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HomeMy WebLinkAboutA5 Tech Memo re Falcon Ridge LS_FINAL_With Addend.9/5/2017 3:59 PM \\rh2.com\dfs\Bothell\Data\REN\117-038\Geo\Tech Memo re Falcon Ridge LS.docx Signed: 9/5/17 Signed: 9/5/17 TECHNICAL MEMORANDUM Client: City of Renton Project: Falcon Ridge Lift Station Replacement and Force Main Improvements Project File: REN 117.038.01.101 Project Manager: Edwin Halim, PE Composed by: Steve Nelson, LEG Reviewed by: Tony Pardi, PE Subject: Falcon Ridge Lift Station Engineering Geology Evaluation Date: September 5, 2017 INTRODUCTION The Falcon Ridge Lift Station, located at the east end of SE 8th Place east of Lake Youngs Way SE in Renton WA, is a duplex fiberglass-enclosed lift station that serves the Falcon Ridge subdivision. The lift station was constructed in 1981 and has now reached the end of its useful life. The City of Renton (City) intends to construct a replacement lift station on land currently owned by the City Parks Department east of the existing lift station. Construction of the proposed lift station will occur while the existing station remains in service. The proposed lift station will have the same capacity as the existing station, which has adequate capacity to serve the area for the foreseeable future. The lift station facility will consist of: • A 10-foot-diameter wet well completed to a depth of 13 feet; • A 4-foot-diameter precast concrete manhole completed to a depth of 8 feet; • An 8-foot by 8-foot valve vault completed to a depth of 9 feet; and • A 10-foot by 9-foot electrical building and emergency generator, both supported on separate, at-grade concrete slab foundations. Technical Memorandum RE: Falcon Ridge Lift Station Engineering Geology Evaluation September 5, 2017 Page 2 9/5/2017 3:59 PM \\rh2.com\dfs\Bothell\Data\REN\117-038\Geo\Tech Memo re Falcon Ridge LS.docx This technical memorandum summarizes the findings of a limited subsurface investigation to observe, characterize, and document earth and groundwater conditions of the site; identify potential geologic hazards and recommendations to mitigate the geologic risks; and provide recommendations for design and construction of the proposed lift station facility. The lift station facility is in the NE ¼ of the NE ¼ of Section 20, Township 23 N, Range 05 E, centered at latitude 47.473021° N, longitude 122.185740 W. The site is located on the end of an east-trending ridge and slope above the Cedar River valley at approximately 375 feet in elevation above mean sea level. The general layout of the site is shown in Figure 1. The area of the proposed lift station is undeveloped and covered by second-growth trees and shrubs. The lift station will be constructed in an excavation cut into the slope. FINDINGS REGIONAL GEOLOGY RH2 Engineering, Inc., (RH2) reviewed geologic maps and descriptions of regional geologic conditions on the Washington State Department of Natural Resources (WADNR) website. RH2 reviewed the driller’s logs for borings and wells completed within 1 mile of the site and recorded at the Washington State Department of Ecology (Ecology) well log website. The surficial geologic unit mapped at the site is glacial till consisting of an unsorted very dense mixture of sand, silt, gravel, and cobbles. The WADNR Interactive Geologic Map, based on the United States Geological Survey National Earthquake Hazards Reduction Program, assigns a Seismic Site Class C, Very Dense Soil, and a very low risk of liquefaction. SITE INVESTIGATION Before site exploration, RH2 reviewed available geologic maps, soil maps, and exploration boring logs for the local area. On May 24, 2017, City staff used an excavator to excavate a test pit to a depth of 14 feet below ground surface. The City subsequently backfilled the test pit with compacted excavated soil using the excavator bucket. The test pit location is shown on Figure 1. RH2 inspected soil samples retrieved from the excavation to identify stratigraphy, composition, texture, structure, and cohesion of native earth materials encountered in the excavation. Appendix A contains the soil test pit log. A review of existing well logs obtained from Ecology for a 1-mile area surrounding the site indicates that groundwater may exist at a depth of more than 80 feet beneath the surface within deeper unconsolidated sediments. Groundwater seepage was observed on the south sidewall at a depth of approximately 9 feet and an estimated rate of less than 1 gallon per minute. Technical Memorandum RE: Falcon Ridge Lift Station Engineering Geology Evaluation September 5, 2017 Page 3 9/5/2017 3:59 PM \\rh2.com\dfs\Bothell\Data\REN\117-038\Geo\Tech Memo re Falcon Ridge LS.docx GEOLOGIC HAZARDS The WADNR website was reviewed for geologic hazards at the lift station site. The information that follows summarizes the geologic hazards and relative risk they pose to the lift station. • Risks from landslides, mass wasting, and flooding are negligible. • The risk of earthquakes of magnitude 5 to 6 during the next 50 years is high (80 percent). • Liquefaction risk is mapped as very low; the soil is very dense. • The risk of persistent groundwater seepage from surrounding native soil into the excavation during site development is negligible, unless construction would occur following a period of sustained rainfall, creating saturated shallow soil. GEOTECHNICAL PROPERTIES