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REVIEWED BLD BLD2020-1129+Geotechnical_Report+10.21.2020_1.39.22_PMFebruary 24, 2020 ES-7076 Select Homes, Inc. 16531 — 131h Avenue West, A107 Lynnwood, Washington 98037 Attention: Ms. Kayla Nichols Subject: Geotechnical Evaluation Proposed Single -Family Residences 1 055 — 7th Avenue South Edmonds, Washington Earth Solutions N W «C Earth Solutions NW LLC Geotechnical Engineering, Construction ❑bservationjesting and Environmental Services Reference: Department of Ecology Stormwater Management Manual for Western Washington Amended December 2014 James P. Minard Geologic Map of the Edmonds East and Part of the Edmonds West Quadrangles Washington, 1983 Edmonds City Code Chapter 23.80 (Geologically Hazard Areas) Dear Ms. Nichols: As requested, Earth Solutions NW, LLC (ESNW) has prepared this geotechnical evaluation report for the proposed single-family residences. As part of our scope of services, we completed a subsurface exploration, in -situ infiltration evaluation, laboratory and engineering analyses, and prepared this written report with our findings and recommendations for the proposed project. Based on our evaluation, the proposed project is feasible from a geotechnical standpoint. Protect Description The subject site is located immediately northeast of the intersection between 7th Avenue South and Fir Street in Edmonds, Washington. The subject site consists of one parcel (Snohomish County parcel number 00619400400600) covering 0.34 acres. Based on our understanding of the proposed redevelopment, the boundary lines will be adjusted to create two new parcels. The property is currently developed with a single-family residence and associated improvements. Site topography descends to the west with about 10 to 15 feet of total elevation change across the property. The subject site is bordered to the north by a single-family residence, to the east by an alley, to the south by Fir Street, and to the west by 7th Avenue South. 15365 N.E. 90th Street, Suite 100 • Redmond, WA 98052 0 (425) 449-4704 • FAX (425) 449-4711 Select Homes, Inc. ES-7076 February 24, 2020 Page 2 We understand the existing single-family residential structure, detached garage, and associated improvements will be removed and two new single-family residential structures will be constructed. We anticipate grading activities will include cuts and fills of up to about five feet to establish the planned building alignments. However, grading plans were not available at the time this report was prepared. We understand infiltration and low impact development methods are being evaluated for stormwater management. Site improvements will also include underground utility installations. At the time this report was prepared, specific building load values were not available. However, we anticipate the proposed residential structures will consist of relatively lightly loaded wood framing supported on conventional foundations. Based on our experience with similar developments, we estimate wall loads on the order of one to two kips per linear foot and slab -on - grade loading of 150 pounds per square foot (psf). If the above design assumptions are incorrect or change, ESNW should be contacted to review the recommendations in this report. ESNW should review the final design to verify the geotechnical recommendations provided in this report have been incorporated into the plans. Subsurface Conditions As part of this geotechnical evaluation, an ESNW representative observed, logged, and sampled three test pits on December 16, 2019, excavated at accessible locations within the proposed development area, using