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CANOD.pdfCity of Edmonds Critical Area Notice of Decision Applicant: Property Owner: Critical Area File # Permit Number: -9 'le, Site Location: JAI i Parcel Number: - 7e Project Description: New [:] Conditional Waiver. No critical area report is required for the project described above. There will be no alteration of a,Critical Area or its required buffer, 2. The proposal is an allowed activity pursuant to ECDC 23,40.220, 23.50,220, and/or 23,80.040. 3, The proposal is exempt pursuant to ECDC 23.40.230. 111,5-lkt/ q F -I Erosion Hazard. Project is within erosion hazard area. Applicant must prepare an erosion and ""'71 1 1�3 sediment control plan in compliance with ECDC 18.30. Critical Area Report Required. The proposed project is within a critical area and/or a critical area buffer and a critical area report is required. A critical area report has been submitted and evaluated for compliance with the following criteria pursuant to ECDC 23.40.160: I The proposal minimizes the impact on critical areas in accordance with ECDC 23.40:120, Mitigation sequencing; 2. The proposal does not pose an unreasonable threat to the public health, safety, or welfare on or off the development proposal site; 3. The proposal is consistent with the general purposes of this title and the public interest; 4. Any alterations permitted to the critical area are mitigated in accordance with ECDC 23,40.110, Mitigation requirements. 5. The proposal protects the critical area functions and values consistent with the best available science and results in no net loss of critical functions and values; and 6. r The proposal is consistent with other applicable regulations and standards. ❑ Unfavorable Critical Area Decision. The proposed project is not exempt or does not adequately mitigate its impacts on critical areas and/or'does not comply with the criteria in ECDC 23,40.160 and the provisions of the City of Edmonds critical area regulations. See attached findings of noncompliance. Pf--Favorable Critical Area Decision. The proposed project as described above and as shown on the attached site plan meets or is exempt from the criteria in ECDC 23.40.160, Review Criteria, and complies with the applicable provisions of the City of Edmonds critical area regulations. Any subsequent changes to the proposal shall void this decision pending re -review of the proposal. Conditions. Critical Area specific condition(s) have been applied to the permit number referenced above. See referenced permit number for specific condition(s). Reviewer Signature Date Appeals: Any decision to approve, condition, or deny a development proposal or other activity based on the requirements of critical area regulations may be appealed according to, and as part of, the appeal procedure, if any, for the permit or approval involved. Revised 12/16/2010 REVISED GEOTECHNICAL ENGINEERING REPORT OLSON RESIDENCE 15500-75 TI PLACE WEST EDMONDS, WASHINGTON PREPARED FOR MR. GEORGE OLSON Main Office 17311 —135" Avenue NE, A-500 Woodinville, WA 98072 (425) 486-1669 FAX (425) 481-2510 (425) 337-1669 Snohomish County July 15, 2011 Mr. George Olson 3528-102 d Place SE Everett, WA 98208 NELSON• ' ASSOCIATES9 INC. 7's E .41 Grw=Q.qms - Revised Geotechnical Engineering Report Olson Residence 15500 - 75`h Place West Edmonds, Washington NGA File No. 8342B11 Dear Mr. Olson: Engineering -Geology Branch 437 East Penny Road Wenatchee, WA 98801 (509) 665-7696 FAX (509) 665-7692 We are pleased to submit the attached report titled "Revised Geotechnical Engineering Report - Olson Residence - 15500 -75th Place West - Edmonds, Washington." This report summarizes the existing surface and subsurface conditions within the project site, and provides geotechnical recommendations for design and construction of the proposed site improvements based on updated plans. Our services were completed in general accordance with the proposal signed by you on March 23, 2011. We previously conducted a preliminary assessment of the property in October 2010 and provided you with verbal opinions regarding the site and potential development, as part of your feasibility evaluation. At that time, we explored the site with two geotechnical borings. Our explorations indicated that the site is generally underlain by silty fine to medium sand near the surface, and silt with sand at depth. We interpreted that the majority of the deposits underlying the sit consist of landslide debris with Whidbey Formation silts at depth. We provided you a preliminary report dated April 27, 2011 based on previous development plans. The current plans have been revised to reduce impacts to the site. This revised report addresses the most recent plans. The site is situated on gently to steeply sloping ground that descends to the right-of-way for the Burlington Northern Santa Fe railroad and the shore of Puget Sound. Current project plans consist of constructing a two-story single-family residence with a daylight basement on the eastern portion of the property. Retaining walls up to nine feet in height are planned. The lower floor slab will be designed as a structural slab and will be entirely supported on auger cast piles. A small soldier pile retaining wall is planned near the northeastern corner of the site to replace a failed ecology block retaining wall. Revised Geotechnical Engineering Report Olson Residence 15500 — 75t' Place West Edmonds, Washington July 15, 2011 NGA File No. 8342B 11 Summary - Page 2 Stormwater handling plans have not been finalized at this time, but we understand that they will include tightlining the runoff down to the bottom of the slope. The site is mapped as part of the large historic/prehistoric Meadowdale Landslide complex and is located in the designated "Zone A" within the lower portion of the slide complex. This report provides information and discussion to fulfill the requirements of the City of Edmonds for construction within this area of Edmonds. It is important to recognize that the site and overall vicinity is part of this ancient landslide, and that development plans and future activity on this property should take that into consideration. It is our opinion that the site is generally compatible with the planned development of a single family residence. It is also our opinion that the slope on the eastern side of 75t1i Place West, also know as Lunds Gulch Road, is relatively stable. Based on the slide debris encountered in our explorations, and close proximity of the planned residence to the steep slopes, we recommend supporting the planned residence on a deep foundation system (drilled piers) extending to the more competent cohesive soils encountered at depth. The piers are intended to provide support for the residence and also provide an effective setback from the steep slopes. Specific recommendations for foundation design and installation are provided in the attached report. A steep slope is located on the uphill side of the property on the other side of 75�' Place West. This slope appears to be relatively stable and does not appear to pose serious risks to the planned improvements. Some debris/mudflow events may occur on this slope from time to time, especially during significant rainfall; however, with the presence of the road below this slope and the distance planned between the residence and the edge of the road, we are of the opinion that debris protection systems are not needed at this time. During our most recent site reconnaissance on March 30, 2011, we observed that a shallow landslide had occurred on the steep portion of the site slopes above the railroad tracks. The slide was located along the southern property line, and seems to have been triggered by water flow off of 75h Place West which made its way towards the steep slope. This water flow appears to have been occurring fairly frequently as evident by the channeling of sediment deposition along the southern property line starting at the road edge. This water flow should be collected and handled within any future drainage system planned as part of this development. Also, the recent landslide should be stabilized to reduce potential adverse impacts on the remainder of the site, and/or the railroad tracks below. We have provided general recommendations for stabilizing the landslide in the attached report. We strongly recommend that all runoff generated within this site, including roof downspouts, driveways, yard and footing drains, and all runoff entering the property from the road, be collected in a tightline and routed to the bottom of the slope. If this alternative is not feasible, the collected water should be pumped into the existing system in the roadway. No water should be infiltrated or dispersed on or near the site slopes. Such activity may cause further sliding within the steep slopes. -- We should be retained to review final grading and drainage plans prior to construction. We also recommend that NGA be retained to provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether or not earthwork and foundation installation activities comply with contract plans and specifications. Revised Geotechnical Engineering Report Olson Residence 15500 — 75h Place West Edmonds, Washington July 15, 2011 NGA File No. 8342B 11 Summary - Page 2 We appreciate the opportunity to provide service to you on this project. Please contact us if you have any questions regarding this report or require further information. Sincerely, NELSON GEOTECHNICAL ASSOCIATES, INC. Khaled M. Shawish, PE Principal Two Copies Submitted cc: James Thomas — Architectural Design Associates John McDonnell — JC McDonnell Engineering PC (via e-mail) INTRODUCTION....................................................................................................................................... I SCOPE.........................................................................................................................................................2 SITECONDITIONS...................................................................................................................................2 SurfaceConditions.....................................................................................................................................2 SubsurfaceConditions...............................................................................................................................3 HydrologicConditions...............................................................................................................................4 SENSITIVEAREA EVALUATION.........................................................................................................5 SeismicHazard..........................................................................................................................................5 ErosionHazard..........................................................................................................................................5 LandslideHazard/Slope Stability..............................................................................................................6 CONCLUSIONS AND RECOMMENDATIONS....................................................................................7 General....................................................................................................................................................... 