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HomeMy WebLinkAboutRS_Geotechnical_Report_180330_v1NELSON GEOTECHNICAL N GA ASSOCIATES, INC. GEOTECHNICAL ENGINEERS & GEOLOGISTS Main Office 17311 — 135'h Ave NE, A-500 Woodinville, WA 98072 (425) 486-1669, FAX (425) 481-2510 June 21, 2013 Peer Frank, LLC c/o Devin Sherrell 800 SW 16th Street Renton, Washington 98057 Geotecimical Engineering Evaluation Shuttle Express Parking Lot and Infiltration 800 SW 16' Street Renton, Washington NGA File No. 875613 Dear Mr. Sherrell: Engineering -Geology Branch 5526 Industry Lane, #2 East Wenatchee, WA 98802 (509) 665-7696 - FAX (509) 665-7692 This letter documents our explorations and provides our opinions and recommendations for the proposed parking lot construction and stormwater infiltration at the existing Shuttle Express office building located at 800 SW 161h Street in Renton, Washington, as shown on the vicinity map in Figure 1. INTRODUCTION The planned improvements will consist of constructing an approximately 16,000 square foot parking lot within the southern portion of the property along SW 161h Street, along with associated underground utilities, and a stormwater management system on the property. The site is currently occupied by a multi- level office building within the central portion of the property with associated parking areas on the north, east and west sides of the building. The existing and proposed site layouts are shown on the Schematic Site Plan in Figure 2. For our use in preparing this letter, you have provided us with an aerial image of the property showing the proposed location of the parking lot area. Stormwater Infiltration Letter Shuttle Express Parking Lot and Infiltration Renton, Washington SCOPE June 21, 2013 NGA File No. 875613 Page 2 The purpose of this study is to explore and characterize the subsurface conditions in specific areas of the site and to provide opinions and recommendations for the proposed parking lot and the potential for stormwater infiltration. Specifically, our scope of services includes the following: 1. Review existing soils and geologic maps of the area. 2. Explore the site subsurface soil and groundwater conditions with backhoe -excavated test pits. Backhoe to be subcontracted by NGA. 3. Perform on-site infiltration testing. 4. Perform laboratory analysis on representative soil samples, as needed. 5. Provide recommendations for stormwater infiltration per 2009 King County Surface Water Design Manual and City of Renton Ammendments. 6. Provide recommendations for pavement subgrade preparation and pavement sections. 7. Provide recommendations for site drainage and erosion control. 8. Provide recommendations for infiltration system installation. 9. Document the results of our explorations, findings, conclusions, and recommendations in a written geotechnical engineering report. SITE CONDITIONS Surface Conditions The site is a relatively level, rectangular shaped lot covering approximately 3.25 acres. The site is bounded to the north by Interstate 405, to the east and west by commercial properties and to the south by SW 161" Street. An existing multi-level office building is located within the central portion of the site with asphalt parking and driveway areas along the north, east and west sides of the building. The proposed parking lot expansion is to be located within a grass lawn area to the south of the office building along SW 16' Street. We did not observe surface water on the site during our visit on June 3, 2013. Subsurface Conditions Geology: The site is mapped on the Geologic May of the Renton Quadrangle King County, Washington, by D.R. Mullineaux (1965). The site is mapped as Alluvium (Qaw). The alluvium deposits are described as sand silt and clay deposits. Our explorations generally encountered undocumented fill soils underlain by silty sand to silt deposits generally consistent with the description of alluvium at depth. NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Letter Shuttle Express Parking Lot and Infiltration Renton, Washington June 21, 2013 NGA File No. 875613 Page 3 Explorations: The subsurface conditions within the site were explored on June 3, 2013 by excavating four test pits with a backhoe. The approximate locations of our explorations are shown on the Schematic Site Plan in Figure 2. A geologist from Nelson Geotechnical Associates, Inc. (NGA) was present during the explorations; collected samples of the soils encountered, and maintained logs of the explorations. The soils were visually classified in general accordance with the Unified Soil Classification System, presented as Figure 3. The logs of the explorations are presented as Figure 4. Each of the test pits exposed a surfrcial layer of topsoil and grass to a depth of 0.2 feet below the existing ground surface. Underlying the topsoil and grass in all of our test pits we encountered approximately 3.8 to 4.8 feet of medium dense to dense, brown gray fine sand with silt, trace gravel and varying amounts of organics which we interpreted as undocumented fill. Below the fill in all of our test pits, we encountered loose/soft, iron -oxide stained, brown gray to gray silty sand and sandy silt with varying amounts of gravel and organics underlain by soft to very soft, blue gray to gray sand, silt with abundant orgaincs that we interpreted as native alluvium soils mapped for the area. All of our test pits were terminated within