ETL 1110-1-163
30 Jun 96
walls. Depending on the depth of the trench, typical
(b) Backfill soil materials. To obtain a low
permeability (typically 1 10-7 cm/sec or less) S-B
excavation equipment may include extended reach
backfill mixture, soils with an appreciable amount of
backhoes, clamshells, or draglines. For depths less
fines (preferably plastic) are necessary. USACE
than about 20 m (70 ft), backhoes are generally most
guide specification CEGS 02444 provides the follow-
efficient. After excavation, the slurry filled trench is
ing gradation criteria for backfill soils:
backfilled with a soil/bentonite/water mixture engi-
neered to create a low-permeability cutoff wall (1
10-7 cm/sec to 1 10-8 cm/sec). U.S. Army Corps
Screen Size or Number
Percent Passing
of Engineers (USACE) guide specification
(U.S. Standard)
by Dry Weight
CEGS 02444, Soil-Bentonite Slurry Trench for
HTRW Projects, should be used in the preparation of
75 mm
(3 in.)
100
plans and specifications for S-B slurry walls.
4.76 mm
(No. 4)
40-80
0.42 mm
(No. 40)
25-60
74 uM
(No. 200)
20-40
c.
Predesign issues.
(c) Bentonite. Bentonite is a natural clay whose
(1) Geologic subsurface investigation. A slurry
principal mineral constituent is sodium montmorillon-
wall should be keyed into an impervious geologic
ite and is characterized by a very large volume
formation along the entire length in order to avoid
increase with wetting. Bentonites which conform to
potential seepage zones under the wall. Borings are
Section 4 of API Spec 13A typically have been
taken along the proposed alignment of a slurry wall,
treated with small amounts of polymers. Bentonites
preferably before the development of plans and speci-
which conform to Section 5 of API Spec 13A have
fications. These borings are spaced 30 to 60 m (100
not been chemically treated. Nontreated (Section 5)
to 200 ft) apart, depending on the geologic uniformity
bentonites are generally more expensive than Section
and thickness of the key layer. Additional borings
4 bentonites. Bentonites conforming to Section 4
may be taken along the alignment during slurry wall
may be used for construction, provided the necessary
construction to verify assumptions made during
permeability is obtained during compatibility testing;
design. During the design phase, samples can be
however, bentonite which conforms to Section 5 of
taken from the slurry wall alignment borings for use
API Spec 13A is generally preferred. During pre-
in backfill optimization and compatibility studies. In
design, several types of bentonite from various
addition, groundwater levels should be measured
suppliers should be obtained for compatibility and
along the alignment of the slurry wall, and if possi-
backfill optimization testing. USACE guide specifi-
ble, seasonal groundwater fluctuations should be
cation CEGS-02444 requires the following properties
documented.
for bentonite used for slurry wall construction:
(2) Soil-bentonite (S-B) backfill testing and
YP/PV Ratio
API Spec 13A
<3
optimization.
Viscometer
>30
<15 cm3
Filtrate Loss
(a) Borrow sources. During predesign investiga-
Moisture
tions, potential borrow sources for backfill material
Content
ASTM D 2216
<10 percent
should be located and sampled. Soils excavated from
the trench may be utilized for backfill soil. This
(d) Water. During pre-design activities, samples
practice saves the time and money of locating, pur-
of the groundwater and samples of tap water to be
chasing, and hauling borrow soil to the site. It also
used for slurry mixing and other operations should be
eliminates the problem of disposing of the excavated
obtained. Generally, 40 liters (10 gallons) of each
trench material. If the in situ soils are not suitable,
should be obtained for compatibility and backfill
due to problems with gradation or gross contamina-
optimization testing. Since a large volume of water
tion, imported borrow is the only viable option. Soil
is normally required for any slurry trench installation,
characteristics such as classification, gradation, water
an adequate source of mixing water must be identi-
content, permeability, and chemical properties should
fied. The guide specification requires water used for
also be evaluated. If a single borrow area does not
mixing slurry and backfill meet the following
have a suitable soil, it may be necessary to mix two
requirements:
different soils to come up with suitable backfill
material.
B-4
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