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Home My WebLink AboutEX08_RS_Geotechnical_Evaluation_Report_Pioneer_Engineers_240729_v1GEOTECHNICAL EVALUATION REPORT PROPOSED 1828 RESIDENCE 1828 NE 20TH STREET RENTON, WASHINGTON For Yin Yin Leong c/o Ms. Tracy Hung 1828 NE 20th Street Renton, WA 98056 Prepared by Pioneer Engineers, Inc. P. O. Box 33628, Seattle, WA 98133 Phone: (206) 427-9118, Fax: (206) 306-2982 josephwu.pei@gmail.com December 30, 2023 Docusign Envelope ID: 104DBDA6-11CD-4791-91FF-7DA2A96942C7 PIONEER ENGINEERS, INC. Geotechnical Engineering Earth Science Water Resources ___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ P. O. Box 33628Seattle, WA 98133 Phone: (206) 427-9118 Fax: (206) 306-2982 josephwu.pei@gmail.com December 30, 2023 Yin Yin Leong c/o Ms. Tracy Hung 1828 NE 20th Street Renton, WA 98056 Subject: Geotechnical Evaluation Proposed 1828 Residence 1828 NE 20th Street Renton, Washington Dear Ms. Hung: Per your request, Pioneer Engineers, Inc. (“PEI”) has explored subsurface conditions with four test pits (“TP-1 through TP-4”) in the area for stormwater infiltration and development at the above subject site in Renton, Washington. The site is located east of the intersection of Northeast 20th Street and Jones Avenue Northeast, as shown in Figure 1. Based on the architectural plans prepared by CHC Architects and the boundary and topographic survey plan by Eastside Consultants, Inc., PEI understands that the development will consist of a one-story wood framed residence with an attached garage, accessible from Northeast 20th Street. Minor grading is anticipated to establish the foundation and access driveway. An investigation of subsurface soil and groundwater conditions is required for evaluation of on-site stormwater infiltration. SCOPE OF WORK The purpose of this geotechnical evaluation is to characterize the subsurface conditions by four test pits and a pilot infiltration test (“PIT”) at TP-3, and use such information obtained for evaluation of stormwater infiltration. To achieve the purpose, the scope of services specifically comprises the following items: Docusign Envelope ID: 104DBDA6-11CD-4791-91FF-7DA2A96942C7 December 30, 2023 Proposed 1828 Residence PEI Project No. G23A20 Page 2 PIONEER ENGINEERS, INC. 1. Explore subsurface conditions with two test pits to a maximum depth of 10 feet. The underlying soils encountered are visually classified; 2. Conduct a PIT at TP-3 and analyze test data; 3. Perform a site reconnaissance to observe and document existing surface features; 4. Review geologic and surficial soil conditions with a geologic or soil publication; 5. Prepare a written report to present the findings and geotechnical recommendations as well as the evaluation of on-site stormwater infiltration. SITE CONDITIONS Surface Conditions The site is a vacant property irregular in shape bounded to the south by Northeast 20th Street, and adjoins private properties on the other three sides. It is measured 26.01 feet along the street, encompassing an area of 1.15 acres. Topographically, the site is located slightly above the toe of a regional slope ascending mildly toward northeast. The ground surface is covered mostly with berry and grass, dotted with grown trees. Geologic Setting A publication, Geologic Map of Surficial Deposits in the Seattle 30’ x 60’ Quadrangle, Washington, prepared by James C. Yount, James P. Minard and Glenn R. Dembroff in 1993 was referenced for the soil conditions at the site. According to this publication, the site is mapped in Recessional outwash (“Qvr”). Qvr was deposited mostly from the meltwater flowing from the retreating glaciers during the last glaciation period. In general, this soil unit is of high permeability and low shear strength since it had not been overridden and consolidated by thick, heavy glacial ice. It is suitable for use to support a light-weight wooded structure, and also is a competent natural