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HomeMy WebLinkAboutRS_Geologic_Assessment_240301_v1 DEVELOPMENT ENGINEERING, PLLC P.O. Box 446 Tacoma, WA 98401 Ph (253) 228-0513 www.de-civil.com September 7, 2017 Schneider Homes, Inc. 6510 Southcenter Blvd #1 Tukwila, WA 98188 Attn: Mr. Harry Schneider Geologic Assessment Report Proposed Short Plat 7XX S 21st Street Renton, Washington PN: 7222000110 Job: 17-166 Schneider.GR INTRODUCTION This letter and design report presents the results of our subsurface explorations 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. SCOPE We understand that the City of Renton is requesting a geotechnical report to address stormwater management and other design issues related to the site development. The purpose of our services is to evaluate the surface and subsurface conditions at the site as a basis for providing engineering recommendations for the development of the site. Specifically, our scope of services for the project included the following: 1. Reviewing the available geologic, hydrogeologic and geotechnical data for the site area. 2. Performing a geological reconnaissance of the site to assess the site’s soil, groundwater and subsurface conditions. 3. Exploring shallow subsurface conditions at the site by monitoring the excavation of two test pits across the site. 4. Evaluating the engineering characteristics of the soils encountered at the site, as appropriate. 5. Providing geotechnical recommendations for site infiltration site grading including site preparation, subgrade reparation, fill placement criteria, suitability of on-site soils for use as structural fill, temporary and permanent cut and fill slopes, drainage and erosion control measures. 6. Providing site-specific seismic criteria based on the subsurface conditions encountered at the site. 7. Preparing a written report summarizing our observations, conclusions and recommendations along with the supporting data. Subsurface explorations or laboratory testing were completed as part of this assessment. 17-166 Schneider.GR September 7, 2017 Page 2 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 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. Subsurface Explorations On September 4, 2017 we visited the site and observed the excavation of three test pits to a maximum depth of 6 feet for the purpose of determining infiltration feasibility for the project, soil samples were collected for later analysis in our lab. The test pits were excavated by a track mounted excavator operated by a licensed earthwork contractor. The test pits were located in the field by our representative by pacing from existing site features such as property corners and adjacent roadways and fences. The approximate location of the test pits are indicated on the attached Site Plan as Figure 2. A geotechnical engineer from our office logged the subsurface conditions encountered in each test pit, obtained representative soil samples, and observed pertinent site features. Representative soil samples obtained from the test pits were placed in sealed plastic bags and taken to our laboratory for further examination and testing, as deemed appropriate. Based on our experience in the area and extent of our explorations in the area it is our opinion that the soils encountered in the test pits are generally representative of the soils at the site. 17-166 Schneider.GR September 7, 2017 Page 3 Because the soils encountered were consistent between the test sites and other exposed site soils and the soil mapping, it is our opinion that no additional soils tests required to evaluate the subsurface conditions at the site. The explorations performed as part of this evaluation indicate conditions only at the specific locations and that the actual condition in other locations could vary. Furthermore, the nature and extent of any such variations would not become evident until additional explorations are performed or until construction activities have begun. Subsurface Conditions Our test pits encountered uniform subsurface conditions that confirmed the general geological mapping stratigraphy. The site soils generally consisted of a minimum of 24 to 36 inches of weathered silty sand and gravel over a dense sandstone layer. Test pit #2 showed signs of a shallow fill layer (approximately 12 to 18 inches). Based on our review of our test pits, the upper weathered site soils in the area of development are consistent with the Beausite soils. The soils encountered were visually classified in accordance with the Unified Soil Classification System (USCS) described on Figure 5. The test pit logs are included as Figures 6. Groundwater Conditions No groundwater seepage was observed in