TUNNEL ENGINEERING

Fig. 20.9 Tangent Piles.
deep. The ease and simplicity of constructing the
final structure within temporary walls must be
balanced against cost savings when they are
incorporated into the final structure. Temporary
excavation support may be steel sheet piles, soldier
piles and lagging, and tangent or secant piles. For
deeper excavations, concrete slurry walls may be
constructed, usually 2 ft, 3 ft, or 4 ft thick, sometimes
incorporating soldier piles or beams (SPTC
walls), and may form part of the final structure.
Steel sheetpile walls, for depths to about 30 to
40 ft, supported by wales and cross bracing. The
walls keep loss of ground to a minimum.
Soldier piles and lagging, made of steel H
beams with wood or concrete lagging. These are
used for greater depth. Lagging must be blocked
tight against the earth to control loss of ground.
Soldier piles may be combined with sheetpiles,
instead of wood lagging, if tight bulkheads are
required. Wales and cross bracing support the
walls.
Concrete slurry walls built in bentonite-slurry
trenches have been used to prevent loss of ground
TUNNEL ENGINEERING
Tunnel Engineering n 20.21
and eliminate or reduce groundwater lowering.
Sections of trenches about 20 ft long are excavated.
The trenches are kept filled with bentonite slurry.
Then, reinforcing cages are lowered into them, and
concrete is placed to fill the trenches, displacing the
slurry. Key sections are formed at the ends of the
trenches. The walls serve as part of the final
structure or as impervious bulkheads.
SPTC or California Wall, a combination of
soldier piles and slurry wall. This was used on
some stations of BART, and as part of the final
structure for much of the Central Artery Project in
Boston. Large wide-flange steel beams are inserted
in slurry-filled bored holes, the space between the
beams is excavated under slurry, and excavation
and pipe holes are filled with concrete. Care must
be used in excavation to have the concrete solidly
keyed into the space between the flanges. The steel
piles in the composite wall act as reinforcing and
permit easy attachment of interior bracing.
The fundamental basis for the design of
excavation support systems is consideration of
how the soil being supported behaves, and perhaps
also how the floor of the trench behaves, since
substantial heave can occur under adverse conditions.
Any movement of the support system can
cause soil movement and hence settlement of
adjacent structures. It is the amount of movement
that can be tolerated at the adjacent structures that
often dictates the type and stiffness of the
excavation support system, with control of
ground-water levels often being crucial. Structures
may have to be designed for both short-term and
long-term effects. One of the primary tools for
this purpose is soil-structure interaction analysis.
Frame structures supported by beam-on-anelastic-foundation
analysis are also commonly
used. In most cases, a two-dimensional analysis is
sufficient, although complex areas may require
three-dimensional modeling, perhaps using finite
element analysis.
Many subway and highway structures have
been built using steel columns and beams with jack
arches, but this is uncommon today. New structures
are generally reinforced concrete box structures
but when the excavation support system also
forms part of the final structure, it may in practice
be difficult to obtain full fixity between the walls
and the slabs. Partial fixity may then be specified,
and perhaps shear connections also provided. For
high load on the roof or large spans, the composite
action of a thin concrete slab on top of steel beams
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