material bolstering stormwater infiltration. Docusign Envelope ID: 104DBDA6-11CD-4791-91FF-7DA2A96942C7 December 30, 2023 Proposed 1828 Residence PEI Project No. G23A20 Page 3 PIONEER ENGINEERS, INC. Subsurface Exploration On December 20, 2023, subsurface conditions were explored with four test pits to a maximum depth of 10 feet. Approximate pit locations were determined with reference to existing surface features, as shown in Figure 2. Subsurface exploration was continuously monitored by an engineer from PEI who documented subsurface soil and groundwater conditions, maintained a log of each pit, and observed pertinent site features. The final test pit logs represent PEI’s interpretation of subsurface conditions explored. The stratification lines in the logs indicate approximate boundaries between soil types where actual transitions may be more gradual in the natural geologic setting. The soils encountered are visually classified in general accordance with Unified Soil Classification System (“USCS”) as shown in Figure 3, with reference to USDA Classification System of Soil Texture Triangle. Subsurface Soils All test pits encountered considerably consistent soil conditions: 8 to 10 inches of topsoil underlain by a thin layer of tan brown, silty, fine sand. Underlying this silty sand was medium- dense recessional outwash of light brown, fine to medium sand with a trace of silt and coarse gravel, occasional cobble. Light gray, medium- to coarse-grained sand with a trace of coarse gravel was last encountered, underlying light brown sand to the termination depths. This cleaner, coarser sand was also interpreted as recessional outwash. More information of soil conditions is presented in Figures 4 and 5. Groundwater Conditions Light to moderate groundwater was encountered at depths varying from 7 to 9 feet in 3 of 4 test pits. Groundwater levels generally fluctuate with seasons, depending on the amount of precipitation and surface runoff, denseness of groundcover, purpose of land use, and other factors. Groundwater levels were at or close to their seasonal high following recent persistent precipitation. Docusign Envelope ID: 104DBDA6-11CD-4791-91FF-7DA2A96942C7 December 30, 2023 Proposed 1828 Residence PEI Project No. G23A20 Page 4 PIONEER ENGINEERS, INC. CONCLUSIONS AND RECOMMENDATIONS Based on the soil and groundwater conditions encountered in subsurface exploration, it is PEI’s opinion that, from a geotechnical engineering viewpoint, the site is appropriate for the development if recommendations in this report are closely followed. The explored area is able to bolster stormwater infiltration for management of on-site stormwater, due to the prevailing existence of Recessional outwash and relatively low groundwater levels. It is recommended that collective stormwater from a pollution-generating impervious surface (“PGIS”) over threshold criteria be treated prior to entering an infiltration facility. Conventional spread footings bearing on medium-dense Recessional outwash should be able to provide adequate support for the proposed residence. It is advisable that grading work be started and completed in the dry season in order to minimize the impacts from precipitation. GRADING AND SITE PREPARATION Site preparation includes clearing and removal of ground features, and implementations of temporary erosion and sediment control (“TESC”) measures. Clearing of ground includes stripping and grubbing of surface vegetation within the construction limits. Occasional overexcavation may be required when local weak soil pockets encountered. Overexcavation should be backfilled with structural fill and compacted to a stable condition. On- site topsoil is unsuitable for use in any structural areas. This soil should be disposed of at approved locations or used solely for landscaping purposes. Prior to starting construction activities, a silt fence should be installed along the lower boundary of the