ETL 1110-1-163
30 Jun 96
the trench should be specified to maximize trench
necessary to key into some low permeability layer.
stability.
These slurry walls are termed "hanging" slurry walls.
(6) Control of groundwater. The control of
(3) Wall thickness. When designing the thick-
groundwater is an integral part of any vertical barrier
ness of the wall, the hydrostatic gradient and the
wall design. To provide positive contaminant con-
permeability of the backfill material must be evalu-
tainment, groundwater is often extracted from the
ated. A wall thickness of 900 mm (3 ft) will meet
inside of a barrier wall to provide an inward hydrau-
most design criteria, although walls up to 2 m (7.5 ft)
lic gradient from the outside of the wall to the inside.
in width have been constructed. Also, a thicker wall
Thus, contaminants in the groundwater may be effec-
will provide a greater margin of safety if inconsisten-
tively contained, since any wall leakage will be
cies exist in the backfill. It is also desirable that the
inward rather than outward.
trench be excavated with one pass of the excavation
equipment.
(7) Site layout. In general, slurry wall construc-
tion requires a great deal of relatively flat, open space
(4) Work platform. If the terrain is highly irreg-
to provide room for equipment, slurry hydration
ular, has steep slopes, or dense vegetation, extensive
ponds, and S-B backfill mixing areas. Mixing areas
site work and grading will be necessary. A S-B
should be located as near the trench as possible, with
slurry wall generally cannot be constructed in areas
adequate utilities available.
with a slope greater than 2 percent along the align-
ment of the slurry wall, unless site grading can be
(8) Construction quality control. As with any
performed or a work platform is constructed to main-
construction project, the preparation of the finest
tain this slope restriction. The width of the work
plans and specifications does not guarantee the instal-
platform must be adequate for the equipment envi-
lation of a quality product. Construction quality
sioned for construction. A work platform width of
control for slurry walls is of utmost importance if a
12 m (40 ft) with the slurry trench located along the
slurry wall is to function as designed. The following
centerline is typically specified (Figure 2). Wider
items are considered to be fundamental to ensuring a
platforms may be needed if S-B backfill will be
quality slurry wall installation:
mixed alongside the trench. Materials to construct
the work platform should exhibit good compaction
(a) Contractor qualifications. It is imperative
characteristics to provide a firm working surface.
that the slurry wall contractor have the necessary
experience and qualifications to do the work. Per-
haps the two most important individuals involved in
the construction of a slurry wall are the slurry trench
specialist, and the excavation equipment operator.
Submittals should be provided for both of these indi-
viduals, prior to construction, showing that they meet
the experience requirements outlined in the specifica-
tions. The Contractor should also submit, prior to
construction, all required plans which describe how
the work will be done. The plans should include four
items: sequence and layout of operations; trenching
operations plan (includes trench excavation, use of
excavated material, trench bottom cleaning, backfill
placement, etc.); preconstruction backfill testing pro-
Figure 2. Typical slurry trench cross section
cedures and results; and quality control sampling and
testing plan.
(5) Trench stability. While the slurry trench
method generally provides adequate trench stability,
(b) Bentonite. The Contractor should submit a
some soils that are extremely loose must be analyzed
bentonite sample and test results from the bentonite
to verify the trench will remain stable during con-
manufacturer for each lot of bentonite. The bentonite
struction. Other construction control practices such
as stockpile locations and equipment operation near
B-7
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