By David Giles
TerraFirma Earth Technologies has completed the installation of temporary dewatering and groundwater treatment systems for Revé, Boulder’s highly anticipated mixed-use development. The Revé project is located at 30th and Pearl St., across from the new Google complex. It has been four years in the making, and projected occupancy is winter 2021. This is Southern Land Company’s first development in the Boulder market.
The Revé Community will offer multiple lifestyle choices including micro, studio, efficiency, one, two, and three bedroom, town home, and live/work residences. The plan contains 244 residences, 24,500 square feet of retail space, 108,700 square feet of office space, and ample green space and bike paths.
With the dewatering system up and running, the excavation of both the north and south parcels has successfully reached its target subgrade of 26 feet below ground surface, despite having to perform the work throughout Boulder’s winter months. The estimated eight-month-long maintenance and operation phase of the dewatering continues and is expected to be complete by Summer 2019.
Winter temperatures required constant monitoring of the deepwells. TerraFirma provided a technician who monitored the wells 24/7, either remotely with camera images every 15 minutes, or on site for the winter months, to be certain the low nighttime temperatures didn’t freeze the pipes and disable the pumps.
According to Southern Land Project Manager Jonathan Harel, “TerraFirma crews were very responsive to any situation that came up. Whenever a situation emerged, they were always willing to put in the time to resolve it, even late in the night.”
Crews kept concrete blankets over the main that connected the wells and added heat trace to be sure nothing froze.
Why So Deep?
As available lands become scarcer, underground parking has become more and more appealing, especially in growing urban areas like Boulder. Additionally, city planners’ and developers’ desires to maintain views and not block sunlight further the advantages of underground parking over above ground parking. For these reasons, Southern Land Company chose to build two levels of subterranean parking beneath the entire development.
Digging deeper, however, can be a difficult and costly task if not properly planned out. One of the most important considerations, especially for excavations extending below the groundwater table, is effective control and lowering of the groundwater prior to excavation. This process is known in the construction industry as “dewatering.”
For site dewatering, Southern Land turned to TerraFirma Earth Technologies, a Houston-based dewatering company with offices in Denver.
The high water table, located just 10 feet below ground surface, along with complex geological conditions at the Revé site, created special challenges for TerraFirma, with important cost considerations for the developer.
“The site of Revé is situated over 2 to 11 feet of man-placed fill, underlain by water bearing, naturally occurring sand, gravel, cobbles, and even some boulders,” said TerraFirma Superintendent Bairn Leonard. “Making dewatering even more of a challenge, bedrock was encountered between 12 to 28 feet below ground surface. With a planned excavation extending to approximately 26 feet below ground surface, well below the top of bedrock, this meant lowering the water table to the top of the confining layer. Committed to having the two parking levels below ground, we proposed multiple possible dewatering methods for the developer to deliberate over. These were supported by the geotechnical engineering report conducted by Terracon, and the Groundwater Characterization Study by Ayres Associates.”
Deepwell Dewatering Wells Chosen Over Other Means of Temporary Dewatering
With the excavation extending well beyond the limits of a traditional vacuum wellpoint dewatering system, and the anticipated flow rates exceeding that of an ejector-well dewatering system, TerraFirma chose to rely on closely spaced, large diameter deepwell dewatering wells as the primary means of controlling the groundwater. The deepwell dewatering wells were installed from ground surface, just outside the excavation’s h-pile and wood lagging support system. With the perimeter of both the south and north parcel excavations exceeding 2,000 lineal feet, a total of 52 deepwell dewatering wells were installed.
To install the wells, TerraFirma utilized a Soilmec 312 hydraulic drilling rig to advance the 30” diameter borehole to the designed termination depth of 32 ft, easily excavating the extensive layer of sand, gravel, cobbles, and boulders.
“Additionally, by utilizing the powerful Solimec 312, we were able to easily penetrate nearly 10 feet into the underlaying bedrock, creating a ‘sump- effect’ along the entire excavation footprint, which allowed us to draw down the water further, closer to the bedrock/alluvial interface,” said Leonard.
