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REVIEWED BLD BLD2021-0929+geo report+7.2.2021_2.58.00_PM+2282217:jneYZiPAKIROM GEOTECH CONSULTANTS, INC_ RECEIVED Jul 26 2021 2401 1 Oth Ave E Seattle, Washington 98102 (425)747-56I8 CITY OF EDMONDS DEVELOPMENT ARTMENRVICES February 11, 2021 JN 20437 Todd and Christi Flynn 8229 Talbot Road Edmonds, Washington 98026 via email: tflynn(a-soundmarkwealth.com & christiflynn(c_comcast.net Subject: Transmittal Letter — Geotechnical Engineering Study Proposed Residence Remodel and Addition 8229 Talbot Road Edmonds, Washington Dear Flynn Family, Attached to this transmittal letter is our geotechnical engineering report for the proposed residence remodel and addition 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 considerations for foundations, retaining walls, subsurface drainage, and temporary excavations and shoring. This work was authorized by your acceptance of our proposal, P-10715, dated September 24, 2020. 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. cc: H2D Architects — Heidi Helgeson via email: heidi(a�h2darchitects.com MKM/JHS:kg Respectfully submitted, GEOTECH CONSULTANTS, INC. James H. Strange, P.E. Associate GEOTECH CONSULTANTS. INC. GEOTECHNICAL ENGINEERING STUDY Proposed Residence Remodel and Addition 8229 Talbot Road Edmonds, Washington This report presents the findings and recommendations of our geotechnical engineering study for the site of the proposed residence remodel and addition to be located in Edmonds. We were provided with preliminary architectural plans. H2D Architects developed these plans, which are dated August 31, 2020. Based on these plans, we understand that a new addition is proposed to extend off the southern side of the existing residence. This addition will contain a garage in its lower floor, with living space above. The new living space will tie into the existing main floor level, which will undergo a remodel. In addition, the existing tall crawlspace along the eastern side of the house is proposed to be excavated and converted to basement space. We are not aware if new structural loads and/or foundations will need to be constructed within the existing footprint, but the roof layout will be modified, which may introduce new structural loading. No work outside of the remodel and addition are proposed at this time for the property. Minimal excavations are planned for a majority of the project, but deeper excavations on the order of 10 feet will be needed to excavate the eastern basement area. 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 irregular shaped site has dimensions of 282.2 to 316.7 feet in the north -south direction, and 101.6 to 189.9 feet in the east -west direction. The site is bounded to the north by a Burlington Northern Santa Fe railway, to the east and west by single family properties, and to the south by Talbot Road. The site is currently developed with a two-story residence located near the northern property line. The residence contains a lower, basement floor that daylights on its southwestern side, with a main floor located above. A garage space is located in the lower floor, which is accessed via a long driveway extending off Talbot Road to the south. A small wooden bridge crosses over Perrinville creek, which runs from the southeastern corner of the site, to a culvert system located southwest of the house. This culvert system pipes the creek under the railroad located north of the site to the Puget Sound. The grade across the site generally slopes downward from the east and south, to the west and north, with a total elevation change of 12 feet across the mapped property bounds. The sloping topography creates a localized low spot through the rough stream alignment that flows through the site, and facilitates the grade drop from the upper, eastern main level entry, to the lower garage entry on the south of the residence. A short, approximately 8-foot-tall steep slope is located off the northwestern corner of the yard north of the residence. The grade continues to carry out past the residence at a moderate rate to the base of the railway, where a rock fill slope leads up to the train tracks. This fill slope again drops back down to the north of the railway, where the grade continues out to the Puget Sound. The City of Edmonds GIS maps portions of the site within an erosion hazard area. No steep slopes are mapped on the property. Perrinville Creek runs through the property, which conveys stormwater runoff from a large tributary area of upslope developments within Edmonds and Lynnwood. The adjacent eastern and western properties are both developed with single family residences. The eastern adjacent residence is situated greater than 5 feet higher than the site grade at the eastern property line and GEOTECH CONSULTANTS. INC. Flynn Family JN 20437 February 11, 2021 Page 2 contains a two-story residence that is not underlain by a basement. This residence is located greater