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REVIEWED RESUB1 BLD2023-1236+GEO LETTER+1.11.2024_3.31.00_PM+3995922GEOTECH CONSULTANTS, INC_ BLD2023-1236 Victor Martinez 21916 — 93rd Place West Edmonds, Washington 98020 via email: victormCaNgross.com RESUB Jan 11 2024 CITY OF EDMONDS DEVELOPMENT SERVICES DEPARTMENT Subject: Geotechnical Engineering Considerations Single -Family Residence ADU Addition Project 21916 — 93rd Place West Edmonds, Washington Dear Mr. Martinez: 2401 1 Oth Ave E Seattle, Washington 98102 (425)747-5618 November 20, 2023 J N 23388 REVIEWED BY CITY OF EDMONDS We understand that an ADU addition is proposed onto the northwestern portion of the existing residence at 21916 — 93rd Place West in Edmonds. The addition will have the same finish floor elevation as the residence, which is near the existing ground surface in the area of the addition. This letter is with regard to the foundation of the new addition. Based on information we have received with regards to foundation work done on the existing residence in 2005, 30 driven pipe piles were used to underpin most of the northern side of the residence, including much of the northwestern portion of the residence that will abut the new addition. The piles were driven to depths ranging from 8 to 15 feet. Because it appears that pipe piles will very likely be needed for the addition, one pipe pile was driven as a test on November 2, 2023 within the addition envelope. The 2-inch-diameter pile was driven with a 140-pound Rhino jackhammer to a refusal rate of 1 inch of driving in one minute. The pile was driven to a depth of approximately 8 feet. Thus, it appears that the use of 2-inch driven pipe piles is very suitable for the foundation of the addition. Recommendations regarding pipe piles are as follows: A 2-inch-diameter pipe pile driven with a minimum 90-pound jackhammer or a 140-pound Rhino hammer to a final penetration rate of 1-inch or less for one minute of continuous driving may be assigned an allowable compressive load of 3 tons. Extra -strong steel pipe should be used for 2- inch-diameter piles. The site soils are not highly organic, and are not located near salt water. As a result, they do not have an elevated corrosion potential. Considering this, it is our opinion that standard "black" pipe can be used, and corrosion protection, such as galvanizing, is not necessary for the pipe piles. If needed, subsequent sections of pipe can be connected with slip or threaded couplers, or they can be welded together. If slip couplers are used, they should fit snugly into the pipe sections. This may require that shims be used or that beads of welding flux be applied to the outside of the coupler. Lateral loads due to wind or seismic forces may be resisted by passive earth pressure acting on the vertical, embedded portions of the foundation. We recommend using a passive earth pressure of 300 pounds per cubic foot (pcf) for this resistance. If the ground in front of a GEOTECH CONSULTANTS, INC. Martinez November 20, 2023 J N 23388 Page 2 foundation is loose or sloping, the passive earth pressure given above will not be appropriate. We recommend a safety factor of at least 1.5 for the foundation's resistance to lateral loading, when using the above ultimate passive value. For any foundation/retaining walls, an active pressure of 35 pcf should be used for the design. LIMITATIONS The conclusions and recommendations contained in this report are based on site conditions as they existed at the time of recent pile driving and past pile driving. This report has been prepared for the exclusive use of representatives Victor Martinez and his representatives for specific application to this project and site. Our conclusions and recommendations are professional opinions derived in accordance with our understanding of current local standards of practice, and within the scope of our services. No warranty is expressed or implied. The scope of our services does not include services related to construction safety precautions, and our recommendations are not intended to direct the contractor's methods, techniques, sequences, or procedures, except as specifically described in our report for consideration in design. ADDITIONAL SERVICES Geotech Consultants, Inc. should be retained to provide geotechnical consultation, testing, and observation services during construction. This is to confirm that subsurface conditions are consistent with those indicated by our exploration, to evaluate whether earthwork and foundation construction activities comply with the general intent of the recommendations presented in this report, and to provide suggestions for design changes in the event subsurface conditions differ from those anticipated prior to the start of construction. However, our work would not include the supervision or direction of the actual work of the contractor and its employees or agents. Also, job and site safety, and dimensional measurements, will be the responsibility of the contractor. During the construction phase, we will provide geotechnical observation and testing services when requested by you or your representatives. Please be aware that we can only document site work we actually observe. It is still the responsibility of your contractor or on -site construction team to verify that our recommendations are being followed, whether we are present at the site or not. We appreciate the opportunity to be of service on this project. Please contact us if you have any questions, or if we can be of further assistance. 1' 1 • Respectfully submitted, GEOTECH CONSULTANTS, IN D. Robert Ward, P.E. Principal 023 GEOTECH CONSULTANTS, INC.