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Infiltration Considerations Letter recd 10-29-15.pdfCONSULTANTS, INC. Phong Le 1123 Maple Avenue Southwest, Suite 110 Renton, Washington 98057 Subject: Infiltration Considerations Letter Proposed Three -Lot Short Plat 18227 80th Avenue West Edmonds, Washington Dear Mr. Le: RECEIVED OCT 2 9 2015 13256 Northeast 20th Street, Suite 16 Bellevue, Washington 98005 (425) 747-5618 FAX (425) 747-8561 March 11, 2015 JN 15071 DEVELOPMENT SERVICESvia email: lep95@yahoo.com We are pleased to present this geotechnical engineering letter for the three -lot short plat to be constructed in Edmonds, Washington. The scope of our services consisted of exploring site surface and subsurface conditions, and then developing this report to provide considerations concerning stormwater infiltration. This work was authorized by your acceptance of our proposal dated February 12, 2015. We were provided with a site plan prepared by Lanktree Land Surveying, Inc. dated October 11, 2014. Based on this plan, we understand that the development will consist of two new residential lots east of the existing site house. A third lot will be on the western side of the property and include the existing house within it. We understand that stormwater infiltration is desired for the project if feasible. If the scope of the project changes from what we have described above, we should be provided with revised plans in order to determine if modifications to the recommendations and conclusions of this report are warranted. SURFACE The rectangular property is bordered to the west by 80th Avenue West, and is otherwise surrounded by residences. The ground surface within the site slopes gently to moderately down toward the east, with a change in elevation of about 25 feet across a horizontal distance of about 160 feet. The west -central portion of the site is developed with a one-story house with a basement that daylights toward the east. The house is accessed by a circular driveway. The area west of the house is mostly covered with grass lawn, with a few mature evergreen trees. The area east of the house is vegetated with mature evergreen and deciduous trees and brush. An apparently man-made pond is located near the east edge of the site. It has a depth of about 5 feet and contained several inches of water at the time of our site reconnaissance. We suspect that low permeability soil or a plastic liner was placed at the base of the pond to retain water. The upper edge of a ravine is located just southeast of the site on adjacent properties. The base of the ravine drains toward the northeast. The top of the ravine is very close to the southeast corner of the site and has an inclination estimated at about 2:1 (H:V). Snohomish County SnoScape shows that the adjacent ravine has a height of about 35 feet. /?C/lTCr'LJ I'rII.ICI II TAkITC IfOrl Le March 11, 2015 SUBSURFACE JN 15071 Page 2 The subsurface conditions east of the existing house were explored by excavating four test pits at the approximate locations shown on the Site Exploration Plan, Plate 2. These locations are in likely areas of infiltration. Our exploration program was based on the proposed construction, anticipated subsurface conditions and those encountered during exploration, and the scope of work outlined in our proposal. The test pits were excavated on February 25, 2015 with a rubber -tired backhoe. A geotechnical engineer from our staff observed the excavation process, logged the test pits, and obtained representative samples of the soil encountered. "Grab" samples of selected subsurface soil were collected from the backhoe bucket. The Test Pit Logs are attached to this report as Plates 3 and 4. Soil Conditions The test pits encountered about one foot of topsoil at the ground surface, which was underlain by a few feet of loose silty sand with gravel to sand with silt and gravel. In Test Pits 1, 2, and 4 this upper material was generally underlain by medium -dense sand soils with varying amounts of silt and gravel. In Test Pit 2, the sand with silt and gravel extended to the base of the exploration at a depth of 10 feet. In Test Pits 1 and 4 the sandy soil was underlain by medium -dense to dense silty sand and sandy silt at depths of approximately 6 to 7 feet. The dense silty sand soil is geologically known as glacial till. Test Pit 3 encountered dense glacial till at a depth of 4.5 feet, below the surficial topsoil and a layer of loose silty sand with gravel. The till extended