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1444 MARIPOSA ROAD <br /> WKA No 2938 05 <br /> August 26, 1999 <br /> Page 6 <br /> Stockpile#2 was generated during the construction of an access ramp and was therefore not <br /> sampled <br /> Vapor Extraction System <br /> In December of 1993 personnel from U S Rentals contacted our office to explain that they <br /> would like to backfill the excavation to eliminate the physical hazard <br /> ' Because contaminated soils were still present in the bottom of the excavation, we proposed that a <br /> network of 4-mch diameter perforated pipes be placed in the bottom of the pit and connected to <br /> vertical risers A workplan addendum was submitted to San Joaquin County on December 1, <br /> 1993 which discussed our proposal to backfill the excavation after installing the proposed vapor <br /> ' extraction network The purpose of this network was to utilize the depth of the existing <br /> excavation to provide a means of vapor extraction should it be required in the future <br /> The vapor extraction system was constructed by placing an approximate three foot layer of clean <br /> pea gravel in the bottom of the excavation to level it Once the pea gravel was in place, three 15 x <br /> 15 foot squares of 4-inch diameter PVC perforated piping were laid horizontally in the <br /> excavation Once risers were attached, pea gravel was used to provide an additional two feet of <br /> cover The pea gravel was then covered with a layer of geotextile then topped with the material <br /> ' originally excavated from the tank pit During the backfill process, the soil was moisture treated <br /> and compacted in one to two foot lifts by the contractor No compaction testing or supervision <br /> was completed by our firm <br /> One riser extended to the surface from each of the horizontal frames (Three total) The <br /> horizontal portion of the framework consisted of a 15 foot square of 4-mch perforated pipe The <br /> risers were also constructed of 4-inch diameter perforated pipe and were constructed using a five- <br /> foot long 12-mch diameter conductor casing filled with pea gravel The conductor casing was <br /> filled with gravel once soil had been compacted around it After the soil was compacted around <br /> the conductor and 1t was filled with pea gravel, it was lifted two or three feet where the process <br /> was repeated The resulting feature consisted of a 4-inch diameter casing surrounded by a gravel <br /> '��W <br /> WALLACE KUHL <br /> Recycled paper &ASSOCIATES INC <br />