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15370-R-GEOT-2017-01-30-Infiltration Eval & Hazard Assessment-Earth Solutions.pdfJanuary 30, 2017 ES-4951 Clear Vision Homes, Inc. 1123 Maple Avenue Renton, Washington 98057 Attention: Mr. Phong Le Earth I Solutions NW«< Earth Solutions NW LLC • Geotechnical Engineering • Construction Monitoring • Environmental Sciences Subject: Summary Infiltration Evaluation and Geologic Hazard Areas Assessment Proposed Short Plat 18227 — 80th Avenue West Edmonds, Washington Reference: Title 18 of the Edmonds Community Development Code (ECDC) Department of Ecology (DOE) 2012 Stormwater Management Manual for Western Washington (SMMWM) As Amended, December 2014 King County Department of Natural Resources and Parks Surface Water Design Manual (KCSWDM), dated April 24, 2016 Geotech Consultants, Inc. Geotechnical Engineering Study Project No. JN-15071, dated June 29, 2016 James P. Minard Geologic Map of the Edmonds East and Part of the Edmonds West Quadrangles, Washington, 1983 Dear Mr. Le: As requested, Earth Solutions NW, LLC (ESNW) has prepared this infiltration evaluation and Geological Hazardous Areas (GHA) assessment for the subject site. The evaluation was performed in general accordance with the scope of services outlined in our proposal dated December 6, 2016 and authorized by you on December 15, 2016. A summary of the subsurface exploration and in -situ testing, as well as geotechnical recommendations for infiltration stormwater management design and review of mitigation recommendations for the identified GHA are provided in this letter. 1805 - 136th Place N.E., Suite 201 0 Bellevue, WA 98005 • (425) 449-4704 • FAX (425) 449-4711 Clear Vision Homes, Inc. January 30, 2017 Proiect Description ES-4951 Page 2 The subject site is located generally north of Seaview Park along the eastern edge of 80th Avenue West in Edmonds, Washington, and is currently developed with a single-family residence and related improvements. We understand that site redevelopment plans include subdividing the property into three lots and constructing two single-family residences and related infrastructure improvements along the eastern, undeveloped portion of the site. We understand that the existing structure is to remain. As part of site redevelopment, native vegetated areas will be established, primarily along the western and southern portions of the site. We anticipate the new structures will be on the order of two to three stories in height and constructed utilizing relatively lightly -loaded wood framing supported on conventional foundations. A drywell will be incorporated into site designs as means of stormwater management for new impervious surfaces. Based on review of available resources and the referenced topographic survey, we understand that a steep slope, located off -site just beyond the southeastern property corner, has been identified as a potential landslide hazard. An erosion hazard area has also been identified within the subject site. Determining infiltration feasibility and providing a long-term design infiltration rate, as well as review of mitigation recommendations for the identified GHA was the primary focus of our investigation. If the assumptions outlined in this section are incorrect or change, ESNW must be contacted to review the recommendations and conclusions provided in this letter. Site Conditions The subject site is located generally north of Seaview Park, along the eastern edge of 80th Avenue in Edmonds, Washington. The approximate location of the property is illustrated on Plate 1 (Vicinity Map). The subject site is comprised of a single tax parcel (Snohomish County Parcel No. 0037080010-1400) totaling approximately 0.85 acre. A steep slope is present directly southeast of the property boundary. The referenced site survey indicates that the inclination of the slope is in excess of 40 percent. Surface The site is bordered to the north, east, and south by single-family residences, with 80th Avenue West bordering the western edge of the site. Site topography has an overall easterly declination with approximately 20 to 25 feet of elevation change. We anticipate that minor grading related to construction of the existing house and associated improvements has occurred on -site; however, appears to be limited in extent. Vegetation primarily consists of grass landscaping features along the western portion of the site that transitions into native brush, bramble, and tree growth as the site progresses eastward. Earth Solutions NW, LLC Clear Vision Homes, Inc. January 30, 2017 Subsurface ES-4951 Page 3 An ESNW representative observed, sampled, and logged the excavation of one test pit within the proposed drywell area, using an excavator and operator retained by our firm. The test pit was completed for purposes of assessing soil conditions, classifying site soils, and evaluating the hydrologic capacity of the site soils. The approximate location of the test pit is depicted on Plate 2 (Test Pit Location Plan). Please refer to the attached test pit log for a more detailed description of subsurface conditions. Soil samples collected at the test pit location were analyzed in accordance with both Unified Soil Classification System (USCS) and United States Department of Agriculture (USDA) methods and procedures. Topsoil was not encountered at the test pit location; however, due to the extent of native vegetation within this portion of the site, topsoil should be anticipated within other locations throughout the site. Fill was encountered at the test pit location extending to an approximate depth of two to two -and -one -quarter feet below the existing ground surface (bgs). The fill was characterized primarily as a silty sand (USCS: SM) in a loose and moist condition. Underlying fill material, native soils were encountered consisting primarily of medium dense to dense, silty sand with trace to significant gravel (USCS: SM). Native soils were encountered primarily in a moist condition and extended to the maximum exploration depth of eight feet bgs. Geologic Setting The referenced geologic map resource identifies Vashon till (Qvt) across the site and surrounding areas. Vashon till typically consists of an non -sorted mixture of clay, silt, sand, pebbles, cobbles, and boulders, all in variable amounts. The referenced WSS resource identifies soils of the Alderwood-Urban land complex (Map Unit Symbol: 5) and Alderwood- Everett gravelly sandy loam series (Map Unit Symbol: 4) across the subject site. Based on the conditions encountered within our subsurface exploration location, soils likely to be encountered on the subject site will generally be consistent with geologic setting described within this section. Groundwater During our fieldwork completed on December 29, 2016, minor groundwater intrusion in the form of isolated perched seepage was observed within the test pit. In general, the perched seepage was observed at approximately five to six 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 wetter, winter months. Based on the time of year that the exploration occurred, in our opinion, the conditions observed likely represent the seasonal high groundwater condition for this site. Earth Solutions NW, LLC Clear Vision Homes, Inc. January 30, 2017 Geologic Hazardous Areas ES-4951 Page 4 As part of our letter preparation, we reviewed the geotechnical engineering report (study) previously prepared by Geotech Consultants and the mitigation recommendations presented for the city -identified potential landslide hazard and erosion hazard. The test pit logs within the referenced study were also reviewed and are provided as an attachment. The following contains the mitigation recommendations and evaluation for each GHA within the referenced study. Landslide Hazard Area A steep slope approximately 35 feet in height is located directly off the southeastern corner of the site. According to ECC 23.80.020B, the steep slope has been identified as a potential landslide hazard. With respect to the location of the steep slope, mitigation recommendations need only apply to lot three. Current site plans include a 25 foot buffer from the top of the slope with a minimum building setback of 40 feet. However, based on our review of the provided test pit logs, it is our opinion that the buffer may be reduced to 20 feet along with a minimum building setback of 15 feet. The buffer and building setback would provide a minimum horizontal offset of 35 from the identified top of slope, which is equal to the height of the slope. In