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HomeMy WebLinkAbout701344025 Geotech Assessment (2003)j IN-DEPTH PERSPECTIVE ~J June 6; 2003 Mr. Roger Larson Architect /Planner 142 Resolute Lane Port Ludlow, Washington 98365 Re: Weaver Residence at Dabob Cove Deflection/Debris Wall Design Project No. 030031-002-O1 Dear Mr. Larson: ;. :~ j N 0'G~ 1 ~ 2003 ~, !' 1` We have read your letters of May 21 and May 27, 2003, including the attachments, which concern the structural design of the new deflection debris wall to be built into the east wall of the Weaver residence repair/remodel. We are not structural engineers; therefore, we do not have any professional. comment on the structural configuration of your wall design. We can comment generally that other walls, which we have seen, and have been designed and built for debris protection purposes have utilized a structural key in the foundation to achieve adequate lateral resistance. In general, a concrete wall incorporated into the house structure has been utilized in other known projects and appears to be a reasonable solution here. Based on your detail 1/B2 for the wall section, we have the following geotechnical comments: • The wall under normal conditions must have 12 feet of clear freeboard to provide the necessary debris catchment protection. The main structure cannot be based on a full-time load against the outside of the walla Your detail appeared to show this area totally backfilled with no catchment and only 10 feet tall. • Three geotechnical loading conditions apply: 1. Normal and earthquake.- no load on outside for the upper 12 feet; 2. Slide static -loading of 60 pounds per cubic foot (pcf) equivalent with soil weight buoyant for the total wall height; and 3. Slide dynamic -loading of 80 pcf equivalent with soil normal weight for the upper 12 feet. Whichever wall type is selected, the easterly face of the wall needs to be 10 feet from the toe of slope to provide the sufficient volume for debris with the 12-foot-high wall. Therefore, if a wall of significant thickness is used, the westerly wall face of the wall will move into the existing house volume. ~~ elOl ~iAv`~~.e~~ 9 k~~o o °~~~~~llilo{ lb"~ _..' i ~., y,.ar S ~mJ ~bl i~ 6C3P 0 B ~ f={-. ~r~vr - ~ o p a Uolnaf ~~ ~ ~`°~ - .~_. _~~~~~,~1~~8 a~,~~~~~~".~2Y~~Ce ° , a~bAA~eCe[, ~° ~ e `~v .m ~ w'S, Mr. Roger Larson June 6, 2003 Project No. 030031-002-01 We hope this has clarified the loading issues for you. We would be available to look over your completed design, if Mr. Weaver concurs for us to do that review. Sincerely, Aspect consulting, LLC /°~~ °3 John L. Peterson, P.E. Associate Geotechnical Engineer jpeterson@aspectconsulting, com cc: Mr. Bill Weaver W:\_GEOTECH\030031 Weaver Slide Remediation\030031-002-01 Weaver Wall Design Comments l.tr.doc Page 2 ~~~~~ -- EXPIRES 6/22/03 ~' ~ ~~~~~®~~~ad~~F~~ !N-DEPTH PERSPECTIVE • -~ ~-: SUBSURFACE EXPLORATION AND GEOTECHNICAL ENGINEERING REPORT Weaver Property Tract 7, Dabob Cove 751 Beach Drive Jefferson County, Washington Prepared for: Bill Weaver Project No. 030031-002-01 • May 12, 2003 Aspect Consulting, LLC - ~, G@~ es A. Peterson " Senior Staff Engineer apeterson@aspectconsulting. com John L. Peterson, P.E. Associate Geotechnical Engineer jpeterson@aspectconsulting. com W:1 GEOTECH1030031 Weaver Slide Remediation\030031-002-01 Weaver Geotech Report.doc T+~ ~o ~o Q~~~1~X 31~~e~~A1 ~.~~9~rtfa o ~~~~Ll r4.-~~_- r,~l ~ ~ .~~- " e: 51Ca.~1~~~9.~ ~ r .~_~,s~*~.s~4` vV`v-r' .` e o ~ ~ ~tal;c i„l ~ol - ~~.. r - ,r _ - _ _-. ~ >< ~ ~ _ s-5r ~ ~~ - . - - ~' -,~~->"- - "fay - - ~F ,..rl~~rz ~+~-.~~:rd~fa~faZ~a~~~YSn~EC~~.+ieGio~~:ieiC~~ele~ile e • s - ~1C1"o' A n _ «- -- EXPIRES 5/22/p 3 -~ ASPECT CONSULTING Contents 1 Project Information ......................................................................................1 . -~ 1.1 Introduction ................................................................................................1 1.1.1 Purpose and.Scope .............................................................................. 1 ~J 1.1.2 Authorization ........................................................................................ 1 1.2 Project and Site Description .......................................................................1 1.2.1 Exploration Borings .............................................................................. 2 `-' 1.3 Subsurface Conditions ............................................................. 3 .................. 1.3.1 Stratigraphy ..........................................................................................3 1.3.2 Ground Water .......................................................................................4 ~y 2 Geologic Hazards and Mitigation ...............................................................5 2.1 Seismic Hazards and Recommended Mitigation ........................................ 5 2.1.1 Surficial Ground Rupture 2.1.2 Liquefaction ..........................................................................................5 2.1.3 Ground Motion ......................................................................................6 .................................. `~ 2.2 Landslide Hazards, Erosion Hazards and Mitigation 2.2.1 Rotational Type Landslides ............................................:..................... 6 2.2.2 Surficial or Debris Landslides ............................................................... 6 ............................................ '° 2.2.3 Landslide Hazard Mitigation ..........,.•......., 7 3 Design Recommendations ..........................................................................8 ......................................................................................... 8 -- 3.1 Site Preparation 3.1.1 Clearing and Filling ............................................................................... 8 3.1.2 Site Disturbance ................................:.................................................. 8 .. 3.2 Structural Fill ..............................................................................................8 3.3 Catchment Wall Recommendation .............................................................9 3.3.1 Retaining wall Criteria ........................................................................... 9 ~~ 3.3.2 Drainage Considerations .................................................................... 10 3.3.3 Retaining Wall Drains ......................................................................... 10 3.4 Project Design and Construction Monitoring .............................:.............. 10 Limitations ......................................................................................................... 12 PROJECT NO. 030031-002-01 •.MAY 12, 2003 ASPECT CONSULTING List of Figures 1 Vicinity Map 2 Site and Exploration Plan List of Appendices A Boring Logs PROJECT NO. 030031-002-01 • MAY 12, 2003 ASPECT CONSULTING 1 Project Information 1.1 Introduction This report presents the results of our subsurface exploration and geotechnical engineering study for the Weaver property located in the Dabob Cove subdivision on the west side of the Toandos peninsula in Jefferson County, Washington. The location of the property is presented on the Vicinity Map, Figure 1. The approximate locations of the subsurface explorations accomplished for this study are presented on the Site and ~. Exploration Plan, Figure 2. In the event that any changes in the nature, design or location of the structure are planned, the conclusions and recommendations contained in this report should be reviewed and verified, or modified, as necessary. 