Significant features of this funnel and gate groundwater control system:
- 1,516 feet of Soil-Bentonite underground barrier wall
- Two (2) modified aggregate filled gates totaling 160 feet of underground permeable reactive (PRB) barrier wall.
- Ten injection wells installed in the PRB gates
Background:
This site is located on Andrews AFB (AAFB) in Northern Maryland, just outside Washington, DC. AAFB is home to the 89th Airlift Wing as well as the 79th Medical Wing of the United States Air Force (USAF). The USAF contracted URS Corporation (URS) through The Air Force Center of Engineering Excellence (AFCEE) to support a remedial action at the LF-05 site, a former landfill on base also known as the Leroy’s Lane Landfill. Geo-Solutions was contracted to perform the underground barrier and PRB installation. Geo-Solutions worked with URS engineers and personnel in Herndon, Virginia, Pittsburgh, Pennsylvania and Salt Lake City, Utah to bring this project to the field.
Geo-Solutions’ work on the project is scheduled to commence after initial site work is completed by URS personnel. The funnel and gate system at Andrews Air Force Base (AAFB) was designed to be installed to an invert elevation of 224 feet above mean sea level, approximately 37 feet below most of the prepared work platform. The three funnel portions of the application were designed to be installed as low permeability soil-bentonite (SB) slurry walls. Two gate sections are to be installed as permeable reactive barriers (PRBs) utilizing the bio-polymer slurry wall method. Excavation depths of up to 41 feet below the ground surface (bgs) are expected to account for deviations in the final elevation of the work pad prepared by URS. Geo-Solutions executed excavation of both the funnel and gate portions of the project to the designed depths using a long-reach hydraulic excavator. The overall construction approach consists of the following definable features of work:
- Mobilization
- Site Preparation – Includes the installation of water handling as well as slurry batching and delivery systems.
- SB Trench Installation – Includes excavation, spoil handling, preparation and placement of bentonite slurry as well as backfill mixing and placement.
- Trench Delineation – Includes the temporary placement of delineating beams between the SB and PRB portions of the trench.
- PRB Trench Gate Installation – Includes excavation, spoil handling, placement of imported sand and gravel fill, degradation of bio-polymer slurry, trench development, removal of the delineation beams and installation of the specified injection wells.
- General Cleanup – Includes the cleanup of Geo-Solutions’ operation.
- Demobilization
Project Objectives:
- Provide the means by which to contain groundwater and leachate collecting and flowing in and around the landfill perimeter
- Provide the ability to control the contained groundwater through two PRB gates
- Install the means to remediate the controlled groundwater via ten injection wells.
- Complete 57,000 vertical square feet of soil-bentonite slurry wall
- Complete 6,000 vertical square feet of permeable reactive barrier slurry wall installed by the bio-polymer slurry method
- Excavate 7,000 cubic yards of material, over 10,000 tons of earth and debris
- Install of a modified gravel and sand mixture exhibiting the exact standard to allow the injected reagents to remain in contact with groundwater for the optimal time of remediation
- Soil-bentonite slurry wall specified permeability of 1×10-7
- Perform a mix design and compatibility testing
- Provide design support
Project Challenges:
- Working underground in a former military landfill
- Maneuvering underground operations past known and unknown subgrade obstructions while maintaining the integrity of the installation
- Working in potentially hazardous subgrade conditions
- Meeting a Fast Track Schedule.
- Mitigating the effects of a high water table on construction
