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HomeMy WebLinkAboutRS_GeoTech_Infiltration_Assessment_180301_v1OEOTEC'H CONSULTANTS, INC. Sazei Design Group, LLC 6608 —110th Avenue Northeast Kirkland, Washington 98033 Attention: Hamid Korasani via email. hamidkorasani@yahoo.com Subject: Infiltration Assessment Report (Updated) Proposed Short Plat 1909 Northeast 14th Street Renton, Washington Dear Mr. Korasani: 24011 Oth Ave E Seattle, Washington 98102 (425) 747-5618 JN 17532 January 2, 2018 This report presents our geotechnical conclusions related to the potential for utilizing infiltration to dispose of storm water from the proposed new short plat to be constructed on the subject lot. The site is located on the southern side of Northeast 14th Street. The lot currently contains a small, single story residence, which will be removed. No detailed plans were available, but from our discussions with you, we are aware that the property will be subdivided for new single-family homes. The subject site generally slopes gently downward from east to west. There are short retaining walls along the west and south property lines, with the neighboring lots being lower than the site. These neighboring lots are developed with homes. On October 3, 2017, an engineer from our firm visited the site to assess the subsurface conditions exposed in test holes excavated in a potential infiltration area located in the front and back yard of the existing lot. The approximate locations of these two test holes are indicated on the attached copy of the site plan. Test hole logs are also attached. The northern test hole (Test Hole 1) was performed in the front yard of the lot and encountered 2.5 feet of loose fill beneath a thin layer of topsoil. The fill was dry and loose, indicating that it was poorly compacted during the original grading of the lot. The fill was underlain by native silty sand that was initially loose, but became dense and intact at a depth of 3 feet. The dense silty sand was fine-grained, and was encountered to the maximum explored depth of 3.5 feet. The south test hole (Test Hole 2) was excavated in the backyard of the lot. Test Hole 2 encountered approximately 1.5 feet of loose fill was encountered beneath a thin layer of topsoil. Loose, native, silty sand with gravel was encountered beneath the fill and was observed to become dense and intact at a depth of 2.5 feet. The dense silty sand was encountered to the base of the test hole at 3 feet. The dense soil encountered in our test holes has been glacially compressed. This soil is mapped on the Washington Geologic Information Portal (Department of Natural Resources) as Glacial Drift. Sazei Design Group JN 17532 January 2, 2018 Page 2 The test holes were excavated during the dry season and no evidence of groundwater seepage was observed. Following extended wet weather, it is not uncommon to encounter subsurface water perched on top of, and within, the silty glacially -compressed soil. Conclusions Based on the results of the test holes and our previous experience in the surrounding area, the site is underlain by fill soils, beneath which is dense, fine-grained silty sand that is mapped as Glacial Drift. A seasonal water table is likely present in at least localized areas perched above the dense silty sand following extended wet weather. The dense silty sand encountered at the subject site is cemented and is essentially impermeable, preventing downward percolation of water, which often causes a perched water table to form following extended heavy rainfall. The dense silty sand encountered in our test holes is very compact and is comprised of fine-grained sand, with a high silt (fines) content. As a result, there are no large or continuous pore spaces in the soil that can transmit water. Often, the impermeable nature of the soil causes a shallow seasonal perched water table to form where the ground surface is not covered by an impervious layer. This is a common problem in the wet season throughout the Pacific Northwest. Infiltration tests conducted in dense, silty, fine-grained soils such as this should yield test rates less than 0.1 inches per hour. Considering the observed soil and wet season groundwater conditions, it is our professional opinion that infiltration of concentrated storm water is not feasible for this site. The silty, fine-grained soils would rapidly clog, even in a well-maintained infiltration system. Large volumes of water infiltrated or dispersed into the upper few feet of soil would migrate lateral toward the west and south retaining walls and the adjacent properties. This would likely result in an increased risk of surface flooding in the downslope lots surrounding the site, and potentially causing increased subsurface water in below -grade spaces beneath those houses. It is our understanding that the City of Renton has expressed a desire for test holes to be at least 4 feet in depth. Our test holes reached refusal in the glacially -compressed soil using hand implements at depths of 3 to 3.5 feet. While the 4 -foot minimum depth is a good general guideline to strive for, it is not always necessary, depending largely on the soil and groundwater conditions that are encountered. As a result, the minimum test hole depth should to be left up to the geotechnical professional's judgement. In the case of this site, the soils encountered in the test holes were fine-grained and glacially compressed at depths of 2.5 to 3 feet. The soil beneath this depth will also have been compressed under the weight of glacial ice, and so it will not become less dense or more permeable with increasing depth. The test holes were extended as deep as was practical using hand implements. Excavating deeper test holes would not reveal more permeable soils, and it would require the use of a backhoe, which would involve extensive site disturbance. Considering the above discussion, it is our professional opinion that the test holes were conducted for this study were sufficiently deep to adequately assess the soil and groundwater conditions. GEOTECH CONSULTANTS, INC. Sazei Design Group January 2, 2018 A 17532 Page 3 Please contact us if you have any questions regarding this letter, or if we can be of further service. Attachments: • Site Plan • Test Hole Logs MKM/MRM: mw Respectfully submitted, GEOTECH CONSULTANTS, INC. - /� Marc R. McGinnis, P. . Principal GEOTECH CONSULTANTS, INC. Legend: Test Hole Location GEOTECH CONSULTANTS, INC. SITE EXPLORATION PLAN 1909 Northeast 14th Street Renton, Washington Job No: Date: Plate: 1 17532 Dec. 2017 No Scale 1 TEST HOLE 1 d: Test Hole was term inate . I at a depth of 3.5 -feet on October 3, 201 7., No groundwater seepage was observed in the. test hole. TEST HOLE -2 Test. Hole was -terminated at a depth of 3 feet on October 3, 2017. No groundwater seepage was observed in the test hole. GEOTECH CONSULTANTS, INC. TEST HOLE LOGS 1909 Northeast 14th Street Renton, Washington Job No: Date: Plate: 17532 1 Dec. 2017 1 1