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HomeMy WebLinkAboutEX07_Geotechnical ReportSouth Sound Geotechnical Consulting November 10, 2018 Mr. Ralraj Mahal 15816 – 147th Court SE Renton, WA 98058 Subject: Geotechnical Engineering Report – Infiltration Assessment 13436 – 156th Avenue SE Short Plat Renton, Washington SSGC Project No. 18090 Mr. Mahal, South Sound Geotechnical Consulting (SSGC) has completed a geotechnical assessment for the planned residential development on above addressed property in Renton, Washington. Our scope of services included completion of one test pit and two infiltration tests on the site, laboratory testing, engineering analyses, and preparation of this report. PROJECT INFORMATION Development plans include subdividing the existing parcel into two separate lots. An existing single- family residence on the property will be removed. Access to the lots will be from a private driveway off of 156th Avenue SE. SITE CONDITIONS The property is located on level to gently westerly sloping ground. The existing residence and separate shop building are in the southern portion of the two planned lots, surrounded by landscaped lawn. SUBSURFACE CONDITIONS Subsurface conditions were characterized by completing one test pit and two infiltration tests on the site on October 22, 2018. Explorations were advanced to final depths between about 2 and 5.5 feet below existing ground surface. Approximate locations of the explorations are shown on Figure 1, Exploration Plan. Logs of subsurface conditions are provided in Appendix A. A summary description of observed subgrade conditions is provided below. Soil Conditions A thin sod layer was observed over native loose silty fine sand with some roots. This surface soil extended to a depth of about 3 feet and is considered alluvium and/or weathered glacial till. Below this upper soil was silty sand with gravel in a medium dense to dense condition, extending to the termination depth of the test pit. This soil is interpreted to be glacial till. Geotechnical Engineering Report SSGC 13436 – 156th Ave. SE Renton, Washington SSGC Project No. 18090 November 10, 2018 2 Groundwater Conditions Groundwater was not observed in the excavations at the time of digging. However, slight mottling was observed near the base of the upper weathered zone in test pit TP-1. The denser glacial till will limit vertical flow of groundwater and can cause perched groundwater conditions during the wetter seasons of the year. Groundwater levels will fluctuate due to seasonal precipitation variations and on- and off-site drainage patterns. Geologic Setting Soils on the property are mapped as glacial till per the Washington State Department of Natural Resources (DNR) geology map of the area. Soils observed in the excavations appear to confirm the mapped soil type. Infiltration Characteristics Infiltration characteristics of upper soils were assessed in general conformance with 2014 DOE Stormwater Management Manual for Western Washington procedures. Two small-scale Pilot Infiltration Tests (PIT) were completed in the upper native soils with approximate location shown on Figure 1. Field and corrected infiltration rates are provided in Table 1. Table 1. Infiltration Rates Test Site and Depth Soil Type Field Infiltration Rate (in/hr) Corrected Infiltration Rate (in/hr) Correction Factors* (CFt/CFv/CFm) PIT-1, 2 ft Silty Sand 4.0 1.1 (0.5/0.7/0.8) PIT-2, 2 ft Silty Sand 6.0 1.7 (0.5/0.7/0.8) *Correction Factors from the 2016 King County Surface Water Design Manual. The field and corrected infiltration rates are considered appropriate for the upper soil tested. However, due to the presence of dense glacial till at fairly shallow depth, these rates are considered representative of mostly horizontal flow through the upper looser soil towards the west. Shallow dispersion systems will be required on this site to facilitate stormwater control. An average design rate of 1.4 inches per hour (in/hr) would be recommended for design of infiltration facilities located in the upper several feet of soils on the site. Other correction factors per the stormwater manual should be applied to the above recommended long-term rate, as required for the type of infiltration system selected. Cation Exchange Capacity (CEC) and organic content tests were completed on a soil samples from the bottom of the infiltration test holes. Test results are summarized in the table below. Geotechnical Engineering Report SSGC 13436 – 156th Ave. SE Renton, Washington SSGC Project No. 18090 November 10, 2018 3 Table 2. CEC and Organic Content