TM 5-818-4/AFM 88-5, Chap. 5
discharge area. Such drainage systems are referred to
5-818-1/AFM 88-3, Chapter 7, EM 1110-2-2902,
as filter drains. The gradation of the granular filter
and EM 1110-2-2502.
material is critical for the functioning of the system.
(1) Exact solutions for surcharge earth pressures
Selection of the proper gradation for the filter materi-
generated by heavy equipment (or other surcharge
al is dependent upon the gradation of the material that
loads) do not exist. However, approximations can be
is being drained. Drainage of silts and clays usually re-
made using appropriate theories of elasticity such as
quires a graded filter made up of several layers of
Boussinesq's equations for load areas of regular shape
granular material with each layer having specific re-
or Newmark's charts for irregular shaped load areas as
quirements for maximum grain size and gradation. De-
given in NAVFAC DM-7. As a conservative guide,
tails on the design of filter drains are presented in TM
heavy-equipment surcharge earth pressures may be
5-818-5/NAVFACP-418/AFM 88-5, Chapter 6.
minimized by specifying that heavy compaction equip-
(1) Selected material. If materials at the jobsite do
ment maintain a horizontal distance from the struc-
not meet the designed filter requirements, select ma-
ture equivalent to the height of the backfill above the
terial must be purchased from commercial sources and
structure's foundation.
shipped to the jobsite. Filter material must be stock-
(2) Compaction-induced earth pressures can cause
piled according to gradation. For graded filter sys-
a significant increase in the permanent lateral earth
tems, the materials must be placed with care to mini-
pressures acting on a vertical wall of a structure (fig.
mize mixing of individual components.
2-5a). This diagram is based on the assumption that
(2) Filter cloths. Both woven and nonwoven filter
the equipment can operate to within 6 inches of the
cloths, which have been found satisfactory for use as a
wall. Significant reductions in lateral pressures occur
filter media for subsurface drains, are available. When
as the closest allowable distance to the wall is in-
granular filter materials are not economically
creased (fig. 2-5b). For an operating distance 5 feet
available, a single wrap of filter cloth around a pipe
from the wall, the induced horizontal earth pressure is
may be used in lieu of a coarser backfill. When
much less than that caused by the backfill. The magni-
available granular filter material is too coarse to satis-
tude of the increase in lateral pressure is dependent,
fy filter criteria for the protected soil, a single layer of
among other factors, on the effective weight of the
filter cloth may be used adjacent to the protected soil.
compaction equipment and the weight, earth pressure
To reduce the chance of clogging, no filter cloth should
coefficient, and Poisson's ratio of the backfill material.
be specified with an open area less than 4 percent and
Compaction-induced earth pressures against walls are
or equivalent opening size (EOS) of less than the No.
also described in TM 5-818-l/AFM 88-3, Chapter 7,
100 sieve (0.0059 inch). A cloth with openings as large
and EM 1110-2-2502.
as allowable should be specified to permit drainage
(3) The designer must evaluate the economics of
and prevent clogging. Additional information on air-
the extra cost of structures designed to withstand very
field drainage is contained in TM 5-820-2/AFM 88-5,
close-in operation of heavy compaction equipment
Chapter 2.
versus the extra cost associated with obtaining re-
(3) Other uses. Filter cloth can also provide pro-
quired compaction of backfill in thin lifts with smaller
tection for excavated slopes and serve as a filter to pre-
compaction equipment. A more economical alternative
vent piping of fine-grained soils. In one project, sand
might be to specify how close to the walls different
was not available for backfill behind a wall and coarse
weights of compaction equipment can be operated.
gravel had to be used to collect seepage. The filter
(4) One method of reducing lateral earth pres-
cloth used to protect the excavated slope served as a
sures behind walls has been to use about 4 feet of un-
filter against piping of the natural silty clay under
compacted granular (sand or gravel) backfill above the
seepage gradients out of the excavated slope after the
base of the wall. Soil backfill can then be compacted in
coarse gravel backfill was placed.
layers above the granular backfill. Compression of the
granular material prevents the buildup of excessive
d. Earth pressures. The rationale design of any
lateral pressures against the wall.
structure requires the designer to consider all loads
acting on the structure. In addition to normal earth
e. Structural backfill. Structural backfill is defined
pressures associated with the effective pressure dis-
as the compacted backfill required over and around a
tribution of the backfill materials, subsurface cut-and-
structure to prevent damage from heavy equipment
cover structures may also be subjected to surcharge
operating over or near the structure. This backfill
loads caused by heavy equipment operating close to
must be compacted using small compaction equip-
the structure and by increased permanent lateral earth
ment, such as mechanical rammers or vibratory-plate
pressures caused by compaction of backfill material
compactors, or intermediate size equipment such as
with heavy equipment. Procedures for predicting nor-
walk-behind, dual-drum vibratory rollers. The hori-
mal earth pressures associated with the effective pres-
zontal and vertical distances from the structure for
sure of backfill materials are discussed in TM
which structural backfill is required should be deter-
2-6
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