Forensic Geotechnical Investigation & Remedial Design

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Project Description

The Costco Wholesale facility was constructed beginning in 1997 at the site of the former New Rochelle Municipal Incinerator facility. Both the interior and exterior of the building began to settle differentially after the site was filled to subgrade levels. Exterior settlement mainly affected pavement surface elevations and subsurface utility performance. Interior settlement adversely affected the structural integrity of portions of the floor slab and steel framing. By 1999, portions of the building had settled up to 45 inches creating safety hazards for both Costco employees and customers.

The building was built on a fill layer ranging in thickness from eight to 29 feet. The fill layer consisted of incinerated municipal trash that was placed on top of a natural peat and organic silt layer between circa 1915 and 1995. Contaminants of concern in the fill layer included metals, SVOCs, VOCs and PCBs.

The building and surrounding site settlement resulted from consolidation of the natural peat and organic silt layer which was induced predominantly by the weight of the redistributed fill and placement of imported new fill to bring the site to current grade. Preliminary efforts to mitigate the effects of differential settlement within the building included two replacements of 12,000 sf of floor slab and raising/replacing interior columns multiple times to re-level roof girders and joists to relieve excess stresses in the steel framing.

Whitestone’s corrective action was designed to eliminate current and anticipated future structural distress and related safety concerns to the public and Costco employees and involved construction of a pile/anchor system to support column foundations in impacted portions of the warehouse building.

Approximately 85 piles and 34 bedrock anchors were installed to support 34 existing column foundations. Both the piles and anchors were drilled and grouted 6.5 feet into the underlying bedrock (located 55 feet to 80 feet below finished floor elevation in the area of the building supported by the piles). Because of suspected contamination in the old fill layer, pile drilling and spoil handling activities were conducted in accordance with a NYSDEC-approved Health and Safety Plan.

The building remained operational during Whitestone’s investigations and corrective action. To protect the safety of the public and Costco employees, temporary barriers were erected around the work area to restrict access and air quality within and adjacent to the work area was monitored constantly for hydrogen sulfide, carbon monoxide, oxygen, and lower explosive limit. An existing underground methane gas venting and monitoring system also was protected and kept operational during corrective action.