site. This fence may be implemented with a highly visible grid fence to delineate the clearing limits. A portion of the existing gravel driveway will be preserved to serve as a construction entrance. The silt fence and the preserved driveway serve as TESC measures during construction, and they should remain in place until full establishment of permanent ESC measures. Docusign Envelope ID: 104DBDA6-11CD-4791-91FF-7DA2A96942C7 December 30, 2023 Proposed 1828 Residence PEI Project No. G23A20 Page 5 PIONEER ENGINEERS, INC. The following strategies and methods should be implemented in the grading phase for enhancements of TESC measures and site stability during construction:  The bare and disturbed ground outside the clearing limits should be protected with a layer of straw mulch (a minimum thickness of 2 inches; about 2 bales per 1,000 square feet of land) during any period of precipitation, in order to minimize soil erosion by storm runoff. Straw needs to be air-dried and free of any undesirable weed or coarse material.  Cut/fill slopes and stockpiles of soils should be covered with durable plastic sheeting weighed down by securely-anchored sand bags if they are to remain unworked for more than 12 hours; other disturbed areas should be covered with straw mulch as addressed above if they are to remain unworked for more than 2 days.  TESC measures in place should have regular inspection weekly and more frequent inspection immediately before, during and after significant precipitation events. STRUCTURAL FILL AND COMPACTION Structural fill is the material placed under footings, on-grade slabs and pavements where it withstands loading. Structural fill should be free of organic, construction debris and other deleterious substances. It should consist of clean soils with individual particles not greater than 3 inches in size. On-site outwash soils containing a significant quantity of free-draining, granular particles can be compacted to meet the criteria in all weather. Recycled concrete may be used as structural fill provided that the requirements of 9-03.12(2) of the latest Washington State Department of Transportation (“WSDOT”) standard specifications are met. Structural fill should be placed per loose lift not more than 12 inches in thickness, and compacted to meet the required percentage of maximum dry density determined by ASTM D1557 (Modified Proctor Method) as summarized in the following table: Docusign Envelope ID: 104DBDA6-11CD-4791-91FF-7DA2A96942C7 December 30, 2023 Proposed 1828 Residence PEI Project No. G23A20 Page 6 PIONEER ENGINEERS, INC. Applicable Area Maximum Dry Soil Density Under Footing 95% Under Driveway and On-Grade Slab 95% for upper 2 feet, 90% below Foundation Wall Backfill 95% for upper 3 feet, 90% below Utility Trench Backfill 95% for upper 4 feet, 90% below Controlled Density Fill (“CDF”) may be used as an alternative for structural fill. CDF (a flowable, self-compacting, rigid-setting and low-density material) is generally used in overexcavation in a foundation or utility trench. Wherever applicable, neither compaction efforts are required to densify this fill, nor density tests needed to ensure the adequacy of compaction efforts. This material’s flowability enables it to access difficult spots and displace standing water in the trench. CDF has a typical slump of 10 inches and a 30-day compressive strength of 200 pounds per square inch (“psi”) or less. Low compressive strength allows CDF for convenient excavation in case of any design alteration during construction. FOUNDATION SUPPORT Foundations are constructed of individual spread footings and/or continuous strip footings bearing on medium-dense Recessional outwash. Individual footings are usually used to support columns, and continuous footings for bearing walls. For footings constructed as stated above, design criteria for these foundations are summarized:  Allowable soil bearing pressure for spread footings