ETL 1110-1-163
30 Jun 96
controlled in the field than in the laboratory, it is
permeability of the S-B backfill over time. After
prudent to attempt to achieve a maximum laboratory
long-term compatibility tests are completed, the
permeability of 5 10-8 cm/sec or less. Laboratory
results are then analyzed to determine if the contami-
test equipment must be constructed of materials that
are chemically resistant to the groundwater contami-
permeability of the S-B backfill in comparison to the
nants. Short-term permeability tests (48 to 72 hr)
control sample.
using fixed-wall or flexible-wall permeameters are
run on various mix designs, varying the amount of
d.
Geotechnical design.
dry bentonite added (ie., 0, 2, and 4 percent). Sam-
ples to be tested should be mixed to achieve a 100- to
(1) Slurry wall alignment. Based on predesign
150-mm (4- to 6-in.) slump. One sample should be
investigations, a slurry wall is often located so that it
run with site tap water as the permeant (control sam-
surrounds a contaminated groundwater plume, or a
ple), and one sample should be run with contaminated
localized "hot spot". Sometimes a slurry wall may be
groundwater as the permeant. It should be noted that
installed to prevent clean groundwater from entering a
when using fixed wall permeameters, the specimen
site. Because of health and safety considerations
should be allowed to set up and then applied head
during construction, it is desirable to locate the slurry
pressures should be increased slowly over several
days, to avoid a sample failure. The most economi-
handling and stockpiling of contaminated materials
cal S-B backfill which achieves a permeability less
excavated from a slurry trench is also expensive.
than or equal to 5 10-8 cm/sec, should be selected
When practical, in order to maintain a continuous
for subsequent long-term testing.
slurry wall, it is desirable to round corners of a slurry
trench rather than to intersect two straight line seg-
(h) Long-term compatibility tests. Long-term
ments. Although depth dependent, a 30-m (100-ft)
compatibility testing consists of running flexible wall
radius curve is usually the shortest radius that most
permeameter tests on a minimum of three S-B back-
excavation equipment can negotiate.
fill material samples utilizing the optimum mix design
obtained during short-term permeameter testing. One
(2) Key depth.
sample should be run with site tap water only as the
permeant (control sample); one sample with contami-
(a) One of the most important elements in a
nated groundwater as the permeant (after one pore
slurry wall design is determining if there is an ade-
volume of site tap water permeant); and one sample
quate "key" into which the slurry wall can be tied.
with a bentonite content 2 percent greater than the
Usually, a slurry wall is keyed into a soil or rock
optimum mix design, with contaminated groundwater
horizon which performs as an aquiclude. When the
as the permeant (after one pore volume of site tap
key material is a soil, the slurry trench should be
water permeant). It is generally recommended that
keyed a minimum of 600 mm (2 ft) into the confin-
ing layer. If the key material is rock, the nature of
S-B backfill material. This testing may take up to
the rock surface must be determined throughout the
3 months, depending on the sample size. It is recom-
alignment of the wall. The wall must extend through
mended that the height-to-width ratio of the sample
areas of broken and/or weathered material into essen-
not exceed 1:1. For example, a 75 mm (3 in.) diam
tially impermeable material in order to minimize
sample should not be longer than 75 mm (3 in.). It is
seepage along the contact. Some options to remove
also recommended that inflow and outflow measure-
weathered rock may include mechanical removal,
ments be taken during long-term tests to ensure that
controlled blasting, and debris removal with airlift
no leakage is in the system. A chemical analysis is
equipment. See ETL 1110-2-282 for more informa-
often run on the effluent that has run through the
tion on rock excavation. Grouting along the contact
samples to see if any changes have occurred in the
may be feasible in some instances; however, grouting
permeant. This data may be used to determine a
of bedrock will greatly increase construction costs
and may create new seepage fractures in the rock
mixture. For example, an increase in the amount of
formation or the S-B backfill.
sodium and a decrease in the amount of calcium in
the permeameter effluent may indicate a displacement
ucts leaked from fuel storage tanks, it may not be
of sodium ions in the bentonite by calcium ions in the
groundwater. This reaction may tend to increase the
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