600 Dupont Street Bellingham, Washington 98225 360.647.1510 March 13, 2020 Island County Public Works Department P.O. Box 5000 Coupeville, Washington 98239-5000 Attention: Dugan McCloskey Subject: Peer Review – 19-1673 and 19-674 Building Permits, Rocha 8230 Lupine Lane Clinton, Washington File No. 0422-155-00 INTRODUCTION AND SCOPE GeoEngineers, Inc. (GeoEngineers) is pleased to present this letter with the results of our geotechnical peer review regarding the proposed residential development at 8230 Lupine Lane in the Clinton area of Whidbey Island, Washington. The parcel number is R32814-210-3800. Our services were performed in general accordance with our On-Call Consultant Agreement PW-1920124 with Island County Public Works (ICPW) and Task Assignment No. 9 dated February 21, 2020. The site is currently undeveloped. A single family (primary) residence and an approximately 225 square-foot tree house are proposed on the upper relatively gently sloping portion of the parcel. A steep slope is located along the eastern margin of the parcel. We understand that the primary residence will have a setback on the order of 85 to 100 feet from the top of the steep slope; the treehouse will have a setback on the order of 15 to 21 feet from the top of the steep slope. The site is designated as a Geologic Hazard Area per Island County Code (ICC) 11.02.140.B because of the steep slope, and the County requested this peer review to address the proposed tree house location because of the close proximity of the tree house to the top of slope. The purpose and scope of our services included: reviewing applicant submitted information provided by ICPW, in-house materials, and published information pertinent to the proposed residential development; performing a brief site reconnaissance; and providing independent assessment of the geotechnical conclusions and recommendations provided by the applicant’s geotechnical professional. ICPW provided the following documentation for our review: ■ ICPW Review prepared by Dugan McCloskey, dated December 2, 2019. (“Subject: 19-1673 and 19-674 Building Permits, Rocha, Build Single Family Residence and Tree House, Parcel No. R32814-210-3800.) Island County Public Works Department | March 13, 2020 Page 2 ■ “Geological Assessment of Slope and Drainage Condition & Processes Operating on Tax Parcel R32814-210-3800”, prepared by Terry Swanson, Ph.D., L.G. (Swanson), dated June 20, 2019. ■ E-mail correspondence between Dugan McCloskey (ICPW), Max Thorsen (co-applicant with Tatiana Rocha), and Swanson (applicant’s geologist). ■ Arborist report prepared by Blake Durr of I-Arbor, LLC, dated October 19, 2019. ICPW DOCUMENT REVIEW Permit Review Document The permit review narrative describes the proposed project, which includes a primary residence and a tree house. Based on review of the e-mail communications from Max Thorsen (applicant), the tree house will be occasionally inhabited. The permit narrative indicates that the primary residence will have a setback on the order of 85 to 100 feet from the steep slope to the east; the tree house will have a setback on the order of 15 to 21 feet from the steep slope to the east. The setbacks are consistent with the recommendations of the Swanson geologic report. The steep slope is identified as “unstable” in the Washington State Department of Ecology “Coastal Zone Atlas”. The permit narrative indicates that the Swanson geologic report describes the site conditions and provides conclusions and recommendations generally consistent with the ICC requirements with the exception of addressing stability the risk to the proposed tree house and slope during a seismic event. There appears to be a conflict with regard to stormwater drainage between the applicant submittals and the geologic report, but the discharge is consistent in that it is to the west of the structures and away from the steep slope. ICPW has required that the applicant record a Geologic Hazard Area covenant with the County Auditor’s office. Geotechnical Report We provide the following discussion, summary of conclusions and recommendations provided in the geologic report by Swanson related to the proposed residential development: ■ The site is located toward the top of a north-south trending upland area at the south end of Whidbey Island, and slopes down to the west and east. The elevation of the top of the upland area is between about 380 to 400 feet. ■ The inclination of the westerly portion of the site beyond the top of the slope (where the primary residence will be constructed) is described as between 5 to 12 degrees (approximately 10 to 22 percent). The upper portion of the slope is well vegetated, includes up to 2- to 3-foot-diameter Douglas fir and cedar trees, and old growth stumps. ■ The steep slope to the east is described in more detail: . The upper 100 to 150 feet of the slope is described as between 28 