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Home My WebLink AboutRS_Geotech Report 711 Renton_190918_v1 Geotechnical and Preliminary Environmental Investigation Report for UST Replacement 7-Eleven Store No. 14441 11505 SE 168th Street, Renton, WA Facility No.: 99786418 UST Site ID: 8705 September 18, 2019 Prepared for: 7-Eleven, Inc. P.O. Box 711 Dallas, TX 75221-0711 Prepared by: Stantec Consulting Services Inc. 11130 NE 33rd Place, Suite 200 Bellevue, Washington 98004 GEOTECHNICAL AND PRELIMINARY ENVIRONMENTAL INVESTIGATION REPORT FOR UST REPLACEMENT 7-ELEVEN STORE NO. 14441 – 11505 SE 168TH STREET, RENTON, WA i Table of Contents ABBREVIATIONS ........................................................................................................................ II 1.0 INTRODUCTION ............................................................................................................ 1.1 2.0 PROJECT DESCRIPTION ............................................................................................. 2.1 3.0 SITE DESCRIPTION ...................................................................................................... 3.2 4.0 SUBSURFACE EXPLORATION ................................................................................... 4.3 4.1 GEOTECHNICAL INVESTIGATION PROGRAM........................................................... 4.3 4.2 ENVIRONMENTAL INVESTIGATION PROGRAM ........................................................ 4.4 5.0 SOIL AND GROUNDWATER CONDITIONS ................................................................ 5.5 5.1 AREA TOPOGRAPHY AND GEOLOGY........................................................................ 5.5 5.2 SOIL CONDITIONS ....................................................................................................... 5.7 5.3 GROUNDWATER CONDITIONS ................................................................................... 5.7 6.0 GENERAL DISCUSSION AND RECOMMENDATIONS ............................................... 6.7 6.1 TEMPORARY EXCAVATIONS ...................................................................................... 6.9 6.2 GROUNDWATER INFLUENCE ON CONSTRUCTION ............................................... 6.10 6.3 EXCAVATION MONITORING FOR POTENTIAL FUEL IMPACTS ............................. 6.10 6.4 CLOSURE .................................................................................................................... 6.11 7.0 REFERENCES ............................................................................................................... 7.1 LIST OF FIGURES Figure 1 Site Location Map Figure 2 Site Vicinity Map Figure 3 Site Plan with Existing UST and Soil Boring Locations Figure 4 Site Plan with Proposed Replacement UST Locations LIST OF APPENDICES Appendix A Soil Boring Logs Appendix B Laboratory Testing Reports – Geotech Appendix C Laboratory Testing Reports - Environmental GEOTECHNICAL AND PRELIMINARY ENVIRONMENTAL INVESTIGATION REPORT FOR UST REPLACEMENT 7-ELEVEN STORE NO. 14441 – 11505 SE 168th STREET, RENTON, WA Introduction ii Abbreviations 7-Eleven DNR Ecology EPA HASP mg/kg MTCA N-value NRCS NEHRP PID psi SPT Stantec TPH USGS UST WAC 7-Eleven, Inc. (the client) Washington Department of Natural Resources Washington State Department of Ecology Environmental Protection Agency health and safety plan milligrams per kilogram Model Toxics Control Act standard penetration resistance using a SPT sampler National Resources Conservation Service National Earthquake Hazard Reduction Program photoionization detector pounds per square inch standard penetration test (a specific soil sampling tool and method) Stantec Consulting Services Inc. total petroleum hydrocarbons United States Geological Survey underground storage tank Washington Administrative Code GEOTECHNICAL AND PRELIMINARY ENVIRONMENTAL INVESTIGATION REPORT FOR UST REPLACEMENT 7-ELEVEN STORE NO. 14441 – 11505 SE 168th STREET, RENTON, WA Introduction 2.1 1.0 INTRODUCTION Stantec Consulting Services Inc. (Stantec) was retained by 7-Eleven, Inc. (7-Eleven) to perform a geotechnical assessment to support upcoming underground storage tank (UST) replacement at 7-Eleven Store No. 14441 located at 11505 SE 168th Street, Renton, King County, Washington (the Site; Figures 1 and 2). The Site is currently occupied by the active 7-Eleven Store No. 14441 with active fueling and a convenience store. The purpose of the geotechnical assessment was to evaluate Site subsurface conditions and to provide this report of geotechnical recommendations for temporary excavations, shoring, and excavation backfill compaction during UST removal/replacement activities. 