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HomeMy WebLinkAboutRS_Geotech Report 2023 081115365 N.E. 90th Street, Suite 100 • Redmond, WA 98052 • (425) 449-4704 • FAX (425) 449-4711 Earth Solutions NW LLC Geotechnical Engineering, Construction Observation/Testing and Environmental Services August 11, 2023 ES-4044.01 Charles Jackson 12815 Northeast 126th Place Kirkland, Washington 98034 Subject: Response to Comments and Geotechnical Update 2223 Northeast 23rd Street Renton, Washington Reference: Earth Solutions NW, LLC Infiltration Evaluation, ES-4044, dated July 15, 2016 City of Renton Engineering Consolidated Comment Email, Jackson Short Plat, dated July 5, 2022 Renton Project Number PRE19-000170 Client Provided, Storm Drainage and Grading Plan, Sheets C1.0 through C2.3, Dated January 18, 2022 Greetings, Charles: In accordance with your request, Earth Solutions NW, LLC (ESNW) has prepared this letter providing a response to comments provided by the City of Renton representatives. The comment from the reviewer is cited, and the ESNW response follows. City of Renton Comment – COR Maps indicates there are sensitive slopes on the property and landslide hazard areas within 50 feet of the project site. The geotechnical report does not address these items. ESNW Response – City of Renton municipal code 4-3-050F (Submittal Requirements and Fees) requires a geotechnical study shall be required where a geologic hazard is present on the site or within 50 feet of the subject site. The site is not described as possessing an erosion hazard on the city mapping for erosion hazards. The site is mapped as possessing a steep slope hazard within the southern portion of the site, but sits at the toe-of-slope which ascends approximately 15 feet to the south. As the subject site is located at the toe-of-slope, there will be no increase in surcharge on the slope which is described on the steep slope map. Charles Jackson ES-4044.01 August 11, 2023 Page 2 Earth Solutions NW, LLC The site is sited within 50 feet of a medium landslide hazard as described for west of the subject project area. ESNW has provided the following assessment where City of Renton Code requires the following: ii. The required studies shall demonstrate the following review criteria can be met: (a) The proposal will not increase the threat of the geological hazard to adjacent or abutting properties beyond pre-development conditions; (b) The proposal will not adversely impact other critical areas; and (c) The development can be safely accommodated on the site. ESNW prepared the referenced report for the site which included a project description, subsurface conditions, geologic setting, groundwater observations, and an infiltration evaluation. ESNW has reviewed the referenced maps demonstrating the subject steep slope complex off- site (west) being of moderate landslide hazard. There are no critical areas mapped for the subject site with the exception of the steep slope in the southern portion of the site which ascends to the south and off-site. The subject site sits at the toe of a slope where in our opinion there is no necessity for analysis of a critical area for the subject site as the planned re-development will not result in an increase in loading on slopes on or around the subject site per ESNW review of the site plans. ESNW anticipates the site soils will adequately infiltrate vertically with little to no lateral migration of the stormwater being introduced to the drywell based on observations of the subsurface. ESNW should be retained to confirm the conditions within three to five feet of the bottom-of- facility are free from silt lensing. ESNW recommends a note be added to the project plans stating subsurface condition confirmation of the drainage characteristics of the receptor soils within infiltration facilities must be undertaken during construction. Were water to migrate laterally, the rockery drainage elements will intercept and vector away from any downslope elements and outlet to an appropriate discharge point. This will act in some ways as redundancy for site drainage. A provision for overflow (catch basin) is provided in the design of the drywell based on our review. In our opinion this is prudent and will aid to minimize the potential for overflow and sheeting of water towards any downslope elements on and around the subject site. In our opinion, if the subject infiltration facility performs as expected there will be no increase in the threat of geological hazard adjacent or abutting properties beyond the pre-development conditions. The proposed infiltration will not adversely impact other critical areas and can be safely accommodated on the site given ESNW is provided the opportunity to observe the installation of the subject facility and confirm the subsurface conditions as mentioned above. City of Renton Comment – The provided geotechnical report was authored in 2016. Please provide an updated report. If conditions have not changed an addendum authored by a professional engineer is acceptable. Charles Jackson ES-4044.01 August 11, 2023 Page 3 Earth Solutions NW, LLC ESNW Response – The subject site has not been changed in such a manner as to render the subsurface information in the referenced infiltration evaluation as non-applicable since the time of site exploration. As the referenced document was an infiltration evaluation it did not