a mini-trackhoe and operator provided by the client. The approximate locations of the test pits are depicted on the Test Pit Location Plan (Plate 2). Please refer to the test pit logs provided as an attachment to this report for a more detailed description of subsurface conditions. Representative soil samples collected at the test pit locations were analyzed in general accordance with the Unified Soil Classification System (USCS) and United States Department of Agriculture (USDA) methods and procedures. Topsoil and Fill Topsoil was observed extending to depths of approximately 4 to 12 inches below the existing ground surface (bgs). The topsoil was characterized by dark brown color and fine organic material. Fill was encountered at all of the test pit locations extending to depths of one and one- half to two and one-half feet bgs. The fill was characterized as silty sand (USCS SM), consistent with reworked native soil. A relic topsoil layer was encountered below the fill at TP-2 and TP-3. Native Soil Underlying the topsoil and fill, native soil conditions at the test pit locations were observed to consist primarily of silty sand (USCS: SM). Silty gravel (USCS: GM) was observed near -surface at TP-2 which transitioned to silty sand at depth. Overall soil relative density increased with depth, extending to the maximum exploration depth of about eight feet bgs. The native soil was generally observed in a weakly cemented condition at about four feet bgs at TP-1. Earth Solutions NW. LLC Select Homes, Inc. February 24, 2020 Geologic Setting ES-7076 Page 3 The referenced geologic map resource identifies Vashon advance outwash deposits (Qva) across the site with Vashon till deposits (Qvt) mapped immediately south and east of the site. Vashon advance outwash is typically a well -sorted sand and gravel deposit, which was formed by streams issuing from the advancing ice sheet. The Vashon advance outwash commonly grades upward into glacial till (mapped to the south and east). Vashon till typically consists of a nonsorted mixture of clay, silt, sand, pebbles, cobbles, and boulders. The till was deposited directly by ice advanced over previously deposited sediment and rocks. In addition, the Web Soil Survey identifies Alderwood-Urban land complex (Map Unit Symbol: 6) as the primary soil unit underlying the subject site. The Alderwood series was formed in glacial till plains. Based on our field observations, native soils on the subject site are generally consistent with the glacial till. Groundwater During our subsurface exploration completed on December 16, 2019, groundwater was observed at each of the test pit locations, at depths of four to seven and one-half feet bgs. Groundwater seepage rates and elevations fluctuate depending on many factors, including precipitation duration and intensity, the time of year, and soil conditions. In general, groundwater flow rates are higher during the winter, spring, and early summer months. Geologically Hazardous Areas Assessment As part of this geotechnical evaluation, the referenced chapter of the Edmonds City Code was reviewed. Based on our investigation and review, there are no geologically hazardous areas present on or adjacent to the site. Foundations The proposed single-family structures can be supported on conventional spread and continuous footings bearing on undisturbed, competent native soil, compacted native soil, or new structural fill. Competent native soils, suitable for support of the foundation, should be encountered beginning at depths of approximately two to three and one-half feet bgs. Where loose or unsuitable soil conditions are encountered at foundation