2.0 PROJECT DESCRIPTION Based on information provided by Barghausen Consulting Engineers’ (BCE) design drawing Sheet G0.1 (dated April 16, 2018), the proposed construction includes the removal of three 12,000-gallon gasoline USTs and ancillary piping followed by the installation of two 15,000-gallon USTs (one as a dual- compartment UST) along with vent piping, product piping, and other equipment. The existing USTs to be removed are located in a single tank pit in the west portion of the site, near the west property line and close to the front of the convenience store building (Figure 3). The existing dispenser area is located partially on top of the northern portion of the UST basin with short existing product piping runs; however, the canopy is to remain during UST removal and replacement activities.The proposed replacement USTs are planned to be installed in a somewhat over-lapping excavation as the existing USTs (Figure 4) with the replacement UST basin being located approximately 6 to 8 feet east of the existing UST basin (a little further away from the western property line) but a little closer to the store building. The new dispenser area will be a short distance northwest (but not on top) of the replacement USTs and an oil-water separator will be installed near the west end of the new dispenser area (Figure 4). Stantec is unaware of the depths of the existing and replacement product piping but piping is normally placed at depths ranging from 2.5 to 3.5 feet. The depth and tank backfill details for the proposed new USTs and excavation area were not provided with BCE’s Sheet G0.1; however, based on the standard 10-foot diameter of a 15,000-gallon UST, we anticipate that the UST removal and replacement work will require excavations extending to depths of 15 feet below ground surface (bgs). Based on recent UST installations at other 7-Eleven facilities, we expect the replacement tank excavation will be backfilled with pea gravel half-way up the new tanks. Imported or suitable onsite structural fill will likely be placed by the contractor up to subgrade elevations and compacted by the contractor to municipality requirements with finish grade achieved by a minimum of 6 inches concrete (at the replacement tank and canopy slabs). GEOTECHNICAL AND PRELIMINARY ENVIRONMENTAL INVESTIGATION REPORT FOR UST REPLACEMENT 7-ELEVEN STORE NO. 14441 – 11505 SE 168th STREET, RENTON, WA Site Description 3.2 Based on a review of the Washington State Department of Ecology (Ecology) Regulated USTs database (accessed by Stantec on September 1, 2019), one of the three existing USTs had been in temporary closure status. Putting a UST into temporary closure indicates that a UST had failed tank integrity testing and, as a result, the potential for leakage from the tank (normally at the spill bucket or a piping connection at the UST; periodically the UST itself) can occur. As such, this project involved environmental sampling. During the time of Stantec’s geotechnical assessment, three types of gasoline (regular unleaded, mid-grade unleaded, and premium/super unleaded) were actively being dispensed, indicating that the three single-compartment USTs were operational. Further review of Ecology and King County assessor records reveals that the onsite building and two of the USTs date back to 1967/1968 with the third UST installed in 1987 and UST grade work in 1998. 3.0 SITE DESCRIPTION The site is located at 11505 SE 168th Street in Renton, Washington at the southwest corner of SE 168th Street and 116th Avenue SE, as shown on Figure 1. The site is bordered on the north by SE 168th Street and further north, by an open undeveloped area but currently occupied by two storage containers (labeled “National Mobile Storage” and “BJF”); on the east by 116th Avenue SE and further east, by several baseball fields; and on the south and west by single-family residences. Per City of Renton Zoning & Comprehensive Plan Maps (accessed online by Stantec on September 2, 2019), the site and the area to the east are zoned “CA” (Community Arterial); the areas to the south and west are zoned “R-1” (Residential 1; described as Low-Density Residential); and the area to the north is zoned “R-8” (Residential 8; described as Medium-Density Residential). At distance from the site, all areas are zoned as Medium- to High-Density Residential. The site consists of one tax parcel identified as King County Assessor’s Parcel #2892600051 (King County Assessor website database; accessed by Stantec on September 2, 2019), and is an approximate 29,717-square-foot, rectangular parcel of developed land. The site is an active 7-Eleven convenience store and retail fueling station with three gasoline USTs in the western portion of the site; a canopied two-dispenser pump fueling area also in the western portion of the site (immediately adjacent to and northwest of the USTs), and a single-story 2,560-square-foot convenience store in the southern portion of the site (Figure 2). The existing UST basin is within 12 feet of the store. Near the west property line and immediately west