This project provided major upgrades to the existing Highspire Wastewater Treatment Plant and included the installation of 30 ft x 50 ft one-story building, an oxidation ditch tank, and a 48 ft diameter x 14 ft, stand-alone final clarifier tank. The oxidation ditch tank was the primary focus for soil improvements and consisted of two tanks having dimensions of 130 ft by 140 ft.
The soil condition consisted of 6.5 to 14 ft of very soft to very stiff silt fill with varying amounts of clay and sand then very stiff to very hard silty clay and silt, underlain by approximately 5 ft of dense to very dense sand on top of rock near a depth of 25 ft. Groundwater was encountered at depths of 15 ft, and a very low bearing capacity of one kip psf was recommended by in the geotechnical report. Also, due to operations from a previous owner, the soils on the site were slightly contaminated with arsenic requiring encapsulation of exposed surfaces during construction.
The Impact® System was recommended by the geotechnical engineer as an Intermediate Foundation support solution in lieu of over-excavate and replace. Due to the soft soils and equipment loads on top of the oxidation tank, Geostructures elected to use grouted Impact® Pier elements to provide a more economical design. A total of 423 twenty-inch diameter grouted Impact® Pier elements were installed to depths of 20 ft below the tank base.