Constructing a dry, decontaminated site foundation for Denver’s first cross-laminated timber mid-rise.
The highly anticipated Platte Fifteen multi-use mid-rise structure, located at 15th and Platte streets in Denver, has reached its final depth of 28 feet below ground surface. Installation of a temporary dewatering system by TerraFirma Earth Technologies is complete and is in the maintenance phase. General contractor Adolfson & Peterson Construction continued excavation in preparation for pouring concrete slabs in late June.
Besides being Denver’s first mid-rise building made from cross-laminated timber (CLT) and offering a reduced carbon footprint, Platte Fifteen is the site of a rarely utilized means of groundwater control known as ejector well dewatering. David Giles, president of TerraFirma Earth Technologies, explained, “Though more costly up front, in the long run, this unusual use of ejector dewatering wells in controlling the groundwater within soils typical to downtown Denver has proven more reliable, and more cost efficient.”
The added benefit of this unique use of the ejector well dewatering system was the lowering of groundwater to the maximum extent possible, allowing excavation and installation of the foundation mat slabs and water proofing membrane in “near-dry” conditions.
“The proper installation of the foundation slabs and waterproofing membrane under near-dry conditions is extremely important to the structure,” said Shiloh Hicks, project engineer. “The concrete slabs and waterproofing membrane create a water-tight foundation, commonly referred to as a ‘bath tub’ foundation. The bath tub-like foundation eliminates the need for the often costly and maintenance-intensive permanent foundation dewatering system typical of Denver’s downtown buildings.”
There are generally three means employed by the dewatering contractor in controlling groundwater: vacuum wellpoints, deep wells, and ejector wells. The most commonly known in the Denver area are deep wells (sometimes referred to as sump wells).
“Most contractors shy away from the ejector wells due to up-front costs; however, they are more often the best solution for Denver soil conditions (water-bearing alluvial soils over shallow bedrock),” Giles said. “We are glad to see that the general contractor, Adolfson & Peterson Construction (AP), made the best decision for dewatering this project. Though ejector wells are typically utilized in much deeper excavations, they are well suited here because of the necessity to lower the groundwater to the very top of the confining bedrock.
“Vacuum wellpoints, though versatile, were ruled out because of the suction limitation, generally 15 feet or less,” Giles said. “Because ejector wells use a continuous, recirculating supply of water through an ejector body to create a vacuum at the bottom of each well, suction limits are not a concern (water is pushed up). Deep wells, though suited for deeper excavations, would have proven ineffective because of the proximity of the relatively shallow bedrock (20 feet below ground surface) in relation to the much deeper subgrade (28 feet below ground surface). Deep wells would have to be so closely spaced to achieve the desired results; they quickly would become cost-inefficient. Additionally, deep wells, which rely on the continuous flow of groundwater to cool the electrically driven submersible pump located near each well bottom, would quickly run dry under the Platte 15 soil conditions, making them a maintenance nightmare. The ejector well, on the other hand, is a self-priming pump. If it runs out of groundwater, it will proceed to pump air without interruption until the groundwater returns.”
According to AP’s Project Manager Shawn Brannon, Platte Fifteen is using the construction manager/general contractor (CM/GC) delivery method, which gave AP the opportunity to employ the “choosing by advantage” process when hiring contractors.
“This ensures a collaborative project team offering the best solutions for the particular project’s requirements, rather than choosing contractors simply by cost,” Brannon said. “TerraFirma was consulted well in advance of groundbreaking. Getting the project on firm ground is critical to the success of a project. We needed TerraFirma’s expertise early in the preconstruction phase. They were able to give us a detailed solution to the site conditions that we could detail out before we broke ground.”
He added that the owner, Crescent Real Estate, applauded the “choosing by advantage” process and that it made the entire team engage wholeheartedly in the preconstruction problem-solving effort, which was a year in the making.
The Platte Fifteen property is being built out right to the property lines. TerraFirma’s Giles said that dewatering of the site was further complicated by the fact that one entire side of the site was inaccessible. “The excavation has a perimeter footprint of approximately 800 linear feet (approximately 200 feet by 200 feet). Two levels of underground parking are planned; the excavation extends up to 28 feet below ground surface.”
TerraFirma installed 64 ejector wells around the site’s three accessible sides — one every 10 linear feet.
Also unique to the project was TerraFirma’s use of the sonic drilling methodology in advancing each borehole several feet into the bedrock. “By employing the use of sonic drilling technology, not only were we able to fully penetrate the water-bearing alluvium overburden, we also were able to penetrate the highly weathered portion of the bedrock, which can often be another source of groundwater,” Giles said. “With the sonic drilling methodology, the 64 ejector wells were installed and ready for operation just three weeks after mobilizing.”
From the ejector wells, the groundwater was directed through a storm drain leading to the Platte River; however, as is true of many construction sites in the downtown Denver area, the preliminary geotechnical and environmental sampling reports revealed groundwater that was contaminated.
“To deal with the contaminated groundwater, TerraFirma teamed up with BakerCorp,” Giles said. “Their extensive experience in the downtown area and knowledge of its groundwater chemistry, as well as their working relationship with the various regulatory agencies involved, has proven invaluable.”
Following approval of the Remedial Discharging Permit Surface Water Permit from the Colorado Department of Public Health and Environment (CDPHE), BakerCorp, in close conjunction with TerraFirma, developed a Remedial Activities Management Plan to reduce the pollutants of concern to below the limits set forth in the permit issued by the CDPHE.
The groundwater treatment plan included introduction of a 10 percent solution of sodium hydroxide into the dewatering influent water to oxidize the metals iron (Fe) and manganese (Mn). This reaction took place in a single, 21,000-gallon flocculation tank. From the flocculation tank, the water was pumped through two, four-unit bag filtration pods to capture the flocculated metals. In addition to Fe and Mn, the permit also required to treat for arsenic and selenium. An ion exchange medium known as Purolite was utilized to reduce the elevated levels of selenium and arsenic. Granular activated carbon was added to the treatment train to remove any volatile compounds encountered, as well as for removal of any residual chlorine in the treated water.
“In the end, pure, drinkable water discharged into the Platte River,” Giles said.
Cory Christensen, project superintendent, said, “The sonic drilling rig utilized by TerraFirma was extremely compact — perfect for the site’s limited accessibility issues. In addition to the rig’s maneuverability, it was quickly able to complete a borehole that extended several feet below the top of the bedrock and easily penetrated the gravelly cobbles overlying the bedrock.”
Platte Fifteen is part one of a three-part development for Crescent Real Estate near the Platte and 15th Street intersection. This first phase is scheduled to open in fall of 2019.
Information provided by TerraFirma Earth Technologies (www.tfearth.com), a Houston-based dewatering firm with offices in Denver. Past deep excavation projects include athletic stadiums, hospitals, high-rise facilities, transportation facilities, airports, tunnels, power plants, dams, waterways, petrochemical plants, and municipal infrastructure sites such as pumping stations and treatment plants; as well as sewer and gas pipelines.