of the existing USTs, there is an approximate 3-foot-high vertical retaining wall beyond which a slight slope rises up to the fenced property line and the westerly- adjacent residential lot is approximately 4 feet higher than the site. The entire site is paved (concrete at the existing USTs and dispenser island area, or asphalt at the remaining areas) with no planters. Access driveways are present at the northwest and southeast corners of the site with both driveways sloping gently up into the site. Except for the site’s northern and eastern perimeters, the site is nearly flat. Along most of the northern side of the site, the site’s paved surface is gently sloped southerly back towards the center and/or eastern portions of the site. As such, surface water flows southerly and easterly across the site’s asphalt-paved surface, being directed towards the southeast driveway where it flows offsite and into the adjacent street’s curbside stormwater grate (located a short distance south of the site’s southeast driveway). There are no catch basins at the site. GEOTECHNICAL AND PRELIMINARY ENVIRONMENTAL INVESTIGATION REPORT FOR UST REPLACEMENT 7-ELEVEN STORE NO. 14441 – 11505 SE 168th STREET, RENTON, WA Subsurface Exploration 4.3 4.0 SUBSURFACE EXPLORATION The subsurface investigation consisted primarily of a geotechnical investigation with a secondary environmental component. Two 30-foot-deep soil borings (GT-1 and GT-2; Figure 3) were drilled, sampled, and logged. Common to both drilling programs, however, were health and safety program/plan (HASP) and pre-drilling utility clearances as outlined below. Prior to field activities, a site-specific HASP was prepared for the drilling and sampling work. The HASP identified potential physical, biological, and chemical hazards associated with the planned field activities and established personal protection standards and mandatory safety practices. The HASP included a list of monitoring equipment, protective clothing and equipment, map and directions to the nearest hospital, and a list of emergency telephone numbers. The HASP was available onsite during the field activities. Stantec personnel and subcontractors working on the Site were required to review, sign, and comply with the provisions set forth in the HASP. Prior to drilling, Stantec contacted One Call, a municipal underground utility location service to identify subsurface municipal utilities located in the public right-of-way (Ticket #19352627). Additionally, Stantec hired GeoMarkOut (a private underground utility location service based in Seattle-Tacoma area, Washington) to clear the proposed borings at the Property; GeoMarkOut performed their non-invasive field work on August 12, 2019. The soil borings were also cleared to depths of approximately 5 feet bgs with an air-knife and vacuum unit on August 21, 2019 (prior to drilling) using equipment provided by Holocene Drilling. 4.1 GEOTECHNICAL INVESTIGATION PROGRAM The geotechnical field investigation program was completed on August 21, 2109 that included drilling and sampling two hollow stem auger 30-foot-deep soil borings (GT-1 and GT-2; Figure 3). Boring GT-1 was located immediately south of the south end of the existing USTs and very close to the front of the convenience store (in between the USTs and the convenience store). Boring GT-2 was located immediately northwest of the existing USTs and west of the dispenser area (between the dispenser area and the small retaining wall near the west property line). The soil borings were drilled by Holocene Drilling with 8-inch-outer-diameter hollow stem augers using a truck-mounted Mobile B-58 drill rig. The exploration was monitored by a Stantec field geologist to provide technical observations of the field program. During drilling, disturbed soil samples were obtained at approximate 5-foot intervals (5, 10, 15, 20, 25, and 30 feet below ground surface [bgs]) using a standard 18-inch-long, 1.5-inch-inner-diameter (2-inch- outer-diameter), split-spoon sampler driven using a 140-pound hammer free falling a vertical distance of 30 inches. The summation of hammer-blows required to drive the sampler the final 12-inches of an 18- inch sample interval is defined as the Standard Penetration Resistance, or N-value. GEOTECHNICAL AND PRELIMINARY ENVIRONMENTAL INVESTIGATION REPORT FOR UST REPLACEMENT 7-ELEVEN STORE NO. 14441 – 11505 SE 168th STREET, RENTON, WA Subsurface Exploration 4.4 The soils encountered were logged in the field during drilling and described in accordance with the Unified Soil Classification System (USCS; ASTM D 2487). Copies of the GT-1 and GT-2 boring logs are provided in Appendix A. The exact locations of soil borings GT-1 and GT-2 were not professionally surveyed; however, their approximate locations were obtained by taping the distances from existing ground-based points at the site, then using Google