any of the test pits. Although based on our observations we anticipate that during periods of heavy rainfall the upper soils would become saturated. Perched groundwater typically develops when the vertical infiltration of precipitation through a more permeable soil is slowed at depth by a deeper, less permeable soil type. We expect that perched groundwater will develop seasonally atop the shallow sandstone layer. Based on the observed mottling and nature of the near surface soils, we anticipate fluctuations in the local groundwater levels will occur in response to precipitation patterns, off-site construction activities, and site utilization. CONCLUSIONS Based on our site observations, subsurface explorations and engineering analysis, it is our opinion that the proposed development is feasible from a geotechnical standpoint. The residences may be supported on new conventional spread footings or floor slabs bearing on competent native soils or on structural fill placed above these native soils. We understand that grading at the site will be minimal, and will consist primarily of excavating the footings for the proposed residence(s), site utilities, and the stormwater facilities. If grading activities will take place during the winter season, the owner should be prepared to import free-draining granular material for use as structural fill and backfill. Proper surface drainage and erosion control measures will reduce the risk for future erosion at the site. Site Preparation Areas to be graded should be cleared of deleterious matter including any existing structures, foundations, abandoned utility lines, debris and vegetation. The portions of the site covered with vegetation should be stripped of any forest duff and organic-laden soils. Based on our test pits we anticipate stripping depths to be on the order of 6 to 18 inches. These materials can be stockpiled and later used for erosion control. Material that cannot be utilized on the site should be removed from the site. Where placement of fill material is required, the stripped/exposed subgrade areas should be compacted to a firm and unyielding surface prior to placement of any fill. Excavations for debris 17-166 Schneider.GR September 7, 2017 Page 4 removal should be backfilled with structural fill compacted to the densities described in the “Structural Fill” section of this report. We recommend that a member of our staff evaluate the exposed subgrade conditions after removal of vegetation and topsoil stripping is completed and prior to placement of structural fill. The exposed subgrade soil should be proof-rolled with heavy rubber-tired equipment during dry weather or probed with a 1/2-inch-diameter steel rod during wet weather conditions. Any soft, loose or otherwise unsuitable areas delineated during proof-rolling or probing should be recompacted, if practical, or over-excavated and replaced with structural fill, based on the recommendations of our site representative. The areas of old fill material should be evaluated during grading operations to determine if they need mitigation; recompaction or removal. Structural Fill All material placed as fill associated with mass grading, as utility trench backfill, under building areas, or under asphalt pavement should be placed as structural fill. The structural fill should be placed in horizontal lifts of appropriate thickness to allow adequate and uniform compaction of each lift. Fill should be compacted to at least 95 percent of MDD (maximum dry density as determined in accordance with ASTM D-1557). The appropriate lift thickness will depend on the fill characteristics and compaction equipment used. We recommend that the appropriate lift thickness be evaluated by our field representative during construction. We recommend that our representative be present during site grading activities to observe the work and perform field density tests. The suitability of material for use as structural fill will depend on the gradation and moisture content of the soil. As the amount of fines (material passing US No. 200 sieve) increases, soil becomes increasingly sensitive to small changes in moisture content and adequate compaction becomes more difficult to achieve. During wet weather, we recommend use of well-graded sand and gravel with less than 5 percent (by weight) passing the US No. 200 sieve based on that fraction passing the 3/4-inch sieve, such as Gravel Backfill for Walls (9-03.12(2)). If prolonged dry weather prevails during the earthwork and foundation installation phase of construction, higher fines content (up to 10 to 12 percent) will be acceptable. Material placed for structural fill should be free of debris, organic matter, trash and cobbles