4328 HOYT AVE Geotech Report 2022-10-25

Geotechnical Engineering Evaluation—REVISED NGA File No.1148519 Staniford Residence Remodel June 23,2020 Everett,Washington Page 6 It is our understanding that the homeowner only wants to underpin the western and portions of the northern and southern exterior foundation lines, as well as all foundation elements for the planned additions. In our opinion,this approach should be feasible from a geotechnical standpoint provided the homeowner is aware of the potential for future differential settlement between the planned underpinned portions of the residence/additions and the unimproved portions of the residence and this information is passed on to any potential buyer if the property should be sold. We recommend that the foundation elements be monitored for signs of settlement after the planned improvements have been made to identify if other foundation elements need to be improved or supported on deep foundation systems. We are available to provide additional consultation if distress to unimproved portions of the residence foundations should arise. We also recommend that all drainage features associated with this residence, such as roof downspouts, yard drains, footing drains, and runoff from any hard surfaces, be investigated and improved such that all runoff generated on site is tightlined into an approved system. This is key to maintaining long-term stability of the site and reducing the potential for further settling. Based on our observations, our understanding of the existing foundation conditions, and the significant undocumented fill and organic-rich soils encountered in our explorations, it is our opinion that the soils will likely experience settlement and subsidence under the proposed loads, and at an increased rate as a result of an earthquake or significant amount of rainfall, which could lead to more distress to the foundation and structure. To mitigate this condition and support the proposed residence additions, we recommend the residence foundation be underpinned by a deep foundation system, such as small diameter pin piles. We recommend the proposed addition foundations be supported on deep foundations consisting of driven pin piles, to transfer the building loads through the loose undocumented fill down to the underlying competent native soils. Your structural engineer should then consult with us to determine the extent of foundation areas to be underpinned, and overall layout of the underpinning system. In addition to underpinning the new proposed additions we also recommend supporting, at a minimum the western portion of the existing residence structure. We should review underpinning plans prior to implementation. NELSON GEOTECHNICAL ASSOCIATES,INC.