7 Erosion Control and Slope Protection Measures.....................................................................................10 StructureSetbacks...................................................................................................................................10 SitePreparation and Grading...................................................................................................................12 Temporaryand Permanent Slopes.......................................................................................................... FoundationSupport .................................................................................................................................13 StructuralFill...........................................................................................................................................13 RetainingWalls.......................................................................................................................................14 StructuralSlab.........................................................................................................................................15 SoldierPile Wall......................................................................................................................................16 PavementSubgrade.................................................................................................................................17 Repairsof Recent Landslide....................................................................................................................18 - Site Drainage...........................................................................................................................................18 USEOF THIS REPORT..........................................................................................................................20 LIST OF FIGURES Figure 1 — Vicinity Map Figure 2 — Site Plan Figure 3 — Cross Sections A -A' Figure 4 — Soil Classification Chart Figures 5 and 6 — Boring Logs Figure 7 — Hand Auger Logs NELSON GEOTECHNICAL ASSOCIATES, INC. Revised Geotechnical Engineering Report Olson Residence 15500 — 75th Place West Edmonds, Washington _ INTRODUCTION This report presents the results of our geotechnical engineering evaluation of the most recent plans for the _ Olson Residence project. The site is located at 15500 — 75`'' Place West in Edmonds, Washington, as shown on the Vicinity Map in Figure 1. We previously prepared a report dated April 27, 2011 for a prior site layout. The purpose of the study is to explore and characterize the surface and subsurface conditions at the site and provide general geotechnical recommendations for site development. For our use in preparing this revised report, we were provided with an undated site plan titled "Foundation Plan," prepared by Architectural Design Associates, PS, dated July 11, 2011. Along with other foundation notes, this plan shows the building layout and pile support configurations. We were also provided with an undated Plan Sheet SP1.0 titled "Site Plan." The plan shows the site topography, property lines, and the planned residence footprint. The site is situated on sloping ground that descends to the right-of-way of the Burlington Northern Santa Fe (BNSF) railroad and the shore of Puget Sound. Project plans consist of constructing a two-story, single-family, daylight basement residence on the eastern portion of the property. Retaining walls up to nine feet in height are planned. Auger cast piles will be used to support the entire structure, as well as an elevated driveway for garage access which will be entered on the upper floor elevation of the residence. The lower floor slab will be designed as a structural slab and will be entirely supported on auger cast piles. A small soldier pile retaining wall is planned near the northeastern corner of the site to replace a failed ecology block retaining wall. Stormwater plans have not been finalized at this time, but we understand they may include tightlining runoff down to the bottom of the slope. The current site layout is shown on the Site Plan in Figure 2. The site is mapped as part of the large historic/prehistoric Meadowdale Landslide area. Slide movement from the large-scale slide complex and small slides within the complex can both affect this property. The residence design has taken this into consideration. YELSON GEOTECHNICAL ASSOCIATES, INC. Revised Geotechnical Engineering Report Olson Residence 15500 — 75"' Place West Edmonds, Washington July 15, 2011 NGA File No. 8342B 11 Page 2 SCOPE The purpose of this study is to explore and characterize the site surface and subsurface conditions, and provide opinions and recommendations for the proposed site development. Specifically, our scope of services includes the following: 1. Review available soil and geologic maps of the area. 2. Explore the subsurface soil and groundwater conditions within the site with two approximately 25 -foot deep geotechnical borings using a portable drill rig. The drill rig was subcontracted by NGA. 3. Meet with you and your representatives and discuss our findings and options for site development. 4. Conduct explorations on the site slopes using hand tools. 5. Map the conditions on the slope and evaluate current slope stability conditions. 6. Perform laboratory classification and analysis of soil samples, as necessary. 7. Provide recommendations for earthwork, foundation support, and slabs -on -grade in accordance with the City of Edmonds standards for development in the North Edmonds ESLHA. 8. Provide recommendations for subgrade preparation. 9. Provide recommendations for retaining walls and shoring. 10. Provide recommendations for debris walls/fences, as needed. - 11. Provide recommendations for site drainage and erosion control. 12. Document the results of our findings, conclusions, and recommendations in a written geotechnical report. _ SITE CONDITIONS Surface Conditions The site is situated on a gentle to steep west -facing slope overlooking Puget Sound. The central portion of the property consists of a gently sloping bench. To the east of the gently sloping bench, a steep but short, slope extends up to 75th Place West. Below the gently sloping bench, the ground surface slopes steeply down to the existing railroad tracks and Puget Sound. There is a very steep slope above 75"' Place West directly above this lot, with silt exposures and signs of soil creep and erosion. A cut along the road has experienced sloughing, but the steep slope above the road appears to be relatively stable. The site is vegetated with brush and deciduous trees. We observed evidence of surface water flow from the road Revised Geotechnical Engineering Report Olson Residence 15500 — 75h Place West Edmonds, Washington July 15, 2011 NGA File No. 8342B11 Page 3 onto the property near the southern property line during our most recent visit on March 30, 2011. An approximate 6 -foot tall block retaining wall that is failing is located just to the east of the eastern property line, directly below the road. It appears that this wall was constructed to support the 75"' Place West. A steep west -facing slope is located along the western portion of the property. This steep slope descends to the BNSF railroad tracks at about 28 to 35 degrees (53 to 70 percent). The slope inclinations are shown on Cross -Section A -A' in Figure 3 Subsurface Conditions Geology: The geologic units for this area are shown on the Geologic Map of the Edmonds East and Part of the Edmonds West Quadrangles, Washington, by James P. Minard (USGS, 1983). The site is mapped -- as Olympia Gravel — Qog, and Whidbey Formation — Qw. The Olympia Gravel deposit generally consists of stratified, fluvial sand and gravel. The Whidbey Formation generally consists of compact, medium to coarse grained sands. Our explorations encountered silty sand and silt material, considered mostly as slide debris. Explorations: The subsurface conditions within the site were explored on October 28, 2010 by drilling two borings using a limited -access drill rig to depths of approximately 21.5 to 26.5 feet below the existing ground surface. We also explored the steep slopes on March 30, 2011 with hand augers to depths of approximately 3.5 to 4.0 feet. The approximate locations of our explorations are shown on the Site Plan in Figure 2. A geologist from NGA was present during the explorations, examined the soils and geologic conditions encountered, obtained samples of the different soil types, and maintained logs of the borings and hand augers. A Standard Penetration Test (SPT) was performed on each of the samples during drilling to document soil density at depth. The SPT consists of driving a 2 -inch outer -diameter, split -spoon sampler 18 inches using a 140 -pound hammer with a drop of 30 inches. The number of blows required to drive the sampler the final 12 inches is referred to as the "N" value and is presented on the boring logs. The N value is used to evaluate the strength and density of the deposit. Revised Geotechnical Engineering Report Olson Residence 15500 — 75"' Place West Edmonds, Washington July 15, 2011 NGA File No. 8342B11 Page 4 _ The soils were visually classified in general accordance with the Unified Soil Classification System, presented in Figure 4. The logs of our borings are attached to this report and are presented as Figures 5 and 6. The logs of our hand augers are attached and are presented as Figure 7. We present a brief summary of the subsurface conditions in the following paragraphs. For a detailed description of the subsurface conditions, the boring and hand auger logs should be reviewed. Borings: At the surface of the explorations, we encountered approximately eight to ten feet of loose to medium dense, gray -brown to gray, fine to medium sand with silt to silty fine to medium sand. This material was underlain by approximately two to five feet of stiff to very stiff, gray silt mottled with iron - oxide staining, and contained possible slickensides which could indicate previous soil movement. The gray silt was underlain by blue -gray and some tan, very stiff to hard, silt to sandy silt. We interpreted the -- soil encountered in the borings to be mostly landslide debris with the Whidbey Formation silts at the bottom of the slide debris. Borings 1 and 2 were terminated at depths of 26.5 feet and 21.5 feet below the existing ground surface, respectively. Hand Augers: Below a surficial topsoil layer, we encountered approximately two feet of soft, brown -gray sandy silt with organics. We interpreted this soil as possible older landslide debris. Below this material, _ we encountered soft to very stiff, blue -gray to gray, sandy silt. We interpreted this soil to be part of the upper portion of the Whidbey Formation. Hand Augers 1, 2, and 3 were terminated in the sandy silt material at depths of 3.5 feet to 4.0 feet below the existing ground surface, respectively. Hydrologic Conditions Groundwater seepage was not encountered in the explorations, although it is our opinion that deep groundwater regimes likely exist below the site. If shallow ground water is encountered on this site, we would expect it to be a perched groundwater condition. Perched water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of underlying, less permeable soils. Perched water does not represent a regional groundwater "table" within the upper soil horizons. Perched water tends to vary spatially and is dependent upon the amount of precipitation. We would expect the amount of perched water to decrease during drier times of the year and increase during wetter periods. Revised Geotechnical Engineering Report Olson Residence - 15500 — 75th Place West Edmonds, Washington July 15, 2011 NGA File No. 8342B11 Page 5 Evidence of surface water was observed from 75h Place West onto the property near the southern property line. This water appears to flow on the ground surface along that side of the property and then disappears into the ground. This water flow was likely the cause of the most recent landslide above the railroad tracks. SENSITIVE AREA EVALUATION Seismic Hazard Undocumented fill and medium stiff to hard cohesive soils were encountered underlying the site. Based on the 2009 International Building Code (IBC), the site conditions best fit the description for Site Class Q — Hazards associated with seismic activity include liquefaction potential and amplification of ground motion by soft or loose geologic deposits. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit beneath the groundwater table. It is our opinion that the medium dense/stiff or better native soils interpreted to underlie the site have a low potential for liquefaction or amplification of ground — motion; however these materials could experience instability as a result of seismic activity. Shallow sloughing failures may occur in the loose surficial soils on the slopes during seismic events. Erosion Hazard The criteria used for determining the erosion hazard for the site soils includes soil type, slope gradient, vegetation cover, and groundwater conditions. The erosion sensitivity is related to the vegetative cover _ and the specific surface soil types, which are related to the underlying geologic soil units. The Soil Survey of Snohomish County Area, Washington, by the Soil Conservation Service (SCS) was reviewed to determine the erosion hazard of the on-site soils. The site surface soils were classified using the SCS classification system as Alderwood-Everett gravelly sandy loams, 25 to 70 percent slopes. These soils are listed as having a moderate hazard of water erosion, and on the steeper portions of the slope the erosion hazard is considered high. These soils should have a low to moderate hazard for erosion in areas that are not disturbed and where the vegetation cover is not removed. • • • Revised Geotechnical Engineering Report Olson Residence 15500 — 75"' Place West Edmonds, Washington July 15, 2011 NGA File No. 8342B 11 Page 6 Landslide Hazard/Slope Stability The criteria used for the evaluation of landslide hazards include soil type, slope gradient, and groundwater conditions. The site slopes moderately to steeply down to the west. Groundwater seepage was not observed on the slopes; however, evidence of water flow was observed along the southern property line from 75"' Place West. This site and the overall site vicinity lies within an ancient landslide area. The site and vicinity have been relatively stable for a very long period of time, and development in the area has taken place in the form of single-family residences and roadways. Although the likelihood of the ancient slide to become active in - the forseable future is very low, extreme environmental conditions coupled with inadequate human practices could re -activate the ancient landslide. Such external factors could include severe and elongated --- weather events and/or significant