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Home My WebLink AboutRS_Mine_Hazard_Letter_240301_v1 DE-CIVIL, PLLC 9601 Key Peninsula Hwy. NW Gig Harbor, WA 98329 Ph (253) 228-0513 www.de-civil.com December 28, 2023 Schneider Homes, Inc. 6510 Southcenter Blvd #1 Tukwila, WA 98188 Attn: Mr. Harry Schneider Geologic Assessment: Mine Hazard Geotechnical Letter Proposed Short Plat 7XX S 21st Street Renton, Washington PN: 7222000110 Job: 17-166 Schneider.MHGL INTRODUCTION This letter summarizes our site observations and addresses the City of Renton mine hazard criteria for the proposed short plat to be located at 7XX S 21st Street in the City of Renton, Washington. The general location of the site is shown on the attached Site Vicinity Map, Figure 1. Our understanding of the project is based on our discussions with yourself, our September 4, 2017 site visit, and our experience in the area. We understand that you are proposing to subdivide the property into 3 lots in order to construct new residences on each of the sites. We anticipate that the proposed residence(s) will be a constructed with conventional wood-framing, supported on spread and continuous foundations. Due to a potential mine hazard indicator being in the vicinity of the site, we understand that a Mine Hazard Assessment is being required by the City of Renton Municipal Code. SCOPE The purpose of our services was to review available mine hazard data and evaluate the site conditions as a basis for assessing potential adverse impacts to and from historical mining activities located in the site area. Our site evaluation was performed in accordance with the City of Renton regulations. Specifically, our scope of services for this project included the following: 1. Visiting the site and conducting a geologic reconnaissance to assess the site’s soil, groundwater and slope conditions. 2. Assessing the mine hazards in accordance with the current Pierce County, Title 18E, Critical Areas and Natural Resource Lands ordinance. Subsurface explorations were completed as part of our previous geotechncial investigation of the property with the results presented in our report dated September 7, 2017, no further test pits were completed as part of the scope for this assessment. SITE CONDITIONS Surface Conditions The subject parcel is located at 7XX S 21st Street in the City of Renton, Washington. The site consists of a single parcel that is rectangular in shape, approximately 290 feet in length (west to east) by 70 feet in width (north to South) and encompasses an area of approximately 0.46 acres. The site is an existing lot 17-166 Schneider.MHGL December 28, 2023 Page 2 of record that is currently undeveloped and is well vegetated with trees, grasses and brush. Access to the site will be from a private driveway from S 21st Street, see Figure 2 Site Plan. The parcel is nearly level with a slight slope to the north, site grades are generally less than 5 percent with less than 5 feet of elevation change across the lot. No surface water or seepage was observed on the site at the time of our first site visit, however we did observe signs of a potentially high groundwater in the upper soils of the site, see below for more information. No evidence of erosion, soil movement, landslide activity or deep-seated slope instability was observed at the site or within 300 feet of the site at the time of our site visit. Site Soils The USDA Natural Resource Conservation Service Web Soil Survey for King County mapped the soils in the area of the site as Beausite gravelly sandy loam (BeC). The Beausite soils are a gravelly sandy loam, formed from glacial deposits overlying sandstone bedrock on slopes of 6 to 15 percent slope and are listed as having a “moderate” erosion potential, see Figure 3 USDA Soil Map. Site Geology The Geological Map of the Renton Quadrangle, King County, Washington by D. R. Mullineaux, 1965 indicates the site is underlain by glacial till (Qvt). These glacial soils were deposited during the latest stage of the most recent Vashon Stade of the Fraser Glaciation, approximately 12,000 to 15,000 years ago. These soils are characterized as mix of silt, sand with gravel which were overridden by the glacial ice. As such, these soils are generally very dense exhibiting high strength and low compressibility characteristics. An excerpt of the above referenced map is included as Figure 4. Based on our site observations the near surface soils at the site are most consistent with the USDA mapped soils. Historical Mine Mapping A review of the “Washington State Coal Mine Map Collection: A Catalog, Index and Users Guide” Open File Report (OFR) 94-7, published by Washington State June 1994 shows that mining activities did take place in the vicinity of the parcel. The nearest mapped mine is the “Old Patton