Different types of backfill materials are used in construction
industry. Backfill materials that are commonly used are described below
with their engineering properties.
1. Coarse-grained soils
Coarse-grained soils include gravelly and sandy soils and range from clayey sands (SC) through the well-graded gravels of gravel-sand mixtures (GW)
with little or no fines. They will exhibit slight to no plasticity. All
of the well graded soils falling in this category have fairly good
compaction characteristics and when adequately compacted provide good
backfill and foundation support.
(a) One difficulty that might arise with soils in
this category would be in obtaining good compaction of the poorly graded
sands and gravels. These poorly graded materials may require saturation
with downward drainage and compaction with greater compaction effort to
achieve sufficiently high densities. Also, close control of water
content is required where silt is present in substantial amounts.
Coarse-grained materials compacted to a low relative density are
susceptible upon saturation to liquefaction under dynamic loads.
(b) For sands and gravelly sands with little or no
fines, good compaction can be achieved in either the air dried or
saturated condition. Downward drainage is required to maintain seepage
forces in a downward direction if saturation is used to aid in
compaction.
Consideration may be given to the economy of adding cement to
stabilize moist clean sands that are particularly difficult to compact
in narrow confined areas. However, the addition of cement may produce
zones with greater rigidity than untreated adjacent backfill and form
“hard spots” resulting in non uniform stresses and deformations in the
structure.
(c) Cohesionless materials are well suited for
placement in confined areas adjacent to and around structures where
heavy equipment is not permitted and beneath and around irregularly
shaped structures, such as tunnels, culverts, utilities, and tanks.
Clean, granular, well-graded materials having a maximum size of 1 inch
with 95 percent passing the No. 4 sieve and 5 percent or less passing
the No. 200 sieve are excellent for use in these zones. However, a
danger exists of creating zones where seepage water may accumulate and
saturate adjacent cohesive soils resulting in undesirable consolidation
or swelling. In such cases, provisions for draining the granular
backfill, sealing the surface, and draining surface water away from the
structure are necessary.
2. Fine-grained soils of low to medium plasticity
In organic clays (CL) of low to medium plasticity (gravelly, sandy, or silty clays and lean clays) and inorganic silts and very fine sands (ML)
of low plasticity (silty or clayey fine sands and clayey silts) are
included in this category. The inorganic clays are relatively impervious
and can be compacted fairly easily with heavy compaction equipment to
provide a good stable backfill.
Soils in the CL group can be compacted in confined areas to a fairly
high degree of compaction with proper water content and lift thickness
control. The clayey sands of the SC group and clayey silts of the ML
group can be compacted to fairly high densities, but close control of
water content is essential and sometimes critical, particularly on the
wet side of optimum water content. Some ML soils, if compacted on the
dry side of optimum, may lose considerable strength upon saturation
after compaction. Considerable settlement may occur.
Caution must therefore be exercised in the use of such soils as
backfill, particularly below the ground water level. Also, saturated ML
soils are likely to be highly susceptible to liquefaction when
dynamically loaded. Where such soils are used as backfill in seismic
prone areas, laboratory tests should be conducted to determine their
liquefaction potential.
3. Rock
The suitability of rock as backfill material is highly dependent upon
the gradation and hardness of the rock particles. The quantity of hard
rock excavated at most subsurface structure sites is relatively small,
but select cohesionless materials may be difficult to find or may be
expensive. Therefore, excavated hard rock may be specified for crusher
processing and used as select cohesionless material.
4. Shale
Although shale is commonly referred to as rock, the tendency of some
shales to breakdown under heavy compaction equipment and slake when
exposed to air or water after placement warrants special consideration.
(a) Some soft shales break down under heavy
compaction equipment causing the material to have entirely different
properties after compaction than it had before compaction. This fact
should be recognized before this type of material is used for backfill.
Establishing the proper compaction criteria may require that the
contractor construct a test fill and vary the water content, lift
thickness, and number of coverages with the equipment proposed for use
in the backfill operation. This type of backfill can be used only in
unrestricted open zones where heavy towed or self-propelled equipment
can operate.
(b) Some shales have a tendency to break down or
slake when exposed to air. Other shales that appear rocklike when
excavated will soften or slake and deteriorate upon wetting after
placement as rock fill. Alternate cycles of wetting and drying increases
the slaking process. The extent of material breakdown determines the
manner in which it is treated as a backfill material. If the material
completely degrades into constituent particles or small chips and
flakes, it must be treated as a soil-like material with property
characteristics similar to ML, CL, or CH materials, depending upon the
intact composition of the parent material. Complete degradation can be
facilitated by alternately wetting, drying, and disking the material
before compaction.
5. Marginal materials
Marginal materials are these materials that because of either their
poor compaction, consolidation, or swelling characteristics would not
normally be used as backfill if sources of suitable material were
available. Material considered to be marginal include fine-grained soils
of high plasticity and expansive clays. The decision to use marginal
materials should be based on economical and energy conservation
considerations to include the cost of obtaining suitable material
whether from a distant borrow area or commercial sources, possible
distress repair costs caused by use of marginal material, and the extra
costs involved in processing, placing, and adequately compacting
marginal material.
(a) The fine-grained, highly plastic materials make
poor backfill because of the difficulty in handling, exercising
water-content control, and compacting. The water content of highly
plastic fine grained soils is critical to proper compaction and is very
difficult to control in the field by aeration or wetting. Furthermore,
such soils are much more compressible than less-plastic and coarse
grained soils; shear strength and thus earth pressures may fluctuate
between wide limits with changes in water content; and in cold climates,
frost action will occur in fine-grained soils that are not properly
drained. The only soil type in this category that might be considered
suitable as backfill is inorganic clay (CH). Use of CH soils should be
avoided in confined areas if a high degree of compaction is needed to
minimize backfill settlement or to provide a high compression modulus.
(b) The swelling (and shrinking) characteristics of
expansive clay vary with the type of clay mineral present in the soil,
the percentage of that clay mineral, and the change in water content.
The active clay minerals include montmorillonite, mixed-layer
combinations of montmorillonite and other clay minerals, and under some
conditions chlorites and vermiculites.
Problems may occur from the rise of groundwater, seepage, leakage, or
elimination of surface evaporation that may increase or decrease the
water content of compacted soil and lead to the tendency to expand or
shrink. If the swelling pressure developed is greater than the
restraining pressure, heave will occur and may cause structural
distress. Compaction on the wet side of optimum moisture content will
produce lower magnitudes of swelling and swell pressure. Expansive clays
that exhibit significant volume increases should not be used as
backfill where the potential for structural damage might exist.
Suitability should be based upon laboratory swell tests.
(c) Additives, such as hydrated lime, quicklime, and
fly ash, can be mixed with some highly plastic clays to improve their
engineering characteristics and permit the use of some materials that
would otherwise be unacceptable. Hydrated lime can also be mixed with
some expansive clays to reduce their swelling characteristics.
Laboratory tests should be performed to determine the amount of the
additive that should be used and the characteristics of the backfill
material as a result of using the additive. Because of the complexity of
soil-additive systems and the almost complete empirical nature of the
current state of the art, trial mixes must be verified in the field by
test fills.
6. Commercial by-products
The use of commercial by-products, such as furnace slag or fly ash as
backfill material, may be advantageous where such products are locally
available and where suitable natural materials cannot be found. Fly ash
has been used as a lightweight backfill behind a 25-foot-high wall and
as an additive to highly plastic clay. The suitability of these
materials will depend upon the desirable characteristics of the backfill
and the engineering characteristics of the products.