15 AUGUST 2005
graded soils falling in this category have fairly good compaction characteristics and when
adequately compacted provide good backfill and foundation support.
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.
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.
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 4.75 mm
(No. 4) sieve and 5 percent or less passing the 75 Micron (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.
Fine-Grained Soils of Low to Medium Plasticity. Inorganic 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 groundwater 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.