202 ALDER ST Geotech Report 2018-01-02 MF Import

1 ' Proposed View It�dye Subsurface F_cplorafion, Geologic Hazards, Inj�lvatian Poten[ial, Elrrnenrary Srhool Reptucemen� and Preliminary Georerhnical£ngineenng Repon Eve�rtt. Washm ton Geolagic Hazards and Mirigarions ' The sediments produced during the recent glaciation of Puget Sound are, howe��er, most conunonly the suspended soils that are carried by site storm water. The fine-grained fraction ' of the glacially derived soil is referred to as "rock flour," which is primarily a silt-sized particle with no electrical charge. These particles, once suspended in water, may have �ettling times in periods of months. , Therefore, the flow length within a temporary sedimeN control trap or pond has virtually no affect on the water quality of the discharge, since sedunent will not settle out of suspension in ' the time it takes to flow from one end of the pond to the other. Reduction of turbidiry from a construction site is almost entirely a function of cover measures and flow control. Temporary , sediment traps and ponds are necessary to control the release rate of the runoff and to provide a catchrt►ent for sand-sized and larger soil particles, but are very ineffective at reducing the [urbidity of the runoff. ' To mi[igate the erosion hazards and potential for off-site sediment transport, we recommend the following: 1 i. ?he winter performance of a site is dependent on a well-conceived plan for control of site erosion and storm water runoff. It is easier to keep the soil on the ground than to � remove it from storm water. The owner and the design team should include adequate ground-cover measures, access roads, and staging azeas in the project bid to give the , selected convactor a workable site. The selected contractor needs to be prepared to unptemen[ and maintain the required measures to reduce the amount of exposed ground. A site maintenance plan should be in place in the event storm water turbidiry ' measurements are greater than the Ecology standazds. 2. AI( TESC measures for a given azea to be graded or otherwise worked should be ' installed prior to any activiry within that area. The recommended sequence of construction within a given area would be to install sediment traps and/or ponds and establish perimeter flow control prior to starting mass grading. t3. During the wetter months of the year, or when large storm events are predicted during the summer months, each work area should be stabilized so that if showers occur, the ' work area can receive the rainfall without excessive erosion or sediment transport. The required measures for an area to be "buttoned-up" will depend on the tune of year and ' the duration the area will be left un-worked During ,he winter months, areas that are I to be left un-worked for more than 2 days should be mulched or covered with plastic. During the summer months, stabilization will usually consist of seal-rolling the ' subgrade. Such measures wil{ aid in the contractor's abiliry ro get back into a work I area after a storm event. The stabilization process also includes establishing remporary ' September 22, 10l0 ASSOCIATED EARTN SGENCES, INC. BwGrtD-KElOIi153n2�Orojrrra�a0lpIZSSII:EiWP Page 10 1