our opinion, a total offset of 35 feet should be considered suitable from a geotechnical standpoint. Erosion Hazard Area Soils of the Alderwood-Urban land complex and Alderwood-Everett series have been mapped across the subject site and surrounding areas. According to ECC 23.80.020A, soils within these series can be considered an erosion hazard. Provided temporary erosion control measures are followed (as directed by the project civil engineer) and permanent erosion control measures including revegetation of exposed, undeveloped soils, the erosion hazard presented onsite can be successfully mitigated. Infiltration Evaluation In accordance with the request made by the City of Edmonds, ESNW performed a Pilot Infiltration Test (PIT) within the proposed drywell facility area in order to evaluate the native soils hydrologic capacity and to determine a long-term design rate. As indicated on the attached laboratory sieve analyses, native soils were classified primarily as silty sand (USCS: SM). The soils were further classified per the USDA Classification System as slightly gravelly loamy sand with a fines content on the order of 26 percent. Based on our field observations and laboratory sieve analysis, soils observed within the test pit location were noted to grade into a slightly gravelly sandy loam at depth. Fines content of the sandy loam soil layer was on the order of 33 percent. An infiltration rate of 2.0 inches per hour was measured at the PIT location. However, infiltration rates measured during field testing are considered short term. In order to obtain a suitable, long-term design rate, appropriate correction factors must be applied to the measured rate. Applicable partial correction factors applied to this testing program are as follows: Earth Solutions NW, LLC Clear Vision Homes, Inc. ES-4951 January 30, 2017 Page 5 • Site Variability (CF„) 1.5 • Degree of Long Term Maintenance (CFm) 3 • Degree of influent control (CFI) 2 Using the above correction factors, a total correction factor (CFT) of 6.5 was calculated. Application of the total correction factor yields a long-term design rate of 0.31 inches per hour. Typical drywell sizing recommendations are based on the USDA classifications of the soils observed at the proposed bottom of facility. The referenced DOE stormwater manual provides sizing criteria for soils considered generally suitable for full infiltration (medium to coarse sands and gravels). Drywells are considered feasible within finer grained soils, but typically are only suitable for limited infiltration applications. Slightly gravelly loamy sand was identified at the proposed bottom of facility. In our opinion, the soils observed at the approximate bottom of facility should be considered suitable for limited infiltration applications. Since the native soils were observed to grade into a sandy loam with increasing depth, it is our opinion that an infiltration facility should be designed accordingly to these soils. In order to provide drywell sizing for the sandy loam, the 2016 KCSWDM was reviewed. As stated in Appendix C.2.3.4 of the KCSWDM, drywells placed within sandy loam must have at least 360 cubic feet of gravel for every 1,000 square feet of tributary impervious surface. It is our opinion that an overflow should be incorporated into facility designs. Discussion Current design parameters of the proposed drywell for use as stormwater management is considered feasible, from a geotechnical standpoint. Based on our review of the provided site plans, we understand that the drywell will extend into the loamy sand identified within the test pit location. However; since the sandy loam will be the primary underlying constrictive layer, drywell sizing should adhere to the specifications set forth in the KCSWDM for this soil type. Minor perched groundwater seepage was observed at approximately five to six feet bgs within the test pit location but should be considered minimal in extent. It is our opinion that an overflow should be incorporated into facility designs and should be directed to an approved discharge location. Uncontrolled water flow should not be allowed to flow over or pond above the identified offsite steep slope area. Closure The recommendations and conclusions provided in this letter 