than 10 feet from the property line. The western adjacent property contains a two-story residence located at a slightly lower elevation than the site and is located greater than 10 feet from the property bounds. Both residences likely bear on conventional foundation systems at a shallow depth. SUBSURFACE The subsurface conditions were explored by drilling three test borings 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 borings were drilled on December 23, 2020 using a track -mounted, hollow -stem auger drill. Samples were taken at approximate 2.5 and 5-foot intervals with a standard penetration sampler. This split -spoon sampler, which has a 2-inch outside diameter, is driven into the soil with a 140-pound hammer falling 30 inches. The number of blows required to advance the sampler a given distance is an indication of the soil density or consistency. A geotechnical engineer from our staff observed the drilling process, logged the test borings, and obtained representative samples of the soil encountered. The Test Boring Logs are attached as Plates 3 through 5. Soil Conditions Test Borings 1 and 2 were drilled near the southwestern and southeastern corners of the existing residence, respectively. Beneath the ground surface, native, sand with gravel was revealed. The sand contained varying amounts of silt, gravel, and organics through the layer depth, and was initially loose, becoming medium -dense beneath depths of 5 to 15 feet. These native soils are inferred to be alluvial soils (soils deposited by lakes, rivers, and streams) and were not glacially compressed. The loose and medium -dense alluvial soils continued to the base of Test Boring 1, where refusal was met due to excessive heave. However, very dense, very silty sand was encountered beneath a depth of feet in Test Boring 2. This very dense soil is glacially compressed and continued to the base of the boring at 41 feet where the boring was terminated due to machine limitations in the upper heaving sand. Test Boring 3 was drilled on the upslope side of the site, near the northern entryway to the residence. Beneath the ground surface, native, medium -dense silt sand was encountered. This layer of silty sand continued to a depth of 12.5 feet and was underlain by the same alluvial sand encountered in Test Borings 1 and 2. In Test Boring 3, the sand layer was mostly medium -dense, becoming dense below a depth of 35 feet. Very dense, glacially compressed silty sand was encountered at the base of the boring at a depth of 40 feet and was similar in composition to what was encountered at the base of Test Boring 2. No obstructions were revealed by our explorations. However, debris, buried utilities, and old foundation and slab elements are commonly encountered on sites that have had previous development. Groundwater Conditions Groundwater seepage was observed at a depth of 7.5 to 10 feet in Test Borings 1 and 2, and at a depth of approximately 20 feet in Test Boring 3. The test borings were left open for only a short time period. Therefore, the seepage levels on the logs represent the location of transient water seepage and may not indicate the static groundwater level. Groundwater levels encountered during drilling can be deceptive because seepage into the boring can be blocked or slowed by the auger itself. GEOTECH CONSULTANTS. INC. Flynn Family February 11, 2021 J N 20437 Page 3 It should be noted that groundwater levels vary seasonally with rainfall and other factors. We anticipate that groundwater levels could fluctuate seasonally following precipitation events, especially due to the creek alignment running through the property. 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. If a transition in soil type occurred between samples in the borings, the depth of the transition was interpreted. The relative densities and moisture descriptions indicated on the test boring logs are interpretive descriptions based on the conditions observed during drilling. SEISMIC CONSIDERATIONS In accordance with the International Building Code (IBC), with regards to the building period and the soils within 100 feet of the ground surface, Site Class Type E (Soft Soil) is most appropriate for this project. As noted in the USGS website, the mapped spectral acceleration value for a 0.2 second (Ss) and 1.0 second period (S1) equals 1.29 g and 0.51g, respectively. The IBC and ASCE 7 require that the potential for liquefaction (soil strength loss) be evaluated for the peak ground acceleration of the Maximum Considered Earthquake (MCE), which has a probability of occurring once in 2,475 years (2 percent probability of occurring in a 50-year period). The MCE peak ground acceleration is adjusted for site class effects (FPGA) and equals 0.48. The site is underlain at approximately 7.5 to 20 feet by loose and medium -dense, saturated, alluvial soils depending on the existing ground elevations. These saturated soils have been demonstrated to have a moderate to