 What
are Slurry Walls?
Slurry walls are underground walls built from the surface. They
are constructed by digging a trench, filling it with liquid slurry,
then displacing the slurry with reinforcement, as needed, and tremie
concrete. The concrete hardens in the trench to form the wall, which
is then used as an excavation support or foundation element. Slurry
walls are also useful as underground water flow cutoff structures
so they are of great value for projects such as landfill containment
and control of contaminated seepage.
Watching the construction of a slurry wall panel can be
amazing. When it is time to install the steel reinforcement,
a crane lifts the massive reinforcement bar cage, which can be more
than 30 meters (100 feet) long and as wide as the slurry wall panel
(up to 6 to 7.5 m, or 20 to 25 feet). It often bows at the middle
under its own weight as it is lifted off the ground. With the spidery
cage of bars dangling in the air, the crane slowly shifts it into
position over the trench. The whole assemblage is then dropped into
the slurry trench and swallowed by the earth. Seeing the construction
work in progress makes you think you are watching something that
cannot possibly succeed. We do the calculations, prepare the specs,
and monitor the slurry, but in actual construction, the operation
seems improbable.
Slurry walls are an important part of underground construction work,
in part because they can be built with relatively little disruption
at grade. Their use evolved from the early 1900s practice of adding
bentonite slurry to well drilling fluids to stabilize uncased drill
holes. From that practice, several other uses of bentonite slurry
methods have emerged, such as:
Jet grouted and deep soil mixed walls do not ordinarily utilize
slurry, but they are otherwise similar in characteristics and usage
to diaphragm, tangent pile, and groundwater cut-off walls. Sometimes,
jet grouting is used in conjunction with slurry walls to provide
watertight connections between tunnels intersecting with slurry
walls (see article by Campo
and Hudson). In modern times, it has often proved useful and
economical to utilize slurry walls for permanent support of excavations
(See articles by Das et
al, and Brenner).
Slurry wall design is very analytical, but also very subjective.
In fact, slurry wall analysis and design is a relatively new and
developing field. There are many different ways to go about the
task. You can sit in a slurry wall design meeting and be very impressed
by the vigor of the analysis and the kind of assumptions that sometimes
support it. The design of slurry walls is a specialty (called Zen
in Thom Neff’s
article) that requires unique knowledge concerning the geotechnical,
structural and construction engineering aspects of the project.
These topics are covered in articles by Dave
Druss, Camille
Bechara and Ken
Merrill.
PB is a Leader in Slurry Walls
A leading designer and constructor of underground facilities, PB
has become a leader in slurry walls. One of our earliest uses of
slurry walls was on the Bay Area Rapid Transit (BART) Market Street
Stations in downtown San Francisco, where we pioneered a variation
of slurry walls using steel W sections instead of reinforcing bars
for primary reinforcement. Since then, PB has been involved in the
planning, design, and construction of slurry walls the world over.
Most have been used on transit and rail projects, although other
uses such as on highway and landfill containment projects are represented.
Our work on slurry walls is summarized on the Table beginning on
the next article.
Slurry walls offer many advantages in infrastructure design and
construction. In particular, they are stiff and hold back the soil
better than soldier piles and lagging or steel sheeting, and they
tend to be more watertight than other methods of excavation support—two
features that help to protect existing buildings adjacent to an
excavation. This type of protection is an essential part of underground
projects today. It is likely to be even more important for future
underground projects that will be built in increasingly developed
urban areas.
Slurry wall technology has continued to advance over the years,
with new types of analysis and design, new types of slurry, and
improved construction procedures. PB’s experience with slurry
walls in many cities around the world has us well positioned to
continue to be at the forefront of this technology in the future.
A word of thanks to our Editor Lorraine Anderson and to this issue’s
technical reviewers, Chris Reseigh, Phil Rice, Lee Abramson and
Robert Rawnsley. Some of the articles deal with specialized technical
analysis and design information. The reviewers had many excellent
suggestions that helped to make the issue more readable for those
not familiar with slurry walls.
Brian Brenner, Senior Professional Associate
PB Network Associate Editor
|
