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The Beth Israel Deaconess (BID) Hospital is located in the corner of Longwood
and Brookline Avenues in Boston Massachusetts. The building consists of a
4-storey low-rise area and a 18-storie high-rise section. A 5-level underground
parking garage was constructed under the building using the up/down
construction method. The Massachusetts College of Arts building (MCA)
previously occupied a major portion of the site. This added to the complexity
of the project since the facade of the MCA building facing Brookline Avenue had
to be incorporated to the new BID building. The foundation perimeter walls were
constructed using the slurry trenching method (slurry walls) with a saw-toothed
configuration.
Settlement control was important since damage to adjacent buildings had to be
minimized. The type of foundations that respective buildings have affects their
settlement during an adjacent excavation (including the up/down construction
method). Buildings with deep foundations like end-bearing piles typically
experience smaller settlements during adjacent excavations from buildings
founded on shallow foundations.
Five major soil strata and two major rock units were encountered in the test
boring program conducted for the BID project namely: 6'-15' miscellaneous fill
(SPT<15 bpf), over 10'-36.5' of loose to very dense medium to fine sand
(30<N<50), 25' to 64' marine clay (BBC), 3'-10' glaciomarine deposits,
and either Argillite or Conglomerate bedrock. The soil conditions at the site
are usual of the glacial past of the Boston area and the soil profile can be
classified as profile B according to Johnson [1989].
The top/down construction method required that deep foundation units were
installed prior to the general excavation. Load Bearing Elements (LBE) were
used to support the basement columns. LBE's were designed as to utilize both
skin friction and end bearing. The reinforced concrete diaphragm (slurry) wall
served both for temporary and permanent earth support of the basement area. The
below grade garage floors provided temporary and permanent bracing of the
slurry walls. The walls were embedded a minimum of 3ft into glacial till in
locations where the walls carried vertical loading from the superstructure. An
allowable pressure of 30ksf was used for slurry wall bearing design. Slurry
wall panels were typically 20' long with most panels at the northern, eastern,
and southern sides of the project 2.5' thick, while some other panels were
3.0ft thick
Horizontal wall movements in this project were moderate to small. Maximum wall
deflections ranged up to 0.88" at LL4. Most of the wall deflections
occurred after the excavation reached below the third or the fourth garage
level. At the final stages in the Brookline Avenue slurry wall the point of
maximum wall movement occurred between the fourth and the base slab levels.
Soil horizontal movements that were monitored next to slurry wall inclinometers
were smaller than wall movements. Typically soil movements matched slurry wall
deflections with the exception of some erroneous inclinometer data. Horizontal
soil movements were observed at considerable depths below the ground surface
despite the large slurry wall embedments (15 to 30 feet). Maximum horizontal
soil movements up to 0.64" occurred at the Brookline Avenue side
inclinometer where wall deflections were the largest throughout the project.
Horizontal soil movements along Longwood Avenue were slightly larger reaching up
to 0.6" ten feet below the ground surface. Slurry wall deflections at the
Longwood panels were also moderate reaching up to 0.84" between the fourth
and the base garage slab. At the base slab elevation the maximum horizontal
soil movements was 0.25" and the corresponding wall deflection was close
to 0.75".
Typical building settlements were small to moderate with the exception of the
MCA facade. The MCA facade settled as much as 1.4" but close to 0.4"
of this settlement occurred before any foundation construction probably due to
the demolition of most of the MCA building. Buildings supported on shallow
foundations closer to the east face of the excavation had moderate settlements.
The southwestern corner of the Kirstein Building settled close to 0.8"
(closer to the excavation) and the Beth Israel garage settled up to 0.5".
The Kirstein building was founded on spread footings while the Beth Israel
garage has pressure injected footings at 20' below ground surface, while other
buildings settled less than 0.2". Overall, performance of the BID
foundation system was satisfactory since most slurry wall deflections and
adjacent building settlements ranged from small to moderate values.
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