the native alluvium soils at depths in the range of 8.5 to 12.0 feet below the existing ground surface. Hydrologic Conditions Groundwater seepage was observed at approximately 11.0 and 7.5 feet below the ground surface in Test Pit 1 and 3, respectively. In addition, zones of iron oxide staining and saturated soil conditions were encountered within all of the test pits between approximately 4.0 to 12.0 feet. In our opinion, the groundwater seepage, iron oxide staining and saturated soil conditions are indicative of the high groundwater table associated with the nearby river. SEISMIC HAZARD The site is shown on the Liquefaction Susceptibility of the Renton Quadrangle, Washington by Stephen P. Palmer, Henry W. Schasse, Dave K. Norman, et. al. (WSDNR, 1994) to have a high liquefaction potential. Liquefaction is the temporary loss of the shear strength of a loose soil deposit (usually submerged clean fine to medium sand) due to a rise in pore water pressures during a seismic event. When subjected to prolonged cyclic dynamic loads, the pore pressures within a saturated sand deposit increase, approaching the total overburden pressure. At that time, a "quick" condition develops causing the soil to temporarily lose its load carrying ability. Depending on the depth to the liquefiable soils and actual loads, surface settlements can result from liquefaction. NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Letter Shuttle Express Parking Lot and Infiltration Renton, Washington June 21, 2013 NGA File No. 875613 Page 4 The majority of the site soils at depth within our explorations consisted of low permeability silts and silty sand. We did not encounter saturated clean sand layers within our explorations. However, clean sand layers under groundwater table may exist at depth underlying the site. Based on the soils encountered within our explorations, we expect the liquefaction potential for the upper site soils to be low. However, there may be some potential of liquefaction of soil layers at depth below the site, but in our opinion we would anticipate the impact to the site would be minimal. ON-SITE INFILTRATION TESTING We conducted two on-site infiltration tests in the proposed parking lot area. The infiltration tests were performed within the upper granular fill and underlying native silty sand soils within the parking lot area. For the infiltration tests, we installed a six-inch diameter vertical tube into the native silty sand soils in Test Pit 2 at a depth of 5.0 feet below the adjacent ground surface and in the upper granular fill soils in Test Pit 4 at a depth of approximately 4.0 feet below the adjacent ground surface. Approximately two inches of pea gravel was placed over the soil in the bottom of the tube to limit disturbance of the soil when water was added to the apparatus. We filled the tube with water and allowed the water to pre-soak the underlying soils before the actual infiltration testing. After the presoak, we conducted timed infiltration tests. The infiltration test results are shown below in Table 1. We have not applied a factor of safety to the in-place infiltration test results shown in the table. Table 1. Field Results - hi -Place Infiltration Test Results Location Approximate depth [ft.] Duration of Test [min.] Head Loss [in.] Measured Infiltration Rate [in/hr TP -2 5.0 52 6.0 6.92 TP -4 4.0 26 6.0 1 13.84 CONCLUSIONS AND RECOMMENDATIONS As stated in the 2009 King County Surface Water Design Manual, a minimum requirement of three feet of permeable soil below the bottom of the infiltration facility, and at least three feet of separation from the bottom of the infiltration facility and the maximum wet -season water table needs to exist. In our opinion, based on the soil and groundwater conditions encountered in our test pits, the site conditions do not meet these minimum requirements for infiltration. NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Letter June 21, 2013 Shuttle Express Parking Lot and Infiltration NGA File No. 875613 Renton, Washington Page 5 It is our opinion that the site subsurface soils are poorly suited for traditional stormwater infiltration trenches. The subsurface soils generally have a high silt content and indications of shallow groundwater within these soils were observed in all of our explorations. It is also our opinion that the use of pervious pavement surfacing to allow runoff from the parking lot to infiltrate into the granular fill layer found within the site is feasible. Specific recommendations for pervious pavement design have been provided in the Pervious Pavement Design subsection of this letter. Pervious pavement requires on-going maintenance in order to prolong the life span of the pavement. Specific recommendations for maximizing the life span of the proposed pervious pavement are included in the Pavement Maintenance subsection of this letter. We should emphasize that on-going maintenance of the pavement is essential for the long-term success of this approach. All recommendations for pavement subgrade preparation and maintenance as presented in this letter should be followed for the parking lot. We are providing a preliminary pavement design; however, this design should be verified by the civil engineer, based on actual anticipated loads and expected performance and life span of the pavement, and should also be checked against the recommendations of the pavement supplier. Pervious