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SUPPORT SYSTEMS FOR DEEP EXCAVATIONS:
Cross-lot/Internal Bracing - Braced Excavations
Cross-lot or internal bracing transfers the lateral earth (and water pressures)
between opposing walls through compressive struts. Rakers resting on a
foundation mat or rock offer another internal bracing alternative. Typically
the struts are either pipe or I- beam sections and are usually preloaded to
provide a very stiff system. Installation of the bracing struts is done by
excavating soil locally around the strut and only continuing the excavation
once preloading is complete. A typical sequence of excavation in cross-lot
braced excavations is shown in Figure 1. The struts rest on a series of wale
beams that distribute the strut load to the diaphragm wall.
Pre-loading ensures a rigid contact between interacting members
and is accomplished by inserting a hydraulic jack as each side
of an individual pipe strut between the wale beam and a special
jacking pblate welded to the strut (Fig. 2, Xanthakos, 1994).
The strut load can either be measured with strain gages or can
be estimated using equations of elasticity by measuring the increased
separation between the wale and the strut. Figure 3 shows the
basic arrangement for the wedging, and the telescoping preloading
methods.
n some earlier projects the struts were not preloaded, and as
a result when the excavation progressed deeper the soil and the
wall movements were large (C1). Thus it has become standard practice
to preload struts in order to minimize wall movements.
Cross-lot bracing makes sense in narrow excavations (60ft to
120ft) when tieback installation is not feasible. The struts can
bend excessively under their own weight if the excavation spacing
is too large. In addition, special provisions have to taken to
account for thermal expansion and contraction of the struts.
The typical strut spacing is in the range of 15ft, both in the
vertical and the horizontal direction. This is larger than the
typical spacing when tiebacks are used, because the pre-loading
levels are much higher. A clear benefit of using struts is that
there are no tieback openings in the slurry wall, thus eliminating
one source of leakage.
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