greater than 6-inches in diameter. The moisture content of the fill material should be adjusted as necessary for proper compaction. Suitability of On-Site Materials as Fill During dry weather construction, any non-organic on-site soil may be considered for use as structural fill; provided it meets the criteria described above in the structural fill section and can be compacted as recommended. If the soil material is over-optimum in moisture content when excavated, it will be necessary to aerate or dry the soil prior to placement as structural fill. We generally did not observe the site soils to be excessively moist at the time of our subsurface exploration program. The native weathered and glacial till soils in the site generally consisted of silty gravel with fine sand. These soils are generally comparable to “common borrow” material and will be suitable for use as structural fill provided the moisture content is maintained within 3 percent of the optimum moisture level. However, because of the high fines content, the till soils encountered across the site will likely be unsuitable during extended periods of wet weather. We recommend that completed graded-areas be restricted from traffic or protected prior to wet weather conditions. The graded areas may be protected by paving, placing asphalt-treated base, a 17-166 Schneider.GR September 7, 2017 Page 5 layer of free-draining material such as pit run sand and gravel or clean crushed rock material containing less than 5 percent fines, or some combination of the above. Temporary Excavations All job site safety issues and precautions are the responsibility of the contractor providing services/work. The following cut/fill slope guidelines are provided for planning purposes only. Temporary cut slopes will likely be necessary during grading operations or utility installation. All excavations at the site associated with confined spaces, such as utility trenches and retaining walls, must be completed in accordance with local, state, or federal requirements. Based on current Washington Industrial Safety and Health Act (WISHA, WAC 296-155-66401) regulations, the shallow upper soils on the site would be classified as Type C soils, whereas the very dense sandstone soils would be classified as Type A soils. According to WISHA, for temporary excavations of less than 20 feet in depth, the side slopes in Type A soils should be laid back at a slope inclination of ¾H:1V (Horizontal: Vertical) and Type C soils should be laid back at a slope inclination of 1½H:1V or flatter from the toe to the crest of the slope. It should be recognized that slopes of this nature do ravel and require occasional maintenance. All exposed slope faces should be covered with a durable reinforced plastic membrane, jute matting, or other erosion control mats during construction to prevent slope raveling and rutting during periods of precipitation. These guidelines assume that all surface loads are kept at a minimum distance of at least one half the depth of the cut away from the top of the slope and that significant seepage is not present on the slope face. Flatter cut slopes will be necessary where significant raveling or seepage occurs, or if construction materials will be stockpiled along the slope crest. Where it is not feasible to slope the site soils back at these inclinations, a retaining structure should be considered. Where retaining structures are greater than 4-feet in height (bottom of footing to top of structure) or have slopes of greater than 15 percent above them, they should be engineered. This information is provided solely for the benefit of the owner and other design consultants, and should not be construed to imply that Development Engineering, PLLC assumes responsibility for job site safety. It is understood that job site safety is the sole responsibility of the project contractor. Foundation Support Based on the subsurface soil conditions encountered across the site, we recommend that spread footings for the new residences be founded on dense to very dense native soils or on structural fill that extends to suitable native soils. Given the presence of fill material on the site it is anticipated that some footings may be located in areas where existing fill material is present. Provided the exposed soils area evaluated and verified by a representative from our office at time of construction to be suitable to support the design loads we do not anticipate that the presence of the fill material on site will be a detrimental factor to the site development. We do not recommend that footings be supported on a mix of sandstone and weathered material. If areas of sandstone are exposed we recommend that those areas are over excavated a minimum of 12 inches and brought back to grade with structural fill. The soil at the base of the footing excavations should be disturbed as little as possible. All loose, soft or unsuitable material should be removed or recompacted, as appropriate. A representative from our firm should observe the foundation excavations to determine if suitable bearing surfaces have been prepared, particularly in the areas where the foundation will be situated on fill material. 