seismic activity. The site falls within "Zone A" of the North Edmonds Earth Subsidence and Landslide Hazard Area Report prepared by Landau Associates for the City of Edmonds. This designation requires that certain features be included (or excluded) in the design. Such features include the restriction of cuts and fills, the need for tightlining runoff into an approved system, the need to design foundations and retaining walls to withstand high lateral earth pressures and potential loss of soil beneath parts of the foundation, and the need to vegetate slopes with deeply rooted drought -tolerant vegetation, and the elimination of any and all irrigation systems. We have addressed all of the requirements in the remainder of this report. We encountered evidence of a potential slope movement as evident by the slickensides in the silt at depths of about 15 to 20 feet below the existing ground surface. Hard, competent silt was encountered below this material and we interpret the core of the slope to consist primarily of relatively stable material. We should note, however, that potential deep-seated slide planes were reported to be up to 100 feet deep below this site. We observed a recent landslide on the southwestern portion of the property during our most recent visit. _ A small wall is located at the toe of the slope along the BNSF Railroad right-of-way. It appears that the slide debris partially filled up the area behind the wall. In general, localized areas of surface instability and surface sliding can occur on steep slopes. Backwasting (movement of near -surface soil) through soil �` • Revised Geotechnical Engineering Report Olson Residence 15500 — 75h Place West Edmonds, Washington July 15, 2011 NGA File No. 8342B11 Page 7 erosion processes and local surface slides is common to steep slopes, particularly where the soils are exposed to weathering. Normal surface erosion and shallow sloughing failures should be expected to occur on the steep slope to the west from time to time. The Puget Sound area has experienced significant rainfall throughout the fall, winter, and early spring. It appears that this recent landslide is related to the rainfall and the water runoff onto the property from 75`s Place West. We recommend that the landslide be repaired, and that the water runoff be diverted to an appropriate system and not be allowed to flow uncontrollably onto the property. We discuss these topics further in the Repairs of Recent Landslide and Drainage subsections of this report, respectively. In this report, we have also provided geotechnical recommendations for deep -foundation support, erosion control, and other development considerations that should reduce the potential impact of site development on the site slopes and the steep slope to the west. CONCLUSIONS AND RECOMMENDATIONS General It is our opinion, from a geotechnical standpoint, that the site is compatible with the proposed improvements, provided that the geotechnical recommendations presented in this report are incorporated into project plans and followed during construction. Also, long-term human activity within the site can adversely or positively impact site stability. The proposed development area appears to be marginally stable under current conditions. It is also our opinion that the slope above the site is currently generally stable. However, the site is mapped within a geologic hazard area, and recent slope instability has occurred on the steep slopes above the railroad track. We also encountered evidence of potential past landsliding within the upper portion of the on-site material in the form of slickensides and low shear strength. To alleviate potential site instability concerns, project plans indicate that the entire residence will be supported on drilled piers extending down into the competent material at depth. This is further described in the Deep Foundations subsection of this report. In this report, we have also provided recommendations for drainage, erosion control, and other -- development considerations intended to reduce the potential impact of development on the site and the steep slope to the west. We should be retained to review final project plans prior to construction and to monitor earthwork and foundation system installation during construction. Revised Geotechnical Engineering Report Olson Residence 15500 — 75h Place West Edmonds, Washington July 15, 2011 _ NGA File No. 8342B11 Page 8 The residence is planned to be about 64 feet away from the steep slope, and the deck is planned to be about 58 feet from the slope. Based on our interpretation of the site, it is our opinion that these setback distances are adequate provided that the recommendations for deep foundations and erosion control management are followed. We further discuss the setbacks in the Structure Setbacks subsection of this report. We understand that the lower floor slab will be designed as a structural slab and will be entirely supported on deep foundation. For slabs -on -grade and other hard surfaces, such as paved areas or walkways that are supported on the existing soil, some risk of future settlement, cracking, and maintenance should be expected. To reduce this risk, we recommend over -excavating a minimum of two feet of the upper soil from the slab and pavement areas and replacing this material with compacted pit run or crushed rock structural fill. This recommendations is only for hard surfaces to be supported on grade and does not apply for the lower floor structural slab. Even with the recommended treatment, some settlement of the underlying loose material should be anticipated. The control of surface and near -surface water is very important for the long-term stability of the site and steep slopes. We highly recommend that temporary and final site grading be designed to direct surface water away from the structures and away from the steep slope. Final drainage plans have not been developed at this time, but we understand that all stormwater generated on the site will be collected in tightlines and transported to the bottom of the slope to the west of the property via a pipe anchored to the slope. No water should be infiltrated or dispersed within the site. We discuss general site drainage in the Site Drainage subsection of this report. — An approximate 6 -foot tall block retaining wall is located just to the east of the eastern property line, directly below the road. It appears that this wall was constructed to support the fill embankment associated with 75`" Place West. It is our opinion that this wall is unstable and could cause problems on the property in the future. We recommend that the City of Edmonds be contacted and the removal of this wall and the replacement with a soldier pile wall be implemented. If this wall cannot be removed, we recommend that a soldier pile wall be placed to the west of the existing wall. This is discussed further in the Soldier Pile subsection of this report. Revised Geotechnical Engineering Report Olson Residence 15500 — 75th Place West Edmonds, Washington July 15, 2011 NGA File No. 8342B 11 Page 9 A surficial landslide occurred on the steep slope. It is our opinion that this slide is shallow and should not affect the residence. However, the landslide should be stabilized as soon as possible to prevent further damage to the slope. We recommend that erosion control matting be secured to the slope and vegetation replanted. We provide specific recommendations in the Repairs of Recent Landslide subsection of this report. - Though a surficial slide recently occurred on the steep slope, in general, the site currently appears generally stable with respect to deep-seated movement. However, the site vicinity is mapped within an - older landslide complex and our explorations encountered evidence of past ground movement. The potential for landslide and erosion hazards will depend on how the site is graded and how surface water and near surface water are controlled. We recommend that grading and site drainage plans be subjected to geotechnical engineering review prior to construction. The soils encountered within our explorations are considered extremely moisture sensitive and will disturb easily when wet. We recommend that construction take place during extended periods of dry weather if possible. If construction takes place during wet weather, additional expenses and delays should be expected due to the wet conditions. Additional expenses could include the need to export on- site soil, the import of clean, granular soil for fill, and the need to place a blanket of rock spalls or crushed rock in the construction traffic areas and on exposed subgrades prior to placing structural fill or structural elements. We recommend that NGA be retained to review final project plans. We also recommend that NGA be retained to provide monitoring and consultation services during construction to confirm that the — conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those -- anticipated, and to evaluate whether or not earthwork and foundation installation activities comply with contract plans and specifications. NELSON GEOTECHNICAL ASSOCIATES, INC. Revised Geotechnical Engineering Report Olson Residence 15500 — 75`h Place West Edmonds, Washington July 15, 2011 — NGA File No. 8342B 11 Page 10 Erosion Control and Slope Protection Measures The erosion hazard for the on-site soils is considered moderate to high, but the actual hazard will be dependent on how the site is graded and how water is allowed to concentrate. Best Management Practices (BMPs) should be used to control erosion. Areas disturbed during construction should be protected from erosion. Erosion control measures may include diverting surface water away from the stripped or disturbed areas. Silt fences and/or straw bales should be erected to prevent muddy water from leaving the site or flowing over the site slopes and the steep slope to the west of the property. Disturbed areas should be planted as soon as practical and the vegetation should be maintained until it is established. The erosion potential for areas not stripped of vegetation should be low to moderate. Also, irrigation systems should not be installed on or near the slope. -- Protection of the slopes should be performed as required by the City of Edmonds. Specifically, we recommend that the slopes not be disturbed or modified through placement of any fill or future structures outside the planned development areas. No additional material of any kind should be placed on the steep slope or any portion of sloping ground, such as excavation spoils and soil stockpiles. Trees may be cut down and removed from the slopes as long as a mitigation plan is developed for maintaining slope stability, such as the replacement of vegetation for erosion protection. A vegetation cover should be preserved on the slopes. Replacement of vegetation should be performed in accordance with the City of Edmonds code. Under no circumstances should water be allowed to concentrate on the slopes. Any sloping areas disturbed during construction should be planted with vegetation as soon as practical to reduce the potential for erosion. Structure Setbacks Uncertainties related to building along the top of steep slopes are typically addressed by the use of building setbacks. The purpose of the setback is to establish a "buffer zone" between the structure and the top of the slope so that ample room is allowed for normal slope recession during a reasonable life span of the structure. In a general sense, the greater the setback, the lower the risk of slope failures to impact the structure. From a geological standpoint, the setback dimension is based on the slope's physical characteristics, such as slope height, slope gradient, soil type, and groundwater conditions. Other factors such as historical slope activity, rate of regression, and the type and desired life span of the development are important considerations as well. • • , Revised Geotechnical Engineering Report Olson Residence - 15500 — 75th Place West Edmonds, Washington July 15, 2011 NGA File No. 8342B 11 Page 11 Based upon the conditions described above, it is our opinion that the potential for shallow sloughing -type failures and small-scale landslides exist on the steep slopes, as seen with the recent landslide on the site. This condition is exacerbated where water is present or where the slopes become locally very steep. Backwasting through sloughing of steep slopes can occur up the slope, such that a loss of ground could occur. The planned residence will be setback approximately 64 feet from the top of the slope, and the planned deck will be about 58 feet from the top of slope. These setback distances are adequate; however, due to the potentially unstable debris found on the site and the history of the area, we have recommended that the residence be supported on deep foundations in the form of drilled piers. We have recommended that the piers advance a minimum of 25 feet below the existing ground surface. Loose material should not be stockpiled in any area between the top of the slope and the residence footprint. Site Preparation and Grading Plans for site grading should be devised such that cuts