poured on medium-dense Recessional outwash should be no more than 1,500 pounds per square foot (“psf”);  Minimum depth to bottom of perimeter footing below adjacent exterior finish grade should be at least 18 inches to reduce the risk of the foundation damage by the frost effect;  Interior footings supporting bearing walls or bracing walls should have a minimum depth of 12 inches from the top of the on-grade floor slab to the bottom of the footing; Docusign Envelope ID: 104DBDA6-11CD-4791-91FF-7DA2A96942C7 December 30, 2023 Proposed 1828 Residence PEI Project No. G23A20 Page 7 PIONEER ENGINEERS, INC. WELLHEAD PROTECTION AREA The site is located in a Zone 1 Modified wellhead protection area. Zone 1 Modified is defined as “the land area situated between a well or well field owned by the City and the three hundred sixty-five (365) day groundwater travel time contour but for the purpose of protecting a high- priority well, wellfield, or spring withdrawing from a confined aquifer with partial leakage in the overlying or underlying confining layers” in accordance with Renton Municipal Code (“RMC”) 4- 3-050G.8. In this zone, collective storm runoff from a PGIS exceeding the threshold area should be treated prior to entering the infiltration facility. STORMWATER INFILTRATION In accordance with Surface Water Design Manual released 6/22/2022 by City of Renton, low impact development (“LID”) is “a stormwater and land use management strategy that strives to mimic pre-disturbance hydrologic processes of infiltration, filtration, storage, evaporation and transpiration by emphasizing conservation and use of on-site natural features, site planning and distributed stormwater management practices that are integrated into a project design.” In recent years, this strategy has been applied primarily to prevent potential pollutants from entering natural water bodies and to reduce the amount of stormwater into a public storm drainage system. To achieve the objectives, LID measures consisting of infiltration trenches, drywells, green roofs, bioretention cells, rain gardens, stormwater harvesting devices and pervious pavements have been widely accepted to enhance safety of water environment, street landscape and urban amenities. These measures have higher application priority than conventional (structural) stormwater management practices such as detention vaults or pipes. They are taken into consideration prior to application of conventional practices. PIT Procedures and Results The infiltration test was conducted at a depth of 2.0 feet in TP-3 with a bottom area of 12 square feet. With an effort of minimizing scour, tap water was carefully dispersed into the pit to soak soils in the infiltration zone for about one hour. A hydraulic head of 12 inches was maintained during this period of time. After soaking, the pit was filled up to have a hydraulic head of 12 inches and the drop of water levels measured over elapsed time to obtain measured infiltration Docusign Envelope ID: 104DBDA6-11CD-4791-91FF-7DA2A96942C7 December 30, 2023 Proposed 1828 Residence PEI Project No. G23A20 Page 8 PIONEER ENGINEERS, INC. rates. Drops of water levels were taken from the readings at a scaled wood stake accurate to half an inch placed in the middle of the pit. This procedure was repeated to obtain the final field infiltration rate after a steady-state rate was reached. Measured water levels versus elapsed time were logged and shown in the table. Soil Elapsed Water Water Level Infiltration Description Time, min. Level, in. Drop, in. Rate, in/hr Light brown, 0 12.0 0.0 fine to medium 15 10.2 1.8 7.2 sand, trace coarse 30 8.5 1.7 6.8 gravel, trace silt, 45 7.3 1.2 4.8 occasional 60 6.0 1.3 5.2 cobble. 