ETL 1110-1-163
30 Jun 96
A recommended compatibility testing program
pH
API RP 13B-1
6 to 8
generally consists of the following:
<200 ppm
Total dissolved
<500 ppm
The free swell test measures the increase in volume
solids
of a bentonite sample when poured into water. Free
Oil, organics,
acids, alkali,
< 50 ppm each
swell is expressed as a percentage of the original
(dry) volume. Two grams (2.2 cm3) of bentonite are
and other
deleterious
slowly poured into 100 ml of water and the volume
substances
of settled solids is recorded after 2 and 24 hr. See
Chloride
ASTM D 512
For Record
United States Pharmacopeia test method USP-NF-
XVII for a test description. Often, two tests are
(e) Bentonite slurry mixture. Once all bentonite
conducted, one using tap water from the site and the
and water samples are obtained, the design of the
other using contaminated groundwater from the site.
bentonite slurry mixture may begin. In general, a
Several bentonites should be evaluated and the results
bentonite content of 6 percent by weight should meet
of the free swell tests used in the selection of a ben-
most requirements. The EPA recommends the fol-
tonite. Bentonite samples which exhibit the greatest
lowing properties for bentonite slurries: viscosity
swell and are the least affected by contaminants are
(measured with a Marsh funnel) greater than 40 sec,
usually chosen for use in the remainder of the com-
unit weight around 1,025 kg/m3 (65 pcf), pH between
patibility testing.
7 and 10, and a bentonite content of 4 to 8 percent.
USACE guide specification CEGS-02444 states that
Another test which is often run to help determine
the bentonite slurry should meet the following crite-
bentonite selection is the "Filter Cake Compatibility
Test." As stated previously, the filter cake is an
ria:
important component of an S-B slurry wall. Filter
cake permeabilities may be as low as 1 10-9 cm/sec.
Viscosity
API RP 13B-1
>40 sec
>1025 kg/m3
Density
For this reason, both filter cake compatibility tests
(64 pcf)
and free swell tests are used to evaluate bentonite
<20 cm3
Filtrate loss
performance. Bentonite slurry (6 percent bentonite
pH
6.5 to 10
by weight and site tap water) from each potential
bentonite source is placed in a fixed-wall permeame-
(f) Initial compatibility testing. The presence of
ter. Slurry is then forced through filter paper overly-
chemical contaminants in soil and/or groundwater
ing a porous stone at the bottom of the chamber with
a chamber pressure of about 7 N/cm2 (10 psi) for
may significantly alter the rate of water movement
through a S-B slurry wall. For example, calcium in
24 hr. During this time, a filter cake of approxi-
soil or groundwater will displace some of the sodium
mately 10 mm (1/2 in.) thick will form on the filter
ions in bentonite. This results in reduced swelling of
paper. The remaining bentonite slurry is then
the bentonite and increased permeability. While the
removed and replaced with either site tap water or
effects of individual chemicals on S-B slurry walls
contaminated groundwater. The water is forced
have been studied and documented, the effects of
through the filter cake with a pressure of about 1 to
2 N/cm2 (2 to 3 psi). Permeabilities are calculated
multiple contaminants, which are common at most
HTRW sites, are largely unknown. The objective of
and plotted for a 48-to 72-hr period. Bentonites
any compatibility/optimization testing is to determine
which exhibit the least variation between tap water
the optimum S-B backfill mix design necessary to
and groundwater should be considered for use. At
achieve an in-place permeability of 1 10-7 cm/sec
the conclusion of the free swell tests and the filter
or less. In addition, the study should determine
cake compatibility tests, one bentonite is chosen for
whether contaminants present in the soil or ground-
the remainder of the compatibility testing.
water will cause long-term changes to the S-B back-
fill. Typically, a compatibility testing program will
(g) Mix design optimization. The purpose of
this phase of testing is to determine the most eco-
last 3 months or longer. Because of the long time
nomical mix of soil, dry bentonite, and bentonite
frame involved, it is often preferable to conduct pre-
slurry which will produce an in-place slurry trench
liminary compatibility tests during the predesign
permeability less than or equal to 1 10-7 cm/sec.
phase and long-term compatibility tests concurrent
Because mixing and placing operations are less
with design.
B-5
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