As part of the redevelopment of Schenectady, a 7 story office and 8 story attached garage building was to be constructed in the downtown business area between and within ten feet of the property lines of two historic churches by BBL Construction Co. of Albany, NY. Overburden in the area is a very loose inter layered deposit of silt and fine sand extending to depths of about 125' where dense glacial till exists. The till extends to bedrock at depths greater than 190'. No basement levels were planned as the groundwater in the area exists at depths between 5 and 10' across the sloping site grades. Older buildings in the area, 1 to 4 stories, are supported upon spread foundations with more recent 5 and 6 story buildings supported upon driven piles end bearing within the tills. Column loads of the new buildings planned at this site range between 300 and 2,500 kips.
The development planned at the site posed several problems in the selection of a suitable foundation. Driven pile installation at this site was considered unfavorable because of the possible densification of the loose silt and sand soils upon which the adjacent masonry churches bear. Friction type piles could not generate the loads necessary for the support of the garage without large column groups and their consequent differential settlement concerns. A partially compensated mat incorporating a basement, while feasible for the office building where column loads range between 250 and 1600 kips, was overly complicated because of the groundwater conditions at the site. The adjacent church steeple rests upon a stone foundation bearing upon a timber mat constructed in the early 1800's beneath the water table. Construction and permanent groundwater depression, downgradient of the MVP site, could thus not be tolerated. To minimize potential effects to the surrounding structures and limit foundation settlements for the heavily loaded columns to tolerable magnitudes, drilled concrete filled shafts were selected for support of the building and garage. The shafts were drilled by HUB Foundations of Cambridge MA. into the deep till soils. Two instrumented Osterberg Load Cell Tests were performed to optimize the shaft socket design. The shafts were installed using the wet, open hole slurry method with synthetic drill fluid employed to stabilize the bores. A total of 83 shafts were installed at the site between depths of 135 and 155'. The shaft diameters ranged between 3' and 6'. |
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