EarthPro imagery to plot the locations of the borings onto Figure 3. Three soil samples (GT-1-5’; GT-1-15’; and GT-2-5’) were collected and transported to Krazan & Associates (Krazan; a construction materials testing firm based in Lynnwood, WA) for sieve analysis testing using ASTM C-136 and ASTM C-117. Results are discussed at the end of Section 5.2. Copies of Krazan’s laboratory test results are provided in Appendix B. 4.2 ENVIRONMENTAL INVESTIGATION PROGRAM The environmental field investigation was completed concurrently with the geotechnical investigation. The purpose of the environmental program was to evaluate the potential presence of petroleum contaminated soil (PCS) that could be exposed during future UST replacement and excavation activities. During drilling, soil samples were collected at approximate 5-foot intervals and were field-screened for potential PCS using visual and olfactory methods and a portable photoionization detector (PID). No field evidence of impact was observed; however, three confirmation samples from each boring were selected for submittal to Test America, a WA-certified chemical analytical testing laboratory: 5 feet bgs (equivalent to base of UST over-spill bucket; 15 feet bgs (equivalent to near UST invert; and 30 feet bgs (bottom of boring). Soil samples for volatile organic analysis were collected in accordance with EPA Sampling Method 5035 using TerracoresTM and extra soil was contained in glass jars that were sealed with Teflon- lined lids. The sample containers were labeled, sealed in ZiplockTM plastic bags, and placed in a chilled cooler with ice for transport to the WA-certified laboratory under proper chain-of-custody documentation. In addition to the soil samples, one groundwater sample was collected from near the bottom of the GT-2 boring. This single groundwater “grab” sample was collected by lowering a clean bailer down through the annulus of the augers, allowing it to fill with groundwater, retrieving the bailer, and filling clean, laboratory- supplied glassware with the sampled groundwater. The groundwater sample containers were labeled, wrapped in bubble-wrap to prevent breakage, sealed in ZiplockTM plastic bags, and placed in a chilled cooler with ice for transport to the WA-certified laboratory under proper chain-of-custody documentation. The single groundwater “grab” sample and six soil samples were analyzed for Hydrocarbon Identification (HCID) using method NWTPH-HCID. Analytical results were compared to WA Department of Ecology Model Toxic Control Act (MTCA) Method A Cleanup Levels (CULs) and are summarized in the following tables. Copies of the Test America laboratory certificates of analyses are provided in Appendix C. GEOTECHNICAL AND PRELIMINARY ENVIRONMENTAL INVESTIGATION REPORT FOR UST REPLACEMENT 7-ELEVEN STORE NO. 14441 – 11505 SE 168th STREET, RENTON, WA SOIL AND GROUNDWATER CONDITIONS 5.5 Soil Sample Chemical Analytical Results (data reported in milligrams per kilogram [mg/kg]; sample date of Aug. 21, 2019) Sample ID TPHg (mg/kg) TPHd (mg/kg) TPHo (mg/kg) GT-1-5’ ND<28.7 ND<57.4 ND<115 GT-1-15’ ND<26.0 ND<51.9 ND<104 GT-1-30’ ND<27.1 ND<54.2 ND<108 GT-2-5’ ND<26.1 ND<52.2 ND<104 GT-2-15’ ND<26.7 ND<53.3 ND<107 GT-2-30’ ND<28.0 ND<56.0 ND<112 MTCA Method A CULs 30 2,000 2,000 For both tables, TPHg, TPHd, TPHo = total petroleum hydrocarbons as gasoline, diesel, and oil, respectively. Groundwater “Grab” Sample Chemical Analytical Results (data reported in micrograms per liter [ug/L]) Sample ID TPHg (ug/L) TPHd (ug/L) TPHo (ug/L) GT-2 ND<143 ND<361 ND<361 MTCA Method A CULs 800 500 500 5.0 SOIL AND GROUNDWATER CONDITIONS 5.1 AREA TOPOGRAPHY, GEOLOGY, AND HYDROLOGY The site is situated in the northeast corner of the SE ¼ of Section 29, Township 23 North, Range 5 East, Willamette Baseline and Meridian, in King County, Washington. The ground surface elevation of the site is approximately 462 feet above mean sea level (msl) and the site itself is generally level. Topography in the immediate site area is also relatively level. Topography in the larger overall site area remains relatively flat to the north but drops gently to the east, south, and west. Topography drops to the west with ground surface elevations down to 25 feet above msl approximately 1.5 miles west of the site. Similarly, GEOTECHNICAL AND PRELIMINARY ENVIRONMENTAL INVESTIGATION REPORT FOR UST REPLACEMENT 7-ELEVEN STORE NO. 14441 – 11505 SE 168th STREET, RENTON, WA SOIL AND GROUNDWATER CONDITIONS 5.6 topography drops down to the northeast with ground surface elevations down to approximately 50 feet above msl at the northwesterly-flowing Cedar River approximately 1.5 miles northeast of the site. The site sits on a raised terrace within the central Puget Lowland which is part of a regional north-south trending trough that extends from southwestern British Columbia to near Eugene, Oregon. The