high potential for liquefaction during a large earthquake. Using analysis procedures developed by Seed, Idriss, et al. we calculated the approximate total ground settlement that could result if liquefaction were to occur in the saturated, loose, and medium -dense soils as the result of the design earthquake. Based on this analysis, it is probable that soil liquefaction could extend down to about 50 feet, following an MCE. Our analysis indicated that total ground settlement of up to approximately 6 to 7.5 inches could result during the MCE. It is important to note that the study of liquefaction and its resulting effects is on -going, as development in areas underlain by saturated alluvium or hydraulic fill has only really occurred to a great extent in the last 30 to 40 years. Recent observations from earthquakes occurring in the State of California and in Japan indicate that ground surface subsidence due to liquefaction tends to occur either over a large area or at concentrated points where sand boils occur. The recommendations presented in this report for reinforcing and connecting all of the foundations and supporting some of the foundation areas on pipe piles are intended to prevent catastrophic foundation collapse during a large seismic event. By preventing catastrophic settlement of the foundations, the safety of the occupants should be protected. The intent is not to prevent damage or ensure continued function of the structures after the design seismic event. CONCLUSIONS AND RECOMMENDATIONS GENERAL 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. GEOTECH CONSULTANTS. INC. Flynn Family JN 20437 February 11, 2021 Page 4 The test borings conducted for this study encountered loose and medium -dense, sands extending to great depths beneath the ground surface. These native soils are alluvial deposits (soils deposited by streams or rivers), which are typical of the area. A significant depth of the alluvial soils are below the groundwater table and are saturated. These loose, wet alluvial soils extend to significant depths, and glacially compressed soils were not encountered until depths of 40 feet in the test borings. Current structures in the area that have been supported on typical conventional shallow foundations systems on top of the alluvial soils can experience significant amounts of post -construction settlement due to consolidation of these loose soils over time. Furthermore, sand in the area that is below the groundwater table is susceptible to liquefaction during a large seismic event. As part of the remodel and new construction for the addition, the potential liquefaction hazards should be addressed. Considering the existing structure, anticipated limited or negligible additional structural loads imparted on the existing foundations, and the anticipated relatively lightly loaded construction of the remodel, it is our professional opinion that the existing residence footings should be retrofitted by installing continuous, heavily reinforced footings (grade beams) adjacent to the existing footings. Any isolated footings should also be attached to the main foundation with grade beams. The continuous footings would be constructed adjacent to the existing residence foundations, and would effectively widen the footings, both decreasing the loads imparted on the soil below by increasing the bearing area of the foundations and strengthening the foundation to resist seismic differential settlements and potential localized loss of bearing due to sand boils. These heavily reinforced foundations should be designed to structurally span a distance of 10 feet without soil support, similar to grade beams. These recommendations are intended to account for the possibility for isolated soil bearing loss as a result of a liquefied sand boil beneath the building footprint. These are typical measures for foundations of new structures build atop alluvial soils. Additional recommendations can be found in the Conventional Continuous Foundations section of this report. The new addition will be constructed south of the existing residence and impart new foundation loads into the unconsolidated alluvial soils and would be predicted to undergo more excessive post -construction settlement with respect to the existing structure (since the existing structure has already experienced nearly all of its primary settlement during its lifetime. To reduce this differential settlement potential, we recommend that the new southern addition foundations be supported on deep foundations consisting of small diameter pipe piles that are driven to refusal in the underlying very dense soils encountered beneath the alluvial deposits. If larger than anticipated new structural loads are planned to be added to the existing residence footprint, the foundations should also be underpinned with pipe piles instead of with grade beams. Recommendations for deep foundations can be found in the Pipe Piles section of this report. Both of