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Corrective Action Plan Page 5 HYDROSTRATIGRAPHY AND GEOTECHNICAL PROPERTIES OF SOILS Hydrostratigraphy The boring logs presented in Appendix A show that the soils beneath the 152 East 11th Street property and adjacent areas are donnantly composed of clays, silty and sandy clays and sands These soil types are complexly interbedded However, over most of the area, from the ground surface to a to a depth that varies locally from some 12 to 14 ft , the soils are dominantly clays with only a few discontinuous lenses of sand Beneath these clays there is a fairly extensive stratum of sands that is typically some 11 ft thick, but which includes lenses of clays and which may be locally discontinuous in areas where silts and clays dominate the matrix Beneath the sand stratum, at a depth that varies between about 21 and 25 ft below the surface and close to the maximum depth of exploration, there is a second stratum of silty clays that appears to be arealy extensive Figure 6 is a site plan showing the 152 East 11th Street property and adjacent areas Figures 7 and 8 are hydrostratigraphic sections along the lines A'-A' and B'-B' shown on Figure 6 The hydostratigraphy shown in the cross sections is an interpretation of the stratigraphic data from the well boring logs with the sediments divided into two classes, the highly permeable sands and the comparatively less permeable silty and sandy clays This presentation makes it possible to reduce the details of the stratigraphy to a tractable degree of complexity by distinguishing between the different soil types based on properties that are of importance to the design of a groundwater remediation system (i a relative hydraulic conductivity and cohesion) Composition of Soils By necessity, the hydrostratigraphy shown on the sections in Figures 7 and 8 is somewhat generalized In actuality, the percentage of silt and sand-size material in the soils classified as "silty and sandy clays" vanes widely from location to location and local lenses of material with little or no clay are present within the strata so classified Some gravel-size material was also found in some areas Similarly, the percentage of silt- and clay-size particles in the materials classified on the sections as "sands" vanes considerably However, the sands are generally composed of fine to medium-course, sub-rounded particles with some silt and are locally very loose (less than 4 blows per foot) When the borings for wells NAV 5, 6, 7 and 8 were drilled, the sand encountered in those holes at a depths between 14 and 16 ft beneath the ground surface flowed into the hollow-stem augers (in one case, trapping the tools) (Ref 9 ) This is a significant finding because such loose sands beneath the water table would readily flow into any excavation that penetrated into them