Puget Lowland is bound to the west by the Olympic Mountains and to the east by the Cascade Mountain Range. In the overall site area (north of Olympia, WA), this lowland is glacially carved with a depositional and erosional history that includes at least four separate glacial advances/retreats and is filled with glacial and non-glacial sediments consisting of interbedded gravel, sand, silt, till, and peat lenses. The raised terrace where the site is located has several lakes (remnant glacial-related retreats) with creeks that drain the lakes and that cut into the terrace materials, and other kame deposits and features (irregular-shaped mounds of sand, gravel, and till that accumulated in depressions formed by retreating glaciers). The main drainages for the area include: Big Soos Creek (with an unnamed tributary located approximately 0.25 miles west and southwest of the site); northwesterly-flowing Cedar River (approximately 1.5 miles northeast of the site); Panther Lake (approximately 2 miles south of the site and a northwesterly-flowing unnamed creek exiting Panther Lake); and the north/northwesterly-flowing Green River (approximately 3 miles of the site). King County iMap shows a wetlands area where the unnamed tributary of Big Soos Creek is located approximately 0.25 miles southwest of the site. The Geologic Map of the Renton, WA Quadrangle (USGS Geologic Quadrangle Map GQ-405, 1965) and the Geologic Map of Washington (WA Department of Natural Resources [DNR] GM-53, 2005) both show that the site is underlain by Pleistocene-age continental glacial drift deposits (Qgd). More specifically, the site is underlain by Upper Pleistocene Vashon-stage Drift that is mapped as Qgt (ground moraine deposits) and described as a thin veneer of ablation (relatively loose and permeable) till overlying lodgment till that consists of very hard, compact, nearly impermeable unsorted sand, silt, clay, and gravel with locally poor surface drainage. As such, the corresponding liquification susceptibility has been mapped as “low” (DNR, Open File Report 92-7, 1992). The site is located in the Duwamish-Green River Watershed and within Water Resource Inventory Area (WRIA) 9, further identified by name as the Duwamish-Green inventory area. No critical aquifer recharge areas (CARAs) exist in the immediate site area (King County iMap website, accessed by Stantec on September 2, 2109). Stantec reviewed available boring logs and well records on Ecology’s Well Records database for the immediate site area. Review of the site-area boring logs for soil borings located at Cascade Village (located 1.5 blocks south of the site with a ground surface elevation approximately 9 feet lower than the site) revealed that most borings were drilled to depths of 14 to 21 feet bgs with native soil consisting of dense sands, gravels, and silts (identified as “till”). One shallow soil boring noted a thin water-bearing horizon at a depth of approximately 11 feet. Another Cascade Village soil boring was drilled to a depth of 35 feet bgs with local groundwater encountered at 28 feet bgs. One groundwater monitoring well was drilled at Cascade Village: it was drilled to a depth of 82 feet bgs with groundwater encountered at 64 feet bgs. GEOTECHNICAL AND PRELIMINARY ENVIRONMENTAL INVESTIGATION REPORT FOR UST REPLACEMENT 7-ELEVEN STORE NO. 14441 – 11505 SE 168th STREET, RENTON, WA GENERAL DISCUSSION AND RECOMMENDATIONS 6.7 5.2 SITE SOIL CONDITIONS Details of the soil conditions encountered during our site investigation are presented on the boring logs in Appendix A. Generalized descriptions of the subsurface conditions encountered at the borings are provided below and is characteristic of till. At Boring GT-1 (located at the south end of the existing USTs and directly in front of the convenience store), moist dense silty sand with trace gravel was encountered to a depth of 10.5 feet bgs. From 10.5 to approximately 14.5 feet bgs, hard dry silt with some fine gravel was present. From approximately 14.5 to 19.5 feet bgs, hard dry silt (no gravel) was present. From approximately 19.5 to 30 feet bgs (maximum depth explored), soil was dense, dry, silty sand and silty sand with some gravel. At Boring GT-2 (located along the northwest side of the existing USTs, close to the site’s low retaining wall and near the site’s western property line), moist dense silty sand with varying amounts of gravel is present throughout the entire length of the boring. A thin water-bearing horizon was encountered at a depth of approximately 17 feet bgs and groundwater was present at the bottom of the 30-foot boring. 