure. In addition to normal earth pressures associated
with the effective pressure distribution of the backfill materials, subsurface cut-and- cover
structures may also be subjected to surcharge loads caused by heavy equipment operating
close to the structure and by increased permanent lateral earth pressures caused by
compaction of backfill material with heavy equipment. Procedures for predicting normal earth
pressures associated with the effective pressure of backfill materials are discussed in Chapter
6.
8-3.2.3.8.1 Surcharge Earth Pressures. Exact solutions for surcharge earth pressures
generated by heavy equipment (or other surcharge loads) do not exist. However,
approximations can be made using appropriate theories of elasticity such as Boussinesq's
equations for load areas of regular shape or Newmark's charts for irregular shaped load areas
as given in UFC 3-220-10N. As a conservative guide, heavy-equipment surcharge earth
pressures may be minimized by specifying that heavy compaction equipment maintain a
horizontal distance from the structure equivalent to the height of the backfill above the
structure's foundation.
8-3.2.3.8.2 Compaction Induced Pressures. Compaction-induced earth pressures can
cause a significant increase in the permanent lateral earth pressures acting on a vertical wall
of a structure (fig. 8-3.5). This diagram is based on the assumption that the equipment can
operate to within 150 mm (6 in) of the wall. Significant reductions in lateral pressures occur as
the closest allowable distance to the wall is increased (Fig 8-3.5). For an operating distance
1.5 m (5 ft) from the wall, the induced horizontal earth pressure is much less than that caused
by the backfill. The magnitude of the increase in lateral pressure is dependent, among other
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