WATER JET CONFERENCE - Waterjet Technology Association

arrier alone requires keying into a naturally occurring impermeable stratum such as clay
or unweathered bedrock. Thus, Block Displacement is economically attractive where the
depth to such a natural stratum is greater than the depth of contaminated soil. Figure 2
shows the tradeoff between Block Displacement and slurry wall for different
combinations of contaminant depth and depth to impermeable stratum.
Block Displacement realizes additional cost savings by isolating the minimum
volume of soil, thus minimizing the fluid handling requirements for subsequent in-situ
and flushing treatment techniques.
Block Displacement can be more economically attractive than all alternative
techniques under suitable site conditions. For the example shown in Table 1, it is 1/3 the
cost of a slurry wall barrier and 1/2 the cost of a well point system. Relative cost of
incineration has been included for reference.
These relative cost figures are based on standard construction costs and have not
considered costs peculiar to a specific hazardous site operation (Walsh, 1982).
Block Displacement at the Whitehouse Site
A demonstration of Block Displacement was recently conducted adjacent to a
Superfund site in Whitehouse, FL. The demonstration "block" was 60 ft. in diameter by
23 ft. deep, and composed of unconsolidated marine sediments (Brunsing and Grube,
1982).
The test program successfully demonstrated the fundamental aspects of the Block
Displacement process. Figure 3 shows a perpendicular cross section of the displacement
of the earth mass. Core samples were taken to verify the continuity of the bottom barrier
in-situ, and barrier thicknesses ranging from 5 to 12 in. were obtained.
Bottom Notching and Fracture
The original demonstration plans called for injection hole bottom notching using
an elaborately plumbed system of high pressure - high volume air, sand and slurry to be
pumped to the base of each of seven casings. A notching nozzle would be inserted to the
bottom of a casing and high pressure and volume air would be pumped in thus displacing
the mud and groundwater and keeping the hole open. Next, sand would be injected into
the line and used to erode a notch radially out from the hole bottom. The volume of
eroded material captured at the surface would give an approximation of the notch
diameter. When the desired notch size had been reached, the air would slowly be replaced
with bentonite slurry until the entire notch and casing were filled. The process would then
proceed with displacing the ground upward by injecting slurry under low pressure into
the notches. The major drawback to this system is that air back pressure control, required
to stabilize the notch opening, is difficult to maintain both during the erosional process
and during slurry backfilling. For this reason, a slurry jet was used for notching. Initially
a 2% bentonite slurry was jetted in a slurry medium in all seven injection holes yielding 2
feet diameter notches in each.
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