subgrade elevations during site preparation activities, compaction of the soils to the specifications of structural fill, or overexcavation and replacement with granular structural fill will be necessary. Structural fill should consist of suitable granular soils compacted to 95 percent of Modified Proctor (ASTM D1557). Compaction of the soil to the levels necessary for use as structural fill will be difficult during wet weather conditions. Organic material exposed at foundation subgrade elevations must be removed and grades restored with structural fill. Earth Solutions NW. LLC Select Homes, Inc. ES-7076 February 24, 2020 Page 4 Provided the structures will be supported as described above, the following parameters can be used for design of the new foundations: • Allowable soil bearing capacity 2,500 psf • Passive earth pressure • Coefficient of friction 300 pcf (equivalent fluid) AM The passive earth pressure and coefficient of friction values included a safety factor of 1.5. A one-third increase in the allowable soil bearing capacity can be assumed for short-term wind and seismic loading conditions. With structural loading as expected, total settlement in the range of one inch is anticipated, with differential settlement of about one-half inch. The majority of the settlements should occur during construction, as dead loads are applied. Seismic Considerations The 2018 IBC recognizes ASCE for seismic site class definitions. In accordance with Table 20.3- 1 of ASCE, Minimum Design Loads for Buildings and Other Structures, Site Class D, should be used for design. In our opinion, the site susceptibility to liquefaction is low. The native soil composition and relative density are the primary bases for this opinion. Drainage Temporary measures to control surface water runoff during construction would likely involve passive elements such as interceptor trenches and sumps. ESNW should be consulted during preliminary grading activities to evaluate seepage areas and provide recommendations to reduce the potential for seepage -related instability. Finish grades should be designed to direct surface water away from structures and slopes. Grades adjacent to buildings should be sloped away at a gradient of either at least 2 percent for a horizontal distance of up to 10 feet or the maximum allowed by adjacent structures. In our opinion, foundation drains should be installed along building perimeter footings. Infiltration Evaluation As indicated in the Subsurface Conditions section, native soils encountered during our fieldwork were characterized primarily as silty sand glacial till deposits. Based on the results of USDA textural analyses, the native soils were classified primarily as sandy loam with fines contents of about 30 to 38 percent. Earth Solutions NW. LLC Select Homes, Inc. ES-7076 February 24, 2020 Page 5 In -situ testing was attempted at TP-1 at a depth of about four feet bgs. However, given the presence of groundwater seepage at a depth of about four feet bgs, the relatively high in -situ fines content of the native soil, and underlying impermeable layers in a cemented condition, infiltration was deemed infeasible. Based on the results of our testing, we do not recommend utilizing infiltration systems for the proposed development. On -site Stormwater Management Pursuant to City of Edmonds stormwater management requirements, implementation of on -site stormwater BMPs are required for proposed developments in accordance with specified thresholds, standards, and lists. The intent of BMP implementation is to infiltrate, disperse, and retain stormwater runoff on site to the extent feasible. The table below