1029 52ND ST SW Geotech Report 2022-10-31

o 4.4 SEISMICITY 4.4.1 General Washington State is situated at a convergent continental margin and is susceptible to subduction zone, intraplate, and shallow crustal source earthquakes. We reviewed published geologic maps for the sitevicinity•to evaluate,seismic activity hazard areas.The site is approximately 1.6 miles east of the north border of the Southern Whidbey Island fault zone(DNR, 2018). 4.4.2 Geologic Hazards The site is generally gently sloping and is underlain at shallow depths by dense to very dense glacial till. Risks associated with landslide hazards,slope stability, liquefaction, lateral spreading, or surficial rupture are low`for'this site. Based on the topography, there are no slopes on site that meet the City Land Use Code Chapter 19.37.080 definition for Geologically Hazard Areas on or adjacent to the site. 5.0 INFILTRATION TESTING Small-scale PITs were performed at two of the test pit locations in general accordance with the 2014 Stormwater Management Manual for Western Washington (DOE, 2014). The test pits were generally rectangular and approximately 3 feet wide by 5 feet long. The PIT was performed near the anticipated bottom of infiltration elements at a depth of 1.5 feet BGS. Soil conditions encountered at the base of the infiltration test consisted of a silty sand with gravel fill material. An electronic pressure transducer and data logger were placed in each test pit to measure groundwater levels at regularshort-term'intervals throughout the saturation period and during the tests. The tests were repeated as time and the infiltration rate permitted. Up to approximately 12 to 18 inches of water was established in each test pit during the tests. The infiltration rate measurednear the.end of each test,which allows for the longest saturation period, is used to calculate the short-term infiltration rate, as summarized in Table 1. Table]. Soil Infiltration Rate Analysis Averaged Measured Test Short-Term Infiltration Soil Type Depth Location Infiltration Rate (feet (inches per hour) Medium dense, silty Sand 1.5 6.0 TP-3 with gravel - Fill Medium dense, silty Sand 1.5 3.7 TP-5 with gravel-Fill 6.0 LABORATORY'TESTING Laboratory testing was conducted on specific soil samples collected from the explorations to assist in the characterization of certain physical parameters of the soil. CEC and organic content tests were completed on samples collected at or near the anticipated base of the LID infiltration C�DESIGN3 3 NWHome-1-01:1 01618