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Most of the site is located within the old Boston peninsula;
however, the northwestern section is located in an area reclaimed
from the sea. Soils in the two sections (3: E-E, D-D) comprise
15' of fill, local pockets of organic silt, 30' to 40' thick clay
(Boston Blue Clay, BBC), 0' to 16' of sand, 3' to 20' of glacial
till, weathered bedrock, and sound bedrock.
All the walls bulged towards the excavation by up to dW=2.2"
at elevations in the clay (LL3 to LL4). The walls deflected the
most near the current excavation level until the LL3 or LL4 slab
was completed. Thereafter, maximum wall deflections did not change
significantly in magnitude and in position.
As the excavation progressed beneath LL3 and LL4 the upper part
of the wall was slightly pushed back into the retained soil. At
the same time, wall deflections beneath the LL3 level steadily
increased. One possible mechanism could be that as the excavation
progressed deeper the lower slabs pick up more of the total lateral
earth load and thus the load that the upper slabs shared decreased.
As a result, the upper slabs slightly expand and the top of the
slurry wall is pushed back. Thermal expansion and creep deformations
of the upper slabs could also have contributed to the deflection
decrease at the top of the wall. When the base of excavation was
reached, there was very little wall bending above the LL3 and
beneath the LL5 slabs. The rock where the slurry walls were keyed
appeared to act as a pin support.
Building settlements were much smaller than surface settlements.
The Meridien Hotel settled up to 0.65". Buildings supported
by deep foundations were essentially unaffected by the excavation.
Surface settlements on the other hand reached up to 2.75"
within 20' from the excavation along Pearl Street, where wall
movements were the largest. Again, a large settlement increase
occurred by the time the LL1 slab was installed. Surface settlements
further away from the excavation started increasing when the excavation
progressed beneath the LL3 slab, as a result of deep soil movements.
Subsurface settlements were much smaller than surface settlements
(0.65" maximum, typically less than 0.4").
The excavation did not affect water levels in the fill, while
slightly lowered the piezometric head in the clay by 10'. However,
dewatering within the site and excavation beneath LL3, decreased
the piezometric head in the rock at one location by 45'.
The excavation did not affect adversely any adjacent structures.
However, deformations were larger than expected because the backfill
material that was used for the LBEs was softer than the surrounding
soil. Thus wall deflections were larger along the eastern wall
(2.0"), where the LBEs formed a parallel to the wall zone
of softer material. A major portion of the deformations occurred
in the initial stages of the excavation, before the roof or the
first level slab was installed. During these first stages, the
lateral earth loads were transferred to the softer LBE backfill
material, through out the wall depth, thus causing large cantilever
movements. Movements were smaller in locations where the LBEs
were further away from wall.
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