the recommended foundation systems are intended to distribute the building loads, reduce the necessary bearing capacity, bridge over any excessively soft areas of soil or localized soil liquefaction (sand boils), and reduce the amount of differential settlement across the building. The use of a heavily reinforced grade beam foundation instead of conventional footings will result in more uniform settlement of the existing building and interior floors as the underlying soils undergo long-term consolidation, and the use of pipe piles will greatly reduce any post construction settlement for the proposed addition. However, settlement -tolerant construction such as wood and metal framing and siding should still be used. Masonry, stucco, tile, and other settlement -sensitive materials should be avoided. The excavations for a majority of the project will be minimal and will be limited to what is needed to construct the new grade beams and pile supported foundations. However, the excavation along the eastern side of the residence for the new basement space will be more extensive. Based on the soil encountered in the test boring drilled east of the existing residence, a temporary excavation of no steeper than a 1:1 (Horizontal:Vertical) should be used. Vertical excavations should not be made on, or near the shared property lines, or near any settlement sensitive structure. Based on the existing property line setback and house location, the excavation may be able to be laid back along the north and south of the residence. However, several large trees exist east of the residence, within the excavation influence zone. Unless these trees can be removed prior to excavation, temporary shoring will be needed along the eastern wall of the residence to facilitate the deeper excavations. The most appropriate shoring system for this area, given site access and soil conditions, would be to utilize closely spaced, driven beam shoring. This rigid shoring system can be GEOTECH CONSULTANTS. INC. Flynn Family JN 20437 February 11, 2021 Page 5 installed with smaller machinery than drilled soldier piles and requires less preparation to facilitate machine access. More information can be found in the Temporary Shoring section of this report. Due to the shallow groundwater table, we do not recommend that concentrated infiltration or dispersion systems be used for the project. Any attempt to utilize a concentrated system will likely lead to early failure of the system, especially during the wet season. All collected stormwater should be tightlined to the appropriate facilities. While the site is mapped as an Erosion Hazard Area, the potential for adverse erosion impacts will be mitigated by properly installed temporary erosion control measures. 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. Silty runoff should not be allowed to flow into the stream alignment on the site. Depending on the time of year of construction, onsite containment may be needed for stormwater runoff. 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, cleaning, 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. As with any project that involves demolition of existing site buildings and/or extensive excavation and shoring, there is a potential risk of movement on surrounding properties. This can potentially translate into noticeable damage of surrounding on -grade elements, such as foundations and slabs. However, the demolition, shoring, and/or excavation work could just translate into perceived damage on adjacent properties. Unfortunately, it is becoming more and more common for adjacent property owners to make unsubstantiated damage claims on new projects that occur close to their developed lots. Therefore, we recommend making an extensive photographic and visual survey of the project vicinity, prior to demolition activities, installing shoring, and/or commencing with the excavation. This documents the condition of buildings, pavements, and utilities in the immediate vicinity of the site in order to avoid, and protect the owner from, unsubstantiated damage claims by surrounding property owners. Additionally, any adjacent structures should be monitored during demolition and construction to detect soil movements. To monitor their performance, we recommend establishing a series of survey reference points to measure any horizontal deflections of the shoring system. Control points should be established at a distance well away from the walls and slopes, and deflections from the reference points should be measured throughout construction by survey methods. 