TerraFirma Earth Technologies, Ltd.

Owner:

Southern Land Company

Architect:

OZ Architecture

General Contractor:

SLC Commercial Construction, LLC

Client:

Jonathan Harel - Project Manager

(617) 834-8193

SLC Commercial Construction, LLC

Scope Of Work

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 January/February of 2021. This is Southern Land Company’s first development in the Boulder market. The Reve Community will offer multiple lifestyle choices including micro, studio, efficiency, 1, 2, and 3 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 ft. 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 of 2019.

Winter temperatures required constant monitoring of the deepwells. According to Southern Land Project Manager Jonathan Harel, TerraFirma provided a technician who monitored the wells 24/7 either remotely with camera images every 15 minutes, or on the site for the winter months to be certain the low nighttime temperatures didn’t freeze the pipes and disable the pumps. “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,” said Mr. Harel. Crews kept concrete blankets over the main that connected the wells and added heat trace to be sure nothing froze.

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 choose 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,” explained Mr. Giles.

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. Giles. 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 ft 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 surround with a select filter material. Mr. Giles said, “Selecting the proper filter material and placement method was very important. 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.”

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,” explained Mr. Giles. “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 Solutions Corp, 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 Solutions Corp 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 a 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. Giles 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 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.