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the PRBs as compared to slurry walls, there are addi-
are critical to wall performance. It is imperative that
tional potential problems with excavating and main-
all of the contaminated water flows through, not
taining trench stability.
around, the treatment zone. The PRB has a fixed
location, and changes in these parameters will limit
B-5. Civil Design
its effectiveness if the residence time is decreased and
Common civil design features for all vertical barrier
untreated. Detailed geological data along the pro-
wall projects which should be considered are
posed PRB site is required to evaluate whether
hydraulic control is achievable. For most sites the
total depth of the PRB is limited to approximately
a. Site access routes. Both public and private
15 m (50 ft), and it is necessary to document the
access routes to the site need to be investigated to
depth to the aquitard which the wall will key into and
ensure they can handle the construction traffic. The
determine the aquitard's physical characteristics.
Contractor should be required to maintain any access
Detailed site stratigraphy using a cone penetrometer
routes including post-construction rehabilitation, if
or other continuous sampling technology is also nec-
necessary. Aggregate surfaced roads may need to be
essary to show the potential effects of site hetero-
built to allow access of construction vehicles or for
geneity on contaminant migration with respect to the
operation and maintenance of the barrier wall.
location chosen for the PRB.
Aggregate surfaced roads should be designed in
(a) Clogging of the wall by precipitants or coat-
accordance with TM 5-822-12. Several U.S. Army
ings may, after a time, decrease the permeability and
Corps of Engineers guide specifications are available
require the replacement of the treatment media.
for the construction of roadways.
There is no good database on wall performance in
this area, and it is impossible to predict with certainty
b. Decontamination facility. The Contractor
how quickly a wall may need to be replaced. Geo-
must decontaminate all vehicles and equipment which
chemical data will assist in determining in situ redox
conditions and chemical parameters that might affect
zone. The contract documents should address the
the speed with which redox reactions take place and
requirements for a decontamination facility and for
whether or not the proposed reactions will be affected
the treatment and disposal of rinsate water. The final
by site chemistry. The major ions can also affect the
disposition of the decontamination facility should also
be addressed. The Contractor should be required to
and, thus, the time it takes for the reactive surface to
submit a plan as part of the Site Health and Safety
become coated or clogged. Treatability and/or pilot
Plan which describes vehicle, equipment, and person-
studies need to be carried out with site waters to
nel decontamination procedures.
assess the needed thickness of the barriers for each
(1) Design criteria. The decontamination facility
site. Each compound has a different degradation rate,
typically consists of 150 to 300 mm (6 to 12 in.) of
and aquifer geochemistry may also affect the perfor-
granular material underlain by a protective geotextile
mance of the wall.
and a geomembrane 1.0 mm (40 mils) in thickness.
More elaborate designs may be used if the decontam-
(b) The most important criteria for the design of
ination facility will be operated for a significant
period of time. To minimize the volume of decon-
sorbed as they move passively through the treatment
tamination water, a temporary cover should be used
zone. The exact method of emplacement will vary
to shed rainfall when the facility is not in use.
depending on the reactive substance to be emplaced
Rinsate water is collected by gravity into a polyethyl-
and any requirements to maintain hydraulic control.
ene or precast concrete storage tank which is typically
A major consideration during emplacement is to
about 3 m3 (100 ft3) in volume. Treatment and dis-
ensure that the barrier has no significant heterogenei-
posal of the rinsate water and sediments should be in
ties which may serve as a preferential flow path.
accordance with all state and Federal regulations.
Alternative materials and methods of emplacement
Federal disposal regulations are described in 40 CFR
are also being developed including cassettes or other
Parts 260-268. Rinsate water is typically disposed of
removable devices to allow easy access to the react-
after onsite treatment or is transported to an offsite
ing agent. Because of the generally greater widths of