TerraFirma Earth Technologies, an AGC of Texas, Highway, Heavy, Utilities, Industrial was selected by general contractor GE Johnson, an AGC of Colorado Building Chapter member, to install a groundwater control system and innovative groundwater collection and treatment system designed by Terracon in the heavily travelled Cherry Creek business district. When ground was broken in February 2014, neither GE Johnson, 100 Saint Paul, LLC (owner), nor the geotechnical consultant Terracon, could predict unforeseen conditions that would require on-the-spot installation of additional dewatering features than were originally designed. TerraFirma worked congruently with the owner, general contractor, and design consultants to come up with an innovative, supplemental dewatering system that averted delays and minimized costs.
According to TerraFirma’s President, David Giles, 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. 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.
“Usually when we pump water from the ground, we pipe it directly to a storm sewer drain,” Giles explains. “However, there was concern that this 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.
“By isolating the supplemental dewatering to just those few areas needing it, we 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.”
Brian Lawrenson, GE Johnson’s superintendent, adds, “The project is situated in the heart of Cherry Creek, with very little or no laydown. Due to the complexity of the site excavation and ever-changing conditions, GE Johnson had TerraFirma install two areas of vacuum-well assemblies to provide additional localized dewatering.”
The final dewatering system included 11 extraction wells, one injection well, 42 mini-vacuum wells, three perimeter high-density polyethylene (HDPE) groundwater discharge manifold lines, and a groundwater treatment system. “It was the first time I have seen such a unique groundwater collection and treatment system design in my 28 years in this business,” says Giles. “It was much easier and less expensive to design and install this at the outset, just to cover the possibility that it would be needed. Had we not planned in advance for the three possible discharge routes and needed them, it would have been expensive to do so later. Terracon deserves credit for a really unique design.”
According to Giles, in the end Terracon’s samplings were all within limits, and the water didn’t have to be redirected away from the sewer. The injection wells will remain in place after completion of the construction and incorporated into a separate, permanent dewatering system.
Headquartered in Houston, TerraFirma helps owners and contractors throughout the United States maintain acceptable groundwater levels for both temporary projects and permanent installations. Past deep excavation projects include athletic stadiums, hospitals, high-rise facilities, transportation facilities, airports, tunnels, power plants, dams, waterways, petro-chemical plants, and municipal infrastructure sites such as pumping stations, treatment plants, as well as sewer and gas pipelines. Call 720-893-0556 or visit www.tfearth.com for additional information.
Owner: 100 Saint Paul, LLC
General Contractor: GE Johnson Construction
Geotechnical Engineer: Terracon
Dewatering Contractor: TerraFirma Earth Technologies