and 32 degrees in one section of the report and 25 to 35 degrees in another section of the report (approximately 52 to 71 percent). No seepage was observed. The slope is well vegetated and includes occasional deciduous trees, 2- and 3-foot-diameter Douglas fir and cedar trees, and old growth stumps. No evidence of sloughing or slope instability, and no evidence of seepage was noted in this upper portion of the slope. The lower part of the slope is slightly steeper, with some areas described as having headward erosion and seepage and as steep as 45 to 60 degrees (100 to approximately 200 percent). The steep portion of the slope is also described as generally well vegetated, with some areas File No. 0422-155-00 Island County Public Works Department | March 13, 2020 Page 3 showing evidence of past shallow slough events and surficial creep, and younger vegetation. The landslide activity is described as shallow and primarily associated with colluvium along the steeper lower slope. The landslides have not caused any headward retreat of the upper slope. ■ The geology of the upper 100 to 150 feet of the slope is described as Vashon glacial till overlying advance outwash sand. A lacustrine silt underlies the advance outwash, acts as an aquiclude and is responsible for some of the seepage that occurs along the slope to the east. The report describes the deeper geologic cross section based on previous work in the Possession Point area. ■ The report conclusions and recommendations include the following: . The western slope has a low slope inclination and is stable. The eastern upper slope “..has been stable over decadal to century time scales based on the presence of mature conifer trees (mainly Douglas fir and western red cedar) and the slope morphology.” A bluff retreat is described “as very little (<0.5 inches per year).” The lower slope is described as susceptible to mass wasting, but concludes that these processes have not affected the upper slope. The seismic discussion does not appear to provide a conclusion other than the greater the setback the greater the margin of safety. A setback of 15 feet from the east slope for the tree house is adequate for the “expected life span (>60 years).” A setback of 50 feet from the east slope is adequate for the primary residence. Any foundation drains and stormwater from the project can be dispersed into the forested topsoil along the west slope. The report also recommends that the vegetation on the steep slopes be maintained as much as possible with limited limbing for view corridors. E-mail Follow-Up Swanson provides additional responses to the seismic stability in an e-mail. The relevant comments are that the steep slope has been stable for “century time scales” based on vegetation and morphology of the slope, and references the most recent Cascadia subduction event (approximately 300 years ago). While he does not appear to provide the distinct conclusion that the tree house location is seismically stable, he concludes that the 15-foot setback is “reasonable.” Arborist Report The arborist evaluated the four Douglas fir trees that are proposed for support of the tree house. The report concludes that the trees are of “normal health and vigor”, are reasonably protected within the forested condition of the slope and upland area, and the proposed hardware attachments will not adversely affect the trees’ health with recommended hardware installation methodologies. SITE VISIT I visited the site on March 4, 2020. The following is a summary of my observations: ■ I accessed the upper site area near the proposed primary residence and tree house, and approximately 100 feet down the eastern steep slope. The four trees for the proposed tree house were identified with a continuous string attached around them. ■ I observed the site conditions to be generally consistent with Swanson’s descriptions. File No. 0422-155-00 Island County Public Works Department | March 13, 2020 Page 4 ■ The foundation excavation had been partially completed for the primary residence. I observed an approximate 1½-foot weathered horizon overlying very dense silty sand with gravel that is representative of Vashon glacial till. Based on probing with a 3-foot long steel “T” probe immediately adjacent the excavation, I could probe relatively easily through the weathered horizon, and encountered abrupt “hard” refusal at the unweathered glacial till interface. ■ I performed similar probing in the upland area around the tree house and upper approximately 100 feet down the eastern slope and encountered the same probe profile, encountered hard refusal at 1 to 1½ feet below the ground surface. I did not observe evidence of perched groundwater, although it could occur along the steep slope during winter. The tree cover is very good such that direct rain impact is limited. I did not observe any evidence of seepage or rilling on the slope.