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Envirocon was awarded a subcontract as a teaming partner to install a permeable reactive barrier (PRB) and slurry wall at the former Kelly AFB in San Antonio, Texas for a Small Business Prime Contractor. The Prime Contractor was awarded this contract from the U.S. Air Force (USAF) under their AFCEE WERC contract. The project included the construction of a slurry wall to direct a TCE-contaminated groundwater plume to the PRB for treatment.
Envirocon provided expertise in working with the client to modify the alignment of the PRB. Although the realignment required a deeper excavation for the PRB, it significantly reduced preparation, restoration, construction complexity, and risks associated with constructing the PRB in a flood plain. In addition, the realignment provided similar benefits for the slurry wall construction, while reducing the original slurry wall length by approximately 100 feet and eliminating a significant grade change.
A PRB consists of a zone of reactive material (i.e., a mixture of zero-valiant iron and sand) installed underground in the path of a contaminated groundwater plume. As the groundwater flows through the zone under its natural gradient, it contacts the reactive material which acts as a passive treatment system. PRBs are designed to provide adequate residence time in the reactive zone for the degradation of the parent contaminants as well as their toxic daughter products.
The PRB was 940-feet long, 3-feet wide, with a maximum depth of 40 feet, keying 2 feet into the "difficult" Navarro Clay aquiclude. The approximate total surface area of the PRB was 37,600 square feet. The PRB excavation was performed using the bio-polymer (BP) technique. BP trenching is a variation of the "slurry trench technique," which involves excavating a narrow trench that is kept full of "BP slurry." The slurry exerts hydraulic pressure against the trench walls and acts as shoring to prevent collapse. Traditional slurry trenches are excavated under bentonite or other clay slurries, which form "filter cakes" on the trench walls and reduce the permeability of the soil. Conversely, BP trenches are excavated under "biodegradable slurries" that do not affect the final permeability of the in-situ soil or permeable backfill media. Envirocon utilized a Cat 365 with an extended stick to perform the BP excavation.
The PRB included three sections backfilled with various granular iron contents, including 40%/60% and 60%/40% mixes of granular iron and sand by volume, as well as a 100% granular iron section. The average height of the granular iron and mixture was approximately 15.5 feet. The reactive backfill mixture was produced on site by loading the required proportions of granular iron and sand into ready-mix trucks with a conveyor system. The mixture was transported to the excavation in ready-mix trucks and discharged into the excavation via a telescoping conveyor and a tremie pipe. After the completion of each PRB segment, the reactive material was "developed" to remove residual BP slurry and to reestablish permeability thereby activating the PRB.
Envirocon received a Letter of Commendation from the Prime Contractor based on our role in making this project such as success for the USAF. The Prime Contractor was awarded a “Blue” rating by the USAF for its excellent performance.
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