and fills are kept to a minimum. Site preparation should consist of excavating the residence footprint and driveway areas down to planned elevations. Site preparation should also consist of stripping any organic topsoil and/or loose/soft soils in areas that will support foundations, slabs -on -grade, pavement, or structural fill. The stripped material should not be stockpiled in any area between the top of the slope and the residence footprint. If the exposed soils are loose/soft, they should be compacted to a non -yielding condition. Areas observed to pump or weave during compaction should be over -excavated and replaced with rock spalls. If significant surface water flow is encountered during construction, this flow should be diverted around areas to be developed and the exposed subgrade maintained in a semi -dry condition. In wet conditions, the exposed subgrade should not be compacted, as compaction of a wet subgrade may result in further disturbance of the soils. A layer of crushed rock may be placed over the prepared areas to protect them from further disturbance. The site soils are considered extremely moisture sensitive and will disturb easily when wet. We recommend that earthwork construction take place during periods of extended dry weather, and suspended during periods of precipitation if possible. If work is to take place during periods of wet weather, care should be taken during site preparation not to disturb the site soils. This can be accomplished by utilizing large excavators equipped with smooth buckets and wide tracks to complete earthwork, and diverting surface and groundwater flow away from the prepared subgrades. Also, Revised Geotechnical Engineering Report Olson Residence 15500 — 75th Place West Edmonds, Washington July 15, 2011 NGA File No. 8342B 11 Page 12 construction traffic should not be allowed on the exposed subgrade. A blanket of rock spalls should be used in construction access areas if wet conditions are prevalent. The thickness of this rock spall layer should be based on subgrade performance at the time of construction. For planning purposes, we recommend a minimum one -foot thick layer of rock spalls. Temporary and Permanent Slopes Temporary cut slope stability is a function of many factors, including the type and consistency of soils, depth of the cut, surcharge loads adjacent to the excavation, length of time a cut remains open and the presence of surface or groundwater. It is exceedingly difficult under these variable conditions to estimate a stable, temporary, cut slope angle. Therefore, it should be the responsibility of the contractor to maintain safe slope configurations since he is continuously at the job site, able to observe the nature and condition of the cut slopes, and able to monitor the subsurface materials and groundwater conditions encountered. The following information is provided solely for the benefit of the owner and other design consultants and should not be construed to imply that Nelson Geotechnical Associates, Inc. assumes responsibility for job site safety. Job site safety is the sole responsibility of the project contractor. For planning purposes, we recommend that temporary cuts in the on-site material be no steeper than two units horizontal to one unit vertical (2H: IV). If groundwater seepage is encountered, we would expect that flatter inclinations would be necessary. Steeper cuts may be feasible if dense soils are exposed. We should be retained to specifically review proposed geometry for significant cuts planned on this site. We recommend that cut slopes be protected from erosion. Erosion control measures may include covering cut slopes with plastic sheeting and diverting surface water runoff away from the top of cut slopes. We do -- not recommend vertical slopes for cuts deeper than four feet, if worker access is necessary. We recommend that cut slope heights and inclinations conform to appropriate OSHA/WISHA regulations. Permanent cut and fill slopes should be no steeper than 3H: IV. However, flatter inclinations may be required in areas where loose soils are encountered. Permanent slopes should be covered with erosion control matting and vegetated. The vegetative cover maintained until established. We should specifically • • ' Revised Geotechnical Engineering Report Olson Residence 15500 — 75`h Place West Edmonds, Washington July 15, 2011 _ NGA File No. 8342B 11 Page 13 review all plans for grading on this project. We do not recommend grading on steep slopes, or placing irrigation systems near the slopes. Foundation Support Current plans indicate that the entire residence will be supported on 16- to 24 -inch reinforced concrete piers, extending a minimum of 25 feet below existing ground surface. -- An open -hole drilling method will likely be feasible based on our field observations, however, if caving conditions are encountered, pile casing will be required. The holes should be cleaned of any slough or -- water prior to pouring concrete. We recommend that the concrete be readily available on site at the time of drilling. The holes should not be left open for any extended period of time, as sloughing debris and/or groundwater seepage into the excavations may hamper pier installation. For piers installed successfully as described above, we recommend using a design axial compression capacity of 25 and 40 tons for 16- and 24 -inch piers, respectively. Lateral resistance on the piers could be _ calculated based on an equivalent fluid density of 200 pounds per cubic foot (pcf) applied to two pile diameters. The upper 15 feet should be neglected for the purpose of calculating the lateral resistance. Structural Fill General: Fill placed beneath foundations, pavements, and other settlement -sensitive structures, or behind retaining walls should be placed as structural fill. Structural fill, by definition, is placed in accordance with prescribed methods and standards and is monitored by an experienced geotechnical professional or soils technician. Field monitoring procedures would include the performance of a representative number of in-place density tests to document the attainment of the desired degree of relative compaction. The area to receive the fill should be prepared as outlined in the Site Preparation and Grading subsection of this report. Sloping areas to receive fill should be benched prior to fill placement. The benches should be level and at least four feet wide. Materials: Structural fill should consist of a good quality, granular soil, free of organics and other deleterious material and be well graded to a maximum size of about three inches. All-weather fill should contain no more than five -percent fines (soil finer than U.S. No. 200 sieve, based on that fraction passing NELSON GEOTECHNICAL ASSOCIATES, INC. Revised Geotechnical Engineering Report Olson Residence 15500 — 75`h Place West Edmonds, Washington July 15, 2011 NGA File No. 8342B 11 Page 14 the U.S. 3/4 -inch sieve). We do not anticipate placement of significant volumes of structural fill for this project. The on-site soils consist of moisture -sensitive silty materials and slide debris. We recommend that the on-site material not be used as structural fill. We should be retained to evaluate the suitability of proposed structural fill materials at the time of construction. Fill Placement: Following subgrade preparation, placement of structural fill may proceed. All filling should be accomplished in uniform lifts up to eight inches thick. Each lift should be spread evenly and be thoroughly compacted prior to placement of subsequent lifts. All structural fill underlying building areas and pavement subgrade should be compacted to a minimum of 95 percent of its maximum dry density. Maximum dry density, in this report, refers to that density as determined by the ASTM D-1557 Compaction Test procedure. The moisture content of the soils to be compacted should be within about two percent of optimum so that a readily compactable condition exists. It may be necessary to over - excavate and remove wet soils in cases where drying to a compactable condition is not feasible. All compaction should be accomplished by equipment of a type and size sufficient to attain the desired degree of compaction. Retaining Walls We understand that retaining walls up to nine feet high will be incorporated into project plans. We recommend that retaining walls be kept as short as possible. The lateral pressure acting on subsurface retaining walls is dependent on the nature and density of the soil behind the wall, the amount of lateral wall movement which can occur as backfill is placed, wall drainage conditions, the inclination of the backfill, and other possible surcharge loads. For walls that are free to yield at the top at least one thousandth of the height of the wall (active condition), soil pressures will be less than if movement is limited by such factors as wall stiffness or bracing (at -rest condition). We recommend that walls supporting horizontal backfill and not subjected to hydrostatic forces be designed using a triangular earth pressure distribution equivalent to that exerted by a fluid with a density of 40 pcf for yielding (active condition) walls, and 60 pcf for non -yielding (at -rest condition) walls. Revised Geotechnical Engineering Report Olson Residence 15500 — 75th Place West Edmonds, Washington July 15, 2011 NGA File No. 8342B 11 Page 15 These recommended lateral earth pressures are for a drained granular backfill and are based on the assumption of a horizontal ground surface behind the wall for a distance of at least the subsurface height of the wall, and do not account for surcharge loads. Additional lateral earth pressures should be considered for surcharge loads acting adjacent to subsurface walls and within a distance equal to the subsurface height of the wall. This would include the effects of surcharges such as traffic loads, floor slab and foundation loads, slopes, or other surface loads. Also, hydrostatic and buoyant forces should be included if the walls could not be drained. We could consult with the structural engineer regarding additional loads on retaining walls during final design, if needed. All wall backfill should be well compacted; however, care should be taken to prevent the buildup of excess lateral soil pressures, due to over -compaction of the wall backfill. This can be accomplished by placing wall backfill in thin loose lifts and compacting it with small, hand -operated compactors within a distance behind the wall equal to at least one-half the height of the wall. The thickness of the loose lifts should be reduced to accommodate the lower compactive energy of the hand -operated equipment. Retaining wall foundations should be supported on a minimum of two feet of rock spalls to reduce the potential for differential settlement of the walls. The active pressure on the wall can be resisted by friction on the bottom of the wall footing and passive resistance on the below -grade portion of the footing. We recommend using a design soil bearing pressure of no more than 2,000 pounds per square foot (psf) along with a friction coefficient and passive resistance of 0.35 and 200 pcf, respectively. Permanent drainage systems should be installed for retaining walls. Recommendations for these systems are found in the Subsurface Drainage subsection of this report. We recommend that we be retained to evaluate the proposed wall drain backfill material and drainage system installation. Structural Slab As mentioned earlier, the lower floor slab will be designed as a structural slab fully supported on deep foundations. We recommend that slabs be underlain by at least six inches of free -draining gravel with less than three percent by weight passing the Sieve #200 for use as a capillary break. We recommend that the capillary break be hydraulically connected to the footing drain system to allow free drainage from under the slab. A suitable vapor barrier, such as heavy plastic sheeting (6 -mil minimum), should be Revised Geotechnical Engineering Report Olson Residence 15500 — 75"` Place West Edmonds, Washington July 15, 2011 NGA File No. 8342B 11 Page 16 placed over the capillary break material. An additional 2 -inch -thick moist sand layer may be used to cover the vapor barrier. This sand layer may be used to protect the vapor barrier membrane and to aid in curing the concrete; however, this sand layer is optional and is intended to protect the vapor barrier membrane during construction. Soldier Pile Wall As mentioned previously, an approximate 6 -foot tall block retaining wall is located just to the east of the eastern property line, but not on the project site. It appears that this wall was constructed to support the Lunds Gulch Road. It is our opinion that this wall is unstable and could cause problems on the property - in the future. We recommend that this wall be removed or stabilized using a soldier pile wall. We recommend that the new soldier pile wall be placed to the west of the existing wall to protect the Olson property against potential failure of the block wall. The soldier pile wall should extend roughly from the northern property line to the southern extent of the