11020 20TH DR SE 2025-01-13 - Laserfiche WebLink

-) i Ivo II I III RcEovE E RILEYGROUP MAY 0 a 2024 1)) CITY OF EVERETT _ April 11, 2024 a s u Permit Services Ryan Rupert 3Stripe Management(Client) JUL 2 6 2024 11217 19th Avenue Southeast — Everett,Washington 98208 CITY OF EVERETT Permit Services Subject: Silver Lake Townhomes-Updated Underpinning Recommendations 11020 20th Drive Southeast Everett,Washington RGI Project No.2013-302-4 Dear Ryan Rupert: As requested, The Riley Group, Inc. (RGI) is providing this letter summarizing our observations and recommendations regarding the settlement occurring at the above address.These recommendations are amended from our letter dated December 27th,2023 after discussions with you(the client)and structural engineers B2 Engineers. Surface Conditions It is our understanding from our discussions with RDH that a repair was made to the area with spall rock in 2020. Spall rock was encountered during a limited excavation in the center of the depression. Depth and edge conditions of the repair are unknown to RGI.Several cavities at the surface immediately adjacent to the structure have noticeably grown in size since our last visit. No cracking in exterior foundations was observed at the time of the exploration. Due to cavities forming at the southwestern corner of the structure,the foundation is undermined and not adequately supported on bearing soil. Subsurface Exploration On December 6, 2023, an RGI representative observed the advancement of 4 borings in and around the observed settlement area southwest of the structure at 11020 20t"Drive Southeast in Everett Washington. The approximate location of the borings and our previous hand augers are shown on Figure 1. RGI was previously on-site on October 20th to evaluate yard subsidence and provided a letter dated October 25, 2023 regarding recommendations for sidewalk and settlement repair, including additional recommended drilling for underpinning evaluation. The additional drilling was to determine the approximate depth of soils suitable for foundation support. The borings encountered approximately 5 to 10 feet of loose to medium dense silty sand with some gravel over dense to very dense glacial till. Underpinning Recommendations We recommend the structure be underpinned with 2-inch diameter steel pin piles to mitigate for the settlement and voids under exterior foundations observed during our site visits. Pin piles can be designed for an allowable capacity of 4-6 kips for 90 and 400 pound hammers,respectively.The allowable capacities apply to dead loads plus design live load conditions. For short-term loads,such as wind and seismic, a 1/3 increase in this allowable capacity may be used. Corporate office:17522 Bothell Way Northeast,Bothell,WA 98011 Tacoma Office:708 Broadway Suite#1008 Tacoma,WA 98402 Phone 425.415.0551•Fax 425.415.0311 www.riley-group.com 1 1 1114