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A 2'-thick perimeter slurry wall provided lateral earth support for the
excavation of this project. Most of the slurry wall was constructed in 20'-long
panels, and was supported by one level of tiebacks. The slurry walls were
typically 36' deep with the toe embedment depth below the excavation base
ranging from 11' to 16'.
The subsurface exploration for this project indicated the presence of six major
strata namely: fill, organic silt, sand, BBC, glacial till and bedrock. Soil
conditions are similar to those in Back Bay corresponding to a soil profile A
according to Johnson [1989].
Two levels of tiebacks supported central panels on long faces. Tieback design
loads ranged from 85 to 130 kips with average of 124 kips. The design line
loads ranged from 10.90 kips/ft to 16.74 kips/ft with the latter being most
common. Loads of around 50 kips were used for second level tiebacks. Estimated
total tieback lengths averaged 40' to 45' with some tiebacks having total
lengths of 35'. All tiebacks picked up load as the excavation progressed and
wall movements increased, until equilibrium was reached.
The overall performance of the lateral earth support system used in this
project was satisfactory. Lateral movements of the slurry wall were moderate to
insignificant (maximum 1.3"). Most of the wall movement occurred prior to
tieback installation, when the wall cantilevered. After the excavation was
completed an additional movement of up to 0.3" occurred when loads were
transferred from the tiebacks to the base mat. Such movements should be
expected since stress transfer from the tiebacks to the floor bracing does take
place.
Wall movements were noticeable despite the fact that the excavation was
relatively shallow. The floating slurry wall of this project translated up to
0.15" near its base as offset surveys indicated. Such translation at the
toe of the slurry wall is to be expected when the wall is floating. One of the
interesting aspects that offset surveys indicated was that wall movements can
vary significantly even between adjacent panels. For example, at one stage one
panel moved back into the soil by as much as an adjacent panel moved towards
the excavation.
Excavation for this project did not affect any adjacent structures. Settlements
increased as the excavation progressed deeper and reached dV=1.05" near
the excavation, while building settlements further away were practically
insignificant.
One panel did cave-in during trenching, due to fill "sliding" into
the trench under the guide walls. This was probably caused by the difficulties
in stabilizing the loose granular fill. Slurry wall water leakage was very
small and temporary (i.e. detensioning and removal of tiebacks, covering of the
sleeve hole).
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