Groundwater Control and Excavation Support for the Salt Lake City Water Reclamation Facility Influent Pump Station

A 105 ft (32.0 m) diameter secant pile support of excavation compression ring and depressurization wells were installed, to control groundwater pressure, basal stability, and permit safe excavation to a depth of 52 ft (15.9 m) inside the ring for below-ground pump station construction. A key challenge was managing groundwater pressures from three confined aquifers within the Lake Bonneville sediments, containing fine sand deposits bracketed by thick lean clay aquitards. To control seepage from the shallower aquifer that is located 72 to 78 ft (22.0 to 23.8 m) below ground surface, secant piles were extended to a depth of 90 ft (27.4 m) in order to penetrate into a low-permeability clay layer for hydraulic cutoff. This decision was driven by the presence of laterally continuous granular soil layers, which would otherwise allow groundwater migration and compromise excavation stability. To prevent uplift forces from a deeper aquifer located 122 to 132 ft (37.2 to 40.2 m) below grade from compromising excavation stability, depressurization wells targeting this layer were installed outside the secant pile ring to lower the piezometric head in this deeper permeable layer in order to protect against a basal blowout failure.