summarizes our evaluation of low impact development methods, as outlined in the referenced stormwater manual, from a geotechnical standpoint. It is instructed in the referenced stormwater manual that BMPs are to be considered in the order listed (from top to bottom) for each surface type, and the first BMP that is determined to be viable should be used. For completeness, however, we have evaluated each listed BMP for the proposed surface types. BMP Viable? Limitations or Infeasibility Criteria Lawns and Landscaped Areas T5.13: Post -construction soil quality and depth (Volume V, Yes None. Chapter 5 Roofs T5.30: Adequate vegetative flow paths will likely not be T5.30: Full dispersion (Volume Maybe* available. V, Chapter 5) T5.10A: The unweathered glacial till soils observed at T5.10A: Downspout full relatively shallow depths and observed groundwater infiltration systems (Volume III, NO conditions exhibit very poor soil infiltration Chapter 3) characteristics and should be considered impermeable for design. The unweathered glacial till soils observed at relatively Bioretention (Volume V, Chapter No shallow depths and observed groundwater conditions 7) exhibit very poor soil infiltration characteristics and should be considered impermeable for design. T5.10B: Downspout dispersion systems (Volume III, Chapter 3) Maybe* Adequate vegetative flow paths are likely not available. Earth Solutions NW. LLC Select Homes, Inc. February 24, 2020 ES-7076 Page 6 BMP Viable? Limitations or Infeasibility Criteria T5.10C: Perforated stub -out Perforated stub -out should be placed within upper connections (Volume III, Chapter Yes weathered soil. 3 Other Hard Surfaces T5.30: Full dispersion (Volume V, Chapter 5) Maybe* Adequate vegetative flow paths are likely not available. The unweathered glacial till soils observed at relatively T5.15: Permeable pavement No shallow depths and observed groundwater conditions (Volume V, Chapter 5) exhibit very poor soil infiltration characteristics and should be considered impermeable for design. The unweathered glacial till soils observed at relatively Bioretention (Volume V, Chapter No shallow depths and observed groundwater conditions 7) exhibit very poor soil infiltration characteristics and should be considered impermeable for design. T5.12: Sheet flow dispersion T5.11: Concentrated flow dispersion (Volume V, Chapter Maybe* Adequate vegetative flow paths are likely not available. 5) * Viability to be determined by storm designer. Limitations The recommendations and conclusions provided in this geotechnical evaluation 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 pit locations may exist and may not become evident until construction. ESNW should reevaluate the conclusions in this geotechnical evaluation report if variations are encountered. Earth Solutions NW. LLC Select Homes, Inc. February 24, 2020 ES-7076 Page 7 Should you require additional information, or have questions, please call. Sincerely, EARTH SOLUTIONS NW, LLC Adam Z. Shier, L.G. Project Geologist Attachments: Plate 1 — Vicinity Map Plate 2 — Test Pit Location Plan Test Pit Logs Grain Size Distribution '24/2020 Henry T. Wright, P.E. Senior Project Manager cc: RAM Engineering, Inc. Attention: Mr. Rob Long, P.E. (Email only) Earth Solutions NW. LLC A r - Wa lntt St[ee[ - walnut 5Lr"i. S� walnutStwt ceaarst:eer. _ r- 10 -znrure sneer ` spruce Srreer. - _ --_ = jam'• 17, .' Ffrri�["i :!c Hr•dr,,k%Nay x' �laurl151r - r es r �__ r f•rrr: f),�[}y GC1 -. 215N S15e�saz[n _ _- �7�-::....1•) � f .. _ . ... :. Pair i[�rvri Pi1P S-reel, Pir:r?