1.1. ~ Purpose and Scope The purpose of-this study was to provide geotechnical design recommendations to be utilized in the design and development of appropriate landslide remediation/mitigation at the property. Our study included reviewing available geologic literature, drilling six exploration borings, and performing geologic studies to assess the type, thickness, distribution and physical properties of the subsurface sediments and shallow ground water conditions. Geotechnical engineering studies were conducted to formulate retaining wall and drainage recommendations. This geotechnical report summarizes our ~y current field work and offers geologic hazard mitigation and geotechnical engineering recommendations based on our present understanding of the project. u x.7.2 Authorization Written authorization to proceed with this study was granted by Mr. Bill Weaver on March 5, 2003. Our study was accomplished in general accordance with our revised `-' scope of work proposal dated February 19, 2003. iv 1.2 Project and Site Description This report was completed with an understanding of the project based on a site plan provided by Mr. Weaver, and by our discussions with Mr. Weaver. At the time of our exploration program, the site was occupied by asingle-family, wood-frame residence that had been damaged by a landslide. Preliminary plans call for the installation of a landslide mitigation structure to protect the house that is planned to be remodeled. `a The property is situated on the waterfront along Dabob Bay. The site is an irregular rectangle in plan, with property dimensions of approximately 110 feet by approximately 140 feet. The upland portion of the site consisted of awest-facing hillside, with `-" inclinations ranging from 1H:1V (Horizontal:Vertical) to SH:1V. Overall vertical relief of the site was on the order of 100 feet. The low tide line of Dabob Bay formed the west property line. The site was bordered to the north by a private community waterfront PROJECT NO. 030031-002-01 • MAY 12, 2003 ASPECT CONSULTING park, to the east by existing homes and to the south by undeveloped, steeply sloping property. Vegetation around the existing residence consisted of grass lawn and landscaped shrubbery. Vegetation on the nearby natural slopes consisted primarily of berry bushes, fern and other low ground cover with scattered second- and third-growth fir trees. A landslide occurred in February 1998 above-the house that resulted in approximately 50 to 100 cubic yards of slide debris that accumulated east of the house between the house and the toe of the slope. The accumulated slide debris originated from a larger slide upslope of the house and did significant damage to the house structure below. Our field study included a surface reconnaissance of the slopes and drilling six exploration borings on the site. The borings were completed adjacent to the existing house and on the slope above the house. The various types of sediments as well as the depths where characteristics of the sediments changed are indicated on the exploration logs presented in the Appendix. The depths indicated on the logs where conditions changed may represent gradational variations between sediment types. Changes logged between sample intervals in our borings were interpreted. Soils were classified in general. accordance with American Society for Testing and Materials {ASTM) procedure D 2488, "Standard Practice for Description and Identification of Soils (Visual-Manual Procedure)". Our explorations were approximately located in the field by measuring from existing site features shown on the site plan provided Mr. Weaver. The conclusions and recommendations presented in this report are based on the six exploration borings completed for this study. Because of the nature of exploratory work below ground, extrapolation of subsurface conditions between field explorations is necessary. It should be noted that differing subsurface conditions may be present due to the random nature of deposition and the alteration of topography by past grading and/or filling. The nature and extent of variations between the field explorations may not become fully evident until construction. If variations are observed at that time, it maybe necessary to re-evaluate specific recommendations in this report and make appropriate changes. 7.2. ~ Exploration Borings The exploration borings were completed by advancing a 4-inch inside-diameter, hollow- stem auger with a tracked drill rig operated under subcontract to Aspect Consulting, LLC on April 1 and 2, 2003. During the drilling process, samples were obtained at generally 5-foot depth intervals. The borings were continuously observed and logged by a geological engineer from our firm. The exploration logs presented in the Appendix are based on the field logs, drilling action, and inspection of the samples secured. Disturbed but representative samples were obtained from the-soil borings using the Standard Penetration Test procedure in accordance with ASTM test method D-1.586. This test and sampling method consists of driving a standard 2-inch outside-diameter split-barrel sampler a distance of 18 inches into the soil with a 140-pound hammer free- falling adistance of 30 inches. The number of blows for each 6-inch interval is recorded and the number of blows required to drive the sampler the final 12 inches is used to calculate the. Standard Penetration Resistance ("N") or blow count. The resistance, or N- PROJECT NO. 030031-002-01 • MAY 12, 2003 ASPECT CONSULTING ._., value, provides a measure of the relative density of granular soils or the relative consistency of cohesive soils; these values are shown on the attached boring logs. The samples obtained from the split-barrel sampler were classified in the field and representative portions placed in watertight containers. The samples were then transported to our laboratory for further visual classification and laboratory testing, as necessary. 1.3 Subsurface Conditions Subsurface conditions at the project site were inferred from the field explorations accomplished for this study, visual reconnaissance of the site and vicinity, and review of applicable geologic literature. The site is characterized by a complex history of landslide activity both prehistoric and recent. In addition to the slide activity that has affected the J existing house, evidence of an adjacent, unrelated prehistoric slide was encountered in the southern borings EB-4 and EB-6. "In place" soils encountered below the most recent landslide activity consisted of prehistoric slide debris that was not active during the most `' recent landslide. The prehistoric slide activity is expressed in the overall topography of the drainage basin that makes up Dabob Cove. ~~ As shown on the boring logs, fill soils from road construction were encountered in borings EB-3, EB-4, and EB-6. The fill soils consisted primarily of loose, silty sand and gravel or sandy silt and gravel with variable amounts of organic material. Recent slide ,~ debris was encountered in EB-l, EB-2, EB-4, EB-5 and EB-6. The recent slide debris consisted primarily of very loose sand with varying amounts of silt and gravel, or soft, moist silt. All borings, except EB-1 and EB-2, encountered prehistoric slide debris below --, the recent slide debris. The older slide deposits consisted of both stiff to hard, silt and medium dense to dense sand. The older slide debris was underlain by medium dense to very dense brown sand in borings EB-1, EB-2, EB-3, and EB-5. These sediments were ~, interpreted as pre-Fraser age, non-glacial outwash sediments. These sediments were encountered to the total explored depths of 16.5 feet in EB-1 and EB-2, 31.5 feet in EB-3, 41 feet in EB-5. The following section presents more detailed subsurface information L organized from the upper (youngest) to the lower (oldest) sediment types. 