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 soil and groundwater conditions observed at the test pit location may exist, and may not become evident until construction. ESNW should reevaluate the conclusion in this geotechnical evaluation letter if variations are encountered. Earth Solutions NW, LLC Clear Vision Homes, Inc. January 30, 2017 ES-4951 Page 6 ESNW should have the opportunity to review final project plans and ensure appropriate geotechnical recommendations have been incorporated into the design. In addition, ESNW should be retained to observe the construction of infiltration facilities on the subject site in order to confirm soil conditions are as anticipated and to provide supplementary geotechnical recommendations where necessary. We appreciate the opportunity to be of service to you and trust this letter meets your current needs. Should you have questions, or require additional information, please call. Sincerely, EARTH SOLUTIONS NW, LLC Chase G. Halsen c t S RTe I, L. G. Staff Geologist oje�t M nager yv f WAsy T Kyle R. Campbell, P.E. Principal Attachments: Plate 1 —Vicinity Map Plate 2 — Test Pit Location Plan Test Pit Log — By ESNW Test Pits Logs — By Others Laboratory Sieve Analyses cc: Mr. Bill Taylor (Email Only) Earth Solutions NW, LLC 5 A11 169TH 1Nr`c ST j►y y Q AIn� A `; ntAOo•lout ° K w t ; ,harm nn !f sNaxit �j 1t� 7a -LWD • yI—' ,:tt. 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VA CQ S l:a f i A i rw+ 196TH T�rC 1` t1�� J�• ` iali:a .� tM.= IF (FI RT it ,r�`�'JcG� G �' XRbrl1 ,Nti, t ;W : {o Y SETA `t,N 6L10 •y� } ;91 A _ too H3rD) PtlrtMlLr7r - =hS+A �.I S•• = to" 3 .0A�.9.$ N • )m, Y s c w{ n �7I(M 41 X Iwo 51 ow f ,• � 7M S� t s !'n # __ mw sI �� j s' V XINMot t�wI v It, s �+ CRUX .« it wly P. so FAIR _F�I�I(M{ r•JDITM'sT�4I1' j r1IN 31 S'h: �i_ _ ;was, 21 11?MYMW= S G f Mu\KAv[ i 1� Pt s f s �. Dir►rs a t ` GAY roLMt1J R N 1 s a i 19' � aR ':ci f r♦ SW � ' R llr ST I .e r >t Reference: NORTH r 1 1 Earth Snohomish County, Washington �hh' NWLLC hnical Engineering, Construction Monitoring Map 455 00� and Environmental Sciences By The Thomas Guide Rand McNally Vicinity Map 32nd Edition Edmonds Short Plat Edmonds, Washington NOTE: This plate may contain areas of color. ESNW cannot be responsible for any subsequent misinterpretation of the information Drwn. CAM Date 01 /19/2017 Proj. No. 4951 resulting from black & white reproductions of this plate. Checked CGH Date Jan. 2017 Plate 1 330 320 tP-1 TP-2 —AII I Lot3 D'L 0v�vLot2v_ TP-3 I TP-1 �I ` \ / ` •� —■— 320-i----- ��-J 340 330 House Deck LEGEND TP-1 Approximate Location of — ■ — ESNW Test Pit, Proj. No. ES-4951, Dec. 2016 TP-1 Approximate Location of — • — Geotech Consultants Test Pit, Proj. No. 15071, June 2016 r— —, L — —� 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. NORTH 0 25 50 100 1 "=50' L I Scale in Feet Test Pit Location Plan Edmonds Short Plat Edmonds, Washington NOTE: This plate may contain areas of color. ESNW cannot be responsible for any subsequent misinterpretation of the information Drwn. CAM Date 01 /19/2017 Proj. No. 4951 resulting from black & white reproductions of this plate. Checked CGH Date Jan. 2017 Plate 2 Earth Solutions NWLLC SOIL CLASSIFICATION CHART MAJOR DIVISIONS SYMBOLS TYPICAL DESCRIPTIONS GRAPH LETTER GRAVEL AND CLEAN GRAVELS 'W �'�� ��•�� GW WELL -GRADED GRAVELS, GRAVEL - MIXTURES, LITTLE OR NO FINES GRAVELLY SOILS (LITTLE OR NO FINES) ° Qo ;Q° o O�o 0 O Q oQ GP POORLY -GRADED GRAVELS, GRAVEL - SAND MIXTURES, LITTLE OR NO FINES COARSE GRAINED SOILS MORE THAN 50% OF COARSE FRACTION GRAVELS WITH FINES °� o Q `° D o 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 S WELL SANDS, GRAVELLY SANDS, LITTLE OR NO FINES LARGER THAN SANDY NO. 200 SIEVE SIZE SOILS (LITTLE OR NO FINES) SP POORLY -GRADED SANDS, GRAVELLY SAND, 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) SIC 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 INORGANIC CLAYS OF LOW TO SILTS FINE GRAINED SOILS AND LIQUID LIMIT CLAYS LESS THAN 50 CL 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 slzE SILTS AND LIQUID LIMIT CLAYS GREATER THAN 50 CI I INORGANIC CLAYS OF HIGH PLASTICITY OH I 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. Earth Solutions NW TEST PIT NUMBER TP-1 1805 - 136th Place N.E., Suite 