•i.x� g inc 5weet - 216rh sk 5authwp _ ATE 217th SlreE[5Qu[hxes[ Forsyth Lane - _ Bella Cnnla Ruacf -_ Earn Street ',�' , Urn I q T�* - Woodway ; Edmonds '= n n WA 10441. -2211h Sireei-Southwest - _ ri Pyaongwnq ya '. tSN'S? eR. Sou[hwey[ to ':'YL �• = o,� J� �o�noeer ome : c .$`rc•r':NOOd s \ L— ds=��' �s6s%l __ __/ec•d FaeS! — Eden°n1: -_- V'.+UCC?�^ray 3 ., WA104 232nd Srreet 5°.uthwes[ _. — 3 __ S Ne:11rfS N 7MIhscree[5[fuLhw- _ - a r r Nottingham Roa6 0 KP1 i I 7i m. Q 3 v m 2'S5[h Place Sou[nWeR ' e36th Srreet Sc Mudrv.mr 0: schvvl 238[h Street 5°u[hives� j 3]xh Ple a¢ 2 738th Stieel, so+nhW Paramour�r r, R j. Petrp7et m s zar[hsvRets-[hwest Terminal m Ea91e lane '�. ,3� 'S - — �FP. is ..::1:•= C rvl nGP 2 o'er. eerYLane n Y NORTH } Reference: . Snohomish County, Washington OpenStreetMap.org Vicinity Map 1055 - 7th Avenue S. Edmonds, Washington NOTE: This plate may contain areas of color. ESNW cannot be responsible for any subsequent misinterpretation of the information Drwn. CAM Date 12/30/2019 Proj. No. 7076 resulting from black & white reproductions of this plate. Checked AZS Date Dec. 2019 Plate 1 I I I I I I I I I I i I � 195 200 205 �—TTP///1 � ---- / I Gara0e I I � I I � House i I � ' I TJP-3 I TP-2 I v I a \ I 205 195 200 LEGEND FIR STREET TP-1 Approximate Location of — ■ — ESNW Test Pit, Proj. No. ES-7076, Dec. 2019 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 existing and / or proposed site features. The information illustrated 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. I I I i I I i I II C77 I I I I I I I I I IL---------- NOT - TO - SCALE Test Pit Location Plan 1055 - 7th Avenue S. Edmonds, Washington NOTE: This plate may contain areas of color. ESNW cannot be ' responsible for any subsequent misinterpretation of the information Drwn. CAM Date 12/30/2019 Proj. No. 7076 resulting from black & white reproductions of this plate. Checked AZS Date Dec. 2019 Plate 2 Earth Solutions NWLLC SOIL CLASSIFICATION CHART MAJOR DIVISIONS SYMBOLS TYPICAL DESCRIPTIONS GRAPH LETTER GRAVEL AND CLEAN GRAVELS �' �� • �•• 46 •� GW WELL -GRADED GRAVELS, GRAVEL - SAND MIXTURES, LITTLE OR NO FINES GRAVELLY SOILS (LITTLE OR NO FINES) ° (Y o B, o DOo D Q oQ Q GP POORLY -GRADED GRAVELS, GRAVEL - SAND MIXTURES, LITTLE OR NO FINES COARSE GRAINED SOILS MORE THAN 50% OF COARSE FRACTION GRAVELS WITH FINES ° o 0 �0 o D GM SILTY GRAVELS, GRAVEL - SAND - SILT MIXTURES RETAINED ON NO. 4 SIEVE (APPRECIABLE AMOUNT OF FINES) GC CLAYEY GRAVELS, GRAVEL - SAND - CLAY MIXTURES MORE THAN 50% OF MATERIAL IS SAND AND CLEAN SANDS SW WELL -GRADED SANDS, GRAVELLY SANDS, LITTLE OR NO FINES �. :. SP POORLY -GRADED SANDS, GRAVELLY SAND, LITTLE OR NO FINES LARGER THAN NO. 200 SIEVE SIZE SANDY SOILS (LITTLE OR NO FINES) SANDS WITH FINES SM SILTY SANDS, SAND - SILT MIXTURES MORE THAN 50% OF COARSE FRACTION PASSING ON NO. 4 SIEVE (APPRECIABLE AMOUNT OF FINES) SC CLAYEY SANDS, SAND - CLAY MIXTURES INORGANIC SILTS AND VERY FINE ML SANDS, ROCK FLOUR, SILTY OR CLAYEY FINE SANDS OR CLAYEY SILTS WITH SLIGHT PLASTICITY FINE GRAINED SOILS SILTS LIQUID LIMIT AND LESS THAN 50 CLAYS CL INORGANIC CLAYS OF LOW TO MEDIUM PLASTICITY, GRAVELLY CLAYS, SANDY CLAYS, SILTY CLAYS, LEAN CLAYS OL ORGANIC SILTS AND ORGANIC SILTY CLAYS OF LOW PLASTICITY MORE THAN 50% OF MATERIAL IS SMALLER THAN NO. 200 SIEVE MH INORGANIC SILTS, MICACEOUS OR DIATOMACEOUS FINE SAND OR SILTY SOILS SIZE SILTS AND LIQUID LIMIT CLAYS GREATER THAN 50 CH INORGANIC CLAYS OF HIGH PLASTICITY OH ORGANIC CLAYS OF MEDIUM TO HIGH PLASTICITY, ORGANIC SILTS HIGHLY ORGANIC SOILS PT PEAT, HUMUS, SWAMP SOILS WITH HIGH ORGANIC CONTENTS DUAL SYMBOLS are used to indicate borderline soil classifications. The discussion in the text of this report is necessary for a proper