7.3. ~ Stratigraphy Fill Road fill was encountered in borings EB-3; EB-4, and EB-6. Thickness was 3 feet in EB-4 and 4 feet in EB-3 and EB-6. As noted on the exploration logs, the fill soils ~~ consisted of loose silty sand and gravel, or very stiff sandy silt and gravel. The fill is the result of historic, as well as recent earthmoving and road construction for the access driveway to the residence and the community park. ,~ Recent Slide Deposits Recent slide deposits were encountered in borings EB-1, EB-2, EB-4, EB-5, and EB-6. .~ .Thickness was 3 feet in EB-1 and E2-2, and S feet in EB-6. As noted on the exploration logs, the recent slide deposits consisted primarily of very loose to loose sand with variable amounts of silt and gravel, and soft to very stiff silt. The slide debris ,J encountered in the upper several feet of EB-1 and EB-2 consisted of a localized debris PROJECT NO. 030031-002-01 • MAY 12, 2003 ASPECT CONSULTING flow that accumulated behind the existing house. The slide debris originated as a semi- intact block/blocks that translated and rotated down slope. The debris then cascaded over a "bench" formed by harder, prehistoric slide debris that remained in place. A combination of ground water seepage and availability of moving soil at the bench above the house and below. the access road resulted in the slurry-type debris flow that impacted the house. The upper, semi-intact slide mass moved approximately 5 feet vertically and approximately 10 feet horizontally before coming to rest in its current position. Older Slide Deposits Older slide deposits were encountered below the recent slide deposits in all borings, except EB-1, EB-2, and EB-3. These soils consisted of medium dense to very dense sand and medium stiff to hard silt. These sediments were deposited as a result of prehistoric landslide activity that occurred at an unknown time in the past. The sediments were disturbed but exhibited stronger general soil properties than the more recent slide debris above. The prehistoric slide debris is consistent with large-scale topographic features of the area that suggest repeated, .historic movement in the area. Characterization of the regional landslide activity and extent of the prehistoric slide debris was beyond the scope of study for slide mitigation related to the existing house. Pre-Fraser Non-Glacial Sediments Intact, pre-Fraser age, non-glacial outwash sand was encountered in all borings, except EB-4. This stratum continued to the entire depth explored in each boring where encountered. The soil consisted of medium dense to very dense, light brown sand with trace amounts of silt and gravel. This stratum can be seen in the hill behind the waterfront parking area north of the subject site. These sediments appeared to be in-place and not appear to have been effected by past slide activity. However, the contact with this geologic unit and the overlying prehistoric slide debris was not readily discernible from the collected samples. Due to the complexity of the landslide environment at the site, it is probable that portions of this unit have also been affected by prehistoric landslide activity. 7.3.2 .Ground Water Ground water was encountered in EB-4 and EB-6 at the time of our field study. The depth to ground water in the exploration borings was measured. at the time of drilling and ranged from approximately 11.5 feet in EB-4 to 15 feet in EB-6. At the time of drilling, seepage was noted in the roadway near the borings, and a shallow ravine with flowing water was located south of the property outside the inferred current slide area. The ground water levels in the area will likely respond rapidly to precipitation, due to the relatively high permeability of the sand encountered. Ground water seepage at the contact between the recent slide debris and the older underlying sediments was likely a primary driving factor for the recent slide activity. The January and February 1999 slides occurred after several months of prolonged heavy precipitation. Increased ground water volumes and seepage are a direct result of precipitation. In addition to precipitation rates and duration, fluctuations in ground water levels will occur due to the time of the year, site usage, irrigation tidal influence and other factors such as off-site usage and water supply systems in the area. Seepage may also occur at random depths and locations in the non-uniform fill soils. PROJECT N0.030031-002-01 • MAY 12, 2003 ~_, 2 Geologic Hazards and Mitigation ASPECT CONSULTING The following discussion of potential geologic hazards is based on the geologic, surface ~~ and ground water conditions observed. Specific hazards addressed include landslide seismic and erosion hazards. ~~ 2.1 Seismic Hazards and Recommended Mitigation Earthquakes regularly occur in the Puget Lowland. The vast majority of these events are small and usually not felt. However, large earthquakes do occur, as evidenced by the 1949, 7.2-magnitude event, the 1965, 6.5-magnitude event, and the recent 2001, 6.8- magnitude event. The 1949 earthquake appears to have been the largest in this area during recorded history. Generally, there are three types of potential geologic hazards associated with large seismic events at the site: 1) surficial ground rupture, 2) liquefaction, and 3} ground ~~ motion. The potential for each of these hazards to adversely impact the proposed project is discussed below. Seismically induced landslides are discussed under the landslide hazard portion of this report. 2. ~. ~ surficial Ground Rupture The nearest known, active fault trace to the project is the Seattle fault, although other ' north-south trending fault structures are likely located in the Hood Canal region that have not been fully characterized. Recent. studies by the. U.S. Geological Survey (e.g., Johnson et al., 1994, Origin and Evolution of the Seattle Fault and Seattle Basin, Washington, `~ Geology, v. 22, p.Zl-74 and Johnson et al., 1999, Active Tectonics of the Seattle Fault and Central Puget Sound Washington-Implications for Earthquake Hazards, Geological Society of America Bulletin, July 1999, v. 111, n. 7, p. 1042-1053) suggest that the trace .v_ of an east-west trending thrust fault may project approximately 12 miles southeast of the project site in the vicinity of Bremerton. The trace of this-fault can be seen south of Blakely Harbor on Bainbridge Island. According to the U.S. Geological Survey studies, the latest movement of this fault was about 1,100 years ago, resulting in about 20 feet of surficial displacement. This displacement can presently be seen in the form of raised, wave-cut beach terraces along Alki Point in West Seattle and Restoration Point at the ~- south end of Bainbridge Island. The recurrence interval of movement along these fault systems is still unknown, although it is hypothesized to be on the order of several thousand years. Due to the suspected long recurrence interval, the distance from the e fault, the potential for surficial ground rupture as a result of faulting is considered low during the expected life of the structure. ~~ 2.1.2 Liquefaction The recent and prehistoric landslide deposits in EB-4, EB-5, and EB-6 have a moderate to high potential for liquefaction (the temporary transformation of sediment into a fluid • ° mass during seismically induced shaking) due to their disturbed, loose state, relatively PROJECT NO. 030031-002-01 • MAY 12, 2003 ASPECT CONSULTING low silt content and potential for ground water seepage and accumulation. Elsewhere, the density and/or silt content of the soils are sufficiently high that liquefaction potential is low. Liquefaction hazard increases in loose sandy soils with increases in ground water levels. 