Portable Sump Wells Supplement Traditional Deepwell Dewatering Wells
TerraFirma’s dewater plan also included the installation of strategically placed portable sump wells within the excavation to supplement the deepwell dewatering wells. These were a very necessary component due to the excavation extending below the rock/alluvium interface, according to Mr. Leonard. The portable sump wells were utilized to handle perched/trapped groundwater within the excavation, as the excavation encroached upon the bedrock/alluvial interface. Each portable sump can consisted of a 12” diameter x 10 foot long SCH 40 PVC factory slotted well screen.
Each sump can was fitted with an electrically driven submersible dewatering pump assembly. The sump wells were drilled in from the top of bedrock, extending approximately 10 feet deep. The sump well was then surrounded with a select filterial material.
“Selecting the proper filter material and placement method was very important,” Leonard said. “By pre-filtering the groundwater prior to collecting it in the common discharge manifold and ultimately through groundwater treatment-filtration equipment installed at ground surface, additional costly treatment measures were avoided by preventing premature clogging and fouling of the treatment equipment. Each sump pump was also fitted with a liquid level control device that turned the pump on and off, based on the water level within the sump, preventing the pumps from running dry and causing premature pump failure.”
Harel was pleased with TerraFirma’s work.
“Their crews are the best guys in the field and a pleasure to work with,” he said. “During constant monitoring of the water contaminant filtration, the limits were occasionally exceeded and TerraFirma always presented multiple options for resolving the issue, creating trust among all stakeholders involved.”
TerraFirma Cleans Up the Water Prior to Discharge into Left Hand Ditch
From the deepwells, the groundwater was directed to Boulder’s Left-Hand Ditch, directly adjacent to the site; however, as is true of many construction sites along Denver’s front range, the preliminary geotechnical and environmental sampling reports revealed groundwater that was contaminated.
“To deal with the contaminated groundwater, we teamed up with United Rentals Fluid Solutions Group,” Leonard said. “Their extensive experience in the Boulder area and knowledge of its groundwater chemistry, as well as their working relationship with the various regulatory agencies involved, has proven invaluable.”
Following the approval of the Remedial Discharging Permit Surface Water Permit from the Colorado Department of Public Health and Environment (CDPHE), United Rentals Fluid SolutionsCorp, 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.
James Weeks, Filtration Productline Manager for United Rentals Fluid SolutionsCorp said, “The variety of hydrogeologic and environmental conditions in the Denver area and associated Rocky Mountain Front Range offer a number of groundwater treatment challenges to meet strict Colorado surface water discharge requirements. We take pride in the ability to provide tailored, cost-effective, groundwater treatment options that are individually designed to meet discharge permit requirements on a site-by-site basis.”
The groundwater treatment plan included the introduction of a of sodium hypochlorite into the dewatering influent water to oxidize the metals Iron (Fe), and Manganese (Mg). Caustic acid was also dripped into the tanks as a means of controlling pH, and further promoting the oxidation of the metals. These reactions took place in two, 21,000-gallon flocculation tanks. From the flocculation tank, the water was pumped through two 12-unit bag filtration pods to capture the flocculated metals. Granular activated carbon was added to the treatment train to remove any volatile compounds encountered, as well as for the removal of any residual chlorine in the treated water.
Mr. Leonard added, “This process has proven effective and cost efficient in achieving the desired results. The system was designed for flows up to 500 gallons per minute. Once the site was dewatered, the system was resized for lower flow rates, significantly reducing the area required to accommodate the equipment on the already very congested site – a benefit to this near zero-lot line project – returning pure, drinkable water to the Left-Hand Ditch.”
Tony Brace, General Superintendent of Colorado, Southern Land, said, “This project, like many others, was different in the field than it was on paper. For example, the discharge manifold was modified in the field as it was being laid. Terrafirma had the flexibility to work with the other trades, giving access to the people that needed it, which kept the project on time.
“TerraFirma is one of the best dewatering subcontractors we’ve ever worked with. They have a ‘get it done’ attitude that produced the results we were looking for, no matter what obstacles we ran into. When field modifications had to be made, their boots-on-the-ground team, led by Superintendent Bairn Leonard, came through without reservation.”
David Giles is President of TerraFirma Earth Technologies. Mr. Giles has a B.S. in Geology from Iowa State University and 33 years of experience in developing and implementing design-build solutions for construction dewatering and groundwater control using deep wells, wellpoints and ejector systems. TerraFirma is a Houston-based dewatering firm with offices in Denver. David Giles may be emailed at email@example.com.