5.3 SITE GROUNDWATER CONDITIONS Groundwater was not encountered in GT-1 with soil samples described as dry to moist. At GT-2, a thin water-bearing horizon at an approximate depth of 17 feet (slightly deeper than the anticipated UST replacement excavation depth of 15 to 16 feet bgs) and local groundwater encountered at approximately 29 to 29.5 feet bgs. Based on these depths, groundwater is not expected to be encountered during the UST replacement activities; however, it is possible as water table elevations can fluctuate over time. Groundwater level depends on a variety of factors that may include seasonal precipitation, irrigation, land use, climatic conditions and soil permeability. Water levels at the time of our field investigation may be different from those encountered during the construction phase of the project. 6.0 GENERAL DISCUSSION AND RECOMMENDATIONS Our borings indicate that the site is underlain by dense silty sand with gravel and/or hard dry interbedded silt (“till”) to depths of 30 feet bgs. These dense soils can represent stable materials which may not slough and may not require support during excavation. However, based on the close proximity of the southern portion of the existing UST basin to the convenience store foundation (approximately 10 to 12 feet away) and on the close proximity of the western portion of the existing UST basin to the low retaining wall (within 2 feet), support of excavation is strongly recommended. It is Stantec’s opinion that temporary excavation support to the full excavation depth (15 to 16 feet bgs) will be necessary to facilitate UST removal and replacement along the southern and western portions of the existing UST basin due to the nearby southerly onsite store building foundation and the westerly GEOTECHNICAL AND PRELIMINARY ENVIRONMENTAL INVESTIGATION REPORT FOR UST REPLACEMENT 7-ELEVEN STORE NO. 14441 – 11505 SE 168th STREET, RENTON, WA GENERAL DISCUSSION AND RECOMMENDATIONS 6.8 retaining wall with its slope. Protecting the southern portion of the existing UST basin will also support the store foundation for the somewhat over-lapping replacement UST excavation which is even closer to the store than the existing UST basin. Between the existing and replacement USTs and the nearby/adjacent structures, the shoring system should be designed to support an open excavation of approximately 16 feet deep, the adjacent store foundation load, and the retaining wall with the upslope behind the wall to the west and the westerly-adjacent residential lot. Vibratory street-based loads are not expected as the existing and replacement USTs are at distance from the neighboring streets. Site vehicle traffic for the public to access the convenience store (if the store remains open during UST replacement work) and construction equipment/heavy-truck traffic associated with UST replacement should be considered. Shoring/Support of Excavation Recommendations Based on lithology, Stantec expects that slide rail shoring or soldier pile and lagging should be suitable for this project. The structural engineer in charge of excavation support design should take into account possible effects of excavation support installation and removal on the adjacent and nearby structures/foundations. Sheet pile support is not recommended. During slide rail shoring or soldier pile-and-lagging support installation and removal, vibrations in subsurface materials can occur and the potential for sloughing is present. If this occurs, natural support of the nearby store foundation to the south and the adjacent retaining wall to the west may be affected. As a result, onsite structures should be monitored before, during, and after UST replacement work by a surveyor in order to determine if any movements occur. In the case of slide rail shoring, temporary open annular space behind the slide rail support may cause sloughing of the loose to medium-dense native soil around the adjacent dispenser-island canopy foundation(s) and/or the store foundation. Temporary soil strengthening (such as but not limited to placement of controlled density fill [CDF] and/or engineered soil [mixing of Portland Cement or grout and water into the native soil]) or “slot-cutting” excavation methods should be considered. As noted in Section 2, it is unknown by Stantec whether the existing dispenser-area canopy and its footings are to remain during UST replacement activities or whether the canopy and its footings will be removed and replaced. If these features are to remain, the general contractor and/or the project structural engineer need to design and implement an appropriate support system. Onsite Structure Monitoring The onsite structures should be monitored before, during, and after UST replacement work by a surveyor in order to determine if any movements occur. Also, existing cracks in structural elements (all onsite features [including the westerly-adjacent retaining wall], nearby sidewalks, pavement) should be documented before construction begins. We do not anticipate any structural movements; however, it is prudent and customary to monitor structures whenever excavations or heavy construction