17-166 Schneider.GR September 7, 2017 Page 6 We recommend a minimum width of 2 feet for isolated footings and at least 16 inches for continuous wall footings. All footing elements should be embedded at least 18 inches below grade for frost protection. Footings founded as described above can be designed using an allowable soil bearing capacity of 2,000psf (pounds per square foot) for combined dead and long-term live loads. For deeper footings supported entirely on the sandstone a bearing capacity of 5,000psf may be used. The weight of the footing and any overlying backfill may be neglected. The allowable bearing value may be increased by one-third for transient loads such as those induced by seismic events or wind loads. Lateral loads may be resisted by friction on the base of footings and floor slabs and as passive pressure on the sides of footings. We recommend that an allowable coefficient of friction of 0.40 be used to calculate friction between the concrete and the underlying soil. Passive pressure may be determined using an allowable equivalent fluid density of 300 pcf (pounds per cubic foot). Factors of safety have been applied to these values. We estimate that settlements of footings designed and constructed as recommended will be less than 1 inch, for the anticipated load conditions, with differential settlements between comparably loaded footings of 1/2 inch or less. Most of the settlements should occur essentially as loads are being applied. However, disturbance of the foundation subgrade during construction could result in larger settlements than predicted. Floor Slab Support Slabs-on-grade, if constructed, should be supported on the medium dense native soils or on structural fill prepared as described above. Any areas of old fill material should be evaluated during grading activity for suitability of structural support. Areas of significant organic debris should be removed. We recommend that floor slabs be directly underlain by a capillary break material with minimum 6- inch thickness of coarse sand, pea gravel, or gravel containing less than 3 percent fines. The drainage material should be placed in one lift and compacted to an unyielding condition. A synthetic vapor barrier is recommended to control moisture migration through the slabs. This is of particular importance where the foundation elements are underlain by the silty till, or where moisture migration through the slab is an issue, such as where adhesives are used to anchor carpet or tile to the slab. A subgrade modulus of 400 kcf (kips per cubic foot) may be used for floor slab design. We estimate that settlement of the floor slabs designed and constructed as recommended, will be 1/2 inch or less over a span of 50 feet. Site Drainage All ground surfaces, pavements and sidewalks at the site should be sloped away from structures. The lot should also be carefully graded to ensure positive drainage away from all structures and property lines. Surface water runoff from the roof area, driveways, perimeter footing drains, and wall drains, should be collected, tightlined, and conveyed to an appropriate discharge point. Based on our site evaluation, it is our opinion that the infiltration of stormwater at the site not feasible on the site. Further, based on our understanding of the current City of Renton Stormwater Management Manual, it will likely be necessary to visit the site and perform additional explorations and possibly at least one full scale “Pilot Infiltration Test” (PIT) between December 1 and April 30 (winter months) in order to determine seasonal groundwater levels and infiltration feasibility. Since our field final work was done in early Suptember we recommend that we revisit our Site Drainage 17-166 Schneider.GR September 7, 2017 Page 7 recommendations once this additional field work is complete. For preliminary design purposes we are not recommending that infiltration be pursued. LID Feasibility The following table presents our recommendations for various Best Management Practices (BMP’s) for various Low Impact Development (LID) BMP’s. BMP Viable Limitations or Infeasibility Criteria Lawn and Landscape Areas T5.13: Post Construction Soil Quality and Depth Yes None. Roofs T5.30: Full Dispersion