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i ' 1444 MARIPOSA ROAD WKA No 2938 06 September 6, 2000 Page 5 Vapor Extraction System In December of 1993 personnel from U S Rentals contacted our office to explam that they would like to backfill the excavation to eliminate the physical hazard Because contaminated soils were still present in the bottom of the excavation, we proposed that a network of 4-inch ' diameter perforated pipes be placed in the bottom of the pit and connected to vertical'risers A workplan addendum was submitted to San Joaquin County on December 1, 1993, which ' discussed our proposal to backfill the excavation after mstallmg the proposed vapor extraction network The purpose of this network was to utilize the depth of the existing excavation to provide a means of vapor extraction should it be required in the future The vapor extraction system was constructed by placing an approximate three-foot layer of clean pea gravel in the bottom of the excavation to level it Once the pea gravel was in place, three 15 x 15 foot squares of 4-mch diameter PVC perforated piping were laid horizontally in the excavation Once risers were attached,pea gravel was used to provide an additional two feet of cover The pea gravel was then covered with a layer of geotextile then topped with the material ' on excavated from the tank pit During the backfill process, the soil was moisture treated and compacted in one to two foot lifts by the contractor Our firm did not perform compaction testing or supervision. 1 One riser extended to the surface from each of the horizontal fames (Three total) The horizontal portion of the framework consisted of a 15-foot square of 4-inch perforated1pipe The risers were also constructed of 4-mch diameter perforated pipe A five-foot long 12-inch ' diameter conductor casing was used during construction of the risers to place an annular gravel pack of pea gravel The conductor casing was filled with gravel once soil had been compacted ' around it After the soil was compacted around the conductor and it was filled with pea gravel, it was lifted two or three feet where the process was repeated The resulting feature consisted of a 4-inch diameter casing surrounded by a gravel pack of clean pea gravel Because the conductor casing was removed during backfill, none remained upon completion This design allowed vapors to be removed from the backfilled material should it be required in the future WALLACE KUHL R-)&-d hnper Er ASSOCIATES INC