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HomeMy WebLinkAbout30b - Geotechnical Report (ZGA) 19023 36th Avenue West, Suite D Lynnwood, Washington 98036 (425) 582-9928 Zipper Geo Associates, LLC Geotechnical and Environmental Consulting Project No. 1688.01 29 August 2016 PACLAND 1505 Westlake Ave. N, Suite 305 Seattle, WA 98109 Attention: Mr. Bill Fortunato Subject: Summary of Field Infiltration Testing Renton Retail 17808 108th Avenue SE Renton, Washington King County Parcel No. 322305-9104 Dear Mr. Fortunato: This report summarizes the field infiltration testing services provided by Zipper Geo Associates, LLC (ZGA) for the proposed retail development at the above-referenced address. Our services have been provided in general accordance with our Proposal P16269 dated 9 August 2016. Written authorization to proceed with our evaluation was provided by PACLAND on 15 August 2016. This letter is an instrument of service and the conclusions presented herein are in respect to the subject property and have been prepared in accordance with generally accepted geotechnical engineering consulting practice. This letter has been prepared for the exclusive use of PACLAND, and its agents, for specific application to the subject property and stated purpose. SITE LOCATION AND DESCRIPTION The project site is a rectilinear parcel comprising approximately 1 acre of land with its long axis oriented east to west between 109th Avenue SE and 108th Avenue SE in Renton, King County, Washington. The western approximately 4/5th of the property is developed with a 2,200 square foot single-story building, occupied by a Skipper’s Seafood, with a surrounding parking lot. The eastern portion of the property is at a slightly higher elevation and consists of a tended grass lot with scattered trees and a fence on its western edge dividing the developed portion of the parcel from the greenspace. The site is bordered to the north and south by developed commercial properties; to the east by 109th Avenue SE and developed residential properties beyond; and to the west by 108th Avenue SE and developed commercial properties beyond. Topographically, the ground surface on the site generally slopes gently downward to the west with a total vertical relief from the east property boundary to the west of about 10 feet. Zipper Geo Associates, LLC Summary of Field Infiltration Testing Project No. 1688.01 29 August 2016 19023 36th Avenue West, Suite D Lynnwood, Washington 98036 (425) 582-9928 Page 2 PROJECT DESCRIPTION A plan provided for our review indicates that proposed development will include one building with paved parking and vehicle access areas. ZGA was not provided with plans for the infiltration facility. You have indicated that the infiltration facility may be approximately 8 feet below existing grade and located north of the proposed building location below parking areas. A proposed location for infiltration testing was outlined on Geotech RFP Exhibit 1.0, issued on 26 July 2016. The test location was moved to the eastern portion of the parcel when it was determined that the Skipper’s facility was still in operation and that the initial test location would impede parking and require resurfacing. The project site is illustrated on Figure 1. SCOPE OF SERVICES The purpose of the limited geotechnical engineering services described in this letter was to determine a field-measured stromwater infiltration rate at the site by completing one field infiltration test. Our scope of services included the following: reviewing the King County 2009 Surface Water Design Manual (Manual) regarding stormwater infiltration testing requirements, arranging for marking of underground utilities, completion of one infiltration test, and preparation of this report. Our scope of services did not include addressing other geotechnical engineering aspects of the proposed site improvements. SUBSURFACE CONDITIONS Soil Conditions Published geologic maps indicate that the property is underlain by glacial till. A geotechnical report was previously completed at the site by PSI (Geotechnical Engineering Report, Firestone Retail Store, 17808 108th Ave. SE, Renton, WA) dated June 1, 2016. Borings completed at the site as part of the PSI report indicate the site is generally underlain by a thin layer of asphalt concrete pavement underlain by about 3.5 to 5 feet of fill consisting of very loose to medium dense, silty sand. Below the silty sand fill, the boring logs disclosed medium dense grading to very dense silty sand with variable gravel content (weathered glacial till grading to unweathered glacial till). Our infiltration test was completed in a test pit located in the eastern, undeveloped portion of the site at the location shown on the attached Figure 1, Site and Exploration Plan. A descriptive log of the subsurface conditions observed at the infiltration test location is presented in Appendix A as test pit TP-1. Please note that per the Manual requirements, the soil descriptions are provided in accordance with US Department of Agriculture nomenclature. Soils observed at the infiltration test location included approximately 4 feet of weathered glacial till material consisting of medium dense, light brown