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HomeMy WebLinkAboutRS_Geotechnical_Infiltration_Evaluation_170727_v1July 13, 2017 ES -5048.02 Harbour Homes, LLC 400 North 34th Street, Suite 300 Seattle, Washington 98103 Attention: Mr. Chris Burrus Subject: Infiltration Evaluation Cedars at the Highlands 14120 - 160th Avenue Southeast Renton, Washington Reference: Earth Solutions NW, LLC Geotechnical Engineering Study Cedars at the Highlands ES -5048, Dated February 23, 2017 Earth Solutions NW, LLC Infiltration Consultation Cedars at the Highlands ES -5048.01, Dated May 16, 2017 Dear Mr. Burrus: Earth I Solutions NWLIC Earth Solutions NW LLC • Geotechnical Engineering • Construction Monitoring • Environmental Sciences In accordance with your request, Earth Solutions NW, LLC (ESNW) has prepared this letter providing a summary of our infiltration evaluation for the proposed residential development at the above-mentioned address in Renton, Washington. Project Description The project area consists of a property located on the east side of 160th Avenue Southeast, and north of the intersection with Southeast 8th Street in Renton, Washington. The site is rectangular in shape; and is currently developed with two barns, and associated improvements. Site plans include the construction of a residential development, and associated improvements, The feasibility of utilizing infiltration of stormwater runoff from impervious surfaces is being investigated as a part of the re -development plans for the subject site, as was required by the City of Renton reviewer. 1805 - 136th Place N.E., Suite 201 0 Bellevue, WA 98005 0 (425) 449-4704 • FAX (425) 449-4711 Harbour Homes, LLC ES -5048.02 July 13, 2017 Page 2 A series of test pits was excavated across the subject site (July 11, 2017) for the purposes of determining the subsurface conditions on the site, and performing pilot infiltration testing (PIT) at depths between four to eight feet in accordance with the City of Renton requirements. The four test pits were advanced to a minimum of ten feet. Suhsurfacp ESNW representatives performed in-situ PIT testing, observed, logged and sampled at four test pit locations across the site. Topsoil Topsoil was encountered at the test pit locations, and was observed in thickness of eight inches. If topsoil is encountered during site grading activities, it is not suitable for use as structural fill nor should it be mixed with material to be used as structural fill. Topsoil or otherwise unsuitable material can be used in landscaping areas if desired. Fill Fill was encountered at all four of the test pit locations. The fill was observed ranging in thickness from one to three feet, and consisted of silty sand with gravel (Unified Soil Classification, SM) in a loose to medium dense condition. Given fill was encountered in all four test pits, fill is likely present in most areas of the site. Native Soil Underlying the topsoil and fill, native soils consisting predominately medium dense to very dense silty sand with gravel (SM), dense gravel with silt and sand (GM), and dense poorly graded sand with silt (SP -SM) were encountered extending to the maximum exploration depth of 11 feet below existing grades. We observed lenses of silt (ML) in the southeastern portion of the site at test pit location TP - 104. The silt lenses were observed below the gravel with sand and silt soils, and in our opinion represent a confining layer in regards to infiltration. Assessment of Infiltration Characteristics We performed in-situ PIT testing in accordance with the City of Renton requirements at four locations across the subject site. Three of the test pits (TP -101, TP -102, and TP -103) were located in areas underlain by glacial till, where no infiltration was observed during our testing protocol. Earth Solutions Nw. LLC Harbour Homes, LLC ES -5048.02 July 13, 2017 Page 3 One of the test pits (TP -104, located in the southeast portion of the site) was observed to expose gravel with sand and silt (GM) soil below fill soil and native soils consisting of silty sand (SM). The gravel with silt and sand was encountered at a depth of six feet, and extended to nine and one-half feet. We preformed infiltration testing at this location and observed limited amounts of infiltration at a depth of eight feet. However, in advancing the test pit deeper than the infiltration test depth, we observed a confining layer of silt (ML) in a six inch thickness