ETL 1110-1-163
30 Jun 96
can make construction more difficult by causing the
must meet all specified requirements and be stored in
groundwater elevation to rise in areas where the wall
a proper manner on site.
has not yet been built. The critical aspect of trench
excavation is ensuring that a proper "key" has been
(c) Water. The water must meet all specified
requirements. The Contractor should be required to
excavated into the confining layer. Most often, it is
condition the water or locate another source if the
relatively easy to determine when the key layer has
been reached based upon bucket cuttings. However,
specified requirements are not met.
if the key layer is difficult to determine from bucket
cuttings, it may be necessary to push drive samplers
(d) Bentonite slurry. The initial bentonite slurry
to obtain samples. Once S-B backfilling operations
must be tested prior to placement in the trench to
have begun, it is recommended that trench excavation
ensure that all required properties are met. Bentonite
precede the toe of the S-B backfill slope by at least
may be mixed in high-shear mixers or mixed and
9 m (30 ft), but not more than 30 m (100 ft). This
hydrated in slurry hydration ponds. In general, a
practice will decrease the length of open trench filled
minimum hydration time of 8 hr will allow the ben-
with slurry, decreasing the risk for caving or slough-
tonite slurry to meet all criteria. Other tests, such as
ing of the trench walls. It is preferable to round the
viscosity, density, and pH, should be run two times
corners of a slurry trench rather than have intersect-
each shift per batching plant during construction.
ing walls. This practice will allow S-B backfill
Slurry in the trench should be tested for viscosity,
materials to flow around the curve. Material excav-
density, sand content, and pH two times each shift, at
ated from the trench should be stockpiled as far as
two locations in the trench (approximately 600 mm
practical away from the trench. If the excavated
(2 ft) below the slurry surface and 600 mm (2 ft)
trench spoils produce odors, it may be necessary to
above the bottom of the trench.)
cover the stockpiles. During excavation, soundings
should be obtained to determine the elevation of the
Bentonite slurry should be introduced into the trench
top of the key layer, the bottom of the excavation,
when excavation begins. The slurry level in open
and the bottom of the trench prior to backfilling. If
trenches should be maintained at a minimum of 1 m
excessive sediments (greater than 50 mm (2 in.)) have
(3 ft) above the groundwater level, and no more than
built up, it will be necessary to clean the trench bot-
600 mm (2 ft) below the top of the working platform.
tom by airlift pumps, or excavation equipment, to
If the density of the slurry in the trench exceeds
1,025 kg/m3 (85 pcf), the excess solids must be
remove the sand and sediment that has settled to the
bottom of the trench. The trench bottom should be
removed or the slurry must be replaced with fresh
cleaned, as a minimum, at the beginning of each
slurry. If the excess solids are not removed, they
shift. The soundings should be obtained approxi-
may settle to the bottom of the trench, creating areas
mately every 6 m (20 ft) along the alignment of the
of higher permeability within the wall.
trench.
(e) S-B backfill material. Mixing S-B backfill
(g) S-B backfill placement. Initial S-B backfill
materials should be done in such a manner that the S-
placement should be performed by one of the follow-
B backfill is consistent. Generally, S-B backfill
ing two methods: placement by lowering S-B back-
materials are mixed either on a separate mixing pad
fill to the bottom of the trench with a crane and
or alongside the slurry trench. It is felt by many that
clamshell bucket or tremie until the surface of the
it is easier to control S-B backfill mixing when done
S-B backfill rises above the surface of the slurry or,
on a separate mixing pad. However, mixing on a
begin excavation at a point outside the limits of work
separate pad requires extra handling of materials and
to allow a S-B backfill face to form prior to reaching
is generally more expensive. It is important that all
the full depth of the required slurry trench. S-B
soil particles are coated with slurry and that clods are
backfill should be placed so that no pockets of slurry
broken down. Quality control test results for perme-
are trapped. Free dropping of S-B backfill is not
ability, slump, density, and moisture content should
permitted. It should be placed on top of the previ-
be supplied by the Contractor as required in the spec-
ously placed S-B backfill and allowed to push out
ifications for the S-B backfill.
along the bottom of the trench, or slide down the
forward face of the S-B backfill slope. Periodic
(f) Trench excavation. The upgradient portion of
soundings should be taken along the face of the slope
the slurry wall is generally constructed first. Con-
structing the down gradient portion of the wall first
B-8
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