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. Flynn Family JN 20437 February 11, 2021 Page 6 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. PIPE PILES 4-inch-diameter pipe piles driven with an 850- or 1,100- or 2,000-pound hydraulic jackhammer to the following final penetration rates may be assigned the following compressive capacities. INSIDE PILE FINAL DRIVING FINAL DRIVING FINAL DRIVING ALLOWABLE DIAMETER RATE RATE RATE COMPRESSIVE (850# hammer) (1,100# hammer) (2,000# hammer) CAPACITY Note: The refusal criteria indicated in the above table are valid only for pipe piles that are installed using a hydraulic impact hammer carried on leads that allow the hammer to sit on the top of the pile during driving. If the piles are installed by alternative methods, such as a vibratory hammer or a hammer that is hard mounted to the installation machine, numerous load tests to 200 percent of the design capacity would be necessary to substantiate the allowable pile load. The appropriate number of load tests would need to be determined at the time the contractor and installation method are chosen. As a minimum, Schedule 40 pipe should be used. The site soils are located near a body of water, and groundwater was encountered at a shallow depth. As a result, they have an elevated corrosion potential. Considering this, it is our opinion that corrosion protection, such as galvanizing, should be used for the pipe piles. Pile caps and grade beams should be used to transmit loads to the piles. Isolated pile caps should include a minimum of two piles to reduce the potential for eccentric loads being applied to the piles. Subsequent sections of pipe can be connected with slip or threaded couplers, or they can be welded together. If slip couplers are used, they should fit snugly into the pipe sections. This may require that shims be used or that beads of welding flux be applied to the outside of the coupler. Lateral loads due to wind or seismic forces may be resisted by passive earth pressure acting on the vertical, embedded portions of the foundation. For this condition, the foundation must be either poured directly against relatively level, undisturbed soil or be surrounded by level compacted fill. We recommend using a passive earth pressure of 250 pounds per cubic foot (pcf) for this resistance. If the ground in front of a foundation is loose or sloping, the passive earth pressure given above will not be appropriate. We recommend a safety factor of at least 1.5 for the foundation's resistance to lateral loading, when using the above ultimate passive value. CONVENTIONAL CONTINUOUS FOUNDATIONS We recommend that continuous footings have a minimum width of 24 inches. 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. The foundations should all be sufficiently reinforced to be able to theoretically span a distance of at least 10 feet without soil support, similar to grade beams. If excessively soft or yielding soils are encountered beneath the existing foundations, they should be removed, and replaced with clean, angular crushed rock such as ballast rock or quarry spalls. Alternately, lean mix concrete could be used to fill in the soft areas, if encountered. An allowable bearing pressure of 1,000 pounds per square foot (psf) is appropriate for the continuous footings. A one-third increase in this design bearing pressure can 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 GEOTECH CONSULTANTS. INC. Flynn Family JN 20437 February 11, 2021 Page 7 footings founded on native soil, will be 2 to 3 inches, with differential settlements on the order of one inch in a distance of 25 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. We recommend using the following ultimate values for the foundation's resistance to lateral loading: PARAMETER ULTIMATE VALUE 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. The above ultimate values for passive earth pressure and coefficient of friction do not include a safety factor. 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: PARAMETER Active Earth Pressure * VALUE 35 pcf Passive Earth Pressure 300 pcf Coefficient of Friction 0.45 Soil Unit Weight 115 pcf 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. This applies only to walls with level backfill. 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 GEOTECH CONSULTANTS. INC. Flynn Family February 11, 2021 J N 20437 Page 8 use the above earth pressures and soil unit weight to back -calculate soil 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 the wall and reinforcing design for a distance of 1.5 times the wall height from corners or bends in the walls, or from other points of restraint. 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 Per IBC Section 1803.5.12, a seismic surcharge load need only be considered in the design of walls over 6 feet in height. A seismic surcharge load would be imposed by adding a uniform lateral pressure to the above -recommended active pressure. The recommended seismic surcharge pressure for this project 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. Retaining Wall Backfill and Waterproofing 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. A minimum 12-inch width of free -draining gravel or drainage composite similar to Miradrain 6000 should be placed against the backfilled retaining walls. The gravel or drainage composites should be hydraulically connected to the foundation drain system. Free draining backfill should be used for the entire width of the backfill where seepage is encountered. The later section entitled Drainage Considerations should also be reviewed for recommendations related to subsurface drainage behind foundation and retaining walls. 