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DEEP EXCAVATION CASE STUDY:
75 State Street, Boston, MA
 This
case study is a review of published report (1).
The 75 State Street building is a 31-story office tower with six
levels of below-grade space utilized primarily for parking. Permanent
and construction earth support was provided by 2.5'-thick, 85'-deep
diaphragm wall constructed with the slurry trench method. The
65-ft deep excavation was constructed by the top/down method,
braced by 5 floor levels. The diaphragm wall extends 20' below
the final excavation grade into the underlying till.
Soil conditions at the structure site are typical of the Boston area located
outside the old colonial shoreline. The overburden soils consist primarily of
fill, Boston Blue Clay, glacial till, and highly altered bedrock 70' to 100'
below the surface. The upper 10ft to 20 ft top of the clay is an
overconsolidated crust, (OCR >2). The overconsolidation ratio decreases
rapidly with increasing depth as the clay becomes almost normally consolidated
(OCR= 1 to 1.1).
The bedrock at the site belongs to the Cambridge Argillite formation and it is
highly altered, containing zones of clay-like kaolinized material. Thickness of
these clay-like "soil" zones may vary from a few inches to hundreds
of feet (2).
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Many adjacent buildings exist near the site. Some of these buildings are
supported on shallow foundations, while within the site a series of
concrete-filled pipe piles from a previous parking garage had to be removed.
The Massachusetts Bay Transit Authority Blue Line tunnel below State Street was
a major concern for this project.
The diaphragm wall was designed to provide a semi-rigid wall system, thus
eliminating the need for underpinning adjacent structures and provide
foundation support for the perimeter columns. In addition, the diaphragm walls
were incorporated into the structure and provided seepage cut off allowing the
lowest floor level to be designed as a fully relieved slab-on-grade (1), since
the underlying bedrock at the site has very low permeability.
The construction process proceeded generally as planned with two exceptions
(1):
a) The slurry caisson method was adopted for installation of the major core
column foundations after several attempts to excavate and dewater the holes
failed.
b) A major problem was pile extraction within the site prior to excavation. The
voids created in the clay by pile extraction plus pre-excavation along the
slurry wall alignment resulted in excessive inward wall movements before the
floor systems could be placed. This clearly illustrated the importance of the
excavation/floor slab construction sequence.
3. Performance
At 75 State Street, where up to 60 ft. of clay underlies the site, horizontal
wall movements ranged from 0.5" to 2". The largest movements took
place where the clay was the thickest and a two level high mechanical room
existed. Adjacent buildings within 5' to 20' from the excavation settled from
1" to 4". However, 50% of this movement occurred prior to the start
of excavation and can be attributed to earlier pile extraction and caisson
installation. Horizontal movements were within the predicted range for an
excavation of this size. However, settlements were significantly larger than
predicted.
4. Reference
1. Dr. James M. Becker, Mark X. Haley, "Up/Down Construction", Design
and Performance of Earth Retaining Structures, Proceedings/Geotechnical
Engineering Division, Cornell University, Ithaca, New York - June 18-21, 1990
2. Humphrey, J. T., Soudemir, C.: "Special Bedrock Conditions in Greater
Boston", Detection of and Construction at the Soil/Rock Interface, ASCE,
Geotechnical Special Publication No. 28., 1991, pg. 45-57.
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