September/October 2022 - 96

Like SB backfill, a laboratory mix
design was developed for the SCCB
slurry using slag cement, Portland
cement, bentonite, site water and
additives. Once a mix was selected, it
was duplicated with addition of site
sand using different rates up to full
saturation to model the effect of native
material on the performance of the
cutoff wall. A self-hardening backfill
served as the trench stabilizing fluid
during construction (like the bentoniteslurry);
however, the self-hardening
slurry is designed with the use of
retarding polymers to consolidate and
set within 8-12 hours following
placement. The self-hardening slurry
then becomes impermeable.
Self-hardening slurry walls are
constructed in the same manner as the
SB backfill method where continuity is
demonstrated by passing the excavator
arm a minimum of 3 ft (0.9 m),
measured horizontally, into the
previous day's panel and removing that
portion of the SCCB solids and replacing
it with the new slurry. SCCB mix
designs for this project required a
hydraulic conductivity of 5x10
- 7
cm/second (2x10 in/second) and a
range of unconfined compressive
strengths from 30 psi (207 kPA) at three
day s t o be twe en 100-300 p s i
(689-2,068 kPA) at 28 days.
-7
Soil bentonite cutoff
wall construction
Bentonite slurry was produced at
the slurry mixer using a jet shear
mixer equipped with monitoring
system allowing for control of the
bentonite/water ratio. The mixer
included a controlled measuring
system for ensuring that dry and wet
constituents were properly proportioned.
The slurried bentonite was
transferred from the mixer into the
adjacent tank where slurry was
circulated and hydrated for eight
hours prior to introduction into the
SCCB batch plant, using a 6 in (15 cm)
trash pump to circulate and agitate
slurry in the tank.
Mixing was achieved by adding
bentonite powder at approximately
6.94% by weight of water (mix design
requirement) while continuously
mixing the water through a low volume
high-pressure nozzle. The mixture was
calibrated to meet minimum Marsh
Funnel Viscosity of greater than 40
seconds and density of 64 lb/cu ft
(1,025 kg/cu m). Bentonite slurry was
pumped from the mixer into an adjacent
tank to hydrate for eight hours prior to
introduction into the SCCB batch plant
using 4 in (10 cm) HDPE pipe.
Bentonite slurry was pumped and
scaled into the SCCB batch plant with a
3 in (7.6 cm) electric centrifugal water
pump from the water storage tank.
Grade 120 Slag, Portland cement and a
strength accelerator additive were
added and directed by the batching
module of the SCCB mixer into the
mixing chamber from three independent
silos. Viscosity modifying
admixture was added in liquid form. In
each silo a viscosity modifying additive
(at 33.3% concentrate solution) was
added through a flowmeter batching
system. The resultant SCCB was mixed
under high shear for two to five
minutes, such that it was not overmixed
and temperature is kept within
specifications. The mixed SCCB was
transferred through a density meter
to the reserve chamber of the batch
plant. The SCCB batch plant data acquisition
system recorded data from
this chamber.
SCCB slurry was introduced into the
trench when excavation began. After
initial slurry setting, free water and all
sloughed trench sidewall material or
disturbed slurry material from the top
of the cutoff wall were removed and
replaced with fresh SCCB slurry.
Deep Mixing Method: The contractor's
subcontractor constructed the
soil cement bentonite cutoff wall to
depths of 85-135 ft (26-41 m) using
three-axis deep mixing equipment. A
subcontractor developed a mix design
us i ng s i t e s o i l s t o de t e rmi ne
cement /bentoni te dosages, and
water/cement and water/bentonite
ratios required to achieve maximum
permeability of 5x10 cm/second
-7
(2x10 in/second) and unconfined
compressive strength of 200-600 psi
(1,379-4,137 kPA) at 28 days.
-7
Three-axis mixing equipment was
used for the cutoff wall. Elements were
installed to overlap one full column
of each adjacent element to maintain
continuity.
After reaching the design bottom
depth of each element, bottom mixing
was performed by raising the mixing
shafts 10 ft (3 m) and then lowering the
shafts to the bottom depth. Bottom
96 * DEEP FOUNDATIONS * SEPT/OCT 2022
September/October 2022

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