Based on the observed soil composition and density, the very dense glacial till may support a structure with an appropriately designed foundation that spreads a load that does not exceed a net allowable bearing capacity of 4,000 pounds per square foot (psf). The following earth pressures are estimated for the moderately dense fine sand, assuming a friction angle of 32 degrees and a unit weight of 110 pounds per cubic foot (pcf). • At rest – 56 psf • Active – 37 psf • Passive – 391 psf RECOMMENDATIONS SITE EXCAVATION FOR FOUNDATIONS AND UNDERGROUND UTILITIES • Based on the subgrade depth of the proposed design, the lift station would be founded on dense, sandy silt with gravel. Excavations to the subgrade should be reviewed by a Licensed Engineering Geologist (LEG) or Professional Engineer with a geotechnical background (PEG), at the lowest excavated subgrade elevation, prior to any subgrade improvements. SUBGRADE PREPARATION • Areas of subgrade that consist of less than 12 inches of loose fill or native soil, or unsuitable fill or native soil, should be over-excavated and backfilled with crushed rock. • Areas of subgrade that consist of more than 12 inches of loose or unsuitable fill/native soil should be over-excavated and backfilled with at least 2 feet of structural fill topped by 6 inches of crushed rock. • Structural backfill should meet the requirements of gravel borrow (or equivalent) per the Washington State Department of Transportation (WSDOT) Standard Specifications 2016, Technical Memorandum RE: Falcon Ridge Lift Station Engineering Geology Evaluation September 5, 2017 Page 4 9/5/2017 3:59 PM \\rh2.com\dfs\Bothell\Data\REN\117-038\Geo\Tech Memo re Falcon Ridge LS.docx 9-03.14(1). Structural backfill should be placed at optimal moisture in 6-inch loose lifts and compacted to a firm and unyielding surface to achieve at least 95 percent of maximum dry density as determined by the modified proctor test (American Society for Testing and Materials (ASTM) D 1557). • The final subgrade should be flat and free of loose earth materials or boulders larger than 4 inches in diameter. The subgrade should be probed to confirm uniform soil density and approved by a LEG or PEG. • Crushed rock should meet the requirements of crushed surfacing base course (CSBC) per the WSDOT Standard Specifications 2016, 9-03.9(3). CSBC should be placed in 6 inch lifts and compacted to a firm and unyielding surface to achieve at least 95 percent of maximum dry density as determined by ASTM D 1557. USE OF EXCAVATED EARTH MATERIALS • Excavated topsoil, which occurs to a depth of approximately 2 feet, is unsuitable for backfill and should be exported offsite or used for landscaping. • Excavated deeper native soil below the topsoil may be stockpiled for reuse as structural fill if the soil contains less than 15 percent fines, the moisture content of the excavated soil is within plus or minus 2 percent of optimum, the soil contains no waste debris, organic soil, or woody material, and all rock materials (cobbles, boulders, and rock fragments) larger than 4 inches in diameter are removed. • Stockpiled earth materials should be protected from wetting or drying that would make it unsuitable as fill. • Earth materials excavated for re-use as fill, as well as all imported fill material, should be tested for moisture content just prior to placement. Structural fill should be within plus or minus 2 percent of its optimum moisture content when placed. SLOPES AND SHORING • The native soil may support temporary slopes at 1H:1V, as long as they are protected from erosion, runoff, and vibration. • The lift station excavation may require shoring below depths of 4 feet if workers will enter the excavation to construct the lift station, and if adjacent roads and utilities require protection. The lift station will be constructed of either precast or cast-in-place concrete, and will become the permanent retaining wall for the cut slope. All permanent faces in native soil should be no steeper than 2H:1V. • All temporary slopes should be reviewed for stability several times a day, or as often as necessary to ensure slope integrity. This shall include reviewing the top of the slope for tension cracks and settlement, as well as erosion. • The native soil is moderately erodible. All temporary slopes should be protected from erosion from direct precipitation, or runoff from adjacent impermeable surfaces. Before Technical Memorandum RE: Falcon Ridge Lift Station Engineering Geology Evaluation September 5, 2017 Page 5 9/5/2017 3:59 PM \\rh2.com\dfs\Bothell\Data\REN\117-038\Geo\Tech Memo re Falcon Ridge LS.docx and during precipitation events, the surface should be covered by plastic sheeting or other techniques that prevent rain splash erosion and rilling. • All permanent slopes and swales should be protected from erosion by hydroseeding, and planting with landscape fabric, coarse bark placement, quarry spalls, or other materials that prevent rain splash erosion and rilling. SEISMIC DESIGN FACTORS • There is a moderate probability that the site will experience a moderate (M 5) earthquake in the next 50 years. • The native soil should be considered as a Site Class C, Very Dense Soil, per International Building Code (IBC) (2016) Site Class definitions. ON-SITE INFILTRATION CAPACITY • No infiltration rate testing was conducted as part of this investigation. The