2.1.3 Ground Motion Based on the site stratigraphy encountered and visual reconnaissance of the site, it is our opinion that earthquake damage to the proposed structure, if founded on a suitable bearing stratum, would likely be caused by the intensity and magnitude of horizontal ground acceleration associated with the event. Structural design of the building should follow current UBC standards and take into consideration stress caused by seismically induced earth shaking. The site would be characterized in the UBC by a Seismic Zone Factor Z = 0.30 and Soil Profile = S~. 2.2 Landslide Hazards, Erosion Hazards and Mitigation The landslide that has affected the site was a rotationaUtranslational landslide and a resulting debris flow. Discussions of these types of slides are presented below. 2.2.1 Rotational Type Landslides The majority of the slide above the house consisted of asemi-intact, rotationaUtranslational landslide. While not observed directly in our explorations, the -slide likely moved along a gently curved or irregular shear surface. The slide transported approximately 500 to 1,000 cubic yards over the bench above the house, where it was deposited as a debris flow against the house. The majority of the slide remained semi- - intact and stopped shortly after initial movement. The active, semi-intact slide debris is at risk for moving in the future under similar triggering conditions. The overall topography of the Dabob Cove property consists of a prominent ravine with a series of relatively flat benches indicative of a large, deep-seated rotational landslide during prehistoric time. With the exception of the current slide area and resulting damage, we saw no evidence of active rotational landslides elsewhere on the site. 2.2.2 Surficial or Debris Landslides The slide debris that impacted the house was essentially a surficial slide debris as the toe of the upper larger slide moved over the bench behind the house. Surficial slides commonly result from a significant increase in the moisture content within the upper colluvium layer on slopes. Increased moisture typically results from periods of extended, heavy precipitation, ground water seepage or concentrated surface water discharge onto a slope. As moisture increases, unit weight of the soil increases and shear strength within the soil decreases. Once shear strength is reduced relative to the force of gravity acting on the soil, rapid downslope movement of the affected soil and underlying vegetation occurs. The slide at the site was not a typical surficial landslide in that the source material was not the typical near-surface weathered soil layer. The impact of the fast- moving slide debris caused the structural damage to the house and accumulated to a depth of approximately 8 feet against the east wall of the house..The slide debris then flowed south and west around the house where it came to rest. PROJECT NO. 030031-002-01 • MAY 12, 2003 ASPECT CONSULTING `. Given the available source of slide debris above the house, the risk of future surficial landslides and future damage to structures is high. Additionally, the risk of damage to site slopes and the existing access road and driveway to the beach park is also considered ,_, high. 2.2.3_ Landslide Hazard Mitigation V The risk of damage to the existing residence structure from landslide activity can be mitigated through installation of a passive retaining wall structure or catchment wall in conjunction with partial regarding of the area and performing drainage improvements. - v An active retaining wall system upslope of the house was considered but due to the complexity of the landslide terrain, and resulting length and depth of a proposed active restraint retaining wall, this alternative was not cost-effective and subsequently was not ~-~ considered feasible. Erosion Hazards and Mitigation • ~ The slope above the existing residence, where most of the construction activity will take place, possesses high erosion potential. To mitigate and reduce the erosion hazard and potential for off-site sediment transport, we recommend the following: 1. Surface water should not be allowed to flow across the site over unprotected surfaces. 2. All storm water from the impermeable surfaces, including roadways and roofs, `y should be tightlined to approved drainage areas or storm water conveyance system and not be directed into the site surface soils. ,~ 3. Areas stripped of natural vegetation during construction should be replanted as soon as possible or otherwise protected. 4. .Silt fences should be placed and maintained around the perimeter of the proposed '~ construction areas throughout the entire construction phase of the project until permanent landscaping and storm water transport facilities have been installed.. ~. 5. Soils that are to be reused around the site should be stared in such a manner as to reduce erosion from the stockpile. Protective measures may include, but are not necessarily limited to; covering with plastic sheeting, the use of low stockpiles in flat r_, areas, or the use of hay bales and/or additional silt fences around pile perimeters.. 6. Provisions of the Jefferson County Grading and Drainage standards should be used on the site. PROJECT NO. 030031-002-01 • MAY 12, 2003 ASPECT CONSULTING 3 Design Recommendations 3.1 Site Preparation Those portions of the existing house and foundations that are not part of the proposed project should be removed. Any buried utilities that are to be abandoned should be removed or relocated if they are under building areas. The resulting depressions should be backfilled with structural fill, as discussed under Section 3.2, Structural Fill. 3.9.1 Clearing and Filling Site preparation of planned building and driveway areas should include removal of all trees, brush, debris and any other deleterious material. Organic topsoil should be removed and the remaining roots grubbed. Areas where loose surficial soils exist due to grubbing operations should be considered as fill to the depth of disturbance and treated as subsequently recommended for structural fill placement. The upper 12 inches of the soils that will be exposed, as a result of stripping and grubbing should be recompacted to a firm and non-yielding:. condition. This recompacted fill will serve primarily as a working surface during construction. Loose to very loose sandy and gravelly soils were present in the existing surficial debris on the site. Because the densities of these soils are variable, random soft pockets may exist-and the depth and extent of stripping can best be determined iri the field by the geotechnical engineer. 