TM 5-818-4/AFM 88-5, Chap. 5
tion equipment to provide a good stable backfill. Soils
plete degradation can be facilitated by alternately wet-
in the CL group can be compacted in confined areas to
ting, drying, and disking the material before compac-
a fairly high degree of compaction with proper water
tion A detailed discussion on the treatment of shales
as a fill material is given in FHWA-RD-78-141.
content and lift thickness control. The clayey sands of
the SC group and clayey silts of the ML group can be
(5) Marginal materials. Marginal materials are
compacted to fairly high densities, but close control of
these materials that because of either their poor com-
water content is essential and sometimes critical, par-
paction, consolidation, or swelling characteristics
ticularly on the wet side of optimum water content.
would not normally be used as backfill if sources of
Some ML soils, if compacted on the dry side of opti-
suitable material were available. Material considered
to be marginal include fine-grained soils of high plas-
mum, may lose considerable strength upon saturation
after compaction. Considerable settlement may occur.
ticity and expansive clays. The decision to use mar-
ginal materials should be based on economical and
Caution must therefore be exercised in the use of such
soils as backfill, particularly below the groundwater
energy conservation considerations to include the cost
level. Also, saturated ML soils are likely to be highly
of obtaining suitable material whether from a distant
susceptible to liquefaction when dynamically loaded.
borrow area or commercial sources, possible distress
Where such soils are used as backfill in seismic prone
repair costs caused by use of marginal material, and
areas, laboratory tests should be conducted to deter-
the extra costs involved in processing, placing, and
mine their liquefaction potential (see para. 17-5 and
adequately compacting marginal material.
17-6, TM 5-818-1/AFM 88-3, Chap. 7).
(a) The fine-grained, highly plastic materials
make poor backfill because of the difficulty in han-
rial is highly dependent upon the gradation and hard-
dling, exercising water-content control, and com-
ness of the rock particles. The quantity of hard rock
pacting. The water content of highly plastic fine-
excavated at most subsurface structure sites is rela-
grained soils is critical to proper compaction and is
tively small, but select cohesionless materials may be
very difficult to control in the field by aeration or wet-
difficult to find or may be expensive. Therefore, exca-
ting. Furthermore, such soils are much more compres-
vated hard rock may be specified for crusher process-
sible than less-plastic and coarse-grained soils; shear
ing and used as select cohesionless material.
strength and thus earth pressures may fluctuate be-
(4) Shale. Although shale is commonly referred to
tween wide limits with changes in water content; and
as rock, the tendency of some shales to breakdown
in cold climates, frost action will occur in fine-grained
under heavy compaction equipment and slake when
soils that are not properly drained. The only soil type
exposed to air or water after placement warrants spe-
in this category that might be considered suitable as
cial consideration.
backfill is inorganic clay (CH). Use of CH soils should
(a) Some soft shales break down under heavy
be avoided in confined areas if a high degree of com-
compaction equipment causing the material to have
paction is needed to minimize backfill settlement or to
entirely different properties after compaction than it
provide a high compression modulus.
had before compaction. This fact should be recognized
(b) The swelling (and shrinking) characteristics
before this type of material is used for backfill. Estab-
of expansive clay vary with the type of clay mineral
lishing the proper compaction criteria may require
present in the soil, the percentage of that clay mineral,
that the contractor construct a test fill and vary the
and the change in water content. The active clay min-
water content, lift thickness, and number of coverages
erals include montmorillonite, mixed-layer combina-
with the equipment proposed for use in the backfill
tions of montmorillonite and other clay minerals, and
operation. This type of backfill can be used only in
under some conditions chlorites and vermiculites.
unrestricted open zones where heavy towed or self-pro-
Problems may occur from the rise of groundwater,
pelled equipment can operate.
seepage, leakage, or elimination of surface evaporation
(b) Some shales have a tendency to break down
that may increase or decrease the water content of
or slake when exposed to air. Other shales that appear
compacted soil and lead to the tendency to expand or
rock-like when excavated will soften or slake and dete-
shrink. If the swelling pressure developed is greater
riorate upon wetting after placement as rockfill. Alter-
than the restraining pressure, heave will occur and
nate cycles of wetting and drying increases the slaking
may cause structural distress. Compaction on the wet
process. The extent of material breakdown determines
side of optimum moisture content will produce lower
the manner in which it is treated as a backfill material.
magnitudes of swelling and swell pressure. Expansive
If the material completely degrades into constituent
clays that exhibit significant volume increases should
particles or small chips and flakes, it must be treated
not be used as backfill where the potential for struc-
as a soil-like material with property characteristics
tural damage might exist. Suitability should be based
similar to ML, CL, or CH materials, depending upon
upon laboratory swell tests (TM 5-818-1/AFM 88-3,
the intact composition of the parent material. Com-
Chapter 7).
3-6
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