Abstract
Problem statement: Permeable Reactive Barriers (PRBs) were one of the most widespread solutions for the remediation of contaminated aquif ers. Although, a variety of excavation methods had been developed, backhoe (hydraulic excavators) were commonly used for the construction of PRBs in North America. Approach: In Europe, the most common method of slurry excavation was with a hydraulic grab and crane. The aim of this study was to compare clamshell and backhoe excavation techniques and to describe the installation of a fu ll scale PRB using a crawler crane equipped with a hydraulic grab. Results: Backhoes had been used on a larger number of PRB installations and permit a rapid rate of excavation and generally require less skill to master. Long stick backhoes were capable of digging as deep as 30 m. Instead, clamshell excavat ors require more skill to use, but were able to excavate to a depth of more than 70 m, with a high degree of precision. Two similar case studies were presented to compare the relative merits of the two excavation techniques. Conclusion/Recommendations: The first describes a funnel and gate system excava ted by long stick backhoe, in the US, whose longest gate is 0.73 m th ick, 68 m long and up to 13 m deep. The latter is a 0.6 m thick, 120 m long and 13 m deep continuous PRB, excavated by crane mounted grab to remediate a chlorinated hydrocarbons plume, in Avigliana, near the city of Torino, in Italy. Comparison of the two techniques is performed on the availability of instrumentation, excavation power and precision, potential for cost savings.
Highlights
Iron-based Permeable Reactive Barriers (PRBs) have evolved from innovative to accepted standard practice for the treatment of a variety of groundwater contaminants, which offers a simple, less costly solution to groundwater cleanup (Gillham and O’Hannesin, 1994)
As the groundwater passes through the PRB, the contaminants are precipitated, adsorbed or degraded by the millimetric zerovalent iron in the PRB with treated groundwater emerging on the down-gradient side
At the end of excavation and filling of the PRB it’s necessary to break down the biopolymer slurry by enzyme recirculation and fill the top of the trench with an impermeable cap
Summary
Iron-based PRBs have evolved from innovative to accepted standard practice for the treatment of a variety of groundwater contaminants, which offers a simple, less costly solution to groundwater cleanup (Gillham and O’Hannesin, 1994). As the groundwater passes through the PRB, the contaminants are precipitated, adsorbed or degraded by the millimetric zerovalent iron in the PRB with treated groundwater emerging on the down-gradient side. This passive type of remediation results in reduced costs due to the semi-permanent installation, lack of external energy input, reduced monitoring requirements, conservation of clean water and continued productive use of the site almost immediately after installation. New constructive techniques were studied and adapted from geotechnical field (Day et al, 1999), PRBs are most often installed using backhoe excavators
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