3000 41ST ST Geotech Report 2025-12-31

PNW Home Builders_, LLC ES-2598.03 May 2, 2013 Page 9 Lateral Spreading Lateral spreading is the lateral displacement of liquefied (loose and saturated) soils that occurs during an earthquake or other intense ground shaking. Lateral spreading is potentially present near slopes_ or free faces and results in generally horizontal movement of those non-liquefied soils above liquefied soils. If liquefaction within the younger alluvium deposits were to occur, it is our opinion that the non- liquefied fill soils would have a, low susceptibility for lateral spreading. The referenced geotechnical report suggests that lateral spreading will generally occur within 100 to 200 feet of the Snohomish River during a seismic event, and that lateral spread displacements. 'are expected to develop east of the subject site. Anticipated lateral spreading would generally be concentrated near and along the river banks. Slab-On-Grade Floors Slab-on-grade floors for residential buildings constructed at this site should be supported on a firm and unyielding subgrade. Where feasible, the existing native soils exposed at the slab-on- grade subgrade level can be compacted in place to the specifications of structural fill. Unstable or yielding areas of the subgrade should be recompacted or overexcavated and replaced with suitable structural fill prior to construction of the slab. A capillary break consisting of a minimum of four inches of free-draining crushed rock or gravel should be placed below the slab. The free- draining material should have a fines content of five percent or less (percent passing the Number 200 sieve, based on the minus three-quarter inch fraction). In areas where slab moisture is undesirable, installation of a vapor barrier below the slab should be considered. if a vapor barrier is to be utilized it should be a material specifically designed for use as a vapor barrier and should be installed in accordance with the manufacturer's specifications. Drainage Groundwater seepage was reported at all five boring locations at depths of approximately 3 to. 14 feet below existing grades at the time of the fieldwork (January 2007). As such, the presence of .groundwater seepage should be anticipated, especially in deeper utility excavations. Temporary measures to control surface water runoff during construction would likely involve interceptor trenches and sumps, as necessary. Based on the soil and groundwater conditions observed at the test sites, dewatering of excavations extending three feet or more below existing grades may be necessary. In our opinion, foundation_drains should be installed along building perimeter footings. A typical foundation drain detail is provided on Plate 4. Finish grades must be designed to direct drainage water away from structures and slopes. Water must not be allowed to pond adjacent to structures or slopes. Earth Solutions NW,LLC