Mine” which shows in two different locations on the three available maps. Two of the maps K-32A & B shows the mine north of the Patton Syncline while the third map K-33A shows the mine south of the Patton Syncline and very near the project site limits. No information was available for the Old Patton Mine in terms of depth or other activity. Generally these mines were active prior to the early 1900’s and were several hundred feet below the surface. See Mine Map, Figure 4x. Mine Hazard Indicators – per City of Renton Municipal Code Title IV 4-3-050 The City of Renton Municipal Code, Title IV defines Mine hazard areas in three categories as follows: i. Low Coal Mine Hazards (CL): Areas with no known mine workings and no predicted subsidence. While no mines are known in these areas, undocumented mining is known to have occurred. ii. Medium Coal Mine Hazards (CM): Areas where mine workings are deeper than two hundred feet (200') for steeply dipping seams, or deeper than fifteen (15) times the thickness of the seam or workings for gently dipping seams. These areas may be affected by subsidence. 17-166 Schneider.MHGL December 28, 2023 Page 3 iii. High Coal Mine Hazard (CH): Areas with abandoned and improperly sealed mine openings and areas underlain by mine workings shallower than two hundred feet (200') in depth for steeply dipping seams, or shallower than fifteen (15) times the thickness of the seam or workings for gently dipping seams. These areas may be affected by collapse or other subsidence. CONCLUSIONS General Based on the results of our data review, site reconnaissance, and our experience in the area, it is our opinion that a mine hazard area does not exist on or within 300 feet of the site. While there is evidence of historical mining activity near the property, we did not observe any surface evidence of subsidence or other failures related to the mining activities. Also, there was no evidence of any mine hazards on or near the site based on our field reconnaissance. LIMITATIONS We have prepared this report for use by Schneider Homes, Inc. and other members of the design team, for use in the permitting and design of a portion of this project. This report may be made available to regulatory agencies or others, but this report and conclusions should not be construed as a warranty of the subsurface conditions. No subsurface explorations were conducted for this assessment. Subsurface conditions can vary over short distances and can change with time. Within the limitations of scope, schedule and budget, our services have been executed in accordance with generally accepted practices in this area at the time this report was prepared. No warranty, expressed or implied, should be understood. We trust this is sufficient for your current needs. Should you have any questions, or require additional information, please contact us at your earliest convenience. Respectfully submitted, Development Engineering, PLLC Glen Coad, PE Owner WGC:wgc Doc ID: 17-166 Schneider.MHGL Attachments: Figure 1: Vicinity Map Figure 2: SCS Soil Survey Figure 3: Geological Map Figure 4x: DNR Mine Map Approximate Site Location Not to Scale DE-CIVIL, PLLC 9601 Key Peninsula Hwy. NW Gig Harbor, WA 98329 Ph (253) 228-0513 www.de-civil.com Site Vicinity Map Schneider SP 7XX S 21st Street Renton, Washington Job No: 17-166 December 2023 Figure 1 DE-CIVIL, PLLC 9601 Key Peninsula Hwy. NW Gig Harbor, WA 98329 Ph (253) 228-0513 www.de-civil.com USDA Web Soil Survey Schneider SP 7XX S 21st Street Renton, Washington Job No: 17-166 December 2023 Figure 2 DE-CIVIL, PLLC 9601 Key Peninsula Hwy. NW Gig Harbor, WA 98329 Ph (253) 228-0513 www.de-civil.com USGS Geologic Map Schneider SP 7XX S 21st Street Renton, Washington Job No: 17-166 December 2023 Figure 3 DE-CIVIL, PLLC 9601 Key Peninsula Hwy. NW Gig Harbor, WA 98329 Ph (253) 228-0513 www.de-civil.com DNR Mine Mapping K-32A Schneider SP 7XX S 21st Street Renton, Washington Job No: 17-166 December 2023 Figure 4a DE-CIVIL, PLLC 9601 Key Peninsula Hwy. NW Gig Harbor, WA 98329 Ph (253) 228-0513 www.de-civil.com DNR Mine Mapping K-32B Schneider SP 7XX S 21st Street Renton, Washington Job No: 17-166 December 2023 Figure 4b DE-CIVIL, PLLC 9601 Key Peninsula Hwy. NW Gig Harbor, WA 98329 Ph (253) 228-0513 www.de-civil.com DNR Mine Mapping K-33A Schneider SP 7XX S 21st Street Renton, Washington Job No: 17-166 December 2023 Figure 4c DE-CIVIL, PLLC 9601 Key Peninsula Hwy. NW Gig Harbor, WA 98329 Ph (253) 228-0513 www.de-civil.com DNR Mine Mapping K-33B Schneider SP 7XX S 21st Street Renton, Washington Job No: 17-166 December 2023 Figure 4d DE-CIVIL, PLLC 9601 Key Peninsula Hwy. NW Gig Harbor, WA 98329 Ph (253) 228-0513 www.de-civil.com DNR Mine Mapping K-33C Schneider SP 7XX S 21st Street Renton, Washington Job No: 17-166 December 2023 Figure 4e