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Geotech Report 7.15.16.pdfEC CONSULTANTS, INC. Phong Le 1123 Maple Avenue Southwest, Suite 110 Renton, Washington 98057 via email. lep95@yahoo. com Subject: Transmittal Letter — Geotechnical Engineering Study Proposed Three -Lot Short Plat 18227 — 80th Avenue West Edmonds, Washington Dear Mr. Le: 2401 10th Ave E Seattle, Washington 98102 (425) 747-5618 FAX (425) 747-8561 June 29, 2016 JN 15071 We are pleased to present this geotechnical engineering report 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 recommendations for general earthwork and design criteria for foundations and retaining walls. We had earlier prepared a letter dated March 11, 2015 with regards to stormwater infiltration. The attached report contains a discussion of the study and our recommendations. Please contact us if there are any questions regarding this report, or for further assistance during the design and construction phases of this project. Respectfully submitted, GEOTECH CONSULTANTS, INC. T�A @A—", Thor Christensen, P.E. Senior Engineer cc: Wilson & Neal PLLC _ Terrance Randall Wilson via email. ferry@wilsonneal.com TRC/DRW: me GEOTECH CONSULTANTS, INC. GEOTECHNICAL ENGINEERING STUDY Proposed Three -Lot Short Plat 18227 — 80th Avenue West Edmonds, Washington This report presents the findings and recommendations of our geotechnical engineering study for the site of the proposed three -lot short plat to be located in Edmond's. We were provided with a topographic site plan dated July 2, 2015 prepared by Lanktree Land Surveying, Inc. We were recently provided with Civil plans for the project prepared by TEC. Based on the civil plans, we understand that the development will consist of three residential lots, with an existing house remaining on the westernmost lot. A residence will be constructed at each of the two eastern lots, which will be accessed via a shared driveway along the northern edge of the site. The plan shows that the eastern residences will be set back at least 25 feet from the southern property line and at least 10 feet from the eastern and northern property lines. A stormwater infiltration system is proposed on the north -central side of the property. A steep slope is located off the southeastern corner of the property. The infiltration system will be at least 100 feet from the steep slope, The easternmost proposed residence will be no closer than 40 feet from the steep slope, although it will mostly be further than 40 feet due to the angle of the top of the steep slope in comparison to the configuration of the residence. 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. SITE CONDITIONS SURFACE The "vicinity Map, Plate 1, illustrates the general location of the site in Edmonds, The rectangular property is bordered to the west by 80"h ,Avenge West, and is otherwise surrounded by residences. The west -central portion of the site is developed with a one-story house with a basement that daylight's 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 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 285 feet. There are several areas near the middle of the site where the inclination of the property is over 15 percent but less than 40 percent. There is a small area in the middle of the site that is inclined at or just steeper than 40 percent, but this slope is only about 6 to 8 feet tall. A steep slope, estimated at 50 percent is located just off the southeastern corner of the property and declines into the neighbor properties. Snohomish County Sno Scape shows that the adjacent ravine has a height of about 35 feet. GEOTECH CONSULTANTS, INC. Le Phong June 29, 2016 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. 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. WOW 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. 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. We completed grain -size analyses on two samples obtained from Test Pit 2. We have attached the grain -size analyses as Plate 5. The grain -size analyses indicate that the samples had silt content of about 8 and 13 percent. GEOTECH CONSULTANTS, INC. Le Phong June 29, 2016 GENERAL JN 15071 Page 3 THIS SECTION CONTAINS A SUMMARY OF OUR STUDY AND FINDINGS FOR THE PURPOSES OF A GENERAL OVERVIEW ONLY. MORE SPECIFIC RECOMMENDATIONS AND CONCLUSIONS ARE CONTAINED IN THE REMAINDER OF THIS REPORT. ANY PARTY RELYING ON THIS REPORT SHOULD READ THE ENTIRE DOCUMENT. The test pits conducted for this study encountered native, medium -dense silty sand and sand with silt within 3 feet of the ground surface. Conventional footing foundations bearing directly on this competent material will provide suitable support to the proposed residences. We anticipate that the apparently man-made pond near the east side of the site will be filled and that the residence on the eastern lot will overlie at least part of the pond. If the pond is filled with structural fill, footings can be placed on the structural fill. The project includes the use of an infiltration system designed by the project civil engineer. This is suitable in our opinion at it will be located at least 100 feet from the steep slope located off the southeastern corner of the site