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 at one to 2 percent 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. Foundation drainage and waterproofing systems are not intended to handle large volumes of infiltrated water. 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 recommended 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 GEOTECH CONSULTANTS. INC. Flynn Family February 11, 2021 J N 20437 Page 9 drainage systems can degrade, subsurface groundwater flow 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 buildup 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. 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 recommends a minimum 10-mil thickness vapor retarder for better durability and long-term performance than is provided by 6-mil plastic sheeting that has historically been used. 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. We recommend that the contractor, the project materials engineer, and the owner discuss these issues and review recent ACI literature and ASTM E-1643 for installation guidelines and guidance on the use of the protection/blotter material. EXCAVATIONS AND SLOPES Temporary excavation slopes should not exceed the limits specified in local, state, and national government safety regulations. Also, temporary cuts should be planned to provide a minimum 2 to 3 feet of space for construction of foundations, walls, and drainage. Temporary cuts to a maximum overall 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. Unshored excavations GEOTECH CONSULTANTS. INC. Flynn Family JN 20437 February 11, 2021 Page 10 should not extend beneath the groundwater table. Based upon Washington Administrative Code (WAC) 296, Part N, the soil at the subject site would generally be classified as Type B. Therefore, temporary cut slopes greater than 4 feet in height should not be excavated at an inclination steeper than 1:1 (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 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). Fill slopes should not be constructed with an inclination greater than 2:1 (H:V). To reduce the potential for shallow sloughing, fill must be compacted to the face of these slopes. This can be accomplished by overbuilding the compacted fill and then trimming it back to its final inclination. Adequate compaction of the slope face is important for long-term stability and is necessary to prevent excessive settlement of patios, slabs, foundations, or other improvements that may be placed near the edge of the slope. 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. TEMPORARY DRIVEN SHORING Cantilevered, driven soldier piles have proven to be an efficient method for providing excavation shoring when onsite soil conditions facilitate their use. The shoring design should be submitted to Geotech Consultants, Inc. for review prior to beginning site excavation. We are available and would be pleased to assist in this design effort. Soldier Pile Installation Soldier pile walls would be constructed after making planned cut slopes, and prior to commencing the mass excavation, by driving steel H-beams to a predetermined depth. As excavation proceeds downward, the space between the piles should be lagged with timber, and any voids behind the timbers should be filled with pea gravel, or a slurry comprised of sand and fly ash. Treated lagging is usually required for permanent walls, while untreated lagging can often be utilized for temporary shoring walls. Temporary vertical cuts will be necessary between the soldier piles for the lagging placement. The prompt and careful installation of lagging is important, particularly in loose or caving soil, to maintain the integrity of the excavation and provide safer working conditions. Additionally, care must be taken by the excavator to remove no more soil between the soldier piles than is necessary to install the lagging. Caving or overexcavation during lagging placement could result in loss of ground on neighboring properties. Timber lagging should be designed for an applied lateral pressure of 30 percent of the design wall pressure if the pile spacing is less than three pile diameters. For larger pile spacings, the lagging should be designed for 50 percent of the design load. Soldier Pile Wall Design Temporary soldier pile shoring that is cantilevered, and that has a level backslope, should be designed for an active soil pressure equal to that pressure exerted by an equivalent fluid with a unit GEOTECH CONSULTANTS. INC. Flynn Family February 11, 2021 J N 20437 Page 11 weight of 35 pounds per cubic foot (pcf). The existing trees above the piles will