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HomeMy WebLinkAboutGeo Tech Report - CAG-25-155TECHNICAL MEMORANDUM 3/1/2024 11:41 AM J:\Data\REN\117-037\Predesign\Kensington Crest\Kensington Crest Generator Pad Geo\REN Kensington Generator Pad Geohazard.docx Introduction RH2 Engineering, Inc., (RH2) has prepared this technical memorandum for the City of Renton’s (City) Kensington Crest Lift Station and Force Main Improvements – Phase 4 Project in Renton, Washington. This technical memorandum summarizes a desktop and reconnaissance geologic assessment of the geology, soil, geotechnical, and groundwater conditions to evaluate potential geohazards and potential impacts of the project on geohazards at the project site. Project Summary The proposed generator pad will be positioned immediately north of Kirkland Avenue SE and across the street from the Shadow Hawk II Condominium and approximately 40 feet west of the Kensington Lift Station (Attachment 1). The Site is in the NW ¼ of the NW ¼ of Section 21, Township 23 N, Range 05 E, centered at latitude 47.473675 degrees north and longitude 122.179761 degrees west at approximately 240 feet in elevation above mean sea level. The Site is located approximately 1.2 miles east of the center of the City of Renton. The general layout of the property is shown in Attachment 1. The Site is on an artificially cut slope on undeveloped forested terrain. The cut slope was prepared to support the construction of Kirkland Avenue and the condominium in 2002. LiDAR imagery obtained from the Washington State Department of Natural Resources (DNR) shows this cut slope Attachment 2. The City will construct a 20-foot by 15-foot concrete pad and a 6-foot-high retaining wall set into a cut slope to support a new emergency generator for the nearby lift station. The generator pad will be constructed by excavating a level surface 10 feet into an existing slope and laying back the cut slope to support the construction of the associated retaining wall. The concrete pad will be poured in place on a layer of compacted crushed rock and permeable Client: City of Renton Project: Kensington Crest Lift Station and Force Main Improvements Project File: REN 0117037.00.0009.0931 Project Manager: Edwin Halim, PE Composed by: Steve Nelson, LG, LHG, LEG Reviewed by: Edwin Halim, PE Subject: Generator Pad Site Geohazard Evaluation Date: March 1, 2024 Signed 03/01/2024 City of Renton March 1, 2024 Generator Pad Site Geohazard Evaluation Page 2 3/1/2024 11:41 AM J:\Data\REN\117-037\Predesign\Kensington Crest\Kensington Crest Generator Pad Geo\REN Kensington Generator Pad Geohazard.docx granular fill will be placed behind the concrete retaining wall to promote drainage behind the wall. Stormwater generated on the site will be directed into existing stormwater structures in Kirkland Avenue SE. Site Conditions RH2 reviewed geologic maps and descriptions of regional geologic conditions on the DNR website reviewed the Natural Resources Conservation Service (NRCS) soil mapping website , and reviewed descriptions of local soil conditions. RH2 reviewed the driller’s logs for borings and wells completed within 1 mile of the Site, which are recorded at the Washington State Department of Ecology (Ecology) well log website. The review of existing geologic information indicates that the area is underlain by dense silty gravelly sand identified as glacial drift, which is commonly encountered as the surficial geologic unit in the project area. NRCS identifies the local soil as Alderwood gravelly sandy loam, which is derived from weathered glacial drift deposits. Boring logs indicate that groundwater may exist at a depth of 100 feet or more beneath the surface. The DNR Interactive Geologic Map, based on the United States Geological Survey National Earthquake Hazards Reduction Program, assigns a Seismic Site Class C to