The project involved construction of 9 floors on top of the existing Macy’s retail store at CambridgeSide Galleria Mall. To support the weight of the additional floors, a drilled mini-pile (DMP) engineering design was incorporated to transition load from the mat foundation to the underlying bedrock. This DMP foundation design had to consider the geologic challenges of a confined aquifer in the bedrock below the Galleria Mall, resulting in potential artesian conditions in the basement garage during installation of DMPs. LRT developed a Dewatering Plan that included depressurization or relief wells installed in the bedrock to temporarily reduce the piezometric head in the glacial till and bedrock below the clay during drilling operations.
LRT designed a water treatment system capable of treating flow rates of up to 200 gallons per minute (gpm) for water generated from both the DMP drilling operations and dewatering/depressurization wells. Specifically, water generated from drilling activities was initially consolidated in a weir tank for pretreatment of elevated pH water via the addition of sulfuric acid, and pretreatment of solids via the addition of coagulant and flocculant. The pretreated water was then pumped to the main water treatment system for further processing prior to discharge. Treated water was discharged under a NPDES Remediation General Permit (RGP). The primary challenges and contaminants of concern included total suspended solids (TSS), solids precipitation and management, hard water, pH adjustment, selenium and arsenic.