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Trench Technology
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Alameda Trench
Los Angeles, CA
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Railroad trench projects are technically feasible because of today’s
advanced methods, machinery, and materials available for designer
and contractor use. Years ago, engineers might have considered the
installation of steel sheet piling and wales along with the use
of tiebacks. And construction at locations with a high water table
might have been stalled due to prohibitive costs, spatial requirements,
and difficulty in handling water intrusion. Current methods employ
a combination of concrete and modern, high-tech machines to speed
construction and assure water tightness, as demonstrated by the
Alameda project. Each trench project may have different conditions
with respect to groundwater, soil types, utilities, and adjacent
structures, but current state-of-the-art wall and trench construction
offers proven techniques for economical and feasible projects.
Trench technology has developed significantly in the past several
years. The technology originated in Europe out of a need to dig
deep trenches in urban areas to facilitate rail transit projects.
These techniques feature low vibration and noise levels that suit
the complexities of construction within urban areas. Various wall
systems for deep trench excavation and retaining systems include
diaphragm wall, secant pile wall, tangent pile or contiguous bored
pile wall, soil-mix wall, and jet-grouted wall. Each system requires
specific types of equipment to attain the desired result, but all
require cast-in-place concrete, bentonite/cement mixtures, soil-cement,
or cement grout to provide the structural rigidity necessary to
retain soils and provide water tightness.
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Contiguous bored pile walls and mat reinforcement
for the Alameda Trench. | The diaphragm wall is formed by excavating the
earth in panels and filling the space with reinforced concrete.
The panels are connected by means of a shear key with rubber water
stops to provide continuity and water tightness. These walls may
be constructed to a thickness of 24 inches and a depth over 150
feet.
A secant pile wall consists of solid concrete
interlocking piles with diameters ranging from 16 to 60 inches depending
on the desired trench wall thickness and varying design parameters.
Piles are spaced fractionally less than the selected pile diameter.
Variations in the design allow for adjacent piles to be hard or
soft. Thus the terms “hard/hard” and “hard/soft”
are used to designate whether the primary or secondary pile has
structural integrity and is reinforced. Economically, the hard/soft
arrangement is best, as it avoids secanting into structural concrete.
The soft material for a hard/soft pile wall comprises bentonite/cement
with or without fly ash.
The contiguous bored pile wall is either tangent
to the adjacent pile or spaced incrementally greater than the pile
diameter. In the case where the pile is spaced to provide a gap,
the soil must be suitable so as not to slough during excavation
of the structure. The gaps are eventually grouted to provide a water
barrier.
Soil-mix and jet-grouted walls differ only in
the equipment and methods used to mix the soils. Both are based
on soil improvement technology. Soil mixing is carried out in-situ
inside the bore holes made by multiple shaft augers in the range
of 20 to 36 inches. Mixing equipment is used to produce a slurry
consisting of cement, bentonite, or other additives to form a pile.
For jet-grouting, rods are positioned at a target depth using a
boring rig. Jets of air, water, and grout are simultaneously used
to progressively erode the native soil and replace it with a soil-grout
(cement) mixture. The jet-grouted piles can be overlapped similar
to a secant pile wall.
References
1. Ryley, M.D., McCaul, C., and Symons, I.F., Trench Construction:
Trial to Study Ground Movement in Boulder Clay, (TRRL research
report 3) Crowthorne, Berkshire: Transport and Road Research Laboratory,
1985, 23 p.
2. Xanthakos, P.P., Slurry Walls as Structural Systems,
2nd ed., New York: McGraw-Hill, 1994, 855 p.
3. Li, K.S., ed., Design and Construction of Diaphragm Wall:
Proceedings, Hong Kong: Centre for Research and Professional
Development, 2001, 134 p.
4. Effective Analysis of Diaphragm Walls, prepared by
SEI/ASCE Technical Committee, Performance of Structures During Construction,
sponsored by Structural Engineering Institute of ASCE. Reston, VA:
American Society of Civil Engineers, 2000, 98 p.
5. Proceedings of Sessions of Geo-Congress 98, Geotechnical
Special Publication No.83, Reston, VA: American Society of Civil
Engineers, 1998, 184 p.
6. Macnab, Alan, P.E., Earth Retention Systems Handbook,
McGraw-Hill, 2002, 531 p.
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