existing block wall. A solider pile wall typically consists of a series of steel H -beams placed vertically at a certain distance from one another (typically six to ten feet). The beams are usually placed in drilled shafts that are filled with concrete or grout. The concrete shafts are typically embedded below the bottom of the planned excavation a distance equals one to two times the height of the cut to be shored, if tie- backs are not used. The steel beams are extended above finished ground surface to provide shoring capabilities for the cut. The beams are typically spanned by pressure treated timber lagging. The H -beam size, shaft diameter, shaft embedment, and pile spacing are dependent on the nature of the soils anticipated in the cut and at depth, cut height, drainage conditions, the need for tie -backs, and the final geometry. The soldier pile wall should be designed by an experienced structural engineer licensed in the State of Washington. The wall should be designed for an active pressure acting on the piles and lagging for design of the soldier piles and should be calculated based on a triangular pressure distribution equivalent to that exerted by a fluid with a density of 50 pcf. The above loads should be applied on the full center -to -center pile spacing above the base of the exposed portion of the wall. These loads could be resisted by passive resistance acting on the below -grade portion Revised Geotechnical Engineering Report Olson Residence _ 15500 — 75h Place West Edmonds, Washington July 15, 2011 _ NGA File No. 8342B 11 Page 17 of the piles. A 50 percent reduction of this value can be applied for the purpose of designing the wall lagging. The passive resistance could be calculated based on a 100 pcf equivalent fluid density acting on two effective pile diameters below the base of the exposed portion of the wall. This value incorporates a factor of safety of 2. The below -grade portion of the wall should not be shorter than twice the wall stick- up height. The soldier pile wall should be installed by a shoring contractor experienced with this type of system. Although we anticipate that an open -hole drilling method will be adequate for installing the soldier piles in the on-site soils, the shoring contractor should be capable of casing the holes as sloughing and/or water -- seepage may be encountered. It might be prudent to perform one or more "test" holes to confirm installation conditions prior to finalizing work plans. Any sloughing or water that may collect in the drilled holes should be removed prior to pouring grout. Grout should be readily available on site at the time the holes are drilled. The holes should not be left open for any length of time, as that may increase the potential for caving and water seepage to impact wall installation. If groundwater seepage is encountered, we recommend that the concrete be tremied from the bottom of the excavations to displace the groundwater to the surface. Extra Portland Cement may also be placed in the bottom of the excavations to reduce the effects of seepage. The spoils from the soldier pile excavations are expected to be moisture -sensitive materials and should be removed from the site. We should be retained to monitor on site activities during the soldier pile wall installation on a full-time basis. The wall should be lagged using pressure -treated timber or concrete panels. Adequate gaps should be maintained between the lagging elements to allow water flow through the face of the wall. Also, all wall backfill should consist of 2 -inch clean drain rock. It is imperative that water not be allowed to pool behind the wall, therefore extreme care should be taken not to contaminate the drain rock with silt or organics. In wet conditions, it might be necessary to use a filter fabric along the back of the drainage -- layer. Pavement Subgrade Pavement subgrade preparation should be completed as recommended in the Site Preparation and Grading and Structural Fill subsections of this report. Depending on the tolerance to pavement NELSON GEOTECHNICAL ASSOCIATES, INC. Revised Geotechnical Engineering Report Olson Residence 15500 — 75t` Place West Edmonds, Washington July 15, 2011 NGA File No. 8342B11 Page 18 cracking, we recommend that the upper two feet of the existing material be removed and replaced with structural fill. The pavement subgrade should be proof -rolled with a heavy, rubber -tired piece of equipment, to identify soft or yielding areas that may require repair prior to placing any structural fill and prior to placing the pavement base course. We should be retained to observe the proof -rolling and recommend repairs prior to placement of the asphalt or hard surfaces. If the existing soil is left in place, the pavement section should be thickened to further reduce the effects of settlement. Repairs of Recent Landslide For the landslide that was recently experienced on the steep slope on the property, we recommended that the exposed soil be covered with heavy duty erosion control matting such as Tensar C350 Turf Reinforcement Mat, or equivalent. Prior to placing the matting, any areas of loose soils should be - removed to expose dense native soil. The matting should be staked at the top of the slope with two to three rows of two- to three-foot long metal rebar that is either bent at the end or has a metal "T" welded to the end. The mat should be laid flush to the slope and staked to the exposed soil on the slope a minimum of every five feet. The slide debris that accumulated behind the small retaining wall should be cleaned out. After the matting netting is placed, we recommended that deep-rooted vegetation be planted on the slope and grass seed be planted to re-establish vegetation growth. All surface water flow should be permanently directed away from the slide area. We should be retained to monitor the repairs of the landslide. Site Drainage Surface Drainage: The finished ground surface should be graded such that stormwater is directed to an appropriate stormwater collection system. Water should not be allowed to collect in any area where footings, slabs, or retaining walls are to be constructed. Final site grades should allow for drainage away from the structure and away from the steep slopes. We suggest that the finished ground be sloped at a minimum gradient of three percent, for a distance of at least 10 feet away from the structure and slopes. Surface water should be collected by permanent catch basins and drain lines, and be discharged into an appropriate discharge system. Under no circumstances should water be allowed to flow uncontrolled over -- the site slopes or excavation walls. Revised Geotechnical Engineering Report Olson Residence 15500 —'75`h Place West Edmonds, Washington July 15, 2011 NGA File No. 8342B11 Page 19 We recommend that stormwater generated on the site, including roof downspouts, footing drains, pavement and yard drains, and water flow from the road, be tightlined to the bottom of the slope to the west. This should entail directing all collected runoff into a main catch basin placed about 15 feet away from the top of the steep slope, from which a 6- to 8 -inch diameter HDPE pipe should be extended to the back of the existing wall along the BNSF right-of-way. A concrete collar should be placed around the pipe between the catch basin and top of the slope to help anchor the pipe. The pipe should be laid on the slope surface but should be anchored using T -posts and metal straps. The pipe should end with a perforated Tee section approximately eight feet long that is capped on both ends and embedded in the crushed rock found behind the existing wall. Subsurface Drainage: If groundwater is encountered during construction, we recommend that the - contractor slope the bottom of the excavations and collect water into ditches and small sump pits where the water can be pumped out of the excavations and routed into an appropriate outlet. We recommend the use of footing drains around the planned structure and behind retaining walls. Footing drains should be installed at least one -foot below planned finished floor elevation. The drains should consist of a minimum four -inch -diameter, rigid, slotted or perforated, PVC pipe surrounded by free -draining material, such as washed rock, wrapped in a filter fabric. We recommend that an 18 -inch - wide zone of clean (less than three -percent fines), granular material be placed along the back of the walls above the drain. Washed rock is an acceptable drain material, or drainage composite may be used instead. The free -draining material should extend up the wall to one -foot below the finished surface. The top foot of backfill should consist of low permeability soil placed over plastic sheeting or building paper to minimize the migration of surface water or silt into the footing drain. Footing drains should discharge into tightlines leading to an appropriate collection and discharge point with convenient cleanouts to prolong the useful life of the drains. Roof drains should not be connected to footing drains. -- Roof drains should also be installed around the site structures. The roof drains should consist of gutters and downspouts collecting stormwater runoff from the roof. The downspouts should discharge to catch basins and 4 -inch minimum diameter, rigid, PVC tightline pipes. The drains should be directed into catch basins and then into the controlled drainage system. The footing and roof drains should discharge via independent (separate) tightlines into catch basins/cleanouts leading to the stormwater system. Surface NELSON GEOTECHNICAL ASSOCIATES, INC. Revised Geotechnical Engineering Report Olson Residence 15500 — 75h Place West Edmonds, Washington July 15, 2011 NGA File No. 8342B11 Page 20 water from the driveway and yard areas should also be collected in a catch basin and tightlined separately to the stormwater system. USE OF THIS REPORT NGA has prepared this report for Mr. George Olson and his agents, for use in the planning and design of the development planned on this site only. The scope of our work does not include services related to construction safety precautions and our recommendations are not intended to direct the contractors' methods, techniques, sequences, or procedures, except as specifically described in our report for consideration in design. There are possible variations in subsurface conditions between the explorations and also with time. Our report, conclusions, and interpretations should not be construed as a warranty of subsurface conditions. A contingency for unanticipated conditions should be included in the budget and schedule. We recommend that we be retained to review final project plans and provide consultation regarding specific structure placement, site grading, foundation support, and drainage. We also recommend that NGA be retained to provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether or not earthwork and foundation installation activities comply with contract plans and specifications. We should be contacted a minimum of one week prior to construction activities and could attend pre -construction meetings if requested. All people who own or occupy homes on or near hillsides should realize that landslide movements are always a possibility. The landowner should periodically inspect the slope, especially after a winter storm. If distress is evident, a geotechnical engineer should be contacted for advice on remedial/preventative measures as soon as possible. The probability that landsliding will occur is substantially reduced by the proper maintenance of drainage control measures at the site (the runoff from the impervious surfaces should be led to an approved discharge point). Therefore, the homeowner should take responsibility for performing such maintenance. Revised Geotechnical Engineering Report Olson Residence 15500 — 75th Place West Edmonds, Washington July 15, 2011 NGA File No. 8342B11 Page 21 Within the limitations of scope, schedule, and budget, our services have been performed in accordance with generally accepted geotechnical engineering practices in effect in this area at the time this report was prepared. No other warranty, expressed or implied, is made. Our observations, findings, and opinions are a means to identify and reduce the inherent risks to the owner. ••• NELSON GEOTECHNICAL ASSOCIATES, INC. Revised Geotechnical Engineering Report Olson Residence 15500 — 75t` Place West Edmonds, Washington July 15, 2011 _ NGA File No. 8342B 11 Page 22 We appreciate the opportunity to provide service to you on this project. If you have any questions or require further information, please call. Sincerely, NELSON GEOTECHNICAL ASSOCIATES, INC. do Bala Do e -Alan y - Project Geologist EXPlR�S Khaled M. Shawish, PE Principal BD:KMS:bd Seven Figures Attached 1 1VICINITY MAP N Not to Scale 1 .1 Project Number NELSON GEOTECHNICAL No. Date Revision By CK ASSOCIATES, INC. 8342611 Olson Residence N A 1 3/25/11 Original DPN BD Vicinity Map GEOTECHNICAL ENGINEERS & GEOLOGISTS Figure 1 17311 -135th A". NE, A-500 Snoh noh County(425) 339-1669 WoodlnvUe, WA 98072 Wenatchee/Chelan (509) 665.7696 1400 m 01 1200 ft Fisher r,3� Project V. Site 152nd St S%+, 1 IV, sodG'wda@ .. 