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IFAI IAA Awards: Dewatering with geotextile tubes in a D.C. tunnel

April 1st, 2018 / By: / Panorama

Completion Date: March 1, 2017

Fabric Name: Flintex™ OS425 Hi-Flo

Fabric Producer: Flint Industries

TITANTube® Hi-Flo geotextile tubes at the Washington, D.C., tunneling project. Photograph courtesy of Flint Industries

In 2013 DC Water in Washington, D.C., awarded the Impregilo-Healy-Parsons joint venture $254 million to design and build the second portion of a large tunnel system. The purpose of the tunneling project was to separately direct wastewater and stormwater to relieve the Anacostia River from combined sewer overflows. This 12,300-foot (3,750-m) long, 23-foot (7-m) diameter tunnel construction approximately 100 feet (30 m) underground also entailed the construction of six water-intake shafts from the surface to the tunnel. It was part of the $2.6 billion Clean Rivers Project to reduce combined sewer runoff to the Anacostia and Potomac rivers, as well as Rock Creek, by 96%. This Clean Rivers Project is the largest project undertaken in Washington, D.C., since the construction of the city’s Metrorail system, the first leg of which opened in 1976. Running north to south, the tunnel connects to the Blue Plains Tunnel. This DC Water project used a 1,248-ton (1,132-tonne) tunnel-boring machine nicknamed “Nannie,” which began work in 2015.

The drilling and mortaring operation generated slurry, a combination of groundwater and residual mortar water. The solids percentage of this mixture varied widely from 1% to 8%. The drilling operation typically ran 24 hours a day, 7 days per week, with very few shutdowns. They pumped intermittently with two shifts per day, and they averaged 10 to 12 hours of pumping time per day. During pumping, the drilling-fluid flow ranged from 100 to 200 gallons per minute (379 to 757 lpm).

The contractor experienced many failed attempts to process drilling fluids from the tunnel. A settling pit could not handle the flow. Because the abrasive solids were damaging to the belts on the belt filter press causing excessive downtime, there was not enough time to run the number of presses needed to reliably process the flow. Conventional geotextile tubes, with an industry-standard dewatering rate and using wastewater-treatment polymer, were also tried, but the dewatering rate was too low to accommodate the tunnel discharge. The contractor had reached an impasse, having experienced environmentally induced shutdowns and fines.

The variability in pumping times created challenges, sometimes creeping up at all hours of the night. Slurry treatment was complicated mostly by the very high pH, usually 12 or greater, which required neutralization by addition of acid prior to poly-merization and often following polymerization. This was necessary to produce effluent acceptable for discharge to the Blue Plains Advanced Wastewater Treatment Plant.

Additionally, there was variation in the material being drilled, such that for part of the project, crews encountered unexpected mercury contamination. Specialized chemical treatment was needed to remove the mercury and make the water safe for discharge to the wastewater treatment facility.

In summer 2016 the successful drilling fluid treatment process was presented to the contractor by the installer, who demonstrated treating the slurry with a custom-blended high-molecular-weight anionic emulsion wastewater treatment polymer followed by dewatering the treated wastewater through geotextile tubes inside roll-off containers. He demonstrated that this method could treat the full flow from the tunneling operation in the space allotted. The contractor was able to recognize the significantly higher dewatering rate, in that he needed a larger pump to move the effluent. The installer used from 6 to 12 high-flow roll-off geotextile tubes on-line at any given time, and during the last few months of the project used 45-foot (14-m) circumference × 50-foot (15-m) long geotextile tubes to accommodate increased flow rates and longer daily pumping periods. The inorganic solids contained a high concentration of calcium, which dewatered rapidly and reached 50% to 60% solids in the high-flow geotextile tubes within two to four days.

This project was completed ahead of schedule in March 2017, and there were no further environmental concerns or shutdowns as crews were able to process the tunnel discharges at a rate commensurate with the construction schedule. With its unique three-dimensional weave, the high-flow geotextile tubes were the solution for dewatering treated drilling fluids on this project. This technology also represented a significant cost savings over other dewatering processes. No other method (conventional geotextile tubes, mechanical dewatering or settling basin) was found to be successful at accomplishing this treatment with the capacity needed.

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