Slurry Walls

Slurry cutoff walls are constructed by excavating narrow trenches under bentonite slurry and then backfilling (displacing) the slurry with higher gravity impermeable material to create subsurface barriers that prevent the lateral flow of groundwater. They are used to contain the migration of contaminated groundwater; reduce the permeability of storage impoundments such as tailings dams, ash ponds, or wastewater treatment ponds; and to prevent under seepage in levee and dam construction. Forgen has constructed more than 21 million square feet of slurry cutoff walls, placing us at the forefront of the industry with regard to capability and experience in this type of construction. Design permeability and strength requirements as well as native soil and groundwater conditions influence the composition of the engineered slurry backfill. Forgen’s slurry wall construction capabilities include the ability to design and construct soil-bentonite, soil-attapulgite, soil-cement, soil- cement-bentonite, cement-bentonite, and slag-cement-bentonite cutoff walls. Additional expertise includes the installation of biopolymer collection trenches and permeable reactive barriers (PRBs).

Biopolymer collection trenches are used to extract or intercept groundwater.  They are used to contain the migration of contaminated groundwater or to dewater areas for construction or excavation. During installation, the trenches are supported by a biopolymer (e.g., guar gum) slurry, eliminating the need for shoring.  The trench is then backfilled with sand or porous media and the biopolymer degraded either naturally or by the addition of enzymes. The native soil and groundwater conditions influence the type and concentration of biopolymer slurry used to maintain the trench. Additional expertise includes the installation of slurry walls and permeable reactive barriers (PRBs).

Permeable reactive barriers (PRBs) are in-situ treatment zones constructed to treat groundwater as it flows through them. The treatment can include sorption/precipitation, chemical reduction or oxidation, and/or biological degradation, allowing PRBs to be specifically designed around the contaminants of concern. Common reactive materials include activated carbon, zero valent iron, limestone, and organic substrates. Often slurry walls are used to direct groundwater flow through PRBs, reducing the size (and cost) to treat a contaminated groundwater plume. Forgen has experience with the installation of PRBs using biopolymer trenching, mandrel placement, and deep soil mixing.  Additional expertise includes the installation of biopolymer collection trenches and slurry walls.

Perth Amboy, New Jersey

  • 226,665 sf soil-cement-bentonite (SCB) cutoff wall
  • Limited work pad area
  • Depth 30-70 ft
  • Close proximity to river
  • Crossing of three active utilities

Forgen was contracted to construct two soil-cement-bentonite (SCB) cutoff walls for a redevelopment company which had assumed the obligation to remediate the property and close out all of the environmental issues. The project was located in Perth Amboy, New Jersey, along the Raritan River. The site was a former steel mill and two SCB cutoff walls were needed, as part of the remedial action, to reduce the rate of contamination leaching into the groundwater and the bordering river.

The project entailed construction of two SCB cutoff walls; a northern diversion wall to divert groundwater around the site, and a southern perimeter wall needed to contain the groundwater at the site. In total, the SCB cutoff walls had an area of 226,665 sf and were 4,635 ft long. The walls, which depths ranged from 30 to 70 ft below ground surface, were constructed using a Komatsu PC1250 extended reach excavator. Scope of work also included construction of a 65 ft wide level working platform, tie-back removal, sheet pile removal, fencing, and site restoration.

Forgen completed the project on time and the client was extremely satisfied with the outcome. Future phases of work at the site include construction of two large distribution centers.

Fort Lupton, Colorado

  • Design-build project
  • Installation of 515,000 sf of slurry wall with depths up to 60 ft
  • Multiple heading for slurry wall
  • Reclamation slope construction of multiple zone fills of 1,000,000 cy
  • Completed on time and on budget

Forgen was contracted by Denver Water for the design-build of a soilbentonite groundwater cutoff wall and mine reclamation for a former sand and gravel pit. Denver Water is the oldest and largest water utility in Colorado, serving 1.4 million people in the City of Denver and many surrounding suburbs. This project was constructed to meet the Colorado State Engineer’s lining criteria for gravel pits established in 1999 and will ultimately be used as a water storage reservoir storing more than 3,000 acre feet of water. The project consisted of a soil-bentonite cutoff wall construction and final reclamation of previously mined high-wall slopes.

