Design and installation of PRB / low permeability cut off wall to protect SSSI
Site Background & History
The site was located at Dunton Green in Kent.
The site was formerly operated as the brick and tile Works. Several large brick pits were excavated across the site. These pits were infilled with building rubble on cessation of the manufacturing operation. The site was subsequently operated as a cold storage facility comprising of eight large warehouse buildings.
The site is located close to Sevenoaks Wildlife Reserve, a SSSI Site is located to the east.
The identified contamination was consistent with previous site usage and comprised of metallic / hydrocarbon pollutants and asbestos. The brick pits contained contaminated fill material which was also polluting the groundwater.
Remediation Strategy
The overall site remediation strategy included free product removal, dewatering and on-site water treatment, bioremediation and the installation of a soil mixed barrier system (PRB).
The soil mixed barrier system (PRB) comprised of an E-Clay permeable reactive barrier section and low permeability sections. The barrier system was developed by Envirotreat to protect the SSSI lakes from the potential impact of contaminated groundwater primarily emanating from the identified source within the large brick pit.
The groundwater within the pit was generally perched and was contaminated with significantly elevated concentrations of Polycyclic Aromatic Hydrocarbons [PAH’s] and Total Petroleum Hydrocarbons [TPH]. The identified risk with the groundwater was as a consequence of levels potentially rising within the pit (due to the probable ingress of surrounding shallow groundwater). The preferred remediation solution was to allow the natural filling of the pit to occur and to address the resulting groundwater plume by containment and treatment utilising a soil mixed barrier system. The low permeability sections were designed to encompass the pit area and to channel the groundwater in the direction of the downstream permeable reactive barrier section, designed to intercept and treat dissolved phase contamination within the groundwater.
Remediation Works
The shallower low permeability sections were installed by trenching techniques. The deeper low permeability and reactive barrier sections were installed by soil mixing techniques utilising a continuous flight auger [CFA]. This application technique produced a series of overlapping soil mixed columns, installed to the required depth. The CFA soil mixed barrier installation is shown below (green = permeable reactive barrier, yellow = low permeability cut off wall).
The CFA barrier installation comprised of 406 columns in total – 358 low permeability columns and 48 permeable reactive columns. The low permeability columns were formed by in-situ mixing of soils with a cementitious / pozzolanic / bentonite slurry. The permeable reactive columns were formed by in-situ mixing of soils with a pillared reactive organoclay (E-Clay)
Validation
Groundwater levels and quality were monitored over a 9 month period.
