Comparison between Clamshell and Backhoe Excavators for the Emplacement PRBs

Permeable reactive barriers (PRBs) are one of the most widespread solutions for the remediation of contaminated aquifers. Over the past 10 years, the use of iron-based PRBs has evolved from innovative to accepted standard practice for the treatment of a variety of groundwater contaminants (ITRC, 2005). Although, a variety of excavation methods have been developed, backhoe (hydraulic excavators) are commonly used for the construction of PRBs in North America. In Europe, the most common method of slurry excavation is with a hydraulic grab and crane. The aim of this study is 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. Backhoes have 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 are capable of digging as deep as 30 m. Instead, clamshell excavators require more skill to use, but are able to excavate to a depth of more than 70 m, with a high degree of precision. Two similar case studies are presented to compare the relative merits of the two excavation techniques. 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.