1 County Park 15&h St S%V 1 :i5 Meadowdale . 164th St S -w 011 1J 1 V r ig�'iSSL�G'63rG!:?(S o r � PUGPUGET 1riRhStS BeverlyAeresO SOUND C Vic` 172nd FI Stip ly4idpiquest 02011 MapQuest Portions 09201 ap 1 .1 Project Number NELSON GEOTECHNICAL No. Date Revision By CK ASSOCIATES, INC. 8342611 Olson Residence N A 1 3/25/11 Original DPN BD Vicinity Map GEOTECHNICAL ENGINEERS & GEOLOGISTS Figure 1 17311 -135th A". NE, A-500 Snoh noh County(425) 339-1669 WoodlnvUe, WA 98072 Wenatchee/Chelan (509) 665.7696 U � O m � x(1) wQ� M ry 0 CL d Project NumberNELSON Q) No. Date Revision i [ 0 (6 E 11 - o U Nt C a r Original N O 0-0) M Figure 2 U 17311 -135th Avo. NE, A600 Snohomish C—ty (425) 337-1669 Woodnvilie, WA 98072 W—= /C (509) 665-7646 C e 14 � O a CU 4- Project NumberNELSON Q) No. Date Revision i [ O E (6 E X X c O a 3/25/11 Original O O 0-0) L U Figure 2 17311 -135th Avo. NE, A600 Snohomish C—ty (425) 337-1669 Woodnvilie, WA 98072 W—= /C (509) 665-7646 C •L MQCL CU 4- CU 4- nOn`` W C ^O`` W O O OO !_ So C O O O 0 0 O X W O a O- E >_ (0 Q O U N LU OL D_ O U Z O Z ay— Q O N U) —i— r m U _ I G �I N'. Project NumberNELSON GEOTECHNICAL ASSOCIATES, INC. No. Date Revision By CK 8342811 Olson Residence Site Plan N GEOTECHNICAL ENGINEERS & GEOLOGISTS 1 3/25/11 Original DPN BD Figure 2 17311 -135th Avo. NE, A600 Snohomish C—ty (425) 337-1669 Woodnvilie, WA 98072 W—= /C (509) 665-7646 (425) 486-1669 / Fax 481-2510 wn�v.nalsongeotech.wm Z ry S C L ZC L > co a) O W 00 0 a) ro Co o co E CU U (6 o U) a>� a C O u) 75 E _T r\1 ,O c a Z ry S C L ZC L Project Number 83421311 Olson Residence Cross -Section A -AI NELSON GEOTECHNICAL ASSOCIATES, INC. GEOTECHNICAL ENGINEERS & GEOLOGISTS a) O I I I I I I I 1 1 1 I I I I I I I I I I I I I I I i l l i C:c6 l l i py N '.0 J N U. C ^ U CU U (6 O p a>� O m U Figure 3 u) 75 E v w 2 o a M C O X O U O a0ia e (1281) uol}ena13 a}ewixoiddy Wbndi_`-. WA 96072 Wa tt .., WA 98601 (425)466-1669/Pax 461-2510 (509)665-76.96 c� o Q Q.. a N O C 14- U N Co N O C (n a.0 UJ •• N x W Q " N _N z U) O N -U to O N N Q c rn .N N p 0 I o a> Cfl L W N Q Q ,W _ fl C', a � T N Mn .2) a� m CL r ® � � M p w U m m a u E a� 3a uJ o o fn Q CL o ro a a> a c p CDCU � C Ca n O N ® z N a ® � C O xI U) C � C I O N N "O N N N N MI W _O T �. .N C: Project Number 83421311 Olson Residence Cross -Section A -AI NELSON GEOTECHNICAL ASSOCIATES, INC. GEOTECHNICAL ENGINEERS & GEOLOGISTS � U I I I I I I I 1 1 1 I I I I I I I I I I I I I I I i l l i C:c6 l l i py N '.0 J N U. C ^ U O O O O �. N oo d O p a>� O m U Figure 3 C:m v w 2 o a M o a0ia e (1281) uol}ena13 a}ewixoiddy Wbndi_`-. WA 96072 Wa tt .., WA 98601 (425)466-1669/Pax 461-2510 (509)665-76.96 c� o a Project Number 83421311 Olson Residence Cross -Section A -AI NELSON GEOTECHNICAL ASSOCIATES, INC. GEOTECHNICAL ENGINEERS & GEOLOGISTS No. Date Revision By CK 1 4/13/11 Original DPN BD Figure 3 17311 -135th Avo, NE, A500 437 East Penny Road Wbndi_`-. WA 96072 Wa tt .., WA 98601 (425)466-1669/Pax 461-2510 (509)665-76.96 IUNIFIED SOIL CLASSIFICATION SYSTEM MAJOR DIVISIONS GROUP SYMBOL GROUP NAME CLEAN GW WELL -GRADED, FINE TO COARSE GRAVEL COARSE- GRAVEL GRAVEL GP POORLY -GRADED GRAVEL GRAINED MORE THAN 50 % OF COARSE FRACTION GRAVEL GM SILTY GRAVEL SOILS RETAINED ON NO. 4 SIEVE WITH FINES GC CLAYEY GRAVEL SAND CLEAN SW WELL -GRADED SAND, FINE TO COARSE SAND SP POORLY GRADED SAND SAND MORE THAN 50 % RETAINED ON NO. 200 SIEVE MORE THAN 50 OF COARSE FRACTION PASSES NO. 4 SIEVE SAND SM SILTY SAND WITH FINES SC CLAYEY SAND FINE - SILT AND CLAY INORGANIC ML SILT - GRAINED LIQUID LIMIT LESS THAN 50 % CL CLAY ORGANIC OL ORGANIC SILT, ORGANIC CLAY SOILS SILT AND CLAY INORGANIC MH SILT OF HIGH PLASTICITY, ELASTIC SILT MORE THAN 50 % _ PASSES NO. 200 SIEVE LIQUID LIMIT 50 % OR MORE CH CLAY OF HIGH PLASTICITY, FLAT CLAY ORGANIC OH ORGANIC CLAY, ORGANIC SILT HIGHLY ORGANIC SOILS PT PEAT NOTES: 1) Field classification is based on visual SOIL MOISTURE MODIFIERS: examination of soil in general accordance with ASTM D 2488-93. Dry Absence of moisture, dusty, dry to the touch 2) Soil classification using laboratory tests Moist - Damp, but no visible water. is based on ASTM D 2488-93. Wet - Visible free water or saturated, _ 3) Descriptions of soil density or usually soil is obtained from consistency are based on below water table interpretation of blowcount data, visual appearance of soils, and/or _ test data. Project Number 8342611 Olson Residence Soil Classification Chart NELSON GEOTECHNICAL %ASSOCIATES, INC. GEOTECHNICAL ENGINEERS & GEOLOGISTS No. Date Revision By CK 1 3/25/11 Original DPN BD — Figure 4 17311 -135th Ave. NE, A5W S-t—d C—ty (425) 337-1669 W—f—Pa. WA 98072 We tcfi.tChe Fan(5W) 6657696 (425) 4 66-1 669 1 Fax 481-2510 vn5w.neiwngeotnch.com L nnnrnYtrr,ate Ground Surface Elevation: —96 feet Soil Profile Description BORING O B-1 Sample Data a� c w m �-76 �jJ U� mV �JEL (D Penetration Resistance (Blows/foot - ) 10 20 30 40 50 Moisture Content (Percent - ) 10 20 30 40 50 rn Piezometer N Installation - 3rown-gray fine to medium sand with silt and trace gravel Ground Water 2 every loose,moist) (Depth in Feet) SP -SM 3ray, silty fine to medium sand with silt and trace gravel 16 medium dense, moist) SM 18 _LGray, silt with fine sand, (very stiff, moist to wet) 11 nottled and contains slickensides (Slide Plane???) ML Blue -gray silt with trace fine sand (very stiff, moist) 20 Contains some slickensides 22 1 �= 35 ML r 40 �r 66 -with interbedded coarse sand a 79 1Boring terminated below existing grade at 26.5 feet on 10/28/10. Groundwater seepage was not encountered. Penetration Resistance (Blows/foot - ) 10 20 30 40 50 Moisture Content (Percent - ) 10 20 30 40 50 rn Piezometer N Installation - Z Ground Water Data 9 (Depth in Feet) co J 5 m 15 ... ... I ........ .I ......... I ......... I ......... I .... E...I F 25 0 LEGEND i� Solid PVC Pipe Concrete M Moisture Content Slotted PVC Pipe Bentonite A Atterberg Limits Depth Driven and Amount Recovered G Grain -size Analysis 1with 2 -inch O.D. Split -Spoon Sampler11= Monument/ Cap Native Soil DS Direct Shear to Piezometer PP Pocket Penetrometer Readings, tons/ft Depth Driven and Amount Recovered * Liquid Limit Silica Sand p Sample Pushed with 3 -inch Shelby Tube Sampler + Plastic Limit V Water Level T Triaxial 1 NOTE: Subsurface conditions depicted represent our observations at the time and location of this exploratory hole, modified by engineering tests, analysis and judgement. They are not necessarily representative of other times and locations. we cannot accept responsibility for the use or interpretation by others of information presented on this log. NELSON GEOTECHNICAL No. Date Revision 13YCK Project Number 8342BII Olson Residence A, ASSOCIATES, INC. 1 3/25/11 Original LSB KMS Boring Log GEOTECHNICAL ENGINEERS Sc GEOLOGISTS Figure 5 SrwKomish C—ty (425) 339-1669 17311 -135th Ava. 98072 A-500 VV n tchee/C"el (509) 665-7696 Pane 1 of 1 W486-1 69,/F 481-2 (425) 466-1669 / Fax 481-2510 wr+/.nalwrgeotech.tom BORING LOG 0 m ®2 co .0 N I T L � Approximate Ground Surface Elevation: —79 feet Penetration Resistance 0' J Soil Profile Sample Data(glows/foot - ) Piezometer 10 20 30 40 50 50+ H Installation - t I Ground Water o °' o °' Moisture Content o Data Description n o o Q o E (Percent -) o (Depth in Feet) m U O 0 10 20 30 40 50 50+ @ l C7 co 3rown-gray silty, fine to medium sand with silt and trace Depth Driven and Amount Recovered A Atterberg Limits Slotted PVC Pipe Bentonite G Grain -size Analysis m 2 Monument/ Ca p Native Soil DS Direct Shear Readings, tons/ft S Depth Driven and Amount Recovered �'c to Piezometer PP Pocket Penetrometer q Silica Sand p Sample Pushed o Liquid Limit p a with 3 -inch Shelby Tube SamplerI Plastic Limit Water Level T Triaxial o 1 NOTE: Subsurface conditions depicted represent our observations at the time and location of this exploratory hole, modified by engineering tests, analysis and judgement. They are not necessarily M representative of other times and locations. We cannot accept responsibility for the use or interpretation by others of information presented on this log. gravel (medium dense, moist) ' NELSON GEOTECHNICAL. No. Date Revision By CK ON C) 8342811 Olson Property ASSOCIATES, INC. N A 1 3/25/11 Original LSB KMS o Boring Log GEOTECHNICAL ENGINEERS & GEOLOGISTS Figure 6 16 _ S, ho,nlsh County (425) 339-1669 17311-135th 7 Woad 411a. WA 96072 Wa t=/CKaln(509) 665-7696 IoAR.1RR9 C a _'� /Gar dAl.�Stn WNN.�BISOMeOMch.COT 7 .. •._. SM 5 ................................................... 5 22 -becomes with silt lenses, and moist to wet 16 Gray silt with trace fine sand (stiff, moist) Plane???) bra 14 10 ................................................... 10 lottled and contains slickensides (Slide s ML 1 �r .................. -15 (Tan fine sandy silt (very hard, moist) 88 -tclean fine sand on top of sample ML 1 2 .............. 20 becomes gray, laminated, with interbedded fine sand 87-11" layers �oring terminated below existing grade at 21.5 feet on 10/28/10. Groundwater seepage was not encountered. 1 25 1 25 ......................... ......................... LEGEND j� Solid PVC Pipe Concrete M Moisture Content Depth Driven and Amount Recovered A Atterberg Limits Slotted PVC Pipe Bentonite G Grain -size Analysis m with 2 -inch O.D. Split -Spoon Sampler 1 Monument/ Ca p Native Soil DS Direct Shear Readings, tons/ft S Depth Driven and Amount Recovered �'c to Piezometer PP Pocket Penetrometer q Silica Sand p Sample Pushed o Liquid Limit p a with 3 -inch Shelby Tube SamplerI Plastic Limit Water Level T Triaxial o 1 NOTE: Subsurface conditions depicted represent our observations at the time and location of this exploratory hole, modified by engineering tests, analysis and judgement. They are not necessarily M representative of other times and locations. We cannot accept responsibility for the use or interpretation by others of information presented on this log. Project Number NELSON GEOTECHNICAL. No. Date Revision By CK ON C) 8342811 Olson Property ASSOCIATES, INC. N A 1 3/25/11 Original LSB KMS o Boring Log GEOTECHNICAL ENGINEERS & GEOLOGISTS Figure 6 _ S, ho,nlsh County (425) 339-1669 17311-135th 7 Woad 411a. WA 96072 Wa t=/CKaln(509) 665-7696 IoAR.1RR9 C a _'� /Gar dAl.�Stn WNN.�BISOMeOMch.COT 7 LOG OF EXPLORATION DEPTH (FEET) USC SOIL DESCRIPTION HAND AUGER ONE 0.0-0.5 TOPSOIL - 0.5-3.5 ML BROWN -GRAY, SANDY SILT WITH TRACE ORGANICS AND SILT LAYERS (SOFT, MOIST TO WET) 3.5-4.0 CL BLUE -GRAY CLAY WITH SLICKENSIDES AND MOTTLED (SOFT, MOIST) SAMPLES WERE COLLECTED AT 2.0, 3.5, AND 4.0 FEET LIGHT GROUNDWATER SEEPAGE WAS ENCOUNTERED AT 3.0 FEET HAND AUGER CAVING WAS NOT ENCOUNTERED HAND AUGER WAS COMPLETED AT 4.0 FEET ON 3/30/11 HAND AUGER TWO 0.0-0.5 TOPSOIL 0.5-2.5 ML BROWN -GRAY, SANDY SILT WITH SILT LAYERS AND ORGANICS (SOFT, MOIST TO WET) 2.5-3.5 ML GRAY, SANDY SILT (VERY STIFF, MOIST) SAMPLE WAS COLLECTED AT 3.5 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED HAND AUGER CAVING WAS NOT ENCOUNTERED HAND AUGER WAS COMPLETED AT 3.5 FEET ON 3/30/11 HAND AUGER THREE 0.0-0.5 TOPSOIL 0.5-2.5 ML BROWN -GRAY, SANDY SILT WITH SILT LAYERS AND ORGANICS (SOFT, MOIST TO WET) 2.5-3.5 ML GRAY, SANDY SILT (VERY STIFF, MOIST) SAMPLES WERE NOT COLLECTED GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED _ HAND AUGER CAVING WAS NOT ENCOUNTERED HAND AUGER MET REFUSAL ON A ROOT AT 3.5 FEET ON 3/30/11 LSB:DPN NELSON GEOTECHNICAL ASSOCIATES, INC. FILE NO: 83421311 FIGURE 7 NELSON O NGAnASSOCIATES, INC. GEOTECHNICAL = & GEOLOGISTS Main Office 17311 — 135"' Avenue NE, A-500 Woodinville, WA 98072 (425) 486-1669 FAX (425) 481-2510 (425) 337-1669 Snohomish County April 3, 2013 Mr, George Olson 3528-102 nd Place SE Everett, Washington 98208 Geotechnical Plan Review and Comment Letter Olson Residence Edmonds, Washington NGA File No. 8342B 11 Dear Mr. Olson: Engineering -Geology Branch 437 East Penny Road Wenatchee, WA 98801 (509) 665-7696 FAX (509) 665-7692 R� R APR - 5 2013 BUILDING DEPAR 11lINHY GAT OF EOMAoNOS This letter presents the results of our geotechnical engineering review and our responses to City of Edmonds comments regarding the plans for your residence project located at 15500 — 75"' Place West in Edmonds, Washington. INTRODUCTION Project plans consist of constructing a new two-story, single-family residence within the eastern portion of the property. The site is situated on sloping ground that descends to the right-of-way of the Burlington Northern Santa Fe (BNSF) railroad and the shores of Puget Sound. The eastern portion of the property where the residence is planned is gently sloping down to the west. The remainder of the site consists of a moderate to steep, west -facing slope. We previously prepared a geotechnical report for the project titled "Revised Geotechnical Engineering Report — Olson Residence — 15500-75"' Place West — Edmonds, Washington," dated July 15, 2011. We also prepared memoranda titled "Olson Property — Tree Management," dated July 13, 2011 and "Olson Residence — Temporary Drainage Recommendations," dated December 7, 2011. For our use in preparing this letter, we were provided with the following documents: 3:1 RFE'l,,=TIL Geotechnical Plan Review and Comment