and is not located in a potential Erosion Hazard Area. However, the proposed residence on Lot 3 (easternmost lot) will be located somewhat close to and slightly downslope of the infiltration system. We recommend that no basement be included in this residence so that there will be no potential for the infiltrated soil to flow eastward and into the basement; there is always some potential for infiltrated water to flow into a nearby basement because its base is below everything around it. Due to its height and inclination, the City of Edmonds code considers the steep slope that is just off southeastern corner of the site to be a Landslide Hazard Area because it is steeper than 40 percent and taller than 40 percent. There are no other Landslide Hazard areas on the property. In addition, based on the type of soils revealed on the property, all slopes within the property that are steeper than 15 percent are potential Erosion Hazard Areas per Edmonds code. Almost all soils in the Puget Sound area are technically an "erosion hazard" because of their normally silty nature. These soils would erode if left unprotected or un -vegetated during periods of precipitation, which is essentially the definition of an erosion hazard. At this site, the civil plans have shown recommendations of how to mitigate the erosion potential when earthwork is occurring and portions of the site will not be vegetated. We believe the plans that include erosion control measures are suitable. After the project is completed, with residences, paving, hardscaping, and landscaping being done, then there is no erosion hazard because the soils are covered or vegetated. As noted earlier, there is an approximate 35 -foot tall steep slope just off the southeastern corner of the property that is considered a Landslide Hazard Area per Edmonds Code (23.80.0206). The project already includes at least a 25 -foot buffer from the top of this slope and minimum building setback of 40 feet. The buffer and the setbacks are generally more than this; these are the distances at the very southeastern corner of the site/proposed residence. In our professional opinion, this buffer and minimum building setback distances are very suitable from a geotechnical engineering standpoint; no larger distances are needed in our opinion. The main reason for this opinion is because dense glacial till soil was encountered in our two test pit explorations located close to the steep slope and thus this soil comprises the core of the slope; this soil has a very low potential for deep-seated slope failures due to its high shear strength. More specific discussions are needed with regards to these recommended distances in accordance with Edmonds Code GEOTECH CONSULTANTS, INC. Le Phong JN 15071 June 29, 2016 Page 4 (23.80.60 and .70). As such, provided the recommendations in this report are followed, the following comments and conclusions with regards to the buffer and building setback distances are made: • Because of the dense core of the steep slope and area above the slope, we strongly believe that the proposed development shall not decrease the factor of safety for landslide occurrences below the limits of 1.5 for static conditions and 1.2 for dynamic conditions. The development will not decrease the stability of the adjacent properties. • The proposed development shall not result in greater risk or a need for increased buffers on neighboring properties. The development will not adversely impact other critical areas • The proposed development will not increase the threat of the geological hazard to adjacent properties beyond predevelopment conditions. • The proposed development not adversely impact other critical areas. • The project is designed so that the hazard to the project is eliminated or mitigated to a level at least equal to predevelopment conditions. • Are certified as "safe" as designed and under anticipated conditions. The erosion control measures needed during the site development will depend heavily on the weather conditions that are encountered. We anticipate that a silt fence will be needed around the downslope sides of any cleared areas. Existing pavements, ground cover, and landscaping should be left in place wherever possible to minimize the amount of exposed soil. Rocked staging areas and construction access roads should be provided to reduce the amount of soil or mud carried off the property by trucks and equipment. Wherever possible, the access roads should follow the alignment of planned pavements. Trucks should not be allowed to drive off of the rock -covered areas. Cut slopes and soil stockpiles should be covered with plastic during wet weather. Following clearing or rough grading, it may be necessary to mulch or hydroseed bare areas that will not be immediately covered with landscaping or an impervious surface. On most construction projects, it is necessary to periodically maintain or modify temporary erosion control measures to address specific site and weather conditions. - The drainage and/or waterproofing recommendations presented in this report are intended only to prevent active seepage from flowing through concrete walls or slabs. Even in the absence of active seepage into and beneath