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October 3,2016 Single-Family Residence L&A kb No. 16-124 Page 8 The surficial topsoil, weathered soil, and advance outwash soil are of moderately-high to high permeability and would allow storm runoff to seep through easily, while the underlying transitional beds clayey silt soil is of extremely low permeability and would perch stormwater infiltrating into the ground. This perched ground water would accumulate and rise in the wet winter months and emerge out of slopes. Buildup of perched groundwater under high seepage hydraulic head would increase groundwater seepage and erode soils in the seepage zone. Prolonged erosion could result in landslide in and above the seepage zone. This is groundwater seepage and erosion induced landslide which often occurs at and around the contact between a highly permeable soil unit (such as the advance outwash) underlain by an impervious soil unit (such as the transitional beds). The potential of this type of landslides can be minimized by installation of interceptor trench drains installed on a bench or benches cut into the contact of the two soil units, as shown on Plate 6. Seismic Hazard The Puget Sound region is in an active seismic zone. The project site is underlain at shallow depth by very-dense cemented glacial till and transitional beds deposits of very- high to high shear strength. Therefore, the potential for seismic hazards, such as landslides, liquefaction, lateral soil spreading, to occur on the site should be minimal if the erosion mitigation, drainage control, site stabilization measures recommended in this report are fully implemented The proposed residence, however, should be designed for seismic forces induced by strong earthquakes. Based on the soil conditions encountered by the test pits, it is our opinion that Seismic Use Group I and Site Class D should be used in the seismic design of the proposed residences in accordance with the 2012 IU 8z ASSOCIATES, INC.