exert surcharge pressures on the wall if it is determined that the trees need to remain in place. Slopes above the shoring walls will exert additional surcharge pressures. These surcharge pressures will vary, depending on the configuration of the cut slope and shoring wall. We can provide recommendations regarding surcharge pressures when the preliminary shoring design is completed. It is important that the shoring design provides sufficient working room to drive and install the soldier piles, without needing to make unsafe, excessively steep temporary cuts. Cut slopes should be planned to intersect the backside of the drilled holes, not the back of the lagging. Lateral movement of the soldier piles below the excavation level will be resisted by an ultimate passive soil pressure equal to that pressure exerted by a fluid with a density of 300 pcf. A reduction factor is included in this passive pressure to account for strain compatibility in regard to pile deflection. For permanent walls, we recommend a minimum factor of safety of 1.5 be applied to overturning and sliding calculations when using this ultimate value (temporary installations may use a factor of safety of 1.2). This soil pressure is valid only for a level excavation in front of the soldier pile; it acts on three times the beam width. Cut slopes made in front of shoring walls significantly decrease the passive resistance. This includes temporary cuts necessary to install internal braces or rakers. The minimum embedment below the floor of the excavation for cantilever soldier piles should be equal to the height of the "stick-up." DRAINAGE CONSIDERATIONS We anticipate that permanent foundation walls may be constructed against the shoring walls. Where this occurs, a plastic -backed drainage composite, such as Miradrain, Battledrain, or similar, should be placed against the entire surface of the shoring prior to pouring the foundation wall. Weep pipes located no more than 6 feet on -center should be connected to the drainage composite and poured into the foundation walls or the perimeter footing. A footing drain installed along the inside of the perimeter footing will be used to collect and carry the water discharged by the weep pipes to the storm system. Isolated zones of moisture or seepage can still reach the permanent wall where groundwater finds leaks or joints in the drainage composite. This is often an acceptable risk in unoccupied below -grade spaces, such as parking garages. However, formal waterproofing is typically necessary in areas where wet conditions at the face of the permanent wall will not be tolerable. If this is a concern, the permanent drainage and waterproofing system should be designed by a specialty consultant familiar with the expected subsurface conditions and proposed construction. A typical shoring drainage detail is attached to this report as Plate 6. 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 footing drain detail is attached to this report as Plate 7. For the best long-term performance, perforated PVC pipe is recommended for all subsurface drains. Clean -outs should be provided for potential future flushing or cleaning of footing drains. Underslab drainage OR Drainage inside the building's footprint should also be provided where (1) a crawl space or slab will slope or be lower than the surrounding ground surface, (2) an excavation encounters significant seepage, or (3) an excavation for a building will be close to the expected high groundwater elevations. We can provide recommendations for interior drains, should they become necessary, during excavation and foundation construction. GEOTECH CONSULTANTS. INC. Flynn Family JN 20437 February 11, 2021 Page 12 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 a few inches of free draining gravel underneath the vapor retarder is also prudent to limit the potential for seepage to build up on top of the vapor retarder. 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 the residence should slope away at least one to 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. 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, or in other areas where the underlying soil needs to support loads. All structural fills 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 soil that will be excavated for the construction of new foundations will not be suitable for use as backfill, due to the fine- grained, silty nature of the soils and poor drainage characteristics. Imported, free -draining clean crushed rock should be used where wall backfill is needed. 