Results Test Location, Sample Number CEC Results (milliequivalents) CEC Required* (milliequivalents) Organic Content Results (%) Organic Content Required* (%) PIT-1, S-1 14.2 ≥ 5 5.4 ≥1.0 PIT-1, S-2 11.8 ≥ 5 4.1 ≥1.0 * Per the 2014 DOE Stormwater Management Manual for Western Washington. CEC and organic content results satisfy DOE requirements at the PIT sites. REPORT CONDITIONS This report has been prepared for the exclusive use of Mr. Ralraj Mahal for specific application to the project discussed, and has been prepared in accordance with generally accepted geotechnical engineering practices in the area. No warranties, either express or implied, are intended or made. The analysis and recommendations presented in this report are based on observed soil conditions and test results at the indicated locations, and from other geologic information discussed. This report does not reflect variations that may occur across the site, or due to the modifying effects of construction or weather. The nature and extent of such variations may not become evident until during or after construction. If variations appear, we should be immediately notified so that further evaluation and supplemental recommendations can be provided. This report was prepared for the planned type of development of the site as discussed herein. It is not valid for third party entities or alternate types of development on the site without the express written consent of SSGC. If development plans change we should be notified to review those changes and modify our recommendations as necessary. The scope of services for this project does not include any environmental or biological assessment of the site including identification or prevention of pollutants, hazardous materials, or conditions. Other studies should be completed if the owner is concerned about the potential for contamination or pollution. N South Sound Geotechnical Consulting P.O. Box 39500 Lakewood, WA 98496 (253) 973-0515 Figure 1 – Exploration Plan 13436 - 156th Avenue SE Renton, WA SSGC Project #18090 Approximate Test Pit Location PIT - 1 TP - 1 PIT - 1 Approximate Infiltration Test Location Scale: NTS Base map from Google Maps PIT-1 TP-1 TP-1 Legend PIT-2 TP-1 Geotechnical Engineering Report SSGC 13436 – 156th Ave. SE Renton, Washington SSGC Project No. 18090 November 10, 2018 A-1 Appendix A Field Exploration Procedures and Test Pit Logs Geotechnical Engineering Report SSGC 13436 – 156th Ave. SE Renton, Washington SSGC Project No. 18090 November 10, 2018 Field Exploration Procedures8 Our field exploration for this project included one test pit and two pilot infiltration tests completed on October 22, 2018. The approximate locations of the explorations are shown on Figure 1, Exploration Plan. The exploration locations were determined by pacing from site features. Ground surface elevations referenced on the logs were inferred from Google Earth Imagery. Exploration locations and elevations should be considered accurate only to the degree implied by the means and methods used. A client provided excavator dug the explorations. Soil samples were collected and stored in moisture tight containers for further assessment and laboratory testing. Explorations were backfilled with excavated soils and tamped when completed. Please note that backfill in the explorations will likely settle with time. Backfill material located in building and pavement areas should be re-excavated and recompacted, or replaced with structural fill. The following logs indicate the observed lithology of soils and other materials observed in the explorations at the time of excavation. Where a soil contact was observed to be gradational, our log indicates the average contact depth. Our logs also indicate the approximate depth to groundwater (where observed at the time of excavation), along with sample numbers and approximate sample depths. Soil descriptions on the logs are based on the Unified Soil Classification System. Project: 13436 - 156th Avenue SE SSGC Job # 18090 EXPLORATION LOGS PAGE 1 OF 2 Location: Renton, WA EXPLORATION LOGS FIGURE A-1 South Sound Geotechnical Consulting TP-1, PIT-1 and PIT-2 Logged by: THR Test Pit TP-1 Depth (feet) Material Description 0 – 0.5 0.5 – 3 3 – 5.5 Sod over silty fine SAND with roots (SM): Loose, dry, brown. Silty SAND with gravel (SM): Loose to medium dense, damp, light brown with slight orange mottling. (Alluvium/Weathered Glacial Till) Silty SAND with gravel (SM): Dense, moist, grayish brown. (Glacial Till) Test pit completed at approximately 5.5 feet on 10/22/18. Groundwater not observed at time