structures, water vapor can migrate through walls, slabs, and floors from the surrounding soil, and can even be transmitted from slabs and foundation walls due to the concrete curing process. Water vapor also results from occupant uses, such as cooking and bathing. Excessive water vapor trapped within structures can result in a variety of undesirable conditions, including, but not limited to, moisture problems with flooring systems, excessively moist air within occupied areas, and the growth of molds, fungi, and other biological organisms that may be harmful to the health of the occupants. The designer or architect must consider the potential vapor sources and likely occupant uses, and provide sufficient ventilation, either passive or mechanical, to prevent a build up of excessive water vapor within the planned structure. Geotech Consultants, Inc. should be allowed to review the final development plans to verify that the recommendations presented in this report are adequately addressed in the design. Such a plan review would be additional work beyond the current scope of work for this study, and it may include revisions to our recommendations to accommodate site, development, and geotechnical constraints that become more evident during the review process. GEOTECH CONSULTANTS, INC. Le Phong JN 15071 June 29, 2016 Page 5 We recommend including this report, in its entirety, in the project contract documents. This report should also be provided to any future property owners so they will be aware of our findings and recommendations. SEISMIC CONSIDERATIONS In accordance with the International Building Code (IBC), the site soil profile within 100 feet of the ground surface is best represented by Site Class Type C (Very Dense Soil and Soft Rock) As noted in the USGS website, the mapped spectral acceleration value for a 0.2 second (Ss) and 1.0 second period (Si) equals 1.298 and 0.50g, respectively. The IBC states that a site-specific seismic study need not be performed provided that the peak ground acceleration be equal to SDs/2.5, where SDs is determined in ASCE 7. It is noted that SDs is equal to 2/3SMs. SMs equals Fa times Ss, where Fa is determined in Table 11.4-1. For our site, Fa = 1.0. The calculated peak ground acceleration that we utilized for the seismic -related parameters (earth pressures and seismic surcharges) of this report equals 0.34g. The site soils are not susceptible to seismic liquefaction because of their dense nature and the absence of near -surface groundwater. CONVENTIONAL FOUNDATIONS The proposed structure can be supported on conventional continuous and spread footings bearing on competent, undisturbed, medium -dense, native soil or on structural fill placed above this competent soil. We recommend that continuous and individual spread footings have minimum widths of 12 and 16 inches, respectively. Exterior footings should also be bottomed at least 18 inches below the lowest adjacent finish ground surface for protection against frost and erosion. The local building codes should be reviewed to determine if different footing widths or embedment depths are required. Footing subgrades must be cleaned of loose or disturbed soil prior to pouring concrete. Depending upon site and equipment constraints, this may require removing the disturbed soil by hand. An allowable bearing pressure of 2,500 pounds per square foot (psf) is appropriate for footings supported on competent native soil. A one-third increase in this design bearing pressure may be used when considering short-term wind or seismic loads. For the above design criteria, it is anticipated that the total post -construction settlement of footings founded on competent native soil will be about one -inch, with differential settlements on the order of one -half-inch in a distance of 30 feet along a continuous footing with a uniform load. Lateral loads due to wind or seismic forces may be resisted by friction between the foundation and the bearing soil, or by passive earth pressure acting on the vertical, embedded portions of the foundation. For the latter condition, the foundation must be either poured directly against relatively level, undisturbed soil or be surrounded by level, well -compacted fill. GEOTECH CONSULTANTS, INC. Le Phong JN 15071 June 29, 2016 Page 6 We recommend using the following ultimate values for the foundation's resistance to lateral loading: Coefficient of Friction 0.45 Passive Earth Pressure 300 pcf Where: pcf is Pounds per Cubic Foot, and Passive Earth Pressure is computed using the equivalent fluid density. If the ground in front of a foundation is loose or sloping, the passive earth pressure given above will not be appropriate. We recommend maintaining a safety factor of at least 1.5 for the foundation's resistance to lateral loading, when using the above ultimate values. FOUNDATION AND RETAINING WALLS Retaining walls backfilled on only one side should be designed to resist the lateral earth pressures imposed by the soil they retain. The following recommended parameters are for walls that restrain level backfill: Where: pcf is Pounds per Cubic Foot, and Active and Passive Earth Pressures are computed using the Equivalent Fluid Pressures. * For a restrained