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T I � r ISite Background Information West Clay Properties Page 4 of 6 r STRATIGRAPHY IDominantly fine-grained soil consisting of silt, clayey silt, silty clay and clay were encountered at depths from the near surface to approximately 210 feet bsg Significant sand units with variable silt to gravel were encountered'between 20 feet and 40 feet bsg, 62 fe'et`to 90 feet bsg and 165 feet to 191 feet bsg The shallow sand unit(20 feet to 40 feet bsg)appears to have limited lateral continuity ' The sand unit encountered at 165 feet bsg appears to have lateral continuity and a documented thickness of greater than 40 feet Lessor, thin, laterally discontinuous sand units were encountered , at approximately 50 and 100 feet bsg at the southern UST No 3 area CPT computer-generated ' boring logs are included in Appendix B Geologic cross sections A-A' through D-D' are,included ' as Figures 5 through 8 t ' HYDROGEOLOGICAL MODEL I f 4 Based on the CPT results, AGE has developed a preliminary subsurface model of the site sediment ' ' stratigraphy comprised of informal zones including a vadose (unsaturated) zone and various hydrogeologic units Through CPT analysis, AGE has identified seven informal zones, a vadose . (unsaturated) zone and six hydrogeologic units as follows ^ 'Vadose Zone - Extending from surface grade to approximately 18 feet bsg, the depth to the water table The vadose zone is predominantly fine-grained material from surface grade to 20 feet bsg The ' zone is characterized by low pore pressures a I , 'Hydrogeologic Unit,I (HU 1), at the top of the current saturated zone, extends from 18 feet bsg to 1 ' a maximum`depth of 40 feet,bsg and is comprised predominantly of clay and fine sand,interbedded " with silt HU1 is characterized by generally low to moderate pore pressures Hydrogeologic Unit 2 (HU2) extends from a minimum,depth of 31 feet bsg to a maximum depth of 72 feet bsg and is composed dominantly bf stiff, fine-grained soils including silt and clay, interbedded with laterally discontinuous fine sand intervals HU2 is characterized by generally 1 moderate to high pore pressures ' Hydrogeologic Unit 3 (HU3) extends from a minimum depth of 62 feet bsg to a maximum depth of ,90 feet bsg, HU3 is composed primarily of laterally continuous silty sand, sand and gravelly sand interbedded"with stiff, fine-grained soil, clayey silt, silt and clay HU3 is characterized by generally ' low pore pressures, and may be classified as a flow unit ; Hydrogeologic Unit 4(HU4) extends from a minimum depth of 83'feet bsg to a maximum depth of ' 170 feet bsg, HU4' is composed primarily of fine-grained soils including clayey silt and silt interbedded with laterally discontinuous silty sand,sand and gravelly sand Minor sandy silt and clay o Advanced GeoEnvironmental,Inc e , I