17 E MARINE VIEW DR 2019 Geotech Report 2025-10-01

Lateral Spreading In addition to liquefaction induced settlements, there is the possibility that lateral spreading of the soils could occur as a result of liquefaction. Lateral spreading involves lateral displacement of large, surficial blocks of non-liquefied soil, as well as the liquefied soil, as the underlying soil layer liquefies and loses strength. Lateral spreading generally develops in areas where sloping ground is present or near a free face, such as in this instance adjacent to the river channel. If liquefaction were to occur within the sands at the site, we anticipate that there would be a moderate potential for lateral spreading to occur, characterized by movement of the soils towards the Snohomish River. In general, the magnitude of lateral spread will decrease with increasing distance between the point of interest and the Snohomish River. During a design earthquake, lateral spread displacements across the site are estimated to be on the order of 5 to 12 feet. The design recommendations for the structural foundations presented in subsequent sections of this report do not take into the account the effects of liquefaction and lateral spreading. Measures to mitigate theses effects include densifying/reinforcing the liquefiable soils or constructing a foundation system that will resist differential settlement or limit lateral movement of the foundation. Remediation would most likely consist of densifying the sand deposits with methods such vibro-compaction or installing stone columns. These mitigation measures are typically very costly and not commonly accomplished for projects of this type. We recommend that Cadman Materials, Inc. (Cadman) be made aware of the risks associated with liquefaction and lateral spreading at this site. We can provide additional recommendations for mitigating the seismic hazards at the site upon request. Fault Rupture The site is located approximately 7 miles northeast of the northern trace of the Southern Whidbey Island fault zone. Based on our knowledge of regional geology in the vicinity of the site,the substantial thickness of glacial and postglacial sediments beneath the site, and the distance to the nearest known fault, we conclude the potential for surface fault rupture is remote. Preload and Surcharge Fill We recommend placing a preload and surcharge fill in the area of the planned soil remediation shed structures and the recycled asphalt building and other settlement-sensitive structures.The purpose of the preload and surcharge fill is to pre-induce a major portion of the settlement that would otherwise occur when the planned stockpiles and structure loads are applied. Preload and Surcharge Height For planning purposes, we suggest a preloading program with heights matching planned stockpiles of 25 feet for the recycled asphalt product pile and soil remediation plant and storage shed buildings. An additional 5-foot-high surcharge is also recommended. Areas surcharged to 30 feet will pre-induce settlement caused by the planned stockpiles.The preload and surcharge height(30 feet) is the thickness of the preload and surcharge fill that is placed above the planned finished grade elevation.The crest of the surcharge should extend laterally at least 5 feet beyond the footprint of the structures in each direction. The preload and surcharge fill should consist of structural fill quality material such as the existing recycled asphalt compacted only to the extent necessary to support construction equipment.Side slopes should be planned no steeper that 1H:1V(horizontal to vertical).The surcharge surface should be crowned slightly to GEOENGINEERS/ AuguSt3,2018 Page8 file No.0643-015 00