to the base of the test pit at a depth of 5 feet. Groundwater Conditions No groundwater seepage was observed in the test pits, which were left open for only a short time period. It should be noted that groundwater levels vary seasonally with rainfall and other factors. We anticipate that perched groundwater could be found between the near - surface looser soil and the underlying denser glacial till and sandy silt soils. The stratification lines on the logs represent the approximate boundaries between soil types at the exploration locations. The actual transition between soil types may be gradual, and subsurface conditions can vary between exploration locations. The logs provide specific subsurface information only at the locations tested. The relative densities and moisture descriptions indicated on the test pit logs are interpretive descriptions based on the conditions observed during excavation. The compaction of test pit backfill was not in the scope of our services. Loose soil will therefore be found in the area of the test pits. If this presents a problem, the backfill will need to be removed and replaced with structural fill during construction. LABORATORY TESTING We obtained several samples during the excavation of the test pits. Of most importance were samples obtained in Test Pit 2, where infiltration appears most feasible. We completed grain -size analyses on two samples obtained from Test Pit 2. We have attached the grain -size analyses as Le JN 15071 March 11, 2015 Page 3 Plate 4. The grain -size analyses indicate that the samples had silt content of about 8 and 13 percent. INFILTRATION CONSIDERATIONS The test pits conducted for this study generally encountered sandy soils near the ground surface that likely are suitable for limited stormwater infiltration. However, in some of the test pits the sand was underlain by glacial till soil and a layer of sandy silt that are not appropriate for infiltration. Another limiting factor is that there is a steep slope located to the southeast of the site that could be destabilized by stormwater infiltration. Therefore, it is our opinion that no infiltration system should be located within 40 feet of the top of the steep slope. Thus, it appears that the only location that is appropriate for infiltration is in the northeast portion of the site. We recommend that an infiltration system in that area introduce water within 5 feet of the ground surface because of the existence of the glacial till and sandy silt soil. We anticipate that a design infiltration rate of 1 to possibly 2 inches per hour could be obtained for the sandy soil in the recommended infiltration location. Full-scale infiltration testing would be needed in specific locations to confirm the infiltration potential. LIMITATIONS The conclusions and recommendations contained in this report are based on site conditions as they existed at the time of our exploration and assume that the soil and groundwater conditions encountered in the test pits are representative of subsurface conditions on the site. If the subsurface conditions encountered during construction are significantly different from those observed in our explorations, we should be advised at once so that we can review these conditions and reconsider our recommendations where necessary. Unanticipated conditions are commonly encountered on construction sites and cannot be fully anticipated by merely taking samples in test pits. Subsurface conditions can also vary between exploration locations. Such unexpected conditions frequently require making additional expenditures to attain a properly constructed project. It is recommended that the owner consider providing a contingency fund to accommodate such potential extra costs and risks. This is a standard recommendation for all projects. This report has been prepared for the exclusive use of Phong Le and his 'representatives for specific application to this project and site. Our conclusions and recommendations are professional opinions derived in accordance with our understanding of current local standards of practice, and within the scope of our services. No warranty is expressed or implied. The scope of our services does not include services related to construction safety precautions, and our recommendations are not intended to direct the contractor's methods, techniques, sequences, or procedures, except as specifically described in our report for consideration in design. Our services also do not include assessing or minimizing