No The site does not have sufficient dispersion area. T5.10A: Downspout full infiltration systems. No Infiltration appears to be infeasible due to a high groundwater table. Bio Retention No This BMP also has limited feasibility for the same general reasons as for full infiltration. T5.10B Downspout Dispersion systems Limited This BMP may be feasible depending on final lot configurations. T5.10C Perforate stub connections Limited Similar to the infiltration systems there will be limited depths of soil for the connection to ensure that it is at least 1 foot above the till layers. Other Hard Surfaces T5.30 Full Dispersion No The site does not have sufficient dispersion area. T5.15 Permeable Surfacing No The soils in the areas of the proposed pavement included a very shallow groundwater depth which does not allow for the required 12 inches of separation between the bottom of the pavement section and the groundwater. Further, given the silty nature of the site soils, permeable surfacing would not be sufficiently supported and would likely fail in a short period of time. Bio Retention No This BMP also has limited feasibility for the same general reasons as for full infiltration. T5.12: Sheet Flow Dispersion T5.11 Concentrated flow dispersion Limited No flooding or erosion impacts are anticipated. However, this BMP may be feasible dependent on the final lot configurations and the available areas for dispersion of runoff. LIMITATIONS We have prepared this report for Schneider Homes, Inc, and other members of the design team for use in evaluating a portion of this project. Subsurface conditions described herein are based on our observations of exposed soils on the parcel. This report may be made available to regulatory agencies or others, but this report and conclusions should not be construed as a warranty of subsurface conditions. Subsurface conditions can vary over short distances and can change with time. 17-166 Schneider.GR September 7, 2017 Page 8 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, express 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 DocID:17-166 Schneider.GR Attachments: Figure 1: Vicinity Map Figure 2: Site Plan Figure 3: SCS Soil Survey Figure 4: USGS Geology Map Figure 5: Soils Classification Chart Figure 6: Test Pit Logs Approximate Site Location Not to Scale DE-CIVIL, PLLC P.O. Box 446 Tacoma, WA 98401 Ph (253) 228-0513 www.de-civil.com Site Vicinity Map Schneider SP 7XX S 21st Street Renton County, Washington Job No: 17-166 September 2017 Figure 1 DEVELOPMENT ENGINEERING, PLLC P.O. Box 446 Tacoma, WA 98401 Ph (253) 228-0513 www.de-civil.com Approximate Test Pit Location Site Plan Schneider SP 7XX S 21st Street Renton County, Washington Job No: 17-166 September 2017 Figure 2 TP-1 TP-2 TP-3 TP-1 Approximate Site Location Not to Scale DE-CIVIL, PLLC P.O. Box 446 Tacoma, WA 98401 Ph (253) 228-0513 www.de-civil.com USDA Web Soil Survey Schneider SP 7XX S 21st Street Renton County, Washington Job No: 17-166 September 2017 Figure 3 Approximate Site Location Not to Scale DE-CIVIL, PLLC P.O. Box 446 Tacoma, WA 98401 Ph (253) 228-0513 www.de-civil.com USGS Geologic Map Schneider SP 7XX S 21st Street Renton County, Washington Job No: 17-166 September 2017 Figure 4 DEVELOPMENT ENGINEERING, PLLC P.O. Box 446 Tacoma, WA 98401 Ph (253) 228-0513 www.de-civil.com Soil Classification Chart Schneider SP 7XX S 21st Street Renton County, Washington Job No: 17-166 September 2017 Figure 5 ` Test Pit TP-1 Location: (See Figure 2) Depth (ft.) Soil Type Description 0 – 0.5 Topsoil 0.5 – 5.0 SM Brown silty sand and gravel (cemented, dense) 5.0 – 6.0 Tan Sandstone (fractured) Terminated at 6.0 feet below ground surface. No caving observed. Evidence of seasonal groundwater at 6 inches. Test Pit TP-2 Location: (See Figure 2) Depth (ft.) Soil Type Description 0 – 1.0 Brown silty SAND with gravel Fill (asphalt and concrete debris) 1.0 – 2.5 SM Brown silty SAND w/ gravel, (weakly cemented, dense) 2.5 – 7.0 ML grey SILT w/sand & gravel, (stiff) 7.0 – 8.0 tan Sandstone (fractured) Terminated at 8.0 feet below ground surface. No caving observed. Evidence of seasonal groundwater at 12 inches. Test Pit TP-3 Location: (See Figure 2) Depth (ft.) Soil Type Description 0 – 0.5 Top soil 0.5 – 2.0 SM Brown silty SAND w/ gravel, (weakly cemented, dense) 2.0 – 4.5 ML grey SILT w/sand & gravel, (stiff) 4.5 – 6.0 tan Sandstone (fractured) Terminated at 6.0 feet below ground surface. No caving observed. Evidence of seasonal groundwater at 12 inches. Logged by WGC, 9/4/17 DE-CIVIL, PLLC P.O. Box 446 Tacoma, WA 98401 Ph (253) 228-0513 www.de-civil.com Test Pit Logs Schneider SP 7XX S 21st Street Renton County, Washington Job No: 17-166 September 2017 Figure 6