wall that cannot deflect at least 0.002 times its height, a uniform lateral pressure equal to 10 psf times the height of the wall should be added to the above active equivalent fluid pressure. The design values given above do not include the effects of any hydrostatic pressures behind the walls and assume that no surcharges, such as those caused by slopes, vehicles, or adjacent foundations will be exerted on the walls. If these conditions exist, those pressures should be added to the above lateral soil pressures. Where sloping backfill is desired behind the walls, we will need to be given the wall dimensions and the slope of the backfill in order to provide the appropriate design earth pressures. The surcharge due to traffic loads behind a wall can typically be accounted for by adding a uniform pressure equal to 2 feet multiplied by the above active fluid density. Heavy construction equipment should not be operated behind retaining and foundation walls within a distance equal to the height of a wall, unless the walls are designed for the additional lateral pressures resulting from the equipment. The values given above are to be used to design only permanent foundation and retaining walls that are to be backfilled, such as conventional walls constructed of reinforced concrete or masonry. It is not appropriate to use the above earth pressures and soil unit weight to back -calculate soil GEOTECH CONSULTANTS, INC. Le Phong June 29, 2016 J N 15071 Page 7 strength parameters for design of other types of retaining walls, such as soldier pile, reinforced earth, modular or soil nail walls. We can assist with design of these types of walls, if desired. The passive pressure given is appropriate only for a shear key poured directly against undisturbed native soil, or for the depth of level, well -compacted fill placed in front of a retaining or foundation wall. The values for friction and passive resistance are ultimate values and do not include a safety factor. Restrained wall soil parameters should be utilized for a distance of 1.5 times the wall height from corners or bends in the walls. This is intended to reduce the amount of cracking that can occur where a wall is restrained by a corner. Wall Pressures Due to Seismic Forces The surcharge wall loads that could be imposed by the design earthquake can be modeled by adding a uniform lateral pressure to the above -recommended active pressure. The recommended surcharge pressure is 8H pounds per square foot (psf), where H is the design retention height of the wall. Using this increased pressure, the safety factor against sliding and overturning can be reduced to 1.2 for the seismic analysis. r Backfill placed behind retaining or foundation walls should be coarse, free -draining structural fill containing no organics. This backfill should contain no more than 5 percent silt or clay particles and have no gravel greater than 4 inches in diameter. The percentage of particles passing the No. 4 sieve should be between 25 and 70 percent. If the native silty sand is used as backfill, a drainage composite similar to Miradrain 6000 should be placed against the backfilled retaining walls. The drainage composites should be hydraulically connected to the foundation drain system. The purpose of these backfill requirements is to ensure that the design criteria for a retaining wall are not exceeded because of a build-up of hydrostatic pressure behind the wall. Also, subsurface drainage systems are not intended to handle large volumes of water from surface runoff. The top 12 to 18 inches of the backfill should consist of a compacted, relatively impermeable soil or topsoil, or the surface should be paved. The ground surface must also slope away from backfilled walls to reduce the potential for surface water to percolate into the backfill. Water percolating through pervious surfaces (pavers, gravel, permeable pavement, etc.) must also be prevented from flowing toward walls or into the backfill zone. The compacted subgrade below pervious surfaces and any associated drainage layer should therefore be sloped away. Alternatively, a membrane and subsurface collection system could be provided below a pervious surface. It is critical that the wall backfill be placed in lifts and be properly compacted, in order for the above -recommended design earth pressures to be appropriate. The wall design criteria assume that the backfill will be well -compacted in lifts no thicker than 12 inches. The compaction of backfill near the walls should be accomplished with hand -operated equipment to prevent the walls from being overloaded by the higher soil forces that occur during compaction. The section entitled General Earthwork and Structural Fill contains additional recommendations regarding the placement and compaction of structural fill behind retaining and foundation walls. The above recommendations are not intended to waterproof below -grade walls, or to prevent the formation of mold, mildew or fungi in interior spaces. Over time, the performance of subsurface drainage systems can degrade, subsurface groundwater flow GEOTECH CONSULTANTS, INC. Le Phong June 29, 2016 JN 15071 Page 8 patterns can change, and utilities can break or develop