The project required installation of a system of vacuum wellpoints directly adjacent to these areas: 10 wellpoints: dewatering sump pit at 58ft. depth; 10 wellpoints: sewage ejector pit and 800 gallon sand/oil separator at 54ft. depth; 8 wellpoints: elevator pit at 62ft. depth; 2 wellpoints: deep pier cap at 61ft. depth; 2 wellpoints: precast box vault at 65ft. depth; and 4 wellpoints: 3000 gallon grease interceptor vault at 64ft. depth.
The geology generally consists of silty to clayey sand fills extending up to 11 ft. below ground surface (bgs), underlain by natural alluvium soils consisting predominantly of clean to clayey sand with sandy clay and gravel lenses. These natural sands extended to depths between 50 and 72 ft. bgs (elevation 5249 to 5274 ft. msl). The alluvium was underlain by bedrock consisting of claystone and sandstone of the Denver Formation.
The original design included widely spaced deepwell dewatering wells meant to maximize drawdown, while minimizing the rate of groundwater flow, in an effort to prevent migration of potentially contaminated groundwater from adjacent properties. There was concern that the ground water was contaminated from three sources: chemicals from a dry cleaning business, hydrocarbons from a previously demolished fuel station, and dissolved metals, which occur naturally in the earth.
The modified system meant adding mini-vacuum wells directly adjacent to the building’s elevator pits and a permanent dewatering sump pit that TerraFirma was able to custom design on the job, making for a unique hybrid of dewatering methods to better suit the actual soil and jobsite conditions.
By isolating the supplemental dewatering to just those few areas needing it, TerraFirma saved precious time and reduced costs. Also, by limiting supplemental dewatering efforts to just those few areas, we reduced the required amount of groundwater to be pumped, thereby reducing the risk of pulling in contaminants identified on adjacent properties during the investigative phase of the project.