3.1.2 Site Disturbance The on-site fill and slide debris deposit soils contain areas with a high percentage of fine- grainedmaterial (silt) or organic matter, which makes them moisture-sensitive (subject to disturbance when wet). The contractor must use care during site preparation and excavation operations so that the underlying soils are not softened. If disturbance occurs, the softened soils should be removed and the area brought to grade with structural fill. Consideration should be given to protecting access and staging areas with an appropriate cover of crushed rock. 3.2 Structural Fill Structural fill will be necessary to establish desired grades, particularly in areas where unsuitable. loose soils have been removed, such as in the vicinity of the proposed retaining wall and residence construction. All references to structural fill in this report refer to subgrade preparation, fill type, placement and compaction of materials as discussed in this section. Different percentages of compaction maybe specified in other sections of this report for other purposes. PROJECT NO. 030031-002-01 • MAY 12, 2003 ASPECT CONSULTING ~. After overexcavation/stripping have been completed, the upper 12 inches of exposed ground should be recompacted to 90 percent of the Modified Proctor maximum density using ASTM D 1557 as the standard. If the subgrade contains too much moisture, adequate recompaction maybe difficult or impossible to obtain and should probably not be attempted. In lieu of recompaction, the area to receive fill can be blanketed with washed rock or quarry spalls to act as a capillary break between the new fill and the wet. ~, subgrade. The use of washed rock or quarry spalls should be considered for areas where filling activity may take place where periodic and/or cyclic inundation may occur during high tide. -° After recompaction of the exposed ground is tested and approved, or afree-draining rock course is laid, structural fill maybe placed to attain desired grades. Structural fill is defined as non-organic soil, acceptable to the geotechnical engineer, placed in maximum ` " 8-inch-thick, loose, horizontal lifts with each lift being compacted to 95 percent of the Modified Proctor maximum density using ASTM D-1557 as the standard. The contractor should note that any proposed fill soils must be evaluated by Aspect Consulting prior to their use in fills. This would require that we have a sample of the materia148 hours in advance to perform a Proctor test and determine its field compaction ,J standard. Soils in which the amount offine-grained material (smaller than No. 200 sieve) is greater than approximately 5 percent (measured on the minus No. 4 sieve size) should be considered moisture-sensitive. Use ofmoisture-sensitive soil in structural fills should be limited to favorable, dry weather conditions. The on-site fill and slide debris deposit soils contained variable amounts of silt, and while considered moisture-sensitive, maybe suitable for use as structural fill, provided they can be demonstrated to compact and perform well. If fill is placed during wet weather or if proper compaction cannot be obtained, a select import material consisting offree-draining gravel and/or sand should be used. Free-draining fill consists ofnon-organic soil with the amount offine-grained ` material limited to 5 percent by weight when measured on the minus No. 4 sieve fraction. 3.3 Catchment Wall Recommendation ~' Based on the slope and access conditions at the site, a catchment wall built along the east wall of the proposed remodeled house will provide protection against future, similar landslide activity. This section of the report presents preliminary design considerations -' and criteria that should be considered in the design of the retaining wall. 3.3.1 Retaining wall Criteria . `' The proposed location of the catchment wall at the back wall of the house would likely result in the wall footings bearing on the in-situ soils encountered in the area in our exploration borings. -For design purposes, we recommend a maximum allowable soil ~"' bearing capacity of 3,000 pounds per square foot (psf} on properly prepared subgrade soil. Subgrade preparation and backfill should be performed in accordance with the recommendations presented in the Section 3.2, Structural Fill. In order to minimize the retaining wall height and required lateral design forces, we recommend incorporating a minimum 10-foot-wide, relatively flat area upslope of the wall, between the wall and the toe of the slope, to act as a run-out/catchment area. The PROJECT NO. 030031-002-01 • MAY 12, 2003 ASPECT CONSULTING run-out area will also facilitate the removal of debris when necessary. Utilizing a 10- foot-wide run-out/catchment area, the wall should extend a minimum of 12 feet above grade level in order to accommodate the volume of material capable of being mobilized. The exposed portions of walls subject to impact loading should be designed to withstand -a dynamic earth pressure of 90 pounds per cubic foot (pcf) to resist the impact of the landslide; and a static equivalent fluid pressure of 60 pcf over the full height of the wall. These two loads would not occur simultaneously.. Apassive resistive footing pressure of 350 pcf can be utilized if the concrete is poured "neat" against the native soil. If re- compacted fill is used far resistance, a value of 250 pcf can be utilized. Any material which accumulates against the wall must be removed as soon as possible to provide continuing protection-for any subsequent even#. 3.3.2 Drainage Considerations Ground water occurrence and surface drainage within the landslide area is complex due to repeated movement and repositioning of potential confining layers. Ground water drainage is not considered an alternative due to the anticipated high cost and limitations on the potential success of installing a ground water drainage system in the slide area. However, to further reduce the effect occurrence of landslide activity, we recommend a comprehensive surface water drainage plan be implemented for the roadway and the slide area in general. The existing culvert below the access road must be.maintained in proper operating conditions. Under no circumstances should this culvert be allowed to overflow. Drainage improvements should include redirecting surface water from the upper road area above the slide to a suitable discharge point near the beach, adequate storm flow ditches on the upslope side of the road and erosion control and energy dissipating outfall structures where appropriate. French drains may also be considered above the slide area to the east to reduce the amount of near-surface ground water flow from entering the slide area. 3.3.3 Retaining Wall Drains The retaining wall should be provided with drainage at the ground surface. Drains should consist of grading the soil behind the retaining wall with sufficient gradient to allow gravity discharge from the center of the wall out to both ends..Roof and surface runoff fiom the residence should not discharge into the foundation drain system but should be handled by a separate, rigid tightline system routed to an appropriate discharge point. In planning, exterior grades adjacent to walls should be sloped downward away from the structure to achieve surface drainage. If grades do not allow gravity flow of the foundation drains into the existing system, a sump maybe required. 3.4 Project Design and Construction Monitoring At the time of this report, site grading, structural plans, and construction methods had not been finalized. We are available to provide additional geotechnical consultation as the project design develops and possibly changes from that upon which this report is based. We recommend that Aspect Consulting perform a geotechnical review of the plans prior to final design completion. In this way, our earthwork and foundation recommendations maybe properly interpreted and implemented in the design. 10 PROJECT N0.030031-002-01 • MAY 12, 2003 ASPECT CONSULTING We are also available to provide geotechnical engineering and monitoring services during construction. The integrity of the foundation depends on proper site preparation and construction procedures. In addition, engineering decisions may have to be made in the field in the event that variations in subsurface conditions become apparent. Construction monitoring services are not part of this current scope of work. If these services are desired, please contact us and we will prepare a cost proposal. PROJECT NO. 030031-002-01 • MAY 12, 2003 11 ASPECT CONSULTING Limitations Work for this project was performed and this report prepared in accordance with generally accepted professional practices for the nature-and conditions of work completed in the same or similar localities, at the time the work was performed. It is intended for the exclusive use of Bill Weaver for specific application to the referenced property. This report does not represent a legal opinion. -No other warranty, expressed or implied, is made. 12 PROJECT NO. 030031-002-01 • MAY 12, 2003 '~. 