D, soft soil, and very low risks of liquefaction. RH2 conducted a site reconnaissance in January 2024. The Site is partially vegetated and bordered on the north by a steep slope and to the south by Kirkland Avenue SE (Photo 1). LiDAR imagery indicates that the cut slope exposes native soil and not imported fill. City of Renton March 1, 2024 Generator Pad Site Geohazard Evaluation Page 3 3/1/2024 11:41 AM J:\Data\REN\117-037\Predesign\Kensington Crest\Kensington Crest Generator Pad Geo\REN Kensington Generator Pad Geohazard.docx Photo 1 – The Site looking north. The undeveloped area north of the Site is naturally sloped at angles ranging from 15 to 40 degrees and covered with dense vegetation consisting of brush (salal, blackberry, sword fern) and second growth Douglas fir, hemlock, and maple trees (Photo 2). City of Renton March 1, 2024 Generator Pad Site Geohazard Evaluation Page 4 3/1/2024 11:41 AM J:\Data\REN\117-037\Predesign\Kensington Crest\Kensington Crest Generator Pad Geo\REN Kensington Generator Pad Geohazard.docx Photo 2 – The Site looking south. No surface water seeps or springs, or wetlands exist on the Site or on the slope immediately north of the generator pad site. The exposed surface on the Site consists of light brown to gray, moderately dense to very dense, silty gravelly sand with cobbles, which is interpreted as glacial drift (Photo 3). City of Renton March 1, 2024 Generator Pad Site Geohazard Evaluation Page 5 3/1/2024 11:41 AM J:\Data\REN\117-037\Predesign\Kensington Crest\Kensington Crest Generator Pad Geo\REN Kensington Generator Pad Geohazard.docx Photo 3 – The Site slope surficial soil. Geohazard Assessment The Site and adjacent areas were identified by DNR as potential landslide areas based on indications of landslide features. Attachment 3 indicates that the Site and the entire Kensington area is underlain by area mapped as ancient landslide with a relict landslide scarp immediately north of the generator pad. City of Renton March 1, 2024 Generator Pad Site Geohazard Evaluation Page 6 3/1/2024 11:41 AM J:\Data\REN\117-037\Predesign\Kensington Crest\Kensington Crest Generator Pad Geo\REN Kensington Generator Pad Geohazard.docx RH2’s site reconnaissance included observing the area for any evidence of active or recent landsliding, structural weakness in the surface soil (fractures, slide planes), geomorphologic features indicating slope weakness or failure (cracks, slumps), evidence of disturbed, tilted, or damaged trees or structures in the area, or hydrologic features indicating potentially saturated soil. No landslide-related features were observed during the site reconnaissance. Inspection of LiDAR imagery shows no indications of an active landslide risk. The project will excavate a mass of soil that will be less than the mass of the replacing concrete pad and wall, and generator. The generator pad will be constructed with sufficient drainage to relieve any accumulating groundwater behind the wall and minim ize the risk of loading the cut slope. Conclusions The Site is identified by DNR as a potential landslide hazard area. Based on RH2’s site reconnaissance and geologic investigations, no landslide indicators are apparent at the site, and the construction of the generator pad will not alter the very low landslide risk at the site. Proper management of excavated soil, cut slopes and stormwater will be included in the design and specifications to minimize alteration of the slope and its low risk of landslide. Attachments Attachment 1 – Kensington Crest LS Proposed Site Plan Attachment 2 – Generator Pad Landslide LiDAR Attachment 3 – Generator Pad Landslide Risk Attachments Attachment 1 Kensington Crest LS Proposed Site Plan 220' 230' 240' 250' 220' 230' 240' 250' 0+00 A 0+40 A RETAINING WALL (BACKGROUND) EXISTING GRADE PROPOSED GENERATOR PROPOSED GENERATOR PAD FINISHED GRADE