10930 HOLLY DR Geotech Report 2025-08-08

Helix Design Group,Inc.—Greenwood Heating&Air Conditioning,Everett,WA April 22,2022 Geotechnical Engineering Report Z0219 3.3 Groundwater Conditions We encountered slow seepage at a depth of 3 to 31/2 feet in test pit exploration TP-1. In our opinion, this seepage is indicative of seasonally perched water,rather than actual groundwater levels. The depth of seepage roughly paralleled the contact between the hydric soils and native glacial till deposits encountered in this region. Groundwater seepage was not encountered in any of our other test pit explorations,which extended to a maximum depth of 5 feet below existing grade. Under natural conditions, runoff waters would perch atop the unweathered.till horizon and move laterally,until being picked up by Swamp Creek or other tributaries,and transported south to Lake Stickney. The project area is located within the Lake Stickney Watershed. We anticipate that seasonally perched groundwater will likely be encountered on site during times of extended wet weather along the contact between weathered and unweathered glacial till deposits. We do not expect groundwater to be encountered in deeper excavations, nor be a limiting factor in the proposed development. Groundwater levels will fluctuate with localized geology and precipitation. 3.4 Infiltration Conditions and Infiltration Rate As indicated in the Soil Conditions section of the report, the site is predominately underlain by glacial till soils. The underlying glacial till soils are generally considered an impermeable surface by most stormwater design manuals, and as indicated by the perched groundwater observed towards the northeast corner of the site,and the presence of soil mottling,it is likely the case within the subject property. Based on the geologic conditions within the subject property, we do not interpret infiltration as being feasible for this project. We recommend that site produced stormwater be diverted to an existing system along Airport Rd,or managed through detention,or another appropriate means. 3.5 Seismic Conditions The site is in the Puget Sound basin which has experienced several earthquakes. A detailed. description of the regional seismicity is beyond the scope of this report;however,previous regional earthquakes can be split into two general categories: 1.) large earthquakes with a moment magnitude greater than 8.0(Mw>8.0),and 2.)modest size earthquakes with a moment magnitude generally less than 7.25(Mw<7.25). In all cases,the thickness of the soil between the bedrock and the ground surface can change (usually amplify) the seismically induced ground motions and therefore the inertial loads acting on surface structures. "Site Class" is a classification system used by the IBC and ASCE 7 to provide some insight to the potential for ground motion amplification. The site class is based on the properties of the upper 100 feet of the soil and rock materials at the site. MGI used a combination of onsite explorations and our review of the. geologic mapping of the site to derive a site class for the site. Based on evaluation and the definitions of Site Class as provided in Table 20.3-1 of ASCE 7-16(as required by the 2018 International Building Code),the soil conditions on this site satisfy the definition of Site Class D-Stiff Soils. Our evaluation assumes the soil conditions encountered in the bottom of our explorations, and those from nearby properties,is similar to or increasing in density/consistency down to 100 feet below ground surface. Migizi Group,Inc. Page 5 of 16 lIII!