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2018 Geotechnical AddendumGO THE GALLI GROUP Geotechnical Consulting JAN 2019 DEVELOPMENT SERVICES COUNTER Scott Schrieber Ron Scott Homes Scott.ronscotthonies@)gmail.com Subject: Geotechnical Report Addendum Parcel 00434208603300 727 Bell Street Edmonds, Washington GEOTECHNICAL REPORT ADDENDUM November 15, 2018 TGG Project 2021-1 The following addendum is meant to accompany a vegetation management program for the above referenced parcel. The Galli Group performed a geotechnical investigation on the undeveloped parcel for a previous owner in November of 2006. This recommendation is submitted as an addendum to our previous report (Geotechnical Report, Phillip and Sybil Butler, 731 Bell Street, Edmonds, Washington, dated November 7, 2006) providing geotechnical recommendations for the proposed activity but also referencing the previous report attached following this letter. Project Description We understand that there is a view easement linked to the adjacent uphill parcel at 731 Bell Street. In addition, we understand that to comply with the terms of that agreement, the uphill owner would like to have some of the taller trees pruned to conditions consistent with that agreement to maintain views. Because the site contains Geologically Hazardous Areas including slope and soil conditions meeting the code definitions for erosion hazard areas and landslide hazard areas (see definitions in ECDC sections 23.80.020 A and B), the proposed activity requires a geotechnical report confirming that the proposed activity will comply with code regulations for the hazardous areas. The proposed activity will include pruning some Maple shoots that have grown up since the time of our original report. Some of these shoots exceed 4 inches in diameter. We did not observe any greater than 6 inches in diameter, but a more detailed assessment should be provided by an arborist. PO Box 30759, Seattle, WA 98113 (206)-525-5097 Geotechnical Report Addendum 727 Bell Street November 10, 2018 Site Conditions Site surface, soil, and groundwater conditions are discussed in sections 3.1 and 3.2 of the attached report. A summary is provided below: • The site consists of a topographic bench between a slope declining at about 30 percent from the uphill, easterly lot (731 Bell Street) approximately 14 vertical feet to the bench and then continuing to descend at about 65 percent, from the bench another 10 vertical feet to the relatively flat lot below and westerly of the topographic bench. Most of the saplings and Maple shoots appear located on the topographic bench portion of the site. The site appears underlain by soil from the Alderwood and Everett series, or described as medium SAND with silt, derived from Advance Outwash deposits. These granular deposits are typically comprised of sand and pebbly gravel placed in meltwater environments proceeding from the advancing glacier thousands of years ago. The upper zone has since been weathered and subjected to grading activities during development. We noted that there was about 6 to 9 feet of undocumented fill on the site, consisting of loose SAND with silt. • The slopes appeared mantled by loose sand interpreted as fill or colluvium (sediment from erosion or grading activities). Rockeries and timber walls provide some of the grade transitions on the site, but they appear rather modest in height and overgrown with vegetation. • In addition to the Maple shoots, the site remains covered by blackberries, some scrub Alder shoots, and a couple older stumps. Where the blackberries thin out, some of the soil is covered with weeds and grass. • Our subsurface boring logs indicated that the site is mantled by loose undocumented fill in the uppermost 5 to 9 feet, underlain by medium dense to dense SAND with silt to depths of about 25 to 30 feet below existing grade. A perched water table was observed at about 21 to 26 feet below grade, but we did not observe springs emerging on the hillside. The sand unit was underlain by very stiff, clayey SILT interpreted as Transitional Beds at about 30 to 33 feet below existing grade. (See attached boring logs for more detail) We evaluated the slope between the 731 lot and the undeveloped lot and calculated that the existing factors of safety for the slope in the condition it was observed in 2006, was about 1.7 for static conditions, and 1.12 for seismic conditions. The most likely failure appeared to be a shallow colluvial failure located within the loose fill soils mantling the slope. We visited the site on November 13, and in our opinion the site conditions have not changed significantly since our original evaluation. Our recommendations for site development included supporting the residence on pile foundations, constructing a soldier pile wall to maintain slope stability during construction, providing a setback of at least 10 feet from the top of existing slopes, and providing a vegetation plan to prevent erosion of slopes on the parcel. Due to the lapse in time, these recommendations should be re-evaluated once a site development plan is presented for the parcel. 