'v'J :t:-vlr f. ,5. = f~+r 13b1 v~(1'l Sz I ~l t ~{i•'~4~'S1t j~ijl ~5~?I•~} `]1 ?`S>s `~. -, ? ~ !r r1 ~~ ~, 1f'6/~ I :.\~\a ~~l . -cf•, ~ ~'~~j,,. ~r ~~ F s ~ 1 ~r ~ ~.~ ~s _ r ~ 1 F3'},~',~'` ~~_ _ ' <,`f~' ~~,.- ~ „•~~,'.,~~r"/`/~ r ~. ~. , !t ~}4 CSI ~~ 1I r1~"~. jlff i i g ~~. ~ ~ r _ } r.r`I~/`~/1 / ril,f .~~ ~ '~ ~tt~>M I~l;_,. ~. 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I' 1r~-~~LfTic~;~ ~ i f `~~1~TJ'~I~, 3 ~r yvc1 c:; 'i• 11~ ( t/r r u`,' ! ~ ~ Li.r~r~ tea' ~ ;~ 1 ` ` ~ - - J ~.IE s~ l7lire ~lif l.!i ~r~~~~~ ~~, ~ S r~~~~ ~ ~fi5.' s`~~ ~5.~ ~~ r! .. .. .I i`~ in51 r \~~f`, t 1r~ ~ I iil ~~~ ~ I I r, I l~ I `t ..~d ` F r/rr~!' r \\ ,~r ~ ,. eoa ~ I r F3' ya'>~. 7/.lS~ 1~ rl' r ,.~ ~r~ / j;`l ' 15::1 r ` 9s ~I~~i :/ +~.-Fi I~ _ F ~~ r, ,g,1 _ Vicinity Map ~ 05!12/03 PROJECT NO. lASp~Cfconsulting ~"e RFC 030031 ® IN-DEPTH PERSPECTIVE Weaver Landslide Remediation "" "MS FIGI/RENO. 179 MaErone Lane Nonh 811 Firet Avenue fW80 Bainhriege l8lanO,WA98110 Searle, WA 98106 RevREe9Y: (108, 790A370 12oe>-~~-~~3 Dabob Cove - Beach Park Parking Area Hood Canal Toe of ~' Beach Slope (1 (1~- i~ Top of Beach Slope Existing Damaged House Surficial ~' Slide Debris Upland Bluff ~RaL~ I ~ ~~e ~ Existing Houses age Exposed Headscarp `_= f~ EBB /' Y ~ C , f~~~ `~ :, ~ %lI ~~ ~ _. ~ Top of ~ /~'' f~: ` ~ `~\` ~ `~. ~ Slope ~~ , ~ ~~_, ` EB-4\ Translational/Rotatio I Slide Area ~ Pre-slide r Road Location / ~ Existing ; Access Road Legend ;~ ~ >~ EB-1 Boring Location g 4 , ` ` c Slide Area _ a ' so ,zo 1~.-_ Feet Site and ExplorationPlan °"'~ o5„z,o3 PROJECT NO. ~ ~Aspectconsulting =~°aR 030031 ~ IN-DEPTH PERSPECTIVE FC 179 M E L N th 811 Fi t A IXd80 Weaver Landslide Remediation a, Ms FIGURE NO. ~ a rone ane or rs venue Bainbrioge IsIaM, WA 99110 SeaWe, WA 98706 (200)780.9370 ,~06,.~29-766 Dabob Cove, Washington - se°ev. _ - ~ < = f i Aspect consultin Ex !oration Lo g Proj t N b E l ti !N-DEPTH PEHSPECTlVE ec um er xp ora on Number Sheet 030031 EB-1 1 of 1 Project Name Weaver SIId2 RemediatiOn Ground Surface Elevation (ft) 0 Location Dabob Bay, WA; Datum Driller/Equipment Davies Drilling/Hollow stem auger, 8" OD, 3-1/4" ID Date Start/Finish 4!1/2003-4/1/2003 Hammer Weight/Drop 140 Ib 130" Hole Diameter (in) 8" This log is part of the report prepared by Aspect Consulting, LLC for the ~ o named protect and should be read together only with that report for ~ ~, °' Y S ~ a E ~ n~ `` ~ complete interpretation. This summary applies only to the location of this exploration and at the time of exploration. Subsurface conditions ma chan e at this location ith th f ti Th d E ~ ~ ° ~' oi BIOWS/FOOL ~ p T ~ , C9 cn y g w e passage o me. e ata presented are a simplification of actual conditions encountered. ;v - o o ~ o m DESCRIPTION a 0 10 20 3a 40 S-1 RECENT SLIDE DEBRIS 1 3" Very.loose; wet, brown, fine to medium SAND (SP) 1 ~ Soft, moist, gray SILT, few sand, trace organics (ML) 2~ FRASIER PRE-VASHON OUTWASH 5'-6' Alternating hard-dense, moist, gray an d brown SILT and SAND; some - _ _ horizontal laminations in silt (~P%ML) ~ - - 6'-6.5"Hard, moist, reddish-brown SANDY SILT, trace gravel (ML) - 5 S-2 25 50.5 50+ Very dense moist, light brown fine to coarse SAND, trace fine gravel; sand predomiharifly`frne to medium "(SP) e- Aspectconsul#in EX (oration Lo g Project N b w-DePrxaeasPECnve um er Exploration Number Sheet 030031 EB-2 1 of 1 Project Name- 1Neaver Slide Remedia#lOn Ground Surface Elevation (ft) 0 Location Dabob Bay, WA; Datum Driller/Equipment Davies DrillinglHollow stem auger 8" OD 3-1/4" ID Date Start/Fimsh 4/1!2003-4/1/2003 Hammer Weight/Drop 140 Ib / 30" Hole Diameter (in) 8" ~ This log is part of the report prepared by Aspect Consulting LLC for the named protect and should be read together only with that report for ~ ~ o ~ complete interpretation. This summary applies only to the location of °' E S o. E a `` > this exploration and at the time of exploration. Subsurface conditions h t hi l t ~ ~' 3 810WS/FOOL ~ , v~ may c ange a t s ocation with the passage of time. The data - ~~ pio o T ~ c9 resented are a sim lification of actual conditions encountered. ~ ~ m DESCRIPTION a o 10 20 30 40 S-1 RECENT SLIDE DEBRIS 1 Soft, moist, gray and brown mottled SANDY SILT with GRAVEL, trace 2 ~ organics (ML) ;~'. FRASIER PRE-VASHON OUTWASH Dense, moist, light brown fineSAND. trace coarse gravel, trace silt: (SP)~_, 5 S-2 ~~ ~ ~.~ -. 14 19 21 - 10 S-3 ~- - •No gravel. faint horizontal laminations 10 Aspect COi SU~tlfi Ex loration Lo 1 g Project N mbe E l ti n b IN-DEPTH PERSPECTIVE u r xp ora Num o er Sheet 030031 :` EB-3`- 1 of 2 Project Name Weaver Sllde RemedlatlOn Ground Surface Elevation (ft} 15 Location Dabob Bay, WA; Datum Driller/Equipment Davies Drilling/Hollow stem auger, 8" OD, 3-1/4" ID .Date Start/Finish 4/1/2003-4/1/2003 Hammer Weight/Drop 140 Ib 130" Hole Diameter (in) 8" This log is part of the report prepared by Aspect Consulting, LLC for the ~ N o _ named protect and should be read together only with that report for o ~ w S ~, a E L o n ~ t° complete interpretation. This summary applies only to the location of this exploration and at the time of exploration. Subsurface conditions ma cha at thi l ti ith th f ti Th d E ~ ~ ° ~' o~ BIOWS/FOOt ~ o T ~ >, O ~ y nge s oca on w e passage o me. e ata resented are a simplification of actual conditions encountered. ;v - o m° ~ m DESCRIPTION a o 10 20 30 40 S-1 ~ ROAD FiLL/RECENT_SLIDE Q,EBRI_S ~ • 12 Very stiff, moist, brown and gray SANDY SILT and GRAVEL (MLG) 14 • , FRASIER PRE-VASHON OUTWASH Medium dense, moist-light brown fine to medium SAND, trace silt; faint 5 S-2 :. horizontal laminae;`=(S~}' 6 11 16 ~ 10 S-3 •Dense $ 14 21 ~ 15 S-4 •Wet at 15.5', sand predominantly fine 18 - 26 ~ Sampl er Typ e (ST): Lab tests:. No Recovery A - Atterberg Limit Logged by: JAP P -Permeability ® 2" OD Split Spoon Sampler M -Moisture Approved by: RFC 3" OD Dames & Moore Split-Spoon Ring Sampler ~ Static Water Level Figure No. A - 4 • WatPr I avel at times of ririllinn fATf)1 Aspectconsuttin Ex loration Lo g Pr t N b E IN-DEPTH PERSPEC77VE- OJec Um er xploration Number Sheet 030031 - EB-3 2 of 2 Project Name Weaver Slide RemediatlOft ~" Ground Surface Elevation (ft) 15 Location Dabob Bay, WA; Datum DrilledEquipment Davies.Drilling/Hollow stem auger, 8" OD 3-114" ID Date StartJFm~sh 4/1/2003-4/1/2003 Hammer WeighUDrbp 140 Ib / 30" Hole Diameter (in) 8" ~ ~ r o This log is part of the report prepared by Aspect Consulting, LLC for the named pro ect and should be read together only with that report for l t i! t ti Thi ~ ~ °' , n ;_ o.Q `` ~ comp e in e erpre on. a s summary applies only to the location of this exploration and at the time of exploration. Subsurface conditions ma chan e at this location with the f ti Th d t E t ~ $ ~' oi BIOWS/Foot o T ~ , c7 ~ y g passage o me. e a a resented are a sim lification of actual conditions encountered. ~- ~ ~ o m DESCRIPTIOiV a ~ o io 20 3o ao S-5 .~ •Very dense, moist, light gray-brown, fine to medium, horizontat orange banding 21'-21.5' 21 33 50+ - 25 S-6 :' •Sand predominantly fine ~1 24 50 50+ - 30 S-7 ~~ ~ ~ 11 30 39 50+ a N , _- QI N a c~ w J w 3 ~, ~ 0 U a m a Aspect lti Ex location Lo consu ng Project N b E l ti N b - IN-DEPTH PERSPECTIVE um er xp ora on um er _ Sheet ~JQQ3~ _'. EB_A. 