Clamshell vs. Backhoe Excavation of Permeable Reactive Barriers

Problem statement: Permeable Reactive Barriers (PRBs) were one of the most widespread solutions for the remediation of contaminated aquifers. 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 full 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 excavators 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 werepresented to compare the relative merits of the two excavation techniques. Conclusion/Recommendations: The first describes a funnel and gate system excavated by long stick backhoe, in the US, whose longest gate is 0.73 m thick, 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.