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AES #1.pdf66129/2005 17:88 4258275424 AESI PAGE 02/65 Associated Earth Sciences, (nc, June 29, 2005 Project No, KE03537B City of Edmonds Community Services Department 121 5`h Avenue North Edmonds, Washington 98020 . Attention: Ms. Jeannine Graf RECEIVED Building Official J U N 3 0 2005 Subject; Geotechnical Plan. Review BUILDING DEPT, Michel Residence 15911 74"' Place- West Edmonds, Washington References: Geotechnical Review — Proposed New Residence Dennis, M. Bruce, P.E., May 27, 2005 Geotechnical Evaluation and Foundation Recommendations Dennis M. Bruce, P.E., March 29,,2001 15911 74t1i Ave West Declaration of Risk Custom Design and Engineering, Inc., May 24, 2005 Storm Drainage Detention Report Donna L. Breske; P.E., May 19, 2005 Grading, Drainage, Paving, TESL plan, Sheet 1 of I Donna L Breske, P.E., dated 5/20/04 (stamped 5-23-05) ,Sheet, P I McGrew Michel Building Design, 5.25,05 Topography Survey, Sheet 1 of 1 C & C Surveying, 9127102 Architectural Plans McGrew Michel, dated 5,25.05 Kirkland 9f I Flfth Avenue, Suite 100 -Kirkland, WA98023 - Phone 4),5 827.7701 -Fax 425 827-5424 Everett 291: 1/z HewK1 Ave„ 5uitc 2, - EveretLWA 987-01 • f' hone 425 259-0522 - Fax 425 252-3449 06/29/2005 17:68 4258275424 AESI PAGE 03/05 Dear Ms. Graf- As raf As requested, Associated Earth Scienccs, Inc. (AESI) has completed a geotechnical review of tike above -referenced geotechnical report and development plans for the subject Michel residence. Our geotechnical review was completed with respect to the Meadowdale Earth Subsidence Landslide .Hazard Area Checklist criteria as contained in the City of Edmonds Ordinance 4 2561 and the Edmonds Community Development Code (ECDC) Chapter 19.05. Project Summary AESI previously completed similar geotechnical reviews for the adjacent Hillman lot and Michel lot at 15915 and 19517 70' Place West, respectively. The above -referenced geotechnical report contains information indicating that both the Hillman and Michel lots are underlain by landslide debris and may be susceptible to deep-seated landsli,ding and foundation settlement, and shallow - seated debris flow landslides originating on the adjacent steep slopes. As such, pile foundation support, debris flow protection, and surface drainage control were recommended for the lots at 15915 and 15917. and are also recommended for the subject 15911 Michel lot. Review of the above -referenced plans indicates that the residence will be constructed by completing a. tiered bench cut within the eastern section, and filling within the western section. The out will be completed within the gently sloping area near the base of the steep bluff slope forming the eastern section of the lot. Maximum cut slope heights will range between approximately 5 to 9 feet, and maximum fill depths will be approximately 2 feet. The residence will be supported. on a pile foundation consisting of4-inch-diameter pipe piles and grade beams. A two-tiered rockery wall with tiers up to 7 feet high will be constructed along the north and east margin,, of the lot to provide driveway access. Review Comments It is AESI's opinion. that the most critical geotechnical issues associated with development of the site consists of landslide hazard mitigation, foundation support within the existing landslide debris. and site grading and drainage. A discussion and our conclusions regarding each issue are presented below, Review comments arc shown in italics. Landslide Hazards The general landslide hazard risk at the site has been defined by the GeoEngineers Landslide Razard Map as ranging between 2 to 30 percent. project geotechD!cal information noted that fractured silt zones were encountered at depths of 9 to 20 feet, in that the site has experienced deep-seated Iandsliding in the past. Medium dense to dense soils located below 25 feet do not appear to have been disturbed by laadsliding. No definite signs of current deep- seated landsliding are present. Gillen these site conditions, no change in the current landslide hazard risk designalion appears necessary, Since the landslide hazard risk does not exceed 30 percent, site development world not be restricted. 