occurs near existing developments. GEOTECHNICAL AND PRELIMINARY ENVIRONMENTAL INVESTIGATION REPORT FOR UST REPLACEMENT 7-ELEVEN STORE NO. 14441 – 11505 SE 168th STREET, RENTON, WA GENERAL DISCUSSION AND RECOMMENDATIONS 6.9 Backfilling and Compaction Recommendations Following shoring placement, UST removal, and replacement UST installation, excavations should be backfilled with structural fill up to subgrade elevations. The upper onsite fill soils may be suitable for use as structural fill; however, we anticipate the need to import material to supplement the onsite soil. It may be possible to use excavated materials from the eastern portion of the replacement UST basin to backfill the western portion of the existing UST basin. If the contractor wants to do this (rather than bringing in import fill), Stantec recommends that Proctor testing be conducted on one to two soil samples when the eastern portion of the replacement UST basin is excavated. In accordance with 7-Eleven backfilling specifications (Item 7.3.8 of the 7-Eleven Construction SEP Protocol/Operations Manual [P/OM], Version 2019), all backfill should be placed in lifts not exceeding six (6) inches in thickness, compacted to a minimum of 90% relative compaction to within one foot of subgrade, and compacted to a minimum of 95% relative compaction up to subgrade (the final 12 inches). Paving Recommendations In accordance with 7-Eleven pavement specifications (Item 10 of Base Bid Form, part of 7-Eleven’s Construction SEP P/OM, Version 2019, Forms 19 DF04A and 19 DF04B) and other similar 7-Eleven UST replacement projects), a minimum of six (6) inch thick, 3000 PSI concrete with #3 rebar at 24-inch spacing in each direction is required at the replacement UST basin. For heavy-traffic facilities, 7-Eleven’s Construction specifications (in particular, Mechanical Plans for 7-Eleven Standard Concrete Specifications and Details, as referenced on other 7-Eleven contractor design drawings for other similar projects) include eight-inch-thick concrete with #3 rebar at 12-inch spacing in each direction and with the concrete poured on top of the tank manufacturer’s specified pea gravel backfill. Concrete canopy slab requirements include a minimum of six (6) inch thick, 3000 PSI concrete with #3 rebar at 18-inch spacing to be placed on 8 inches aggregate base that is placed on 12 inches of 95%- compacted subgrade material (Mechanical Plans for 7-Eleven Standard Concrete Specifications and Details). Typical asphalt paving details include 3 inches of asphaltic concrete on top of 6 inches of crushed stone base (aggregate base size = one inch maximum) on top of 12 inches of 95%-compacted subgrade material and normally includes conformance with TYPAR 3401 as part of the 7-Eleven asphalt paving requirement. 6.1 TEMPORARY EXCAVATIONS Based on our understanding of the project, we anticipate that the excavation work will include local cuts to depths of approximately 16 feet or less for the overlapping UST removal and replacement locations. We anticipate that excavation work associated with fuel lines, vent lines, and other existing or future utility features will generally be less than 4 feet deep. GEOTECHNICAL AND PRELIMINARY ENVIRONMENTAL INVESTIGATION REPORT FOR UST REPLACEMENT 7-ELEVEN STORE NO. 14441 – 11505 SE 168th STREET, RENTON, WA GENERAL DISCUSSION AND RECOMMENDATIONS 6.10 All temporary excavations should be in accordance with the Washington Administrative Code (WAC) Part N – Excavation, Trenching, and Shoring, King County requirements, and/or City of Renton requirements. As noted above, the temporary slopes should be visually inspected daily during construction activities and the inspections should be documented in daily reports. The contractor is responsible for maintaining the stability of the temporary cut slopes and reducing slope erosion during construction. The temporary cut slopes should be covered with visqueen to help reduce erosion during wet weather with berms placed around the top of the excavation to minimize surface water runoff onto the excavation slopes, and the slopes should be closely monitored until the permanent retaining systems or slope configurations are complete. Soil should not be stockpiled, materials should not be stored, and equipment should not be operated within 10 feet of the top of any temporary cut slope. Soil conditions may not be completely known from the geotechnical investigation. In the case of temporary cuts, the existing soil conditions will not be completely revealed until the excavation work exposes the soil. The maximum inclination of the temporary slopes will need to be evaluated by the geotechnical or structural engineer so that any supplemental recommendation can be made. Soil and groundwater conditions can be highly variable. The contractor and/or the project structural engineer are responsible for developing and designing temporary shoring systems, as needed. 