201 Bellevue, Washington 98005 PAGE 1 OF 1 Telephone: 425-449-4704 Fax: 425-449-4711 CLIENT Clear Vision Homes. Inc. PROJECT NAME Edmonds Short Plat PROJECT NUMBER 4951 PROJECT LOCATION Edmonds, Washington DATE STARTED 12/29/16 COMPLETED 12/29/16 GROUND ELEVATION TEST PIT SIZE EXCAVATION CONTRACTOR NW Excavating GROUND WATER LEVELS: EXCAVATION METHOD AT TIME OF EXCAVATION --- LOGGED BY CGH CHECKED BY CGH AT END OF EXCAVATION -- NOTES Surface Conditions: brush, brambles AFTER EXCAVATION --- W a. W U = a 1— W E g TESTS ' U = a p MATERIAL DESCRIPTION WW a� v) qa0 Q Z (� U) 0 Black silty SAND, loose, moist (Fill) -concrete pipe debris (surficial) - MC = 34.20% SM -root intrusions to 5' -glass bottle debris _ 2.3 Brown silty SAND, loose to medium dense, moist MC = 18.80% Fines = 22.90% [USDA Classification: slightly gravelly loamy SAND] -becomes gray, minor to moderate iron oxide staining -becomes dense 5 MC = 22.20% _ Fines = 32.30% SM -minor perched groundwater seepage at 5' and 6' [USDA Classification: slightly gravelly sandy LOAM] MC = 22.00% -sand lens —6-12" thick a o -heavy iron oxide staining MC = 13.30% Test pit terminated at 8.0 feet below existing grade. Groundwater seepage encountered at 5.0 and 6.0 feet during excavation. Caving observed from 4.0 to 6.0 feet. Bottom of test pit at 8.0 feet. Jae I1 TEST PIT 1 5 10 F1 10 Topsoil over; Description I' s Brown SAND with silt, fine to coarse -grained, moist, loose SM Brown silty SAND, fine to coarse -grained, moist, medium -dense . .! SM I I Brown SAND with silt and gravel, fine to coarse -grained, moist, medium -dense * Test Pit terminated at 8 feet on February 25, 2015. * No groundwater seepage was observed during excavation. * No caving observed during excavation. 10 GS sw SM TEST PIT 2 Description Topsoil over; Brown silty SAND with gravel, fine to coarse -grained, moist, loose -becomes medium -dense and gray -brown Gray SAND with silt and gravel, fine to coarse -grained, moist, medium -dense to densel * Test Pit terminated at 10 feet on February 25, 2015. * No groundwater seepage was observed during excavation. * No caving observed during excavation. GEOTECH CONSULTANT'S, INC. TEST PIT LOG 18227 - 80th Avenue West Edmonds, Washington Job Date: Logged by: I plate: 15071 1 June 2016 TRC 3 '1 10 5 10 e '0'a �o` ,�e`��0�, �G5 0 G J TEST PIT 3 Topsoil over; Description Brown silty SAND with gravel, fine to coarse -grained, moist, loose -becomes medium -dense and gray -brown -becomes dense (TILL) * Test Pit terminated at 5 feet on February 25, 2015. * No groundwater seepage was observed during excavation. * No caving observed during excavation. 1 ��e eQ`r`��o`o��e���ae��e CGS 0 TEST PIT 4 Description Topsoil over; I l: Brown SAND with silt and gravel, fine to coarse -grained, moist, loose ISM SW Brown SAND with silt and gravel, fine to coarse -grained, moist, medium -dense SM MML~ — Dust- r— o� wn mottled gray -brown SILT wit san , f ne o coarse -grave , moist t, _medium -dense-- SM Rust -brown mottled gray -brown silty SAND with gravel, fine to coarse -grained, moist, rnedium-dense * Test Pit terminated at 8 feet on February 25, 2015. * No groundwater seepage was observed during excavation. * No caving observed during excavation. GEOTECH CONSULTANT'S, INC. TEST PIT LOG 18227 - 80th Avenue West Edmonds, Washington Job Date: Logged by: I plate: 15071 June 2016 TRC 4 Earth Solutions NW, LLC GRAIN SIZE DISTRIBUTION 1 1805 - 136th PL N.E., Suite 201 Bellevue, WA 98005 Telephone: 425-449-4704 Fax: 425-449-4711 CLIENT Clear Vision Homes. Inc. PROJECT NAME Edmonds SP PROJECT NUMBER ES-4951 PROJECT LOCATION Edmonds U.S. SIEVE OPENING IN INCHES I U.S. SIEVE NUMBERS I HYDROMETER 6 4 3 2 1.5 1 112 3 4 6 810 14 16 20 30 40 50 60 100 140 200 100 95 90 85 - 80 75 70 rl 65 — 2 � 60 w � 55 - m of 50 z LLTH F- 45 — z w 40 — w a 35 30 25 —1+ 20 - 15 — - 10 — - 5 0 TIT- 100 10 1 0.1 0.01 0.001 GRAIN SIZE IN MILLIMETERS COBBLES GRAVEL SAND SILT OR CLAY coarse fine�coa�rsermedium fine Classification Cc Cu USDA: Brown Slightly Gravelly Loamy Sand. USCS: SM. Specimen Identification • TP-1 3.00ft. m TP-1 5.00ft. USDA: Gray Slightly Gravelly Sandy Loam. USCS: SM. Specimen Identification D100 D60 D30 D10 LL PL PI %Silt %Clay • TP-1 3.Oft. 19 0.333 0.111 22.9 m TP-1 5.Oft. 9.5 0.422 32.3