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15 March 2002 AGE-NC Project No 97-0249 Page 3of10 23 HYDROLOGIC UNITS Based on CPT data results and companson with soil descriptions from borings previously advanced on the site, AGE has developed a preliminary subsurface model of the site soil stratigraphy comprised of a vadose (unsaturated) zone and six hydrologic units (Figures 3, 4 and 5) Hydrologic units may be defined as significant vertical intervals of soil having significant lateral extent and having similar hydrologic characteristics (i a pore pressure, hydraulic conductivity, transmissivity, etc ) over its vertical and horizontal extent Although general soil distribution may vary at each boring, AGE interprets the CPT data collected from soil boring P-18 to represent the general hydrologic units beneath the site(Figure 5) The model presented is interpretational and will be modified as more data is collected The seven vertical zones identified are as follows (depths are approximate and can vary by several feet across the site) • Vadose Zone (VZ) - Located between surface grade and 10 feet bsg and characterized by variable low to high pore pressures, comprised primarily by stiff fine grained silty sand although less permeable silty clay/clayey silt occurs locally • Hydrologic Unit I (HUI) - Located between 10 feet and 35 feet bsg, characterized by fluctuating low to high pore pressures and comprised primarily by semi-permeable silty sand and sandy silt although less permeable silty clay/clayey silt occurs locally O Hydrologic Unit 2 (HU2) - Located between 35 feet and 60 feet bsg Generally characterized by low pore pressures between 35 feet and 55 feet bsg and comprised pnmanly by permeable, silty sand and fine to medium sand although less permeable silt and clay occur locally The interval between 55 feet and 60 feet bsg is generally characterized by medium to high pore pressures and comprised primarily by laterally continuous-stiff fine grained Isandy silt and silty sand • Hydrologic Unit 3 (HU3) - Located between 60 feet and 90 feet bsg Generally characterized by low pore pressures and localized medium pore pressures between 60 feet and 85 feet bsg rand comprised primanly by stiff fine grained silty sand and sandy silt although minor permeable silty clay/clayey silt occurs locally The interval between 85 feet and 90 feet bsg is generally characterized by variable medium to high pore pressures and comprised primarily by less permeable, stiff fine grained silt and clay • Hydrologic Unit 4 (HU4) - Located between 90 and 110 feet bsg Generally characterized I by low pore pressures between 90 feet and 105 feet bsg and comprised primarily by permeable silty sand and fine sand although minor permeable clay and clayey silt occurs locally The interval between 105 feet and 110 feet bsg is generally characterized by low and medium pore pressures and localized high pore pressure intervals and comprised primarily by stiff fine grained sandy silt, silt and clay Advanced Geo En viron mental,Inc