at a depth of nine and one-half feet. The confining layer limits the capacity of the underlying subgrade to infiltrate. Furthermore, stormwater will travel laterally on a confining layer such as the one observed at test pit TP -104. This lateral migration of water may create flooding related issues on the neighboring property to the south of the subject site since it is sited lower in elevation than the subject site. In our opinion, based on the soil conditions observed at the test sites, and past experience on similar sites, the subject property is not a good candidate for infiltration of stormwater runoff. This conclusion is based on the highly cemented nature of the native soils observed within the potential infiltration locations across the majority of the subject site, the stratified nature of soil adjacent to the property line, proximity of neighboring structures, and presence of near -surface groundwater seepage in areas underlain by glacial till soil. Glacial till material, and the silt lenses observed in the southern portion of the site, represent a confining layer of soil, which is a limiting factor when investigating stormwater infiltration. In our opinion, due to the presence of a confining layer and groundwater seepage, alternative measures should be considered for stormwater management on the subject site. I imitatinnc The recommendations and conclusions provided in this geotechnical report are professional opinions consistent with the level of care and skill that is typical of other members in the profession currently practicing under similar conditions in this area. A warranty is not expressed or implied. Variations in the soil and groundwater conditions observed at the test locations may exist, and may not become evident until construction. ESNW should reevaluate the conclusions in this geotechnical engineering study if variations are encountered. Earth Solutions NW, LLC Harbour Homes, LLC July 13, 2017 ES -5048.02 Page 4 We trust this information meets you needs. If you have any questions or require additional information, please call. Sincerely, EARTH SOLUTIONS NW, LLC Stephen H. Avril Project Geologist Attachments: Test Pit Logs Test Pit Location Plan WAs� Kyle R. Campbell, P.E. Principal Earth Solutions NW, LLC Test Pit Logs ES -5048.02 TP -101 0'-1' SM Brown Silty Sand with Gravel, Loose, Moist (Fill) 1'-2.5' SM Brown Silty Sand with Gravel, Dense, Moist (Weathered Till) 2.5'-11' SM Brown Silty Sand with Gravel, Very Dense, Moist (Unweathered Till) TP -102 0'-3' SM Brown Silty Sand with Gravel, Loose, Moist (Fill) 3'-4' SM Brown Silty Sand with Gravel, Dense, Moist (Weathered Till) 4'-11' SM Brown Silty Sand with Gravel, Very Dense, Moist (Unweathered Till) TP -103 0'-8" TPSL Topsoil, Loose, Moist 8"-2' SM Brown Silty Sand with Gravel, Medium Dense, Moist (Fill) 2'-4' SM Brown Silty Sand with Gravel, Dense, Moist (Weathered Till) 4'-11' SM Brown Silty Sand with Gravel, Very Dense, Moist (Unweathered Till) TP -104 0'-2' SM Brown Silty Sand with Gravel, Loose, Moist (Fill) 2'-4' SM Brown Silty Sand with Gravel, Dense, Moist (Weathered Till) 4'-6' SM Brown Silty Sand with Gravel, Very Dense, Moist (Unweathered Till) 6'-9.5' GM Brown Gravel with Silt and Sand, Dense, Moist 9.5'-10' ML Brown Silt, Stiff, Moist 10'-11' SP -SM Brown Poorly Graded Sand with Silt and Gravel, Dense, Moist Earth Solutions NW, LLC I �'tPlde Tract B Workshop Serf .dive Are, Shed � I TP -5 TP -2 - t l4 TP -4 — TP -3 TractA TP -11 — I — ■ — Storm Drama( )o j LEGEND TP -1 Approximate Location of — — ESNW Test Pit, Proj. No. ES -5048, Jan. 2017 Subject Site Existing Building NOTE: The graphics shown on this plate are not intended for design purposes or precise scale measurements, but only to illustrate the approximate test locations relative to the approximate locations of 1 existing and / or proposed site features. The information illustrated I is largely based on data provided by the client at the time of our study. ESNW cannot be responsible for subsequent design changes or interpretation of the data by others. NOTE: This plate may contain areas of color. ESNW cannot be responsible for any subsequent misinterpretation of the information resulting from black & white reproductions of this plate. 0 50 100 200 1"=100' Scale in Feet Earth Solutions NW irc Test Pit Location Plan Cedar of the Highlands Renton, Washington Drwn. MRS I Date 02/10/2017 1 Proj. No. 5048 Checked SHA Date Feb. 2017 Plate 2