't ~ ~f 2 Project Name Weaver Slide Rernediation - Ground Surface Elevation (ft) 25 Location Dabob Bay, WA; Datum DrillerlEquipment Davies Drilling/Hollow stem auger, 8" OD, 3-1/4" ID Date Start/Finish 4/1/2003-4/1/2003 Hammer Weight/Drop 140 Ib 130" Hole Diameter (in) 8" This log is part of the report prepared by Aspect Consulting, LLC for the ~ ~ o named pro ect and should be read together only with that report for t ~, ° ' ~ ~ ~ complete in erpretation. This summary applies only to the location of - - E ' S o- o- ~ ~ this exploration and at the time of exploration. Subsurface conditions ma chan e at this lo ati ith th f ti Th t d L ~ ' ° ~ o~ BIOWS/FOOL p T ~ ~ >, c7 rn g y on w c e passage o me. e a a resented are a simplification of actual conditions encountered. ~ - o m° ~ m DESCRIPTION Q o 10 20 30 40 S-1 . - 0 ROAD FILL _-----~-- --- 7 ° Loose, moist to wet, brown and black SILTY SAND and GRAVEL 2 0 (SM-GM) 0 0 RECENT SLIDE DEBRIS Loose, moist, mottled gray, brown and tan SILTY SAND, few organics (SM) 5 S-2 a _ . . . 6 (a PREHISTORIC SLIDE DEBRIS Loose, wet, light brown fine to medium SAND (SP) 10 S-3 ._ 2 4 O 15 S-4 •Medium dense, light gray-brown 4 6 9 ~ Samol er Tvoe fSTI: Lab tests: No Recovery A - Atterberg Limit P -Permeability ® 2" OD Split Spoon Sampler M -Moisture 3" OD Dames & Moore Split-Spoon Ring Sampler ~ Static Water Level 1 Water Level at tim Logged by: JAP Approved by: RFC Figure No. A - 5 Aspect ~ti Ex !oration Lo colsu f,9 Pr j t N b E l ti N b 1N-DEPTH PERSPECTIVE o ec um er xp ora on um er Sheet 030031 EB-4 2 Qf 2 Project Name Weaver Sljde RemedlatlOn Ground Surface Elevation (ft) 25 Location Dabob Bay, WA; Datum Driller/Equipment Davies Drilling/Hollow stem auger, 8" OD, 3-1/4" ID Date StarUFinish 4/1/2003-4/1!2003 Hammer WeighUDrop 140 Ib / 30" Hofe Diameter (in) 8" This log is part of the report prepared by Aspect Consulting, LLC for the d h l ~ ~ ~ L o named protect an s ou d be read together only with that report for complete interpretation This summary applies only to the location of ~, °-' E w n o. ~ `` . this exploration and at the time of exploration. Subsurface conditions h thi l i i t ~ a ~' o BIoWS/Foot o T E >, may c ange at s ocat on w th the passage of time. The data - ~ ~ C7 cn resented are a sim lification of actual conditions encountered. o ~ m DESCRIPTION _ Q o 10 20 30 40 S-5 .~ 13 16 25 S-6 :..~.• 3 13 ~ Very stiff, moist, gray and light brown mottled SILT {ML} 30 S_~ .Gray, blocky, fractured ~ 13 Bottom of boring at 31.5'. Backfilled with bentonite chips and cuttings. 35 Sampler Typ e {ST ): Lab tests: 0 N Oi .~ a c~ ui a J LLl Q 3 ® No Recovery A - Atterberg Limit Logged by: JAP P -Permeability ® 2" OD Split Spoon Sampler M -Moisture Approved by: RFC 3" OD Dames & Moore Split-Spoon Ring Sampler ~ Static Water Level Figure No. A - 5 ~ Water I eval at Lima of ririllinn lATI~I ect t i As EX /oration Lo consu t ng p Pro t N b Ex loratio Numb Sh t !N-DEPTH PERSPECTIVE jec um er 030031 p n. er -~ EB-5 ; ee 1 of 2 Project Name Weaver Slide ReftledlatlOft Ground Surface Elevation (ft) 37 Location Dabob Bay, WA; Datum Driller/Equipment Davies Drilling/Hollow stem auger, 8" OD, 3-1/4" ID Date Start/Finish 4/1/2003-4/212003 Hammer Weight/Drop 140 Ib / 30" Hole Diameter (in) 8" This log is part of the report prepared by Aspect Consulting, LLC for the d h ld b d t th l ith th t rt f d ~ ~ r o repo or ou e rea oge er on y w a name protect an s complete interpretation. This summary applies only to the location of ~ °-' E Y a a ~ ` this exploration and at the time of exp/oration. Subsurface conditions t hi l i i h f i Th d L o 3= BIOWS/Foot ~ S E 0 >, may change at t s ocat on w t the passage o t me. e a a ~a o~ p T ~ c7 cn presented are a simplification of actual conditions encountered. m° ~ m DESCRIPTION Q o 10 20 30 40 S-1 RECENT SLIDE DEBRIS - • - -- - 1 Very loose, moist, light gray fine to medium SAND (SP)`; g S-2 -. = : 2 1 2 ~. ~ Soft, moist, light gray-brown SILT, trace sand; blocky (ML) 10 S-3 2 • 1 15 S_4 •Light brown grading to gray, fractured, slickensides 2 • t 20 S-5 PREHISTORIC SLIDE DEBRIS 14 • Hard, moist, olive gray to gray SILT; fractures, slickensides (ML) 23 0 0 N N 7 a c~ ui 0 J Q U1 3 0 m U w a Sampler Type (ST): ~aD ~C~~~• © No Recovery A - Atterberg Limit Logged by: JAP P -Permeability 2" OD Split Spoon Sampler M -Moisture Approved by: RFC 3" OD Dames & Moore Split-Spoon Ring Sampler ~ Static Water Level Figure No. A - 6 1 Water Level at time of drilling (ATD) Aspect consultin Ex -Ioration Lo g Project N b E l ti N b IN-DEPTH f EfiSPECTIVE um er xp ora on um er Sheet 03003 ~ EB-5 2 Of 2 Project Name Weaver Slide Remediation Ground Surface Elevation (ft) 3 7 _ Location Dabob Bay, WA; Datum DrilledEquipment Davies Drilling/Hollow stem auger, 8" OD, 3-1/4" ID Date Start/Flnlsh 4/1/2003-4/2/2003 Hammer Weight/Drop 140 Ib / 30" Hole Diameter (in) 8" This log is part of the report prepared by Aspect Consulting, LLC for the d t d h l b ~ ~ ~ L o ~ name pro ec an s ou d e read together only with that report for complete Interpretation. This summary applies only to the location of ~, °' E S a a `` ~ this exploration and at the time of exp/oration. Subsurface conditions ma chan e at this location with th f ti Th d r ~ ° ~' o~ BIOWS/FOOL o ~ N , C7 ~n y g e passage o me. e ata resented are a sim lification of actual conditions encountered. ~- o o ~ m DESCRIPTION a o 10 20 30 40 S-6 •Gray - - FRASIER PRE-VASHON OUTWASH - Very dense, moist, light brown fine to medium sand (SP) 30 S-7 ~~ - ~~ 50+ „ 35 S-8 ~= - •Horizontallaminae 26 50+ ~„ Bottom of boring at 41'. Backfilled with bentonite chips and cuttings to 10'. Placed 1.25" PVC 10-slot screen 10'-5', 1.25" PVC blank 5'-surface. 8-12 silica sand filter pack 11'-4'. Cuttings to surface. 45 Sampl er Type (ST ): La tests: ® No Recovery A - Atterberg Limit P -Permeability Logged by: JAP ® 2" OD Split Spoon Sampler M -Moisture Approved by: RFC 3" OD Dames & Moore Split-Spoon Ring Sampler ~ Static Water Level Figure No. A - 6 ~ 1A/for 1 oval ~+ fimo of rl rillinn /6Tr11 a m a c~ ui o J w 3 0 U w a ~_ a i ti Aspect Ex location Lo . consu ng Project N mb r E l ti N b IN-DEPTH PERSPECTIVE u e 030031 xp ora on um er EB-6 -: Sheet 1 of 2 Project Name Weaver Sllde Remedlatl011 Ground Surface Elevation (ft) 31 Location Dabob Bay, WA; Datum Driller/Equipment Davies Drilling/Hollow stem auger, 8" OD, 3-1/4" ID Date Start/Finish 4/212003-4/2/2003 Hammer Weight/Drop 140 Ib / 30" Hole Diameter (in) 8" This log is part of the report prepared by Aspect Consulting, LLC for the ~ o named protJect and should be read together only with that report for ti l t f t Thi li l h l i f ~ ° ' ~ n. L a ~ ` comp e e m erpre a on. s summary app es on y to t e ocat on o this exploration and at the time of explocation. Subsurface conditions - E t o 3, BIOWS/FOOL ~ S E 0 >, may change at this location with the passage of time. The data ~a o~ p T ~ C~ cn resented are a simplification of actual conditions encountered. m° ~ m DESCRIPTION a 0 10 20 30 40 S-1 0 FILL • 4 ° Loose, moist, light brown SILTY SAND with GRAVEL; roots (SM-GM} 3 Q __ _ _.. _,,-.___. 0 0 PREHISTORIC SLIDE DEBRIS 5 Medium dense, moist, light gray-brown, fine to medium SAND, trace silt 5 14 .J _ ~ l~ _ Very stiff, moist, light gray-brown SILT;, blocky, fractured (ML) 10 S 3 7. 