include a seismic site class determination or foundation design parameters. As such, seismic and foundation design recommendations are presented below. Seismic Class Determination The 2018 IBC recognizes the most recent edition of the Minimum Design Loads for Buildings and Other Structures manual (ASCE 7-16) for seismic design, specifically with respect to earthquake loads. Based on the soil conditions encountered at the test pit locations, the parameters and values provided below are recommended for seismic design per the 2018 IBC. Parameter Value Site Class D Mapped short period spectral response acceleration, SS (g) 1.434 Mapped 1-second period spectral response acceleration, S1 (g) 0.492 Short period site coefficient, Fa 1.000 Long period site coefficient, Fv 1.808 Adjusted short period spectral response acceleration, SMS (g) 1.434 Adjusted 1-second period spectral response acceleration, SM1 (g) 0.889 Design short period spectral response acceleration, SDS (g) 0.956 Design 1-second period spectral response acceleration, SD1 (g) 0.593 *Assumes medium dense soil conditions, encountered to a maximum depth of five feet bgs during the field exploration, remain medium dense to dense to at least 100 feet bgs. Based on our experience with the project geologic setting across the Puget Sound region, soil conditions are likely consistent with this assumption. Foundations Based on the subsurface information in the referenced ESNW letter, the proposed residential structure can be supported on conventional spread and continuous footings bearing on competent native soils, competent existing fill or new structural fill. Based on the soil conditions encountered at the test sites, competent native soils suitable for support of foundations should be encountered at depths of approximately two feet below existing grades in most areas. Where loose or unsuitable soil conditions are exposed at foundation subgrade elevations, compaction of the soils to the specifications of structural fill, or overexcavation and replacement with structural fill, may be necessary. ESNW should be retained during construction to confirm the suitability of the new foundation subgrade and offer recommendations as necessary. Charles Jackson ES-4044.01 August 11, 2023 Page 4 Earth Solutions NW, LLC Provided foundations will be supported as described above, the following parameters can be used for design of new foundations: Allowable soil bearing capacity 2,500 psf Passive earth pressure 300 pcf (equivalent fluid)  Coefficient of friction 0.40 A one-third increase in the allowable soil bearing capacity can be assumed for short-term wind and seismic loading conditions. The above passive pressure and friction values include a factor- of-safety of 1.5. With structural loading as expected, total settlement in the range of one inch and differential settlement of about one-half inch is anticipated. The majority of the settlements should occur during construction, as dead loads are applied. Limitations The recommendations and conclusions provided in this letter are professional opinions consistent with the level of care and skill that is typical of other members in the profession currently practicing under similar conditions in this area. Our recommendations are based on the information available at the time of this letter preparation. A warranty is not expressed or implied. We trust this letter meets your current needs. If you have any questions, or if additional information is required, please call. Sincerely, EARTH SOLUTIONS NW, LLC Stephen H. Avril Kyle R. Campbell, P.E. Project Manager Senior Principal Engineer Attachment: Earth Solutions NW, LLC Infiltration Evaluation, ES-4044, dated July 15, 2016 cc: Merrick Lentz Architect Attention: Jason Beagle 08/11/2023 Drwn. Checked Date Date Proj.No. Plate Earth Solutions NWLLC Geotechnical Engineering,Construction Monitoring EarthSolutionsNWLLC EarthSolutions NW LLC and Environmental Sciences Vicinity Map N.E.23rd Street Renton,Washington GLS 07/13/2016 4044 BJP July 2016 1 NOTE:This plate may contain areas of color.ESNW cannot be responsible for any subsequent misinterpretation of the information resulting from black &white reproductions of this plate. Reference: King County,Washington Map 626 By The Thomas Guide Rand McNally 32nd Edition NORTH SITE Drwn. Checked Date Date Proj.No. Plate Earth Solutions NWLLC Geotechnical Engineering,Construction Monitoring EarthSolutionsNWLLC EarthSolutions NW LLC and Environmental Sciences Test Pit Location Plan N.E.23rd Street Renton,Washington MRS BJP 07/13/2016 July 2016 4044 2 NORTH NOTE:This plate may contain areas of color.ESNW cannot be responsible for any subsequent misinterpretation of the information resulting from black &white reproductions of this plate. NOTE:The graphics shown on this plate are not intended for design purposes or precise scale measurements,but only to illustrate the approximate test locations relative to the approximate locations of existing and /or proposed site features.The information illustrated is largely based on data provided by the client at the time of our study.ESNW cannot be responsible for subsequent design changes or interpretation of the data by others. NOT -TO -SCALE LEGEND Approximate Location of ESNW Test Pit,Proj.No. ES-4044,June 2016 Subject Site Existing Building N.E. 22ND STREET TP-1 TP-2 TP-3 TP-1 N.E. 23RD STR EET CAMASAVENUEN.E.