to tan with some oxidation, sandy loam with gravel and some organics in the form of tree roots. The weathered till was underlain by unweathered glacial till consisting of dense, tan to light brown, sandy loam with gravel. The test pit was terminated at a depth of approximately 8 feet in dense, tan, unweathered glacial till. Zipper Geo Associates, LLC Summary of Field Infiltration Testing Project No. 1688.01 29 August 2016 19023 36th Avenue West, Suite D Lynnwood, Washington 98036 (425) 582-9928 Page 3 Groundwater Conditions No groundwater was observed during our exploration, which extended to a depth of approximately 8 feet below existing site grade at the infiltration test location. We did not observe soil coloration mottling suggesting that seasonal groundwater may be higher than the observed condition. PSI did not report observing groundwater in their explorations, which were advanced to a depth of up to 21 feet below the current grade of the paved portion of the property. However, PSI notes that “…PSI believes that small amounts of perched water was observed in borings B-3 and B-4. This perched water was observed within sand lenses within the till soils and may be present in other portions of the site seasonally or after significant precipitation events.” Groundwater conditions should be expected to fluctuate due to seasonal precipitation variation, irrigation, site utilization, and other factors. INFILTRATION TESTING RESULTS A single small-scale Pilot Infiltration Test (PIT) was completed at a depth of approximately 8 feet below existing site grade in general accordance with the procedures described in the Manual. The approximate test location is illustrated on Figure 1. Based on the results of our testing, the field-measured infiltration rate was determined to be approximately 0.21 inches per hour. For design, corrections (or reductions) to the measured infiltration rate should be made for testing, geometry, and plugging per the Manual to arrive at a design infiltration rate. CLOSURE We trust that this letter meets your current needs. Please do not hesitate to contact us should you have any questions. Respectfully submitted, Zipper Geo Associates, LLC Robert A. Ross, P.E. Principal Enclosures: Figure 1 – Site and Exploration Plan Appendix A – Field Testing Procedures and Log ITP-1FIGUREJob No.Zipper Geo Associates, LLC19023 36th Ave. W.,Suite DLynnwood, WA, 98036SHT. of11SITE AND EXPLORATION PLAN1688.01DATE: AUGUST 20161RENTON RETAIL INFILTRATION TESTING17808 108th Ave. SERenton, WashingtonSCALE IN FEET0303015LEGENDREFERENCE: UNDATED CAD FILE PROVIDED BY PACLANDITP-1INFILTRATION TEST ANDPIT LOCATION APPENDIX A FIELD EXPLORATION PROCEDURES AND LOGS APPENDIX A FIELD EXPLORATION PROCEDURES AND LOGS 1688.01 Our field exploration for this project included one test pit completed on August 19, 2016. Approximate exploration locations are shown on the Site and Exploration Plan, Figure 1. Exploration locations were determined by measuring distances from existing site features with a fiberglass tape relative to an ALTA survey CAD file of the site provided PACLAND. As such, the exploration locations should be considered accurate to the degree implied by the measurement method. The approximate ground surface elevation at each exploration location was determined by interpolating the information provided on the ALTA survey. The following sections describe our procedures associated with the exploration. Descriptive logs of the explorations are enclosed in this appendix. Test Pit Explorations An independent contractor working under subcontract to our firm excavated the test pits through the use of a rubber-tired backhoe. An engineering geologist form our firm continuously observed the test pit excavations and logged the subsurface conditions. After we logged each test pit, the operator backfilled each with excavated soils tamped into place. Some settlement of the backfill should be expected over time. The enclosed test pit logs indicate the vertical sequence of soils and materials encountered in each test pit, based primarily on our field classifications and supported by our subsequent laboratory testing. Where a soil contact was observed to be gradational or undulating, our logs indicate the average contact depth. We estimated the relative density and consistency of in situ soils by means of the excavation characteristics and by the sidewall stability. Our logs also indicate the approximate depths of any sidewall caving or groundwater seepage observed in the test pits, as well as all sample numbers and sampling locations. Test Pit TP-1 Location: See Site and Exploration Plan, Figure 1 Approx. Ground Surface Elevation: 409 feet Project: Renton Firestone Project No: 1688.01 Date Excavated: 19 August 2016 Depth (ft) Material Description Sample NC %M Testing 1/2 foot or less of sod. Medium dense to dense, light brown to tan with some oxidation, sandy loam with some gravel, some organics above 2 feet consisting of roots less than 1/2 inch in diameter. [Weathered Till] Dense, tan to light brown, sandy loam with some gravel. [Unweathered Till] TP-1 completed at approximately 8 feet. No groundwater seepage observed at time of excavation. 1 2 3 4 5 6 7 8 9 10