leaks. Therefore, waterproofing should be provided where future seepage through the walls is not acceptable. This typically includes limiting cold joints and wall penetrations, and using bentonite panels or membranes on the outside of the walls. There are a variety of different waterproofing materials and systems, which should be installed by an experienced contractor familiar with the anticipated construction and subsurface conditions. Applying a thin coat of asphalt emulsion to the outside face of a wall is not considered waterproofing, and will only help to reduce moisture generated from water vapor or capillary action from seeping through the concrete. As with any project, adequate ventilation of basement and crawl space areas is important to prevent a build up of water vapor that is commonly transmitted through concrete walls from the surrounding soil, even when seepage is not present. This is appropriate even when waterproofing is applied to the outside of foundation and retaining walls. We recommend that you contact an experienced envelope consultant if detailed recommendations or specifications related to waterproofing design, or minimizing the potential for infestations of mold and mildew are desired. The General, Slabs -On -Grade, and Drainage Considerations sections should be reviewed for additional recommendations related to the control of groundwater and excess water vapor for the anticipated construction. SLABS -ON -GRADE The building floors can be constructed as slabs -on -grade atop competent native soil or on structural fill. The subgrade soil must be in a firm, non -yielding condition at the time of slab construction or underslab fill placement. Any soft areas encountered should be excavated and replaced with select, imported structural fill. Even where the exposed soils appear dry, water vapor will tend to naturally migrate upward through the soil to the new constructed space above it. This can affect moisture -sensitive flooring, cause imperfections or damage to the slab, or simply allow excessive water vapor into the space above the slab. All interior slabs -on -grade should be underlain by a capillary break drainage layer consisting of a minimum 4 -inch thickness of clean gravel or crushed rock that has a fines content (percent passing the No. 200 sieve) of less than 3 percent and a sand content (percent passing the No. 4 sieve) of no more than 10 percent. Pea gravel or crushed rock are typically used for this layer. As noted by the American Concrete Institute (ACI) in the Guides for Concrete Floor and Slab Structures, proper moisture protection is desirable immediately below any on -grade slab that will be covered by tile, wood, carpet, impermeable floor coverings, or any moisture -sensitive equipment or products. ACI also notes that vapor retarders such as 6 -mil plastic sheeting have been used in the past, but are now recommending a minimum 10 -mil thickness for better durability and long term performance. A vapor retarder is defined as a material with a permeance of less than 0.3 perms, as determined by ASTM E 96. It is possible that concrete admixtures may meet this specification, although the manufacturers of the admixtures should be consulted. Where vapor retarders are used under slabs, their edges should overlap by at least 6 inches and be sealed with adhesive tape. The sheeting should extend to the foundation walls for maximum vapor protection. If no potential for vapor passage through the slab is desired, a vapor barrier should be used. A vapor barrier, as defined by ACI, is a product with a water transmission rate of 0.01 perms when tested in accordance with ASTM E 96. Reinforced membranes having sealed overlaps can meet this requirement. GEOTECH CONSULTANTS, INC. Le Phong JN 15071 June 29, 2016 Page 9 The General, Permanent Foundation and Retaining Walls, and Drainage Considerations sections should be reviewed for additional recommendations related to the control of groundwater and excess water vapor for the anticipated construction. EXCAVATIONS AND SLOPES Excavation slopes should not exceed the limits specified in local, state, and national government safety regulations. Temporary cuts to a depth of about 4 feet may be attempted vertically in unsaturated soil, if there are no indications of slope instability. However, vertical cuts should not be made near property boundaries, or existing utilities and structures. Based upon Washington Administrative Code (WAC) 296, Part N, the soil at the subject site would generally be classified as Type A. Therefore, temporary cut slopes greater than 4 feet in height should not be excavated at an inclination steeper than 0.75:1 (Horizontal:Vertical), extending continuously between the top and the bottom of a cut. The above -recommended temporary slope inclination is based on the conditions exposed in our explorations, and on what has been successful at other sites with similar soil conditions. It is possible that variations in soil and groundwater conditions will require modifications to the inclination at which temporary slopes can stand. Temporary cuts are those that will remain unsupported for a relatively short duration to allow for the construction of foundations, retaining