understanding of the nature of the material presented in the attached logs. r Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 TEST PIT NUMBER TP-1 PAGE 1 OF 1 PROJECT NUMBER ES-7076 PROJECT NAME 1055 - 7th Avenue S. DATE STARTED 12/16/19 COMPLETED 12/16/19 GROUND ELEVATION TEST PIT SIZE EXCAVATION CONTRACTOR Client -Provided GROUND WATER LEVELS: EXCAVATION METHOD AT TIME OF EXCAVATION -- LOGGED BY AZS CHECKED BY HTW AT END OF EXCAVATION -- NOTES Depth of Topsoil & Sod 12": grass AFTER EXCAVATION - W 0- W TESTS = Q O MATERIAL DESCRIPTION p 2Z � Q U 0 Dark brown TOPSOIL (Fill) TPSL _ 1.0 Brownish red silty SAND, loose to medium dense, moist to wet (Fill) 12.5 SM -brick - MC = 18.70% Gray silty SAND with gravel, medium dense, moist - SM 3.5 -light groundwater seepage at 3.5', weakly cemented, iron oxide staining Gray silty SAND, dense, moist _ MC = 12.80% Fines = 30.00% [USDA Classification: slightly gravelly sandy LOAM] 5 SM -weakly cemented nnr. = 1 n Fn°i 7.5 Test pit terminated at 7.5 feet below existing grade. Groundwater seepage encountered at 3.5 feet during excavation. No caving observed. r Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 TEST PIT NUMBER TP-2 PAGE 1 OF 1 PROJECT NUMBER ES-7076 PROJECT NAME 1055 - 7th Avenue S. DATE STARTED 12/16/19 COMPLETED 12/16/19 GROUND ELEVATION TEST PIT SIZE EXCAVATION CONTRACTOR Client -Provided GROUND WATER LEVELS: EXCAVATION METHOD AT TIME OF EXCAVATION -- LOGGED BY AZS CHECKED BY HTW AT END OF EXCAVATION -- NOTES Depth of Topsoil & Sod 12": grass AFTER EXCAVATION - W W = wTESTS Q O MATERIAL DESCRIPTION QZ U 1.0 SM 12.0 TPSL L' '-1' 3.5 MC = 6.10% GM _ 5 MC = 17.00% 5.0 SM hm, = 11 Rno/ 7.5 Dark brown TOPSOIL (Fill) Gray silty SAND, loose to medium dense, moist (Fill) Dark brown TOPSOIL Gray silty GRAVEL, dense, moist Gray silty SAND, medium dense, moist to wet -groundwater table Test pit terminated at 7.5 feet below existing grade. Groundwater table encountered at 7.0 feet during excavation. No caving observed. r Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 TEST PIT NUMBER TP-3 PAGE 1 OF 1 PROJECT NUMBER ES-7076 PROJECT NAME 1055 - 7th Avenue S. DATE STARTED 12/16/19 COMPLETED 12/16/19 GROUND ELEVATION TEST PIT SIZE EXCAVATION CONTRACTOR Client -Provided GROUND WATER LEVELS: EXCAVATION METHOD AT TIME OF EXCAVATION -- LOGGED BY AZS CHECKED BY HTW AT END OF EXCAVATION -- NOTES Depth of Topsoil & Sod 4": grass AFTER EXCAVATION - W W = wTESTS Q O MATERIAL DESCRIPTION QZ U TPSL 0.4 Dark brown TOPSOIL (Fill) Gray silty SAND, medium dense, moist (Fill) SM 1.5 TPSL •' ':A2.0 Dark brown TOPSOIL Gray silty SAND with gravel, medium dense to dense, moist MC = 14.60% Fines = 38.30% -iron oxide staining SM [USDA Classification: gravelly fine sandy LOAM] 4.0 Gray silty SAND, medium dense, moist MC = 16.10% 5 SM -increased fines MC = 13.70% Fines = 33.90% 8.0 -groundwater table [USDA Classification: slightly gravelly fine sandy LOAM] Test pit terminated at 8.0 feet below existing grade. Groundwater table encountered at 7.5 feet during excavation. No caving observed. itMEN MEN I ME, I I'm, mmiiiiiilmmiuililmmiiililis NINE I uu 1 u 1 0.1 0.01 0.001 GRAIN SIZE IN MILLIMETERS 3 i COBBLES GRAVEL SAND SILT OR CLAY coarse fine coarse I medium fine Specimen Identification Classification Cc Cu • 1 TP-01 4.00ft. USDA: Gray Slightly Gravelly Sandy Loam. USCS: SM. m TP-03 2.50ft. USDA: Gray Gravelly Fine Sandy Loam. USCS: SM with Gravel. ♦ TP-03 8.00ft. USDA: Gray Slightly Gravelly Fine Sandy Loam. USCS: SM. Specimen Identification D100 D60 D30 D10 ILL PL PI %Silt %Cray • TP-01 4.Oft. 9.5 0.237 0.075 30.0 m TP-03 2.5ft. 37.5 0.275 38.3 ♦ TP-03 8.Oft. 19 0.201 33.9