the potential for biological hazards, such as mold, bacteria, mildew and fungi in either the existing or proposed site development. ADDITIONAL SERVICES Geotech Consultants, Inc. should be retained to provide geotechnical consultation, testing, and observation services during construction. This is to confirm that subsurface conditions are consistent with those indicated by our exploration, to evaluate whether earthwork and foundation construction activities comply with the general intent of the recommendations presented in this Le March 11, 2015 JN 15071 Page 4 report, and to provide suggestions for design changes in the event subsurface conditions differ from those anticipated prior to the start of construction. However, our work would not include the supervision or direction of the actual work of the contractor and its employees or agents. Also, job and site safety, and dimensional measurements, will be the responsibility of the contractor. During the construction phase, we will provide geotechnical observation and testing services when requested by you or your representatives. Please be aware that we can only document site work we actually observe. It is still the responsibility of your contractor or on-site construction team to verify that our recommendations are being followed, whether we are present at the site or not. The following plates are attached to complete this report: Plate 1 Site Exploration Plan Plates 2 - 3 Test Pit Logs Plate 4 Grain -Size Analyses We appreciate the opportunity to be of service on this project. Please contact us if you have any questions, or if we can be of further assistance. Respectfully submitted, GEOTECH CONSULTANTS, INC.. $)& L ��' Thor Christensen, P.E. Senior Endineer W. D. Robert Ward, P.E. Principal cc: Dan Del Duca via email: bmdc57@gmail.com TRC/DRW:at 6 9IL-06 ON 8602 40jen 6Mg9 :ajeld :ejep :ON 70/1 UOIBU S M `spuouap3 }s9M anuany 4108 - LZZB N7v,rld NOUNHO'IdX3 ails om `sINvizrnsxoa u011e00-1 ;id Jsal 0 ' 4"aNAsrtrBUTmka iw'u W 'M'k H9'� �+.. y.w ». n � � fMA'M hdfg0 it ^+yU�vf Py +P1tl4,4 W�f„ --. F ANKM i pce7us ar Saar - �'„- ua r mnu� xu�w �s"p ✓'� ��s�ri w.usrw � )y YbJ41'rA''yA0. .WWW” o .0 .. �p 17FL1.-0' h� kr"- ups �� Ih WO WI y C 4 .' r nrvsi ^�maA+rY�d«,rr�; I y��a�rKrrarw �,� � h � n M1, p r Y � �� _ � �g r 4CN✓rerv4 y,�, 5 j � � C #ItyY rAry � ��, 4„.{ �`4 ,, '�� ' Jt'tt"� 'C1v:a k�A"W'1. �L W T r�V +;;� � q �,�➢y�ry+" �! ¢� �f r��a ' bpi^ � •�, «'� +f p a pp i� I rairtl:Wmirtldr✓ � ' '��•, 4 WoI CP7+t {r � •�prs�w�4 p�geA Mnxxxs oxV 101 r -a s� ��.tt4 �� w 47.E ti -dl �� 1=.7 �"'� WSY 4 In,es vsb9 dt a��a r r ua 9p xm r'M fi ACi"N� x aWY.e+n "I .,.. f "'�I'" r11 . qW f. o �c�u•nrrot�^ % � m 5:.1 •� �v»tir Ir R 10 A 10 o TEST PIT 1 0 Description Topsoil over; SWBrown SAND with silt, fine to coarse-grained, moist, loose M Brown silty SAND, fine to coarse-grained, moist, medium -dense W ....... .... _ ... w.....� ..,..,,.u,..�..�.�� syy ;'°'' Brown SAND with silt and gravel, fine to coarse-grained, moist, medium -dense SM Brown silly SAND, fineto coarse -rine ri oist dense TILL * Test Pit terminated at 8 feet on February 25, 2015. * No groundwater seepage was observed during excavation. * No caving observed during excavation. SM sW sM TEST PIT 2 Topsoil over; Description 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 dense * Test Pit terminated at 10 feet on February 25, 2015. * No groundwater seepage was observed during excavation. * No caving observed during excavation. GO'T"C CONSULTAN'T'S, INC. TEST" PIT LOG 18227 - 80th Avenue West Edmonds, Washington Job Date: Logged by:Plate: 15071 1 March 20151 TRC 2 5 10 5 10 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. TEST PIT 4 Description Topsoil over; Brown SAND with silt and gravel, fine to coarse-grained, moist, loose SM ... sw ; � Brown SAND with sitl and gravel, fine to coarse-grained, moist, medium -dense SM 1 Rust�braw ro e to dd r e g rained, "mro1st, Slid ," Rust -brown mottled gray -brown silty SAND with gravel, fine to coarse-grained, moist, * Test Pit terminated at 8 feet on February 25, 2015. * No groundwater seepage was observed during excavation. * No caving observed during excavation. GT "TECH CONSULTANTS, INC. TEST PIT LOG 18227 - 80th Avenue West Edmonds, Washington Job Date: Logged by: P/afe: 15071 1 March 2015,1 TRC 3 NONE i `rrrrw�rrr�rrrr�wrw��wrrwrw�rrrr�rrrrrrrwwwi�r��ur MENOMONEE - rr�.. ��rririr►rrww ��wrrruwrrwwwwi�wiii�� IFAMEMEMEN MEMENNEWME, SIEVE ANALYSIS GEOiTECH 18227 - 80th Avenue West