1528 GRAND AVE Geotech Report 2022-10-25

GeoTest Services, Inc. Report Date 7/17/2019 Anderson Residence- Everett, Washington GTS Project No.19-0384 saisaim groundwater or groundwater seepage is present, and the trench is not properly dewatered, the soil within the trench zone may be prone to caving, channeling, and running. Trench widths may be substantially wider than under dewatered conditions. Stormwater Infiltration Potential Based on the presence of predominantly granular materials and the lack of a restrictive layer that would otherwise impact infiltration facilities, it is our opinion that the on-site infiltration of stormwater is feasible for this project site. The proximity of the slope to planned construction, however, presents elevated risks to the slope should significant amounts of water be collected, held,and infiltrated near the top-of- slope condition. GTS recommends that we be allowed to review stormwater concepts with the specific goal of limiting infiltration and/or dispersion concepts near the top- of-slope condition. Test Pit Gradation Results From the explorations excavated in the areas of interest, two representative soil samples were selected and mechanically tested for grain size distribution and infiltration rate calculation according to the soil grain size analysis method, Section 3.3.6 of the 2012 Storm water Management Manual for Western Washington (SMMWW),amended December,2014.The test results and calculated infiltration rates are provided in Table 1 below. Table 1 Preliminary Infiltration Results Based,on Grain Size Analysis Test Pit ID Uncorrected Ksat Calculated Ksat & Depth Geologic Unit Infiltration Rate Infiltration Rate [i n/h r] [i n/h r] HA-2 @ 2 ft Fill 18.4 5.3 HA-2 @ 10 ft Recessional 60.2 17.3* Outwash HA-3 @ 6.5 ft Recessional 81.3 23.4* Outwash Notes: -Ksat = Initial Saturated Hydraulic Conductivity -Correction Factors Used: CFv= 0.8, CFt=0.40,CFm =0.9,Total Correction Factor =0.288 *GTS does not recommend using infiltration rates greater than 10 inches per hour for the grain size analysis approach. Should infiltration rates greater than 10 in/hr be desired, GTS recommends performing a Pilot Infiltration Test to establish design rates. 15