Forgen teamed with Deere & Ault Consulting, Inc. of Longmont, Colorado to first begin with a conventional design during the RFP stage and then bring the design phases of 30 and 60 percent to final design for the cutoff wall construction. The design phase consisted of permit research, SWPPP, drilling program, topographical surveying, slope stability analysis, and final design reports. The cutoff wall was constructed in two separate construction seasons utilizing two Komatsu PC1250 excavators equipped with extended arms and booms capable of reaching depths of 80 ft and greater. The project included the installation of 515,000 sf of slurry wall with depths up to 60 ft and an average depth of 57 ft. The installed wall exhibited a hydraulic conductivity of 1 x 10-8 cm/sec and passed the Colorado State Engineer’s design requirements for lining of gravel pits. Work associated with the slurry wall included clearing and grubbing, construction of a temporary working platform, maintaining dewatering of the gravel pit at 7,000 gpm, temporary fencing, and coordination with the mine operator throughout construction.

Forgen partnered with Denver Water to complete the design-construction of the reclamation slopes of the previously mined high-wall and pit floor. The reclamation embankment included a detailed design effort to utilize existing soils/spoils from mining, bedrock floor, soils that were available in the mine complex, and imported soils to complete the work. The reclamation embankment placed three different types of material to complete the zone fills in order to stabilize the slopes. The zone fills totaled more than 1,000,000 cy of placement utilizing an assortment of equipment, including a fleet of ten 627 scrapers, excavators, 815 compactor, GPS-enabled dozers/motor graders, and the assorted support equipment. The reclamation fill included preparing the foundation for embankment placement to depths up to 10 ft into the claystone bedrock. Scope of work also included the final restoration of the site by placing a rim road around the newly constructed water storage reservoir along with replacement of perimeter fencing, restoration of drainage features, and seeding of slopes.

The project was completed on time, on budget, and without any recordable injuries. This project was also recognized by Denver Water as the 2017 Project of the Year and Forgen was awarded the sister project at the same mine complex that will begin in the first quarter of 2018 and complete construction in December of 2018.

Sacramento, California

Forgen continued a history of award-winning performance, receiving the American Society of Civil Engineers (ASCE) 2010 Outstanding Project of the Year Award for the Natomas Levee Improvement Program. The Natomas Basin is part of the Sacramento River Flood Control Project (SRFCP), an integrated system of levees, nearby bass channels and dams, constructed to protect communities in the Sacramento Valley from flooding. The levees were constructed with unsuitable materials that proved to be extremely porous when subjected to sustained high flows.

In 2007, the Sacramento Area Flood Control Agency (SAFCA) initiated the Natomas Levee Improvement Program. This program was designed to correct levee seepage and to bring the 42-mile Natomas Basin perimeter levee system into compliance with applicable standards for urban area protection. Great Lakes E&I team members have participated in four phases of this program dating back to the program’s inception. The most recent two phases (2009 and 2010) are described below.


Forgen was subcontracted to construct a 547,500 sf soil-bentonite cutoff slurry wall to a depth of 75 ft below the crown of the Natomas Cross Canal South Levee. The cutoff slurry wall was built along 7,300 lf of the levee with a maximum specified permeability of 1×10-7 centimeters per second. Excavated soils and imported sand were utilized to meet a maximum fine content of 60 percent. Forgen constructed the cutoff slurry wall using a company-owned Komatsu PC 1250 excavator equipped with a custom built boom and stick capable of excavating to a depth of 85 ft. The wall was completed on an accelerated schedule between June and August 2009 to ensure that the general contractor could complete the larger scale levee rehabilitation by the stipulated completion date of October 31, 2009. Forgen worked around the clock, six days per week to meet the schedule, completing the project within budget and without incident.

In the fourth phase of SAFCA’s multiyear levee improvement project in the Natomas Basin, Forgen constructed an 18,300 lf soil-bentonite cutoff slurry wall along the Sacramento River East Levee adjacent to the Garden Highway. The wall was excavated to depths between 27 ft and 63 ft below grade. All excavated soils were placed along work padding adjacent to the trench for use in the soil-bentonite backfill mixture. Excavated soils containing fine content greater than 60 percent were amended with imported sand. Amendment ratios were carefully controlled to maintain specified design permeability and to eliminate the need for further bentonite addition. The project was completed on time, within budget, and without incident between July and September 2010.