glacial till has low permeability, which suggests a low suitability for on-site infiltration. ATTACHMENTS Figure 1 – Site Plan Appendix A – Test Pit Log FIGURE 1 TEST PIT LOCATIONFIGURE 1SITE PLANFALCON RIDGE LIFT STATIONCITY OF RENTON APPENDIX A Page 1 of 1 REN Falcon Ridge test pit log.docx Test Pit/Exploration Log REN TP1 Exploration Name Falcon Ridge Lift Station Replacement Project East end of SE 8th Place Renton, WA Location Steve Nelson, LEG Inspected By May 24, 2017 Date REN 117.038.01 Project No. City-provided Excavator, 2-foot bucket City of Renton Backhoe and Operator Depth Description Sketch/Photo 0 to 2.0 feet Silty SAND (SM) with gravel; light brown; fine to coarse sand; non-plastic fines; fine to coarse subrounded gravel; few cobbles; abundant roots; compact; moist (topsoil/weathered till). 2.0 to 9.0 feet Sandy SILT with gravel (ML); light gray; non-plastic fines; fine to coarse sand; fine to coarse subrounded gravel; few cobbles; trace roots; very dense; moist (glacial till). 9.0 to 14 feet Gravelly Sandy SILT (ML); gray; non-plastic fines; fine to coarse sand; fine to coarse subrounded gravel; few cobbles; very dense; moist (glacial till). Sidewalls stable unless disturbed. Exploration backfilled with excavated material. 3/9/2018 3:47 PM J:\DATA\REN\117-038\GEO\TECH MEMO RE FALCON RIDGE LS ADDENDUM.DOCX TECHNICAL MEMORANDUM Client: City of Renton Project: Falcon Ridge Lift Station Replacement and Force Main Improvements Project File: REN 117.038.01.101 Project Manager: Edwin Halim, PE Composed by: Steve Nelson, LEG Reviewed by: Tony Pardi, PE Subject: Coal Mine Hazard Evaluation Date: March 9, 2018 INTRODUCTION The Falcon Ridge Lift Station, located at the east end of SE 8th Place east of Lake Youngs Way SE in Renton, Washington, is a duplex fiberglass-enclosed lift station that serves the Falcon Ridge subdivision. The lift station was constructed in 1981 and has now reached the end of its useful service life. The City of Renton (City) intends to construct a replacement lift station on land currently owned by the City Parks Department adjacent to and east of the existing lift station. Construction of the proposed lift station will occur while the existing station remains in service. The proposed lift station will have the same capacity as the existing station, which has adequate capacity to serve the area for the foreseeable future. The replacement lift station facility will consist of a 10-foot-diameter wet well completed at a depth of approximately 13 feet. SITE CONDITIONS The lift station site (the Site) is located within an area designated by the City as a Coal Mine Hazard Area. Surficial geologic units at the Site encountered during a previous site investigation consist of very dense sandy silt with gravel identified as unweathered glacial till (RH2 Engineering, Inc., Technical Memorandum Falcon Ridge Lift Station Engineering Geology Evaluation, September 5, 2017). Mapping by Washington State Department of Natural 3/09/18 3/09/18 Technical Memorandum RE: Coal Mine Hazard Evaluation March 9, 2018 Page 2 3/9/2018 3:47 PM J:\DATA\REN\117-038\GEO\TECH MEMO RE FALCON RIDGE LS ADDENDUM.DOCX Resources (WADNR) indicates that the till and other glacial deposits extend to a depth of approximately 200 feet in the area. WADNR identifies Tertiary-age bedrock underlying the glacial units known as the Renton Formation, which underlies Falcon Ridge and is exposed on the north slopes of the ridge above the Cedar River valley. The Renton Formation consists of Tertiary-age sandstone, siltstone, clay, and coal that was uplifted and folded by tectonic forces and then subsequently buried and eroded by glacial processes. Inspection of the WADNR mapping and cross-sections indicates that the Renton Formation is 200 feet to more than 500 feet below the Falcon Ridge site. The nearest outcrop of the Renton Formation is approximately 400 feet to the north and 200 feet lower in elevation of the Site. The attached figure shows the mapped outline of the Renton Formation, the extent of former underground coal mining, and the Site. EVALUATION OF COAL MINE RISK Former coal mine operations could pose a risk to surface structures and utilities where underground coal mining extended close to or reached the surface and created a potential for land subsidence or sinkholes to form over the former coal mine excavations. According to mapping of the former Renton Coal Company Mine available on the WADNR website, underground coal mining extended no closer than 1,300 feet west of the Site and at depths of at least 200 feet below Falcon Ridge. The May 2017 geologic investigation for the proposed lift station encountered only dense glacial till, and no indication of subsidence or weakness in the till was observed in the 14-foot-deep soil exploration excavation. Former coal mine activities do not pose a significant risk of ground subsidence that could negatively impact the construction and operation of the Falcon Ridge Lift Station. ATTACHMENTS Renton Falcon Ridge Coal Mine Risk Map Attachments Sources: Esri, HERE, DeLorme, USGS, Intermap, INCREMENT P, NRCan, Esri Japan, METI, EsriChina (Hong Kong), Esri Korea, Esri (Thailand), MapmyIndia, NGCC, © OpenStreetMap contributors,and the GIS User Community, Washington Division of Geology and Earth Resources Washington Geologic Portal N Renton Falcon Ridge 0 0.15 0.30.075 Miles