Terrafirma Earth Technologies, Ltd. partnered with Hensel Phelps Construction Company on the construction of the new Denver Justice Center and Detention Facility project. Terrafirma designed and installed a deepwell dewatering system to ensure the integrity of the soils for the new construction.
The project consisted of the construction of the Denver Justice Center (DJC), located in downtown Denver, Colorado. The DJC will include a 320,000 GSF Criminal Court Facility, a 1500 bed 320,00 GSF Detention Center, and a subsurface pedestrian tunnel connecting the two structures.
The Court Facility building footprint is approximately 420ft long by 120ft wide, with a basement elevation of 5212 msl; however, deeper excavations are required for foundation support structures, as well as for the north and south basement pits.
The Detention Center is approximately 460ft long by 200ft wde in plan view. The excavation for the Detention Center will extend to 5200 msl, penetrating the reported piezometric water surface.
A tunnel pedestrian/utility tunnel approximately 250 ft long x 20 ft wide connects Court Facility and the Detention Center at approximately midpoint of both structures. The top of slab for the tunnel slopes downward to the east from El (+) 5212 msl at the Court Facility to elevation 5206.67’ msl at the Detention Center.
The existing ground surface at the site ranges from approximate elevation 5214’ to 5228’ msl. Subsurface conditions consist of up to 10 feet of fill (predominately clayey sand/sandy clay), overlying natural alluvial soils (predominately fine to coarse grain sand/gravels). Claystone and occasional interstratified sandstone/claystone bedrock, also known as the “Denver Blue” was encountered beneath the alluvial soils, ranging in depth from 9 ft below ground surface (bgs) in the north west corner of the site, to 46 ft bgs on the south east corner of the site. The piezometric water surface measured in the borings ranged between elevations 5206’ to 5208’ msl over a majority of the site, and dropping of to 5198 msl in the far North West corner of the site.
TerraFirma designed the dewatering system utilizing ten closely spaced deepwell dewatering wells, an average of 50’ center to center, installed within over-sized bore holes along the excavation perimeters, approximately 4 to 6 feet outside the earth retention system. The deepwell dewatering wells consist of 12” PVC well materials placed within an oversized, 28” to 34” diameter bore hole. The bore holes fully penetrated the upper pervious and semi-pervious alluvial soils, and terminate 10’ to 15’ into the denser, more impervious zone of the Denver Formation. Electrically driven submersible 7.5 HP pump/motor assemblies were placed near the bottom of each well. Discharge water form each well is directed, via a common discharge header system (HDPE) surrounding the site, on grade, to a portable settlement tank, and then into the storm sewer system. The discharge water was tested by a local laboratory to ensure that it met all safety standards.
The primary function of the deepwells is to relieve excess hydrostatic pressures in the surrounding/underlying foundation soils. Additionally the wells were installed in a manner to assist in the capture and/or minimization of any perched/trapped water that may exist within the upper natural alluvial overburden, and/or the “fill” soils.