2021 Geotechnical Report Addendum 2 The Galli Group Geotechnical Report Addendum 727 Bell Street November 10, 2018 Code Compliance Requirements For development activity on the site, including pruning activity, the proposed activity must comply with sections 23.80.060 A and 23.80.070 A3 of the ECDC for Geologically Hazardous Areas. ❑ SHARE A. Alterations of geologically hazardous areas or associated buffers may only occur for activities that: 1. Will not increase the threat of the geological hazard to adjacent properties beyond predevelopment conditions; 2. Will not adversely impact other critical areas; 3. Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than predevelopment conditions; and 4. Are certified as safe as designed and under anticipated conditions by a qualified engineer or geologist, licensed in the state of Washington. Discussion The proposed activity will likely include access to the site by foot traffic and possibly a small chipper to mulch the branches removed during pruning. The removed branches will be mulched and spread over exposed soil areas on the site. Exposed soil areas from foot traffic will be mulched by hand after the work is completed. The proposed pruning activity will not increase the threat of the hazard to adjacent properties beyond predevelopment conditions. It will not impact other critical areas. Provided our recommendations that follow are incorporated into the proposed activity the hazard will be mitigated to levels equivalent to or less than predevelopment conditions. The pruning activity is safe as designed. C SHARE A. Erosion and Landslide Hazard Areas. Activities on sites containing erosion or landslide hazards shall meet the requirements of ECDC 23.80.060, Development standards — General requirements, and the specific following requirements: 2021 Geotechnical Report Addendum 3 The Galli Group Geotechnical Report Addendum 727 Bell Street November 10, 2018 3. Alterations. Alterations of an erosion or landslide hazard area, minimum building setback and/or buffer may only occur for activities for which a hazards analysis is submitted and certifies that: a. The alteration will not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions; b. The alteration will not decrease slope stability on adjacent properties; and c. Such alterations will not adversely impact other critical areas; Discussion The proposed activity will not include stump removal or significant soil disturbance. The anticipated change in the deciduous canopy will not be significant, and had no influence on runoff conditions during the wet season. The proposed activity as discussed above and including recommendations below will not increase the surface water discharge or sedimentation to adjacent properties beyond existing predevelopment conditions. The alteration will not decrease slope stability on adjacent properties, and the pruning activity will not adversely impact other critical areas. The existing factor of safety for landslide occurrences is currently above 1.5 and the proposed activity will not alter that. The proposed activity will not alter the factor of safety for the seismic condition either, which according to our calculations in 2006 was estimated at 1.12. Given the anticipated critical failure surface (shallow colluvial failure less than 5 feet deep) this factor of safety seems reasonable for the undeveloped lot and existing conditions. The proposed activity will have no significant impact on the current slope stability. Recommendations We recommend the following measures to maintain erosion control and current site stability: 1. The pruning work on the site should be overseen and directed by a licensed arborist. If the arborist recommendations result in removal of any stumps or trees larger than 6 inches in diameter, The Gatti Group should be consulted to provide specific recommendations related to said activity. 2. We recommend accessing the site by foot to limit damage to existing vegetation. 3. We recommend chipping and shredding all removed branches on site and spreading them on areas of exposed soil or on the upper slope to protect exposed soils or areas with 2021 Geotechnical Report Addendum 4 The Galli Group Geotechnical Report Addendum 727 Bell Street November 10, 2018 limited vegetative cover. 4. All soils exposed by the pruning and maintenance activity must be mulched with straw or fertile mulch prior to project completion. Provided these recommendations are incorporated into the pruning and maintenance activity, it is our professional opinion that the activity will have minimal impact on the site and will not adversely impact geologically hazardous areas on site or adjacent parcels. Limitations The recommendations in this letter and references to the attached report are intended for the proposed maintenance activity only and not for development of the site for future residential construction. Future site development should be evaluated considering the site -specific plan and the report should be reviewed considering that site development plan. Please contact the undersigned with any questions regarding this addendum. Sincerely, THE GALLI GROUP SVOLTf,B __.\ ' 2931y�Q Q,� Paul L. Stoltenberg, P.E. Senior Geotechnical Engineer 11-15-2018 Attached: Geotechnical Report, Phillip and Sybil Butler, 731 Bell Street, Seattle, Washington, Nov 7, 2006, prepared by The Galli Group. 2021 Geotechnical Report Addendum 5 The Galli Group