6.2 GROUNDWATER INFLUENCE ON CONSTRUCTION Groundwater was not encountered at boring GT-1; however, at GT-2, a thin perched water-bearing horizon was encountered at a depth of approximately 17 feet and local groundwater was encountered to a depth of approximately 29 to 29.5 feet bgs during the recent drilling at the site. Both of these depths (17 and 29 to 29.5 feet bgs) are below the anticipated UST excavation depths of 15 to 16 bgs. Seasonal conditions may cause a rise in groundwater, especially after periods of heavy rainfall. As such, groundwater may enter into the excavation at a depth of approximately 15 or 16 feet if the thin water-bearing horizon rises and/or if the excavation is deepened below 15 to 16 feet bgs. If water is encountered during UST removal and replacement (either groundwater or percolated surface water that seeps into the tank pit), the amount of water required for excavation dewatering can range from installing a network of temporary excavation dewatering wells around the excavation areas to simply dewatering from one or several temporary sumps directly from the open excavations. The contractor is responsible for the design of any needed dewatering plans. 6.3 EXCAVATION SOIL MONITORING FOR POTENTIAL FUEL IMPACTS Results of chemical analyses on soil samples collected during recent drilling did not reveal fuel impacts; however, the contractor should be ready to handle any fuel-impacted soil that may be encountered during excavation. Stantec should be notified if field evidence of fuel impacts is encountered so that soil samples can be collected and analyzed. Any potentially-impacted soil should be stockpiled separately from clean material, and placed on and covered by visqueen. GEOTECHNICAL AND PRELIMINARY ENVIRONMENTAL INVESTIGATION REPORT FOR UST REPLACEMENT 7-ELEVEN STORE NO. 14441 – 11505 SE 168th STREET, RENTON, WA GENERAL DISCUSSION AND RECOMMENDATIONS 6.11 6.4 CLOSURE This report was prepared for the exclusive use of 7-Eleven, Inc. and their appointed contractors. Any use of this report or the material contained herein by third parties, or for other than the intended purpose, should first be approved by Stantec. The recommendations in this report are based on assumed continuity of soils with those of our test holes. GEOTECHNICAL AND PRELIMINARY ENVIRONMENTAL INVESTIGATION REPORT FOR UST REPLACEMENT 7-ELEVEN STORE NO. 14441 – 11505 SE 168th STREET, RENTON, WA References 7.1 7.0 REFERENCES Barghausen Consulting Engineers (BCE), Dimension Group, 2018. Design Drawing Sheet G0.2, Fueling Architectural Site Plan, 11505 SE 168th Street, Kent, Washington; dated April 16. Received from Fi Companies (7-11 subcontractor) on July 23, 2019. King County, WA, 2019. Tax Assessor online files (website accessed by Stantec on September 2, 2019). Natural Resources Conservation Service (NRCS) Maps, 2019. Surface soil website accessed by Stantec on April 9, 2019. Renton Zoning and Comprehensive Plans, 2019. City online files (website accessed by Stantec on September 2, 2019. United States Geological Survey (USGS), 1965. Geologic Map of the Renton Quadrangle, King County Washington; Geologic Quadrangle May GQ-405 by D. R. Mullineaux. United States Geological Survey (USGS), 2014 and 1949. 7.5-Minute Topographic Quadrangle of Renton, Washington. United States Geological Survey (USGS), 2019. Earthquake Hazards Program (EHP); website accessed by Stantec on April 9, 2019. WA Department of Ecology, 2019. Well Logs (website accessed by Stantec on November 28, 2018). WA Department of Ecology, 2019. Regulated UST Database (website accessed by Stantec on September 1, 2019). WA Department of Natural Resources, 2019. National Earthquake Hazard Reduction Program (NEHRP) Site Class and Liquefaction Susceptibility Maps of King County, WA; Open File Report 2004-20, Maps 17A and 17B, Sheets 33 and 34 of 78; website accessed by Stantec on July 29, 2019. WA Department of Natural Resources, 2005. Geologic Map of Washington State (WA Division of Geology and Earth Resources Geologic Map GM-53) compiled by J. Eric Schuster. WA Department of Natural Resources, 2002. Geologic Map of Washington – Northwest Quadrant (WA Division of Geology and Earth Resources Geologic Map GM-50, scale 1:250,000). WA Department of Natural Resources, 1992. Preliminary Maps of Liquefaction Susceptibility for the Renton and Auburn 7.5-Minute Quadrangles, Washington (WA Division of Geology and Earth Resources Open File Report 92-7) by Stephen P. Palmer. GEOTECHNICAL AND PRELIMINARY ENVIRONMENTAL INVESTIGATION REPORT FOR UST REPLACEMENT Appendix A APPENDIX/DIVIDER TITLE Appendix Subtitle GEOTECHNICAL AND PRELIMINARY ENVIRONMENTAL INVESTIGATION REPORT FOR UST REPLACEMENT Appendix A A.1 Appendix A [Enter the body text here] A.1 HEADING 8 [Enter the body text here] A.1.1 Heading 9 [Enter the body text here]