Pavement Design We recommend that the pervious pavement section for the parking lot be a minimum of 6 inches thick and underlain by a minimum of 12 inches of clean crushed rock. This pavement section is preliminary and assumes only light traffic would be imposed on the pavement. Permanent design should be the subject to specific analysis prepared by the civil or traffic engineer and approval by the supplier. Based on the low permeability soils encountered throughout the site at depth, we recommend that the pervious pavement section be underlain by a minimum of 12 inches of 3/4 -inch clean crushed gravel with a minimum void ratio of 15 to 20 percent. The gravel should contain no more than 3 percent fines by weight in order to readily infiltrate into the underlying soils. The gravel layer should help facilitate infiltration, but will also aid in providing storage for infiltrating water. This layer should only be lightly compacted. The subgrade below the crushed rock should be stripped of grass and topsoil material to expose the granular fill soils prior to placing the crushed rock. These soils should generally be encountered approximately one foot below the existing ground surface. The subgrade below the gravel layer should be scarified to a depth of six inches and graded level. The exposed subgrade should not be heavily compacted or contaminated with silt, as these conditions may reduce the infiltration capability of NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Letter Shuttle Express Parking Lot and Infiltration Renton, Washington June 21, 2013 NGA File No. 875613 Page 6 this material. Also, construction traffic on the exposed subgrade should be avoided. We should be retained to observe subgrade preparation prior to placing the crushed rock layer. Based on our observations and on-site infiltration testing along with the varying nature of the underlying soils throughout the site, we recommend that a design infiltration rate of 0.5 inches per hour be used for the granular fill material encountered near the ground surface. Additional recommendations and information regarding pervious pavements are discussed on Pages 97 through 111 in the Low Impact Design Technical Guidance Manual for Puget Sound, (Puget Sound Action Team & Washington State University, May 2005). The existing underlying fill and crushed rock surface should be proof -rolled with a heavy, rubber -tired piece of equipment, to identify soft or yielding areas that require repair, however, the clean gravel layer should only be lightly compacted. We should be retained to observe the proof -rolling, as well as the placement of the gravel layer, prior to placement of pervious pavement. After installation, pervious pavements should be thoroughly swept and pressure -washed periodically to minimize siltation potential as discussed in the following section. Pavement Maintenance Typical methods for maintaining paved parking areas should be applied to pervious pavement. These methods include power sweeping, blowing, vacuuming, and high-pressure washing. Keeping debris from accumulating on top of and within the pervious pavement is important to maintaining the design infiltration capacity of the system. Should the pavement become partially clogged with debris, permeability could possibly be re-established with the use of vacuuming or pressure washing. Stormwater from sources outside the pavement should never be directed onto the pervious pavement or subgrade. Fines and organic material from off-site sources could potentially promote clogging of the pavement section. Following the recommendations provided in this letter should help reduce the migration of silt and debris into the pavement section. USE OF THIS LETTER This letter was prepared for Peer Frank, LLC and their agents, for their use in planning and budgeting the above -referenced project only. Our services included an evaluation of the infiltration capability of the site soils at specific locations, and should not be considered as an in-depth geotechnical study. This letter may be used for bidding and estimating purposes, but our letter, conclusions, and interpretations should NELSON GEOTECHNICAL ASSOCIATES, INC. Storrawater Infiltration Letter Shuttle Express Parking Lot and Infiltration Renton, Washington June 21, 2013 NGA File No. 875613 Page 7 not be construed as a warranty of the subsurface conditions. The subsurface conditions between explorations may vary. A contingency for changed conditions should be incorporated into the project plans. Within the limitations of scope, schedule, and budget, our services have been performed in accordance with generally accepted geotechnical engineering practices in effect in this area at the time this letter was prepared. No other warranty, expressed or implied, is made. Our observations, findings, and opinions are a means to identify and reduce the inherent risks to the owner. M•. NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Letter Shuttle Express Parking Lot and Infiltration Renton, Washington June 21, 2013 NGA File No. 875613 Page 8 We appreciate the opportunity to provide service to you on this project. If you have any questions or require further information, please call. Sincerely, NELSON GEOTECHNICAL ASSOCIATES, INC. Lee S. Bellah, LG Project Geologist Khaled M. Shawish, PE Principal LSB:KMS:kmn Four Figures Attached Three Copies Submitted NELSON GEOTECHNICAL ASSOCIATES, INC.