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The concrete diaphragm wall serves as both the temporary excavation
support wall and permanent foundation wall. The slurry wall was
designed to resist temporary and permanent static lateral pressures
from soil, water, and lateral surcharge loads from building #197
to the north, and other adjacent surface loadings.
Cross-lot bracing was used for the lateral support of the foundation
walls of this project. Three levels of temporary prestressed (jacked)
steel struts and corner braces supported the slurry wall. The
struts were hollow cylinder steel pipes with a diameter ranging
from 24" to 34" and 3/8" to 0.5" wall thickness.
Steel HP sections were in the connections between struts supporting
the north and south wall (Fig. 4.29). The horizontal forces jacked
on the struts were transferred on the wall by means of walers.
Most wall movements at the Flagship Wharf project were moderate
but along the northern slurry they were large reaching dW=2".
The caissons of building #197 added significantly more surcharge
on the northern slurry wall, and as a result, the southern wall
was pushed back into the retained soil by as much as 1" at
the top. Wall movements at opposite panels, braced by common struts,
showed opposing trends (0). That is when a panel moved towards
the excavation the opposite panel moved back into the soil. The
concave bending seen in the slurry walls of this project is indicative
of the cross-lot bracing construction sequence.
Settlement control was important in the north side of the project
due to the proximity of building #197 that was supported by a
series of caissons. Settlements up to dV=1.8" occurred along
the section containing the caissons but the settlements were not
transmitted to the exterior walls or the floors of the building
(1). Wall movements and settlements tied very well together.
A cave-in occurred during trenching of a panel, at a small depth
in the fill layer during excavation without slurry. Clearly if
trenching was carried out under slurry this cave-in might have
been avoided. The cave-in caused additional problems with the
vertical construction of the trench. Construction of this panel
took a longer time since numerous underground obstructions were
encountered.
Data for strut loads as measured from strain gages showed that
a quick 0.5" movement at I-2 occurred as a result of a jacking
box failure that resulted in the sharp load loss of the L3 strut
during April 1989 (2). The structural factor of safety of the
struts was only Fs=1.2 and as a result 50% of the total wall movement
occurred after the excavation base was reached (indicating some
creep movement in the bracing). The upper and lower level struts
picked up only 70% of the design loads, that were in the order
of 500 tons for the 3rd level (2). The load at second level struts
picked up as the excavation progressed to the third level (El
-38ft).More frequent readings indicated that daily variations
in temperature caused expansion and contraction of the struts
that showed up as variations in the strain gage readings.
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