75 5.0 1.0 4.0 165 0.0 5.0 3.3 The lowest measured infiltration rate of 3.3 in/hr is selected from the above values. It reasonably represents the infiltration rate in the upper Recessional outwash in the explored area. The exploration depth was further extended three feet from the bottom after fully drained, and no groundwater mounding observed. Design Infiltration Rate In accordance with the aforementioned design manual, correction was made to account for factors (types of field infiltration tests, Ftesting; the depth from the infiltration facility bottom to the groundwater table, Fgeometry; flow channels plugged by fine particles in soil mass, Fplugging) that may affect long-term performance of on-site stormwater infiltration. The correction for the measured infiltration rate is summarized: Idesign = Imeasured x Ftesting x Fgeometry x Fplugging Ftesting = 0.5 (small- or large-scale PIT) Fgeometry = 4(D/W) + 0.05 = 4(3/3) + 0.05 = 4.05 > 1.0, Use Fgeometry = 1.0 D: depth from the bottom of the proposed facility to the maximum wet season water table or nearest impervious layer, whichever is less = 3.0 feet Docusign Envelope ID: 104DBDA6-11CD-4791-91FF-7DA2A96942C7 December 30, 2023 Proposed 1828 Residence PEI Project No. G23A20 Page 9 PIONEER ENGINEERS, INC. W: width of facility = 3.0 feet (assumed) Fplugging = 0.85 (loamy sandy to sand) Idesign = Imeasured x Ftesting x Fgeometry x Fplugging = (3.3 in/hr)(0.5)(1.0)(0.85) = 1.40 in/hr The design infiltration rate is 1.40 in/hr for a storm facility to be constructed at a depth of 2 feet, a maximum ponding depth of 12 inches and a width of 3 feet. PEI recommends that an infiltration trench or multiple be used for on-site stormwater management. Infiltration Trench The trench should consist of a level 6-inch perforated PVC pipe with a (cleanout) stand pipe of same size and material at its end. The level pipe should be surrounded by 0.75- to 1.5-inch washed rock wrapped with non-woven geotextile such as Mirafi 140N, where the bottom area left open to receptor soil. Receptor soil surface should be scarified or grooved prior to placement of washed rock. The trench should be setback a minimum of 5 feet from the residence without a basement or a crawl space; at least 10 feet from the residence with a basement or a crawl space; a minimum clearance of two time the trench width from an additional trench. LIMITATIONS This report has been prepared for the specific application to this project for the exclusive use of Ms. Tracy Hung and her authorized personnel. PEI’s geotechnical recommendations are based on the soil and groundwater conditions encountered in the test pits, PIT results, local experience and engineering judgment. The recommendations are professional opinions derived in a manner consistent with the level of care and skill ordinarily exercised by other members of the profession currently practicing under similar conditions in local areas. No warranty, expressed or implied, is made. Docusign Envelope ID: 104DBDA6-11CD-4791-91FF-7DA2A96942C7 December 30, 2023 Proposed 1828 Residence PEI Project No. G23A20 Page 10 PIONEER ENGINEERS, INC. Respectfully submitted, PIONEER ENGINEERS, INC. Joseph Wu, P.E. Engineering Consultant Five figures attached. Docusign Envelope ID: 104DBDA6-11CD-4791-91FF-7DA2A96942C7 VICINITY MAP PROPOSED 1828 RESIDENCE 1828 NE 20TH STREET Geotechnical Engineering · Earth Science · Water Resources RENTON, WASHINGTON PROJ. NO. G23A20 DATE 12/30/23 FIGURE 1 PIONEER ENGINEERS, INC. NTS NE 20th St Ke n n e w i c k Pl N E Jo n e s A v e N E Li n c o l n P l N E Ke n n e w i c k P l N E Ab e r d e e n A v e N E NE 20th St SITE Ab e r d e e n P l NE 23rd St NE 21st St Docusign Envelope ID: 104DBDA6-11CD-4791-91FF-7DA2A96942C7 SITE EXPLORATION PLAN PROPOSED 1828 RESIDENCE 1828 NE 20TH STREET Geotechnical Engineering · Earth Science · Water Resources RENTON, WASHINGTON PROJ. NO. G23A20 DATE 12/30/23 FIGURE 2 PIONEER ENGINEERS, INC. Scale: 1" = 50'± Approx. Test Pit NE 20TH ST TP-1TP-2 TP-3 TP-4 Docusign Envelope ID: 104DBDA6-11CD-4791-91FF-7DA2A96942C7 PROPERTY FROM STANDARD PENETRATION TEST SILTY OR CLAYEY SOILS Relative Density Friction Angle %φ 0 - 15 15 - 35 26 - 30 35 - 65 28 - 35 65 - 85 35 - 42 USCS CHART PROPOSED 1828 RESIDENCE 1828 NE 20TH STREET Geotechnical Engineering · Earth Science · Water Resources RENTON, WASHINGTON PROJ. NO. G23A20 DATE 12/30/23 FIGURE 3 Hard > 50 85 - 100 38 - 46 Very Dense > 4.00 15 - 30 > 30 0.50 - 1.00 1.00 - 2.00 2.00 - 4.00 8 - 15 4 - 8 Very Stiff Medium Stiff Stiff Very Soft Description Soft2 - 4 < 0.25 0.25 - 0.50 Unconfined Strength qu, tsf Loose Medium Dense Dense < 2 Blow Count N PIONEER ENGINEERS, INC. SANDY SOILS N Blow Count 0 - 4 4 - 10 10 - 30 Description 30 - 50 Very Loose Docusign Envelope ID: 104DBDA6-11CD-4791-91FF-7DA2A96942C7 TEST PIT NO. TP-1 Logged By:JW Date:12/20/23 Depth brown Sample (N) W USCS Soil Description Blows/ Other Test ft. Type No. ft. % 8" Topsoil. SM Tan brown, silty, fine SAND, trace coarse gravel, occasional cobble, loose, moist. (Transitional Soil: Loamy Sand) SP-SM Light brown, fine to medium SAND, trace silt, trace coarse gravel, occasional cobble, medium-dense, moist. (Recessional Outwash; Loamy Sand to Sand) B 5 SP Light gray, medium to coarse SAND, trace coarse B gravel, medium-dense, very moist. (Recessional Outwash; Sand) 10 Test pit terminated @ 9 ft; light groundwater and minor caving encountered @ 9 ft during exploration. TEST PIT NO. TP-2 Logged By:JW Date:12/20/23 Depth Sample (N) W USCS Soil Description Blows/ Other Test ft. Type No. ft. % 9" Topsoil. SM Tan brown, silty, fine SAND, trace coarse gravel, occasional cobble, loose, moist. (Transitional Soil: Loamy Sand) SP-SM Light brown, fine to medium SAND, trace silt, trace coarse gravel, occasional cobble, medium-dense, moist. (Recessional Outwash; Loamy Sand to Sand) B 5 SP Light gray, medium to coarse SAND, trace coarse gravel, medium-dense, very moist. (Recessional Outwash; Sand) B 10 Test pit terminated @ 10 ft; light groundwater and minor caving encountered @ 7 ft during exploration. LEGEND: SS - 2" O.D. Split-Spoon Sample GROUNDWATER: Seal ST - 3" O.D. Shelby-Tube Sample Water Level B - Bulk Sample Observation Well Tip TEST PIT LOGS 1 & 2 PROPOSED 1828 RESIDENCE 1828 NE 20TH STREET Geotechnical Engineering · Earth Science · Water Resources RENTON, WASHINGTON PROJ. NO. G23A20 DATE 12/30/23 FIGURE 4 2 2 PIONEER ENGINEERS, INC. 1 1 Docusign Envelope ID: 104DBDA6-11CD-4791-91FF-7DA2A96942C7 TEST PIT NO. TP-3 Logged By:JW Date:12/20/23 Depth brown Sample (N) W USCS Soil Description Blows/ Other Test ft. Type No. ft. % 10" Topsoil. 1 SM Tan brown, silty, fine SAND, trace coarse gravel, occasional cobble, loose, moist. (Transitional Soil)PIT @ 2 ft. SP-SM Light brown, fine to medium SAND, trace silt, trace coarse gravel, occasional cobble, medium-dense, moist. (Recessional Outwash; Loamy Sand to Sand) 5 Pilot infiltration test ("PIT") conducted @ 2 ft; no groundwater mounding occurred within 3 ft extension after testing water fully drained. No occurrence of caving. 10 TEST PIT NO. TP-4 Logged By:JW Date:12/20/23 Depth Sample (N) W USCS Soil Description Blows/ Other Test ft. Type No. ft. % 10" Topsoil.1 SM Tan brown, silty, fine SAND, trace coarse gravel, occasional cobble, loose, moist. (Transitional Soil) SP-SM Light brown, fine to medium SAND, trace silt, trace coarse gravel, occasional cobble, medium-dense, moist. (Recessional Outwash; Loamy Sand to Sand) B 5 SP Light gray, medium to coarse SAND, trace coarse gravel, medium-dense, very moist. (Recessional Outwash; Sand) B Test pit terminated @ 7 ft; moderate caving @ 5 ft, moderate groundwater encountered @ 7 ft during exploration. 10 LEGEND: SS - 2" O.D. Split-Spoon Sample GROUNDWATER: Seal ST - 3" O.D. Shelby-Tube Sample Water Level B - Bulk Sample Observation Well Tip TEST PIT LOGS 3 & 4 PROPOSED 1828 RESIDENCE 1828 NE 20TH STREET Geotechnical Engineering · Earth Science · Water Resources RENTON, WASHINGTON PROJ. NO. G23A20 DATE 12/30/23 FIGURE 5 1 PIONEER ENGINEERS, INC. 2 Docusign Envelope ID: 104DBDA6-11CD-4791-91FF-7DA2A96942C7