walls, or utilities. Temporary cut slopes should be protected with plastic sheeting during wet weather. It is also important that surface runoff be directed away from the top of temporary slope cuts. Cut slopes should also be backfilled or retained as soon as possible to reduce the potential for instability. Please note that sand or loose soil can cave suddenly and without warning. Excavation, foundation, and utility contractors should be made especially aware of this potential danger. These recommendations may need to be modified if the area near the potential cuts has been disturbed in the past by utility installation, or if settlement -sensitive utilities are located nearby. All permanent cuts into native soil should be inclined no steeper than 2:1 (H:V). Water should not be allowed to flow uncontrolled over the top of any temporary or permanent slope. All permanently exposed slopes should be seeded with an appropriate species of vegetation to reduce erosion and improve the stability of the surficial layer of soil. Any disturbance to the existing slope outside of the building limits may reduce the stability of the slope. Damage to the existing vegetation and ground should be minimized, and any disturbed areas should be revegetated as soon as possible. Soil from the excavation should not be placed on the slope, and this may require the off-site disposal of any surplus soil. DRAINAGE CONSIDERATIONS Footing drains should be used where: (1) crawl spaces or basements will be below a structure; (2) a slab is below the outside grade; or, (3) the outside grade does not slope downward from a building. Drains should also be placed at the base of all earth -retaining walls. These drains should be surrounded by at least 6 inches of 1 -inch -minus, washed rock that is encircled with non -woven, geotextile filter fabric (Mirafi 140N, Supac 4NP, or similar material). At its highest point, a perforated pipe invert should be at least 6 inches below the bottom of a slab floor or the level of a crawl space. The discharge pipe for subsurface drains should be sloped for flow to the outlet point. Roof and surface water drains must not discharge into the foundation drain system. A typical drain GEOTECH CONSULTANTS, INC. Le Phong JN 15071 June 29, 2016 Page 10 detail is attached to this report as Plate 6. For the best long-term performance, perforated PVC pipe is recommended for all subsurface drains. As a minimum, a vapor retarder, as defined in the Slabs -On -Grade section, should be provided in any crawl space area to limit the transmission of water vapor from the underlying soils. Crawl space grades are sometimes left near the elevation of the bottom of the footings. As a result, an outlet drain is recommended for all crawl spaces to prevent an accumulation of any water that may bypass the footing drains. Providing even a few inches of free draining gravel underneath the vapor retarder limits the potential for seepage to build up on top of the vapor retarder. No groundwater was observed during our field work. If seepage is encountered in an excavation, it should be drained from the site by directing it through drainage ditches, perforated pipe, or French drains, or by pumping it from sumps interconnected by shallow connector trenches at the bottom of the excavation. The excavation and site should be graded so that surface water is directed off the site and away from the tops of slopes. Water should not be allowed to stand in any area where foundations, slabs, or pavements are to be constructed. Final site grading in areas adjacent to a building should slope away at least 2 percent, except where the area is paved. Surface drains should be provided where necessary to prevent ponding of water behind foundation or retaining walls. A discussion of grading and drainage related to pervious surfaces near walls and structures is contained in the Foundation and Retaining Walls section. Water from roof, storm water, and foundation drains should not be discharged onto slopes; it should be tightlined to a suitable outfall located away from any slopes. GENERAL EARTHWORK AND STRUCTURAL FILL All building and pavement areas should be stripped of surface vegetation, topsoil, organic soil, and other deleterious material. The stripped or removed materials should not be mixed with any materials to be used as structural fill, but they could be used in non-structural areas, such as landscape beds. Structural fill is defined as any fill, including utility backfill, placed under, or close to, a building, behind permanent retaining or foundation walls, or in other areas where the underlying soil needs to support loads. All structural fill should be placed in horizontal lifts with a moisture content at, or near, the optimum moisture content. The optimum moisture content is that moisture content that results in the greatest compacted dry density. The moisture content of fill is very important and must be closely controlled during the filling and compaction process. The allowable thickness of the fill lift will depend on the material type selected, the compaction equipment used, and the number of passes made to compact the lift. The loose lift thickness should not exceed 12 inches. We