PROPOSED GENERATOR PAD EXISTING GRADE FINISHED GRADE SECTION B PROFILE 220' 230' 240' 250' 220' 230' 240' 250' 0+00 B 0+30 B FILL PROPOSED GENERATOR RETAINING WALL BOLLARD 0 0 A 0 4 0 A 0 00 B0 30 B0 SHADOW HAWK II CONDO, PH. 1 REC. NO. 20030826002165 KIRKLAND AVE SE SE 8TH ST K I R K L A N D A V E S E APPROX 20' JD EH Fe b 2 2 , 2 0 2 4 Fe b 2 3 , 2 0 2 4 RE N 11 7 - 0 3 7 RE N L S 4 - D - K E N - C 0 2 . D W G SHOWN C101 2 5 DWG NO.: DA T E DE S C R I P T I O N RE V I S I O N S BY RE V I E W EN G I N E E R : SA V E D A T E : RE V I E W E D : PL O T D A T E : CL I E N T : FI L E N A M E : SHEET NO.: JO B N O . : NO . 0"1"2" DRAWING IS FULL SCALE WHEN BAR MEASURES 2" SCALE: KE N S I N G T O N C R E S T L S P R O P O S E D SI T E P L A N SECTION A SECTION B CI T Y O F R E N T O N LI F T S T A T I O N A N D F O R C E M A I N IM P R O V E M E N T S - P H A S E 4 CI T Y O F R E N T O N LI F T S T A T I O N A N D F O R C E M A I N IM P R O V E M E N T S - P H A S E 4 PRELIMINARY PRELIMINARY PROPOSED SITE PLAN PROPOSED 3/4" CONDUIT CONTAINING (1) 2-CONDUCTOR SHIELDED CABLE. GROUT CORE AT STRUCTURE. RESTORE PAVEMENT AND CONCRETE SURFACE TO EXISTING OR BETTER CONDITION PER DETAIL ON DWG NO. G001. NOTES PROTECT EXISTING GUARDRAIL. PROTECT EXISTING TREES. PROPOSED STANDBY GENERATOR WITH CONCRETE PAD HYDROSEED DISTURBED AREA BOLLARD, TYP OF 5 RETAINING WALL APPROX 50' APPROX 30'APPROX 30' 15' CONSTRUCTION EASEMENT 5' PERMANENT EASEMENT A A B B Attachment 2 Generator Pad Landslide LiDAR Extreme care was used during the compilation of this map to ensure accuracy. However, due to changes in data and the need to rely on outside information, the Department of Natural Resources cannot accept responsibly for errors or omissions, and therefore, there are no warranties which accompany this material. 2/27/2024 10:18 AM Esri Community Maps Contributors, City of Renton, King County, WA State Parks GIS, © OpenStreetMap, Microsoft, Esri, TomTom, Garmin, SafeGraph, GeoTechnologies, Inc, METI/NASA, USGS, Bureau of Land Management, EPA, NPS, US Census Bureau, USDA, USFWS, Washington Geological Survey, Sources: Esri, Airbus DS, USGS, NGA, NASA, CGIAR, N Robinson, NCEAS, NLS, OS, NMA, Geodatastyrelsen, Rijkswaterstaat, GSA, Geoland, FEMA, Intermap and the GIS user community Coordinate System: NAD 1983 HARN StatePlane Washington South FIPS 4602 Feet ¯ Approximate Scale : City of Renton Kensington Generator Pad Lidar Additional Information 0 0.01 0.03 0.04 0.06 Miles1:2,929 Notes Generator Pad site Attachment 3 Generator Pad Landslide Risk Extreme care was used during the compilation of this map to ensure accuracy. However, due to changes in data and the need to rely on outside information, the Department of Natural Resources cannot accept responsibly for errors or omissions, and therefore, there are no warranties which accompany this material. 2/27/2024 10:14 AM Esri Community Maps Contributors, City of Renton, King County, WA State Parks GIS, © OpenStreetMap, Microsoft, Esri, TomTom, Garmin, SafeGraph, GeoTechnologies, Inc, METI/NASA, USGS, Bureau of Land Management, EPA, NPS, US Census Bureau, USDA, USFWS, Washington Geological Survey, Sources: Esri, Airbus DS, USGS, NGA, NASA, CGIAR, N Robinson, NCEAS, NLS, OS, NMA, Geodatastyrelsen, Rijkswaterstaat, GSA, Geoland, FEMA, Intermap and the GIS user community, Washington Geological Survey, 2023, Washington State Landslide Inventory Database-GIS data, June 2023: Washington Geological Survey Digital Data Series 29, version 1.1. https://fortress.wa.gov/dnr/geologydata/publications/data_download/ ger_portal_landslide_database.zip Coordinate System: NAD 1983 HARN StatePlane Washington South FIPS 4602 Feet ¯ Approximate Scale : City of Renton Kensington Generator Pad Landslide Risk Additional Information 0 0.01 0.03 0.04 0.06 Miles1:2,929 Notes Generator Pad site Landslide Scarp Landslide Deposits