06/29/2005 17:08 4258275424 AESI PAGE 04/05 The upper 15 feet of soil appears to consist of unconsolidated landslide debris deposits with entrained peat and woody debris. This would indicate that the site does experience debris flow types of Iandslides within the site's colluvial slope soils, which, reach onto the proposed building site. To mitigate potential debris flow landsliding, we propose extending the garage foundation wall at least 4.feei above finished grades along the east side. To mitigate the potential for foundation damage due to the effects of deep-seated landsliding, we propose that the foundations should extend through the .sheared, fractured silt zone and gain bearing in the undisturbed soils 25 feet below the existing ground surface. The deep foundations would also be required to supporr the residence above the upper 15 feet of unconsolidated debris deposits. Foundation Support As mentioned above. the upper 15 Feet of soil within the proposed building area appears to consist of unconsoIidated landslide debris deposits with entrained peat and woody debris. Thus, this material wotdd not provide suitable bearing support for conventional spread footing foundations, due to the potential for long -terra differential settlements. To mitigate these poor bearing conditions, deep foundations bearing in the undisturbed soils as recommended above should be utilized. Review of the project structural engineering plans by Custom Design and Engineering, Sheets s- 5, stamped May 25, 2005 show that the residence will be supported on 4 -inch -diameter driven pipe piles. Notes should be added to these sheets calling out that pile hearing should be within the undisturbed soils at a minimum depth of 25 feet below the existing ground surface and he inspected by the project geotechnical engineer. Additionally, allowable axial compressive capacities of the pipe piles, testing requirements, and coupling details or notes should he indicated on the plans. Site Gradintr and Drainage Im2rovernents Given the variable consistency and loose to medium dense, unconsolidated, condition of the surfiicial site soils, the planned excavation slopes are likely to be unstable and may require laying back the excavation face at 1.5H:1 V (Horizontal: Vertical), or shallower as necessary. Notes should be added specifying that the project geotechnical engineer .should be on .site to evaluate the .suitability of any temporary slope cuts over 4 feet high. The above -referenced plans show a proposed iwo-tiered rockery system with labeled tier heights up to 7 feet high and calculated tier heights rap to 9 feet high based on the grading plan. A City of E•dnzonds "Tiered Rockeries Detail" is included on the grading plan with a note indicating that the rockery tiers over 8 feet high roust be designed and stamped by a licensed professional engineer, or if the City determines (liar special conditions exist, such as the site being located in a critical area or the Meadowdale Earth Subsidence Landslide Hazard Area. In our opinion, the .site is located within the Meadowdale Earth Subsidence Landslide Hazard Area, and the rockeries appear to ,face cuts over 8 feet high. AESI recommends site-specific engineering design of any rocker), over 4 feet high located within the site's disturbed landslide debris soils. The rockeries should be designed and stamped by a licensed professional engineer and include geogrid reinforcemenr as part of the design. 06/29/2005 17:08 4258275424 AESI PAGE 65/05 Site. drainage improvement should consist of controlling surface wafter flow and ground water seepage from the base of the adjacent slope. Add surface drainage Swale to divert surface water away from top of rockeries and bottom of slopes, especially near east side of the proposed garage. The drains wirhin the rockeries located adjacent to the driveway parking area should not be used .for this purpose, because the drain rock would be vulnerable to infill by slope sedimentation. City code ollows,for non -building at -grade surface improvements within the 15 - ,foot building setback. Thus, a shallow graveled surface swale could be placed above the rockery within the flatter portion of the 15 -foot building .setback. This drainage _feature would be intended to intercept slope surface floor, and .shallow ground water .seepage prior to flov1 reaching the rockery and building site. The swale should be approximately 3 feet wide by 1.5 feet deep. Conclusion In summary. it is AESI's opinion that if the conditions discussed in this review are included with the construction plans for the proposed Michel residence, the requirements of Chapter 19.05 of the ECDC should be substantially satisfied. This letter has been prepared for the exclusive use of the City of Edmonds Community Services Department, Engineering Division and their agents, for specific application to this project. Within the limitations of scope, schedule, and budget, our services have been performed in accordance with generally accepted geotechnical engineering and engineering geology practices in effect in this area at the time our review was completed. Noother warranty, express or implied, is made. We Appreciate the opportunity to be of service to the City of Edinonds on this project. Should you have any questions regarding this letter or other geotechnical aspects of the project, please call us at your earliest convenience. Sincerely, ASSOCIATED EARTH SCIENCES, INC. Kirkland, Washington <�T .Y� n r � 1- G. Aaron McMichael, P.E., P.E.G. Associate Engineer 9