of excavation. Approximate surface elevation: 446 feet Pilot Infiltration Test PIT-1 Depth (feet) Material Description 0 – 0.5 0.5 – 2 Sod over silty fine SAND with roots (SM): Loose, dry, brown. Silty SAND with gravel: Loose to medium dense, damp, light brown. (Alluvium/Weathered Glacial Till)(Sample S-1 @ 2 feet) Test completed at approximately 2 feet on 10/22/18. Groundwater not observed at time of excavation. Approximate surface elevation: 447 feet Pilot Infiltration Test PIT-2 Depth (feet) Material Description 0 – 0.5 0.5 – 3 Sod over silty fine SAND with roots (SM): Loose, dry, brown. Silty SAND with gravel: Loose to medium dense, damp, light brown (SM). (Alluvium/Weathered Glacial Till)(Sample S-1 @ 3 feet) Test completed at approximately 3 feet on 10/22/18. Groundwater not observed at time of excavation. Approximate surface elevation: 443 feet Geotechnical Engineering Report SSGC 5411 – 132nd Avenue East Edgewood, Washington SSGC Project No. 18059 July 18, 2018 B-1 Appendix B Laboratory Testing and Results Geotechnical Engineering Report SSGC 13436 – 156th Ave. SE Renton, Washington SSGC Project No. 18090 November 10, 2018 B-1 Laboratory Testing Select soil samples were tested for organic content and cation exchange capacity (CEC) by Northwest Agricultural Consultants of Kennewick, Washington. Results of the laboratory testing are included in this appendix. 2545 W Falls Avenue Kennewick, WA 99336 509.783.7450 www.nwag.com lab@nwag.com Sample ID Organic Matter Cation Exchange Capacity PIT-1, S-1 5.38% 14.2 meq/100g PIT-2, S-1 4.13% 11.8 meq/100g Method ASTM D2974 EPA 9081 South Sound Geotechnical Consulting PO Box 39500 Lakewood, WA 98496 Report: 46725-1 Date: November 8, 2018 Project No: 18090 Project Name: Mahal - 156th Ave SE UNIFIED SOIL CLASSIFICATION SYSTEM Criteria for Assigning Group Symbols and Group Names Using Laboratory TestsA Soil Classification Group Symbol Group NameB Coarse Grained Soils More than 50% retained on No. 200 sieve Gravels More than 50% of coarse fraction retained on No. 4 sieve Clean Gravels Less than 5% finesC Cu  4 and 1  Cc  3E GW Well-graded gravelF Cu  4 and/or 1  Cc  3E GP Poorly graded gravelF Gravels with Fines More than 12% finesC Fines classify as ML or MH GM Silty gravelF,G, H Fines classify as CL or CH GC Clayey gravelF,G,H Sands 50% or more of coarse fraction passes No. 4 sieve Clean Sands Less than 5% finesD Cu  6 and 1  Cc  3E SW Well-graded sandI Cu  6 and/or 1  Cc  3E SP Poorly graded sandI Sands with Fines More than 12% finesD Fines classify as ML or MH SM Silty sandG,H,I Fines Classify as CL or CH SC Clayey sandG,H,I Fine-Grained Soils 50% or more passes the No. 200 sieve Silts and Clays Liquid limit less than 50 inorganic PI  7 and plots on or above “A” lineJ CL Lean clayK,L,M PI  4 or plots below “A” lineJ ML SiltK,L,M organic Liquid limit - oven dried  0.75 OL Organic clayK,L,M,N Liquid limit - not dried Organic siltK,L,M,O Silts and Clays Liquid limit 50 or more inorganic PI plots on or above “A” line CH Fat clayK,L,M PI plots below “A” line MH Elastic SiltK,L,M organic Liquid limit - oven dried  0.75 OH Organic clayK,L,M,P Liquid limit - not dried Organic siltK,L,M,Q Highly organic soils Primarily organic matter, dark in color, and organic odor PT Peat A Based on the material passing the 3-in. (75-mm) sieve B If field sample contained cobbles or boulders, or both, add “with cobbles or boulders, or both” to group name. C Gravels with 5 to 12% fines require dual symbols: GW -GM well-graded gravel with silt, GW -GC well-graded gravel with clay, GP-GM poorly graded gravel with silt, GP-GC poorly graded gravel with clay. D Sands with 5 to 12% fines require dual symbols: SW -SM well-graded sand with silt, SW -SC well-graded sand with clay, SP-SM poorly graded sand with silt, SP-SC poorly graded sand with clay E Cu = D60/D10 Cc = 6010 2 30 DxD )(D F If soil contains  15% sand, add “with sand” to group name. G If fines classify as CL-ML, use dual symbol GC-GM, or SC-SM. HIf fines are organic, add “with organic fines” to group name. I If soil contains  15% gravel, add “with gravel” to group name. J If Atterberg limits plot in shaded area, soil is a CL-ML, silty clay. K If soil contains 15 to 29% plus No. 200, add “with sand” or “with gravel,” whichever is predominant. L If soil contains  30% plus No. 200 predominantly sand, add “sandy” to group name. M If soil contains  30% plus No. 200, predominantly gravel, add “gravelly” to group name. N PI  4 and plots on or above “A” line. O PI  4 or plots below “A” line. P PI plots on or above “A” line. Q PI plots below “A” line.