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MITIGATION MEASURES/ CONDITIONS OF APPROVAL: <br /> 1. Prior to the issuance of the Clearing & Grading Permit, BLD-2021-0322, the applicant shall have the <br /> geotechnical assessment prepared by MTC, INC revised to include the following conclusions and <br /> recommendations per the Peer Review Report, dated August 31, 2021 prepared by GeoEngineers: <br /> 1.1. MTC used a PGA of 0.391g for pseudostatic analyses; it is our conclusion that the current <br /> American Society of Civil Engineers (ASCE) 7-16 PGA per Office of Statewide Health Planning <br /> and Development (OSHPD) for the site is 0.489g. MTC performed some research and found <br /> references that suggest that the PGA can be reduced by various factors to 0.4 times this value. It <br /> is our opinion that the more appropriate value to be using is 0.5 times 0.489g or 0.244g. <br /> 1.2. Without geogrid reinforcement along the rock fill slope face, the results of MTC's pseudostatic <br /> analyses showed less than the minimum 1.2 required by AMC Chapter 17.70 for some of the <br /> modeling runs. MTC states that the critical surfaces representing the potential failures"were <br /> small masses in the fill slope." The same slopes showed greater than 1.2 for the seismic <br /> condition with geogrid reinforcement, which was used in their addendum report. Limit equilibrium <br /> analyses do not provide sufficient information to determine the deformation geometry or impacts <br /> of such movement. Considering the recommended minimum 10-foot setback for the homes, and <br /> potential for rockfall hazard to the residences below the site, it is our opinion that meeting the <br /> minimum 1.2 FOS for seismic conditions is appropriate. It appears that geogrid reinforcement of <br /> the slope face will be required to meet this criterion. <br /> 1.3. MTC applied a 3,000 psf uniform surcharge loading for the house at the top of the slope in their <br /> modeling. Typical residential structures do not apply this magnitude of loading, and it is very <br /> likely that this surcharge loading is adversely influencing the static and pseudostatic results. We <br /> suggest it would be worthwhile to reevaluate this surcharge loading. <br /> 1.4. MTC recommended placing a geotextile between the rock fill material and the landscape,fill <br /> placed along the 1.5H:1V final slope. While MTC recommends a high permeability geotextile, we <br /> suggest considering whether the use of a geotextile in this situation could lead to interface <br /> stability and loss of the landscape fill during a major storm event. If some lateral stability is <br /> desirable, consideration could be given to using a geocell system along the face. <br /> 1.5. MTC recommended that the rock fill material be tamped in place with the construction <br /> equipment. It is our opinion that this is not adequate placement procedure considering the <br /> adverse consequences for foundation support and slope stability if adequate strength and <br /> interlocking of the rock fill material is not achieved. It is our opinion that the rock fill material <br /> should be placed in accordance with conventional Rock Embankment Construction procedures <br /> per Washington State Department of Transportation (WSDOT) Standard Specifications Section <br /> 2.03.3(14)A. • <br /> • <br /> 1.6. We recommend that MTC provide conclusions regarding suitability of the existing rock fill <br /> material that has already been placed. We also recommend that they determine if the <br /> topsoil/colluvium was left in place prior to placing the existing rock fill material; if this is the case, <br /> provide an opinion whether the existing rock fill material is acceptable in-place considering <br /> potential impacts to slope stability and foundation support for the homes. <br /> • <br /> 1.7. MTC recommends mitigation of the rockfall hazard during construction and incorporating a <br /> mitigation method into the project for the post construction condition. We observed large rock <br /> fragments (boulders) on the gentle sloping bench of the eastern lot(lot 15) during our site <br /> reconnaissance. We recommend that the mitigation strategy(s) with appropriate design <br /> documentation be included in the project plans submitted to the City. <br /> BLD-2021-0033—MJB Properties, LLC SEPA MDNS <br /> Page 2 of 3 <br />