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, but should be thinner if small, hand -operated compactors are used. We recommend testing structural fill as it is placed. If the fill is not sufficiently compacted, it should be recompacted before another lift is placed. This eliminates the need to remove the fill to achieve the required compaction. The following table presents recommended levels of relative compaction for compacted fill: Beneath slabs or 95% walkways Filled slopes and 90% behind retaininq walls 95% for upper 12 inches of Beneath pavements subgrade; 90% below that level 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). GEOTECH CONSULTANTS. INC. Flynn Family February 11, 2021 LIMITATIONS J N 20437 Page 13 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 borings 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 borings. 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 Todd and Christi Flynn and their 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 In addition to reviewing the final plans, 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 - 5 Test Boring Logs Plate 6 Typical Shoring Drain Detail Plate 7 Typical Footing Drain Detail GEOTECH CONSULTANTS. INC. Flynn Family February 11, 2021 J N 20437 Page 14 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. MKM/JHS:kg Respectfully submitted, GEOTECH CONSULTANTS, INC. 2/11 /21 James H. Associate Strange, Jr., P.E. GEOTECH CONSULTANTS. INC. NnRTH j: GEOTECH CONSULTANTS, INC. E dmon ds 7 � 1 .� A I •� �'� G�114: w •k I-6On! , 1 Point Park' Pa wk 11d •• Park 'I• 6. f- SITE I -- i- —FE M CIL111U ake Terrace �. Brier- F (Source: King County Map) VICINITY MAP 8229 Talbot Road Edmonds, Washington Job No: Date: Plate: 20437 1 Jan.2021 1 1 1 NnRTH i /r yJ M 000 I ITEM N6 b E REG. N� 93Q 4 4 4 .4 4 4, 4 4 4 1, 1_ 4 Legend: Test Boring Location GEOTECH CONSULTANTS, INC. \ SITE EXPLORATION PLAN 8229 Talbot Road Edmonds, Washington Job No: Date: Plate: 20437 1 Jan. 2021 No Scale 1 2 4i 10 15 20 25 30 BORING 1 �'�°� G Description Topsoil 4 1 Brown and red -brown SAND with gravel, fine-grained, moist, loose 3 2 -becomes dark -brown with a large root 3 3 -becomes gray -brown, very moist to wet, gravelly 1 5 4 -becomes gray with trace organics, wet 19 5SP -becomes very wet, with trace silt � **30 6 (**Blow counts overstated due to heaving sand) **31 7 . * Test boring was terminated at 30 feet on December 23, 2020 due to excessive heave. * Groundwater seepage was encountered at 14 feet during drilling. GEOTECH CONSULTANTS, INC. BORING LOG 8229 Talbot Road Edmonds ,Washington Job Date: Logged by. Plate: 20437 1 Jan.2021 1 MKM 3 ;l 10 15 20 25 30 35 40 45 BORING 2 ° 4°SSG Description Topsoil 3 1 Brown slightly gravelly SAND with trace organics, fine-grained, very moist, loose 16 2 -becomes gray -brown with rusting, very moist to wet, medium -dense I 13 3 1':: -becomes gray with trace silt, very wet, 17 4 -becomes brown with thin, dark -brown lenses I 19 5 -becomes gray 24 6 SP -becomes gravelly 18 7 I 19 8 17 9� 50 101 1,61, f I Uray mottled oranae. very silty SAND. f'ne-ara'ned. very monst, very densE 6" * Test boring was terminated at 41 feet on December 23, 2020. * Groundwater seepage was encountered at 7.5 feet during drilling. GEOTECH CONSULTANTS, INC. BORING LOG 8229 Talbot Road Edmonds ,Washington Job Date: Logged by: Plate: 20437 Jan.2021 MKM 4 BORING 3 4, '�� 5G Description Topsoil 17 1 Light -brown silty SAND with roots, very fine-grained, dry, medium -dense 5 13 2� -becomes slightly gravelly 18 3� SM -becomes gray 10 18 4 -reduced silt content 15 13 5 Gray slightly gravelly SAND, fine-grained, moist, medium -dense 20 22 6 -becomes fine to medium -grained, wet 25 24 7SP ' 30 22 8 35 39 9 -becomes slightly silty, dense 40 50 SM Gray mottled orange, slightly gravelly, very silty SAND, very fine-grained, moist, 100 very dense 6" * Test boring was terminated at 41 feet on December 23, 2020. * Groundwater seepage was encountered at 20 feet during drilling. 45 GEOTECH CONSULTANTS, INC. BORING LOG 8229 Talbot Road Edmonds ,Washington Job Date: Logged by. Plaf 20437 Jan.2021 MKM :5] Vapor retarder , Non -woven filter fabri, Washed rock or pea g 4" pi (h 2" PVC wee (Pour into Attach weep pipe to d Pierce waterproofing of drainage composite Note - Refer to the report for additional considerations related to drainage and waterproofing. GEOTECH CONSULTANTS, INC. SHORING DRAIN DETAIL 8229 Talbot Road Edmonds, Washington Job No: Date: Plate: 20437 1 Jan.2021 1 1 6 Slope backfill away from foundation. Provide surface drains where necessary. Tightline Roof Drain (Do not connect to footing drain) Backfill (See text for requirements) O c� Nonwoven Geotextile = Filter Fabric O Washed Rock LL Possible Slab (7/8" min. size) o e.a o e.a a o 0 o.Q O O O oo<oa Q.o<oe.Q._000e.o..000p.o•.o>oo.Q 0 0 0 0 0 0 0 0 0 0 o ao Do0°� o ooQ°p o°o Ooa°p o°o Ooa°p p°o Oo6°p o'a Do 000 0 0 0 0 0 Q°. Q Qv0-o 0a00OOo 4" min. ������ Vapor Retarder/Barrier and Capillary Break/Drainage Layer (Refer to Report text) 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 prevent buildup of water that bypasses the perimeter footing drains. (2) Refer to report text for additional drainage, waterproofing, and slab considerations. GEOTECH CONSULTANTS, INC. FOOTING DRAIN DETAIL 8229 Talbot Road Edmonds, Washington Job No: Date: Plate: 20437 1 Jan.2021 1 1 7