recommend testing the fill as it is placed. If the fill is not sufficiently compacted, it can be recompacted before another lift is placed. This eliminates the need to remove the fill to achieve the required compaction. GEOTECH CONSULTANTS, INC. Le Phong June 29, 2016 JN 15071 Page 11 The following table presents recommended relative compactions for structural fill: Where: Minimum Relative Compaction Is the ratio, expressed in percentages, of the compacted dry density to the maximum dry density, as determined in accordance with ASTM Test Designation D 1557-91 (Modified Proctor). Structural fill that will be placed in wet weather should consist of a coarse, granular soil with a silt or clay content of no more than 5 percent. The percentage of particles passing the No. 200 sieve should be measured from that portion of soil passing the three -quarter -inch sieve. 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. The recommendations presented in this report are directed toward the protection of only the proposed structures from damage due to slope movement. Predicting the future behavior of steep slopes and the potential effects of development on their stability is an inexact and imperfect science that is currently based mostly on the past behavior of slopes with similar characteristics. As with any steep slope in the Puget Sound area, the movement of near -surface soils that overlie the dense, "core" soil of the slope can occur on before, during, or after the development of property. This movement generally occurs during times of extreme precipitation. This movement would not adversely affect the structures on the subject site because of the distance they are away from the steep slope. The owner of any property containing, or located close to steep slopes must ultimately accept the possibility that the near -surface soil can move. 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 GEOTECH CONSULTANTS, INC. Le Phong JN 15071 June 29, 2016 Page 12 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 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 Vicinity Map Plate 2 Site Exploration Plan Plates 3 - 4 Test Pit Logs Plate 5 Grain -Size Analysis Plate 6 Typical Footing Drain Detail 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. qwunK Thor Christensen, P.E. D. Robert Ward, P.E. Senior Engineer Principal TRC/DRW:mc GEOTECH CONSULTANTS, INC. GEOTEC = . CONSULTAN'T'S, INC. (Source: Microsoft MapPoint, 2013) VICINITY MAP 18227 - 80th Avenue West Edmonds, Washington Job No: I Date: I Plate: 15071 June 2106 1 1 Legend: [j Test Pit Location GEOTECH CONSULTANTS, INC. SITE EXPLORATION PLAN 18227 - 80th Avenue West Edmonds, Washington Job No: I Date: Plate: :2] 15071 1 June 2106 No Scale I 5 10 5 10 e TEST PIT 1 GC Description Topsoil over; --- ------- - . ........ . ...... ............. � Brown SAND with silt, fine to coarse-grained, moist, loose SIM . ........ .... _ _ ... . sm i, Brown silty SAND, fine to coarse-grained, moist, medium -dense SW Brown SAND with silt and gravel, fine to coarse-grained, moist, medium -dense SM .. ..-------- - Brown sil SAND fine to coarse- rained moist dense TIlbt.... * Test Pit terminated at 8 feet on February 25, 2015. * No groundwater seepage was observed during excavation. * No caving observed during excavation. \i,, 5 J5G AA 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. G OTEC CONSULTANTS, INC. TEST PIT LOG 18227 - 80th Avenue West Edmonds, Washington Job Date: I Logged by: Plate: 15071 June 2016 TRC 3 61 10 OON 5 10 O '\ 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 Topsoil over; Description SM ;11 Brown SAND with silt and gravel, fine to coarse-grained, moist, loose Sw SM ML Brown SAND with silt and gravel, fine to coarse-grained, moist, medium -dense Kust-brown me SM medium -dense gray -brown 511 F with sand, tone to coa Silty aANU wan gravel, * Test Pit terminated at 8 feet on February 25, 2015. * No groundwater seepage was observed during excavation. * No caving observed during excavation. GEO"T ECH CONSULTANT'S, INC. TEST PIT LOG 18227 - 80th Avenue West Edmonds, Washington Job Date: ILoggedby: Plate: 15071 June 2016 TRC 4 cn 0 m 00 d N _41 CA M� l C- C 7 0 1 CL N O O (i) r•r v < cn c cn CD � :° � cn c cn Sieve Opening (mm.) Sieve Opening (US standard) #— Test Pit 2, 6 feet —4*— Test Pit 2, 8.5 feet iuu ao )o ;o Slope backfill away from foundation. Provide surface drains where necessary. Washed Rod (7/8" min. size) Backfill (See text for requirements) Nonwoven Geotextile Filter Fabric Tightline Roof Drain (Do not connect to footing drain) Possible Slab ". c�� �a �� ^�:, �� „•�°a � ea „•�o a� ��'; �� �� ,� spa � �w��, r� 4" Perforated Hard PVC Pipe (Invert at least 6 inches below slab or crawl space. Slope to drain to appropriate outfall. Place holes downward.) NOTES: (1) In crawl spaces, provide an outlet drain to bypasses the perimeter footing drains. (2) Refer to report text for additional drainage, GEO ECH. CONSULTANTS, INC. Vapor Retarder/Barrier and Capillary Break/Drainage Layer (Refer to Report text) prevent buildup of water that waterproofing, and slab considerations. FOOTING DRAIN DETAIL 18227 - 80th Avenue West Edmonds, Washington Job No Date: Plate: 15071 June 2106 1 6