Letter Olson Residence Edmonds, Washington NGA File No. 8342B 11 April 3, 2013 Page 2 ® Plan Review Comments - Plan Check #2012-0585 — Project: Olson SFR — Project Address: 15500 75`x' Place W, prepared by the City of Edmonds Building Division, dated November 14, 2012. ® Geotechnical Peer Review — Earth Subsidence and Landslide Hazard Area — PRE2012-0032 — Olson SFR -15500 75`x' Place West — Edmonds, Washington, prepared by Landau Associates, dated November 19, 2012. ® Plan Review Comments for Plan Check #2012-0858 — New SFR at 15500 75'x' Place West, prepared by the City of Edmonds, dated November 20, 2012. ® Plan Review Corrections, prepared by the City of Edmonds, dated November 20, 2012. ® Memo: Olson Residential Construction Proposal — 15500 75`x' PI W, prepared by the City of Edmonds, dated November 21, 2012. We were also provided with the following Plan Sheets: A1.0, A3.0, A3.2, S 1.0, and S 1.1 of the plans titled "George & Ginger Olson — Olson Residence," prepared by Architectural Design Associates, dated August 20, 2012; Sheets Cl through C5 of the plans titled "George Olson SFR," prepared by JC McDonnell Engineering dated March 29, 2013; Undated, untitled Plan Sheet SP1.0; and Sheets L1.0 and L2.0. The site falls within "Zone A" of the North Edmonds Earth Subsidence and Landslide Hazard Area Report prepared by Landau Associates for the City of Edmonds. This designation requires that certain features be included (or excluded) in the design. Such features include the restriction of cuts and fills, the need for tightlining runoff into an approved system, the need to design foundations and retaining walls to withstand high lateral earth pressures and potential loss of soil beneath parts of the foundation, and the need to vegetate slopes with deeply rooted drought -tolerant vegetation, and the elimination of any and all irrigation systems. PLAN REVIEW We have reviewed the geotechnical aspects of the most recent plans and found the plans to be in general compliance with our recommendations as presented in our previous geotechnical report. The residence is Geotechnical Plan Review and Comment Letter Olson Residence Edmonds, Washington NGA File No. 8342B 11 April 3, 2013 Page 3 set -back roughly 55 feet from the existing steep slope which is in line with the recommendations of the report. The entire structure, as well as an elevated driveway for garage access, will be supported on 21, 18 -inch diameter, 25 -foot deep auger cast piles as recommended in our report. Originally, the lower floor was to be supported on a slab -on -grade, but the current plans show that there will be a crawl space underneath the entire structure. Slabs -on -grades are not planned for this project. Cuts up to approximately nine feet were previously planned, but current plans indicate that only minor grading will be made on this site. There is currently a deteriorating block retaining wall located within the City of Edmonds Right -of -Way along the eastern side of the property. We previously recommended that this wall be removed and replaced with a soldier pile wall. Current plans show no grading, structures, or retaining walls are planned near this wall. As long as this area is not disturbed during construction, in the forseeable future, this wall should not need to be replaced. It should be monitored and repaired/replaced at a later date as needed. Depending on the time of the year, temporary drainage and erosion control during construction will consist of a level spreader and/or straw wattles located no closer than 10 feet from the top of the slope. Silt fences are not planned to be used for this project. Permanent drainage will consist of all footing drains, downspouts, and the channel drain for the driveway being directed to a detention pipe which will be located directly to the west of the new residence. The detention pipe will be connected to a Drisco pipe which will be directed down the slope towards the existing retaining wall along the railroad tracks. The pipe will be secured at the top of the slope with a deadman anchor, and will be anchored to the slope every five feet. The drain outlet will be installed according to Burlington Northern & Santa Fe Railroad Company specifications as shown on Plan Sheet C4. In the past, water from the existing road has flowed onto this site. To help prevent this condition, a new drainage system including a catch basin in the road will be installed along the property line to the south of the residence. The pipes carrying runoff from the road will be connected to the main drainage system below the planned detention pipe. A shallow landslide, as documented in the geotechnical report, was cause by the runoff from the road reaching the slide area. Specific recommendations for repairing the slide were provided in the report. The current plans call for repairing the slide in accordance with the report under the supervision of NGA. Geotechnical Plan Review and Comment Letter Olson Residence Edmonds, Washington CITY REVIEW COMMENT RESPONSES NGA File No. 8342B 11 April 3, 2013 Page 4 In general, most of the review comments made by the City of Edmonds in the above mentioned documents centered around discrepancies between the project descriptions found in the geotechnical report and the plans that were submitted for review, such as the soldier pile wall that was mentioned in the report by not shown on the plans; previous substantial grading; previous slab -on -grade; drainage plans; and temporary erosion control plans. All these issues have since been resolved and clearly addressed by the most recent plans. LANDU ASSOCIATES REVIEW COMMENTS LETTER In their November 19, 2012 review memorandum, Landau Associates made several review comments regarding our geotechnical report and some geotechnical aspects of the plans. The Landau Associates comments are listed below along with our responses to each comment: 1. We recommend that NGA revisit the landslide risk conditions at this site and also provide a clear statement of risk and their assessment of the stability of the site and whether it is in their professional opinion that the site meets the criteria for "Stable" as defined in ECDC 19.10.020.0. (including the estimated probability of earth movement). Response: With the studies of the ESLHA and the drainage improvements to reduce the risk of landsliding, the City of Edmonds is allowing cautious and appropriate development within this area. The landslide hazard for this site is mitigated by the house design and drainage and erosion control plans for this project in the form of minimal grading and deep foundation support, as well as, drainage and grading improvements, to reduce the potential for landslides on this property. The drainage improvements are designed to collect surface water runoff and near surface groundwater and remove it from the site. Minimal grading will be carried out on this site, and the entire residence will be supported on a deep foundation system. Most of the surface water flowing onto the site, as well as the water collected in downspouts, footing drains, and on other hard surfaces, will be collected into a drainage system that will direct the runoff down to the toe of the slope near the railroad tracks. The residence will be founded on piers that extend 25 feet below the ground surface and therefore will not be supported on the near surface soils. By incorporating all of the above in the project plans, the Olson residence has been designed to reduce the potential for landsliding on this specific site, and to resist landslides if they were to Geotechnical Plan Review and Comment Letter Olson Residence Edmonds, Washington NGA File No. 8342B 11 April 3, 2013 Page 5 occur. By following similar methods as required by the City of Edmonds, other development in the overall ESLHA zone is also reducing the risk of landsliding, and thus, the overall risk of a catastrophic landslide that would impact a large portion of this area is being reduced as more sites are developed. However, if deep-seated large-scale landsliding were to occur within the overall site vicinity due to extreme environmental factors outside the control of any one homeowner, such as prolonged extreme wet weather and/or significant earthquakes, many residences in this area could be damaged or destroyed. It is not practical for an individual property owner to eliminate the potential for deep-seated large-scale landsliding in this historic slide area. In our opinion, the local community and the City of Edmonds acting together have significantly reduced the potential for landslides in this area. In our opinion, based on the site conditions found in our explorations and the large-scale studies of the ESLHA by others, we consider this site stable and estimate that there is currently no more than 30 percent probability of a slope failure in a 25 -year period within the Olson property. This probability should be further reduced after the residence has been constructed in accordance with the plans and the recommendations in our report. We should emphasize that the shallow landslide that took place along the western edge of the site was largely caused by uncontrolled runoff that originated on the road above the site. This condition has been addressed by the plans. 2. We recommend that the notes be revised appropriately oto reflect the current geotechnical engineer of record] Response: Nelson Geotechnical Associates, Inc. is listed as the geotechnical engineer of record in the Engineers Notes section on Sheet Cl. 3. The stockpile location does not satisfy the recommendations for the geotechnical report. The location of stockpiling should be modified to accommodate the geotechnical recommendations. Response: Stock piles have been removed from Sheet Cl. Grading should be minimum, and any excavated soils will be removed promptly from the site. No significant stockpiles of soils are planned. ,=GEOTECHNICAL Geotechnical Plan Review and Comment Letter Olson Residence Edmonds, Washington NGA File No. 8342B 11 April 3, 2013 Page 6 4. We recommend that the plans include the location and structural details for the soldier pile retaining wall. Response: The soldier pile retaining wall is no longer part of the current plans and has been removed. CONCLUSIONS AND RECOMMENDATIONS It is our opinion that the provided plans have addressed all of the geotechnical issues that were raised in our geotechnical report and by the City of Edmonds. The plans have been revised to reduce the impacts to the site and steep slope. It is also our opinion that the site will be more stable after the residence and anticipated design and erosion control systems are implemented. The site soils are considered extremely moisture sensitive and will disturb easily when wet. We recommend that earthwork construction take place during periods of extended dry weather, and suspended during periods of precipitation if possible. If work is to take place during periods of wet weather, care should be taken during site preparation not to disturb the site soils. This can be accomplished by utilizing large excavators equipped with smooth buckets and wide tracks to complete earthwork, and diverting surface and groundwater flow away from the prepared subgrades. Also, construction traffic should not be allowed on the exposed subgrade. A blanket of rock spalls should be used in construction access areas if wet conditions are prevalent. The thickness of this rock spall layer should be based on subgrade performance at the time of construction. For planning purposes, we recommend a minimum one -foot thick layer of rock spalls. We recommend that all erosion control and drainage systems be monitored on a regular basis, especially during periods of heavy precipitation, and maintained as warranted to ensure continued functionality. We should be promptly contacted should any signs of distress to the slope or the drainage system be observed. MINIMUM RISK STATEMENT Provided that the recommendations in this letter and the geotechnical report dated July 15, 2011 are followed during construction, the areas disturbed by construction should remain stable. Therefore, the risk of damage to the proposed development or to adjacent properties form soil instability should be minimal, and the proposed grading and development should not increase the potential for soil movement. Geotechnical Plan Review and Comment Letter NGA File No. 8342B 11 Olson Residence April 3, 2013 Edmonds, Washington Page 7 C�IC�T.