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f t <br />Natalia Subbotnikova [EH] <br />From: <br />Robert McClellon [EH] <br />Sent: <br />Monday, September 11, 2006 8:37 AM <br />To: <br />Natalia Subbotnikova [EH] <br />Subject: <br />FW: Passive Soil Vent Plan <br />-----Original Message ----- <br />From: Jay_Kamine@URSCorp.com [mailto:Jay_Kamine@URSCorp.com] <br />Sent: Monday, September 11, 2006 7:34 AM <br />To: Robert McClellon [EH] <br />Cc: Judy McCampbell <br />Subject: RE: Passive Soil Vent Plan <br />Robert <br />The following responses are provided to address your comments below and are <br />based on our conversation of September 8, 2006: <br />1. Although an impermeable membrane was considered to be installed for <br />the passive soil vapor extraction trench, there were several factors <br />that support our decision for the passive soil vapor extraction <br />trench to be installed without a impermeable membrane and included <br />the following: <br />Of the 10 gas -migration monitoring probes located along the <br />perimeter of the site, methane has been measured above 5 percent <br />in only three of them (VW -4, VW -5, and VW -6) since the start of <br />the quarterly monitoring activities. The passive soil vapor <br />extraction trench was designed to extend beyond these probes. <br />Based on the information collected during our investigations at <br />the site, landfill material was found to be within approximately 8 <br />feet of the property boundary. Since the trench was to be <br />constructed within the site boundary, there was a potential for <br />construction debris to be encountered along portions of the trench <br />during trench construction. Several construction methods were <br />considered and included: traditional excavation methods and <br />one -pass excavation methods. The traditional excavation method <br />was selected since there was a potential for construction debris <br />to be encountered during trench construction and the size and type <br />of construction material within the landfill was largely unknown. <br />The one -pass trenching method has been ineffective if large <br />materials are encountered during trench construction. Several <br />traditional construction methods were considered and included: <br />sheet piling, excavation from the surface of the site, and <br />excavation from a bench at 7 feet below the ground surface. <br />Installation of sheet piles has been unsuccessful where large <br />construction debris was encountered. Portions of the soil <br />material near the surface of the site along the trench location <br />appeared to be non -cohesive and this non -cohesive material could <br />collapse into the excavation during trench construction so it was <br />decided to excavate the trench from a bench at 7 feet below the <br />ground surface. Based on the geotechnical properties of the soil <br />at the site, a minimum of 2 (horizontal) to 1 (vertical) slope <br />from the edge of the property to the bottom of the bench was <br />established for safety considerations. Since there was a <br />potential for the trench to be installed within the landfill <br />material, the passive soil vapor extraction trench was designed to <br />be installed without a impermeable membrane. If a permeable <br />membrane was installed, any methane being generated from the <br />