|
Slurry Wall History
The first diaphragm walls were tested in 1948 and the first full scale slurry
wall was built by Icos in Italy in 1950 (Puller, 1996) with bentonite slurry
support as a cut-off wall. Icos constructed the first structural slurry wall in
the late 1950s for the Milan Metro (Puller, 1996). Slurry walls were introduced
in the US in the mid 1960s by European contractors. The first application in
the US was in New York City [1962] for a 7m diameter by 24m deep shaft (Tamaro,
1990), that was followed by the Bank of California in San Francisco (Clough and
Buchignani, 1980), the CNA building in Chicago (Cunningham and Fernandez,
1972), and the World Trade Center in New York (Kapp, 1969, Saxena, 1974). The
majority of diaphragm wall projects in the US are located in six cities Boston,
Chicago, Washington DC, San Francisco and New York.
Diaphragm walls are extensively used in the Central Artery/Tunnel
project (CA/T) in Boston, Massachusetts (Fig. 3). Work in the
CA/T involves many cut and cover tunnels constructed under the
existing artery. Some of the deepest T- slurry walls, extending
120' below the surface have been constructed for the Central Artery
(Lambrechts et al., 1998).
Diaphragm (Structural) Wall Applications
Earth retention walls for deep excavations, basements, and tunnels.
High capacity vertical foundation elements.
Retaining wall-foundations
Retaining wall-water control
Used in top-down construction method as permanent basement walls
Slurry Wall for Cut-Off Wall Applications
Water and seepage control for deep excavations
Cut-off curtains
Contaminated groundwater / seepage control
Gas barriers for landfills
LIMITATIONS OF SLURRY WALLS
Slurry wall construction requires the use of heavy construction
equipment that requires reasonable headroom, site area, and considerable
mobilization costs. In limited headroom conditions smaller cranes
can be used and the technique can be altered to “remote backfill
mixing”, where the excavated soil is transported and mixed
to a remote location, and then is returned as backfill.
Cement-bentonite slurry walls also provide another alternative.
In this method, the trenches are excavated under a slurry that
later solidifies and create the permanent barrier/backfill.
Also, one should check that used bentonite slurry and soil-bentonite
slurries are able to withstand chemical attacks from the insitu
soils. In such a case, alternate slurry materials such as attapulgite
and treated bentonites can be used. Other backfill compositions
may be used when deemed appropriate (soil-attapulgite and soil-bentonite
with geomembrane inserts). When required, cement-bentonite and
soil-cement-bentonite can provide greater strengths.
SLURRY WALL COSTS
Slurry wall construction cost for cut-off barries is considerably
cheeper than diaphragm wall construction for deep excavations.
The differences arise mainly from construction method differences.
In cut-off walls construction is much quicker as a continuous
trench is excavated and backfilled and reinforcement cages are
seldomly used. In contrast, in diaphragm walls the wall perimeter
is constructed panel by panel and reinforcement cages are almost
always used.
Figure 1: Typical construction sequence of slurry walls : (A)
Trenching under slurry, (B) End stop inserted (steel tube or other),
(C) Reinforcement cage lowered into the slurry-filled trench,
(D) Concreting by tremie pipes.
Figure 2: Diaphragm Wall Trenching equipment, (A) Mechanical clamshell
in front and hydraulic clamshell in the back, (B) Smaller size
mechanical clamshell
Figure 4: Central Artery Tunnel Project, Boston MA (Ladd et al.,
1999) where slurry walls have been extensively used.
|