�i77 We recommend that NGA be retained to provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether or not earthwork activities comply with contract plans and specifications. We appreciate the opportunity to provide service to you on this project. Please contact us if you have any questions regarding this letter or require further information. Sincerely, NELSON GEOTECHNICAL ASSOCIATES, INC. B a D doye-Al�'li roject Geologist Khaled M. Shawish, PE Principal Three Copies Submitted BD:KMS:kms NELSON GEOTECHNICAL ASSOCIATES, INC. % f � Main Office 17311 — 135"Avenue NE, A-500 Woodinville, WA 98072 (425) 486-1669 FAX (425) 481-2510 (425) 337-1669 Snohomish County DATE: July 1, 2013 TO: George Olson CC: FROM: Ai so T 1 INC. GEOLOGISTSGEOTECHNICAL ENGINEERS & Jim Thomas — Architectural Design Associates, Inc. Khaled Shawish, PE RE: Olson Property Existing Block Retaining Wall Edmonds, Washington NGA File No. 8342B13 Engineering -Geology Branch 437 East Penny Road Wenatchee, WA 98801 (509) 665-7696 FAX (509) 665-7692 1"13 I p( �ha' �X ,•F' W^1sPy� / �qC', 'amu, J-, This memo provides our opinions regarding the existing block retaining wall located within the City of Edmond's 75`x' Place West Right -of -Way, adjacent to your property. We previously provided a geotechnical evaluation report for your future single-family residence project title "Revised Geotechnical Engineering Report" dated July 15, 2011. In that report we concluded that this existing block wall was in a deteriorating condition and posed risk to your development plans. That opinion was based on your plans at the time the report was prepared, which included a permanent cut near this wall and improvements within a close proximity to the wall. We recommended at that time that a soldier pile wall be constructed along the property line to buttress the deteriorating wall and provide permanent support for the planned grading. You have since revised your development plans which omitted alterations to the area adjacent to the existing wall and all improvements in that area. The soldier pile wall is no longer part of the plans. We have provided a more recent plan review letter for the current plans dated April 3, 2013 in which we stated that since no structures or grading were planned adjacent to the block wall, there was no need for replacing the wall for the benefit of your project. We stated that periodic monitoring of the wall with the Memorandum Existing Block Retaining Wall Olson Property Edmonds, Washington NGA File No. 8342B13 July 1, 2013 Page 2 understanding that the City may have to repair/replace the wall in the future should be sufficient at this time. We have been provided with a copy of a technical memorandum by Landau Associates, Inc. dated June 7, 2013 in which they expressed concern that the block wall is not being sufficiently addressed by current development plans, or by any plans by the City for dealing with the wall. This memo recommended that you and the City collaborate to assess the degree of hazard this wall poses to your property and/or the right-of-way and evaluate whether additional actions beyond monitoring the wall are needed. As mentioned in our previous review letter, with the alteration of your development plans to avoid any disturbance to the wall or vicinity, it is our opinion the wall will continue to perform as it has for many years, and therefore periodic monitoring of the wall should be sufficient to allow ample time for decisions to be made regarding repairs/replacement of the wall, long before a total collapse of the wall. Even if the wall were to abruptly fail, such failure should not impact the improvements planned on your property or hinder access of emergency vehicles. It is important to keep in mind that this wall is located entirely within the City right-of-way and thus any future alterations to the wall should be planned and carried out by the City. The City should make such assessment as, unlike your property, failure of this wall could impact the road and access to properties located beyond the location of this wall. If the City elects to further evaluate the need for repairing/replacing the wall, we would be happy to consult directly with the City regarding such plans. We trust this memorandum should satisfy your needs at this time. Please contact us if you have any questions or require additional services. LANDAU ASSOCIATES TECHNICAL MEMORANDUM IMIGEOWMAL RE50UiCE5 TO: Leif Bjorback, Building Official City of Edmonds Development Services Department, Building Division FROM: Dennis R. Stettler, P.E. tJ/Z CO-- DATE: June 7, 2013 RE: SUPPLEMENTAL GEOTECHNICAL PEER REVIEW 6 EARTH SUBSIDENCE AND LANDSLIDE HAzARD AREA - BLD20120858 OLSON SFR -15500 75TH PLACE WEST - MIS�N S EDMONDS, WASHINGTON A cdl-�56 (�(— This technical memorandum provides our supplemental geotechnical peer review for the permit submittal package that was submitted to the City of Edmonds (City) for the proposed referenced development within the Earth Subsidence and Landslide Hazard Area (ESLHA) of North Edmonds, Landau Associates previously completed a peer review for this project and summarized our observations and recommendations in a Technical Memorandum dated November 19, 2012. Several submittal packets pertain to this project. The submittal reviewed for our previous peer review in 2012 consisted of two document packets—one stamped "Received August 23, 2012" and one stamped "Received October 1, 2012," The former packet was submitted for completeness review by the City; the latter packet contained a replacement set of architectural -structural drawings, an updated structural calculations packet, and an executed agreement with BNSF for stormwater tightline construction and discharge onto the railroad's right-of-way. The City completed their review of the submittal package and provided review comments to the applicant in November, 2012. The applicant revised the design and resubmitted the documents for reviews with a letter dated April 4, 2013; the submittal was suspended shortly after that date so that additional revisions could be made. Revisions to the amended design, primarily relocating the structure to provide for adjusted building setbacks from the property lines were made, along with changes to grading to reduce or eliminate the need for walls, and to support the lower floor as a structural floor on grade beams instead of slab -on -grade. Other changes were made to the storm drainage configuration. These changes were documented in transmittal letters dated April 4 and May 2, 2013. The current review focuses on the most recent of the revised documents from either April 4 or May 2, 2013. The purpose of the current supplemental geotechnical peer review is to review revised portions of the submittal package and assess its compliance with City development and building permit requirements as contained in Edmonds Community Development Code (ECDC) Chapters 19.10 and .23.80, and our previous review comments from our Technical Memorandum dated November 19, 2012. This 130 2nd Avenue South ® Edmonds, WA 98020 . (425) 778-0907 m fax (425) 778-6409 ® www.landauinc.com geotechnical peer review was accomplished in accordance with Task Order No. 13-08 of Landau Associates' On -Call Geotechnical Engineering Services Agreement with the City. We have received and reviewed the following new or resubmitted information forwarded by the City for review: • Resubmittal of selected plans for The Proposed Olson Residence, 15500 75"' Pl. SW, BLD20120858, letter from Architectural Design Associates, PS. to City of Edmonds, dated May 2, 2013. • Olson Residence, 15500 75"' Pl. SW, Plan Check No. 2012-0858, letter from Architectural Design Associates, PS. to City of Edmonds, dated April 4, 2013. • Site Plan (Sheet SPLO) listed in sheet index of Architectural Plan Set (above) but not bound; encountered loose in submittal packet. Updated for revised setback, but still dated July 11, 2011. • Architectural Plan Sheets A1.0 and AL 1, prepared by Architectural Design Associates, dated August 20, 2012, but stamped by City of Edmonds as Resubmittal on May 3, 2013. • Architectural/Structural Plan Set (14 Sheets, including architectural and structural plans, elevations, and details) prepared by Architectural Design Associates, resubmittal dated April 4, 2013. • Landscape Plan Set and Tree Hazard Evaluation packet, including Re -vegetation and Tree Mitigation Plan (drawing L1.0), Significant Tree Inventory and Tree Preservation Plan (drawing L2.0). Harmsen & Associates. Plans dated August 13, 2012; revision I dated March 29, 2013. • Resubmittal for Olson Residence, 15500 75"' Place West, Landscape Architecture response to November 2012 comments, letter prepared by Harmsen & Associates, dated March 29, 2013. • Civil Plan Set (5 Sheets, CI through C5, including Winter TESC & Grading Plan, Grading and TESC Details, Detailed Drainage Plan, Notes & Details, and Utility Plan W. Pump. • Geotechnical Plan Review and Comment Letter, Olson Residence, Edmonds, Washington. Letter prepared for Mr. George Olson by Nelson Geotechnical Associates, Inc., dated April 3, 2013. The following sections provide our specific geotechnical peer review comments. GEOTECHNICAL REPORT The geotechnical plan review and comment letter addresses Landau Associates' previous review comments. We understand that our previous review comments on the structural plans regarding the location and details of a proposed soldier pile and lagging wall no longer apply, as the proposed wall has been excluded from the project. However, the Revised Geotechnical Report prepared by Nelson Geotechnical Associates (7/15/2011) describes a "failed" or "failing" approximate 6 ft high block retaining wall that the proposed soldier pile and lagging wall was intended to replace. The report also states "it is our opinion 617/13 P:1074Vl71W3OWileRoom\R1OlsonSupplementalPeerReview tmAocx LANDAu /ASSOCIATES that this wall is unstable and could cause problems on the property in the future. " By contrast, the Geotechnical Plan Review and Comment Letter prepared by Nelson Geotechnical Associates (4/3/2013) states that the wall is a deteriorating block wall located within City of Edmonds right-of-way and recommends that the wall be monitored and repaired/replaced at a later date as needed. We are concerned that the wall has been described as unstable, failing, or failed and it appears that this wall could potentially present either a hazard to the property or to the public on the public right-of-way (or both) if the wall was to suddenly fail. Monitoring may not be effective in alleviating this hazard. We recommend that the integrity of this wall and consequences of failure be further evaluated by either the applicant, the City, or the two parties in a collaborative approach to assess the degree of hazard to the property or to the public and to evaluate whether monitoring is sufficient, or whether mitigation, repair, or replacement alternatives should be implemented: S CIVIL PLANS AND STORM DRAINAGE STUDY The revised civil and storm drainage plans appear to have addressed the previous review comments, including appropriate reference to the current geotechnical engineer, clarification of a drawing symbol, and prohibiting temporary soil stockpiles on site. We notice that the reference to no stockpiling on the site and removing excess excavated material from the site on Sheet Cl also contains the note: "Any soil removed from the site must be hauled to a City approved site. " We would expect that the City does not really care where the excess soil is taken, as long as it is taken to a site that is legally allowed to accept the soil in accordance with any applicable requirements and. regulations regarding disposal, or filling and grading. We recommend that the City not approve any specific disposal site. LANDSCAPE DESIGN Harmsen & Associates has updated the plans and indicated a total of 23 replacement trees in order to conform to the required 2:1 replacement ratio, rather than the 21 replacement trees indicated on previous plans. It appears that one of our earlier review comments may have been misinterpreted. In our review letter we commented that "Automatic sprinklers or other irrigation systems are not specified on the landscaping plans." What was meant by that comment was that the absence of automatic or irrigation systems is highly desirable, since such systems are not allowed by the City in the North Edmonds Earth Subsidence and Landslide Hazard Area as stated in ECDC 19.10.070. The design team may have misinterpreted our comment and attempted to add the provision that automatic sprinklers or irrigation systems could be added. Note 12 on Drawing L1.0 states in part that "The Owner may install automatic 617113 P:10741171to30�FileRoom\RkOlsonSupplementaiPwReHew Im.dou LANDAU ASSOCIATES 3 sprinklers or irrigation systems." The note regarding automatic sprinkler or irrigation systems should be removed SE5 l Fwk� 44"S This technical memorandum has been prepared for use by the City of Edmonds in evaluating the adequacy of resubmitted permit submittal documents related to the proposed Olson single family residence at 15500 75`h Place West. The focus of this review was the geotechnical aspects of the application. The purpose of the review was to assess the adequacy of the application documents for compliance with City requirements contained in ECDC 23.80 and ECDC 19.10 and conformance with conventionally accepted geotechnical engineering practices. This geotechnical peer review by Landau Associates does not lessen the requirements for the applicant's geotechnical consultant and other design professionals to, prepare an appropriate design for the site conditions. We appreciate this opportunity to be of service to the City. Please contact us if you have any questions or if we may be of further service. 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