13 16 FRASIER PRE-VASHON OUTWASH Dense, wet, light gray, fine to medium SAND (SP) 15 S-4 15 16 20 S_5 •Medium dense, trace gravel : _ 16 15 • Sampl er Ty pe (ST): Lab tests: ~j No Recovery A - Atterberg omit Logged by: JAP P -Permeability ® 2" OD Split Spoon Sampler M -Moisture Approved by: RFC 3" OD Dames & Moore Split-Spoon Ring Sampler ~ Static Water Level Figure No. A _ 7 ~ Water Level at time of rlrillina (ATD1 Aspect c n utti Ex loration Lo o s n g project N mb E l ti N b IN-DEPTH PERSPECTIVE u er 030031 ora on xp um er EB-6 Sheet 2 of 2 project game Weaver Slide Remediation Ground Surface Elevation (ft) 31 Location Dabob Bay, WA; Datum Driller/Equipment Davies Drilling/Hollow stem auger, 8" OD, 3-1l4" ID Date Start/Finish 4/212003-4/2/2003 Hammer Weight/Drop 140 Ib / 30" Hole Diameter (in) 8" This log is part of the report prepared by Aspect Consulting, LLC for the d h l ~ o named protect an s ou d be read together only with that repor# for l t i i Thi l ~ ° ' ~ ~ t - comp e e nterpretat on. s summary app ies only to the location of - E ~ n a ~ ~ this exploration and at the time of exploration. Subsurface conditions t ~ ° ' om BIOWS/FOOL o _ T E >, may change at this location with the passage of time. -The data ;v- o ~ c9 cn resented are a sim I~cation of actual conditions encountered. ~ ~ m DESCRIPTION a o 10 20 30 40 S-6 := •Very dense 50+ 21 • 30 30 S-7 ~ .:~: ~ ~ •Dense ~ 9 21 25 ~ Very stiff, moist, olive grading to gray SILT, trace sand; silt blocky, fractured (ML) 35 S-8 9 18 40 S-9 •Gray 0 ~ 13 Bottom of boring at 41.5'. Backfilled with cuttings to 33'. Placed 1.25" PVC blink 40'-32', 1.25" PVC 10-slot screen 32'-22', 1.25" PVC blank 22'-surface. 8-12 silica sand filter pack 33'-19'. Bentonite pellets and chips.l9'-16.5'. Cuttings to surface. 45 Samoler Tvoe fST ): La tests: ® No Recovery A - Atterberg Limit Lagged by: JAP P -Permeability ® 2" OD Split Spoon Sampler M -Moisture Approved by: RFC 3" OD Dames & Moore Split-Spoon Ring Sampler ~ Static Water Level Figure No. A _ 7 ~ Water I oval at times of rlrillinn (ATfl1 _ 1~1~~rs ~iad~narr~ic~ inc. MB geotechnical and environmental science and engineering May 10, 2001 Mr. Lloyd Powell 725 Market Street ` Kirkland, Washington 98033 Re: Geotnchnical Slope Evaluation 751 Beach Drive Jefferson County, Washington Dear Mr. Powell: This letter report presents the results of our geotechnical evaluation of your property located at 751 Beach Drive in Jefferson County, Washington. The purpose of our work was to observe existing slope conditions and generally determine the relative slope stability risk associated with reconstruction of a residence on the property. Our work was conducted in general accordance with our letter of agreement dated March 15, 2001- and included a review of site reference mapping, discussions with you and Mr: Al Scalf of the Jefferson County Planning Department, a site visit, and preparation of the letter report. BACKGROUND During the winter of 1998-99, heavy to record rainfall levels were recorded in the Puget Sound region and numerous landslides occurred within the Beach Drive community and throughout Dabob Cove. One of the landslides severely damaged your residence on Beach Drive. The landslide destroyed the north end of the structure and soil debris filled a portion of the standing, damaged north end of the residence. Since the time of the landslide, we understand you have performed minor modifications to the property including securing the residence by boarding up door and window areas. Additionally, grading operations (performed by others) have roughly reestablished the driveway. Some landslide soil and debris removal/stockpiling has also occurred. Several drainage ditches were excavated in the vicinity of the residence and surface water runoff from the ditches was routed to the south into an existing drainage course. Landslide debris removal and regrading has been performed along Beach Drive. Roadway drainage improvements were installed (by others) and included reestablishing drainage ditches, installing large diameter culverts, and incorporating erosion control measures. SITE DESCRIPTION The subject property is known as Tact 7 of the Dabob Cove Tracts and fronts in the Dabob Cove shoreline. The property is generally rectangular in shape with the principle direction oriented north to south. The site is accessed from Beach Drive which parallels the east side - of the property. The damaged residence is located adjacent to and below Beach Drive. ROLLING' BAY i~/IERCANTILE BUILDiVCr ? 125 SIIAIRISE DRIVE BAINBRIDGE ISLAND WA 98110 ~ (206) 842-6073 _ Poviell 01904-5 May 10, 2001 page 2 of 5 SOIL AND GEOLOGY Mapping- of the area was reviewed for this report included soil mapping (U.S. Department of Agriculture Soil Conservation Service, Soil Survey of Jefferson County Area, Washington, 1975) and' geologic and coastal zone mapping (State. of Washington Department of Ecology Coastal Zone Atlas, Volume 11, Jefferson County, July 1978). Coastal zone geologic mapping indicates the property and local area are underlain by glacially derived sediments.' Glacial outwash deposits composed primarily of sand and gravel overlie undifferentiated stratified deposits typically composed of silt and clay. Soil mapping indicates the site soils vary in composition, -but in general, consist of a surficial topsoil horizon of gravel, sand, silt, and organic material reflecting their origin from the underlying glacial deposits. Coastal zone mapping indicates that the site and local slope are located within a "very critical slope area" along. the north margin of a large area designated an "unstable recent slide." Numerous "unstable recent slide" areas are also identified on coastal zone mapping along the Dabob Cove shoreline in the vicinity of the subject property. SITE OBSERVATIONS ~ ~ The site was evalua_ ted by conducting a reconnaissance of the property and observing slope conditions on April 11, 2001. At the time of our site visit, we observed general topography, the winter 1998-99 landslide area, soil and groundwater conditions, and vegetation. A brief description of our observations are presented below. Topo r~aphy The subject property is part of a coastal slope that extends over 500 vertical feet from the crest of the slope near Coyle Road down to the Dabob Cove shoreline. Locally, the coastal slope ranges from a 1/3 to 1/2-mile horizontal distance from crest to toe. The coastal slope topography ranges from steep, near-vertical grades to localized areas of gentle slopes and relatively level benches. The subject property is located at the toe of the coastal slope, adjacent to the shoreline. The site slopes approximately 10 to 15 feet down below Beach Drive with variable grades including near-vertical exposures of landslide debris. On-the subject property, the residence is looted on a relatively level area, likely created by original site development and recent grading following the landslide. West of the residence, grades drop down to the shoreline approximately 30 feet with average slope angles of 40 to 50 degrees. Local Landslide Area The winter of 1998-99 landslide effecting the property includes both adeep-seated rotational landslide mass and a debris flow. The existing, deep-seated _rotational landslide mass is defined by a series of scarps or shear surfaces that create astair-step pattern up the slgpe above the subject property and residence. -The slope face scarps are 10 to 15 feet in height with the uppermost scarp located several hundred feet east of the subject property. This uppermost scarp appears to define the east limit of the 1998-99 deep-seated landslide mass movement. In addition to the deep-seated rotational movement, evidence of a debris flow was observed during our site visit. East-northeast of the residence and immediately north of the deep-seated landslide area, the origin of a debris flow was identified. A 5 to 10 foot thick soil mass ` Myers Biodynamics, Inc. Powe1101904-5 May 10; 2001 page 3 of 5 apparently moved rapidly (flowed) downslope onto Beach Drive and the subject property. The debris flow completely destroyed the north end of the residence and landslide debris filled the remaining standing portion of the structure. Immediately south of the debris flow, 'the landslide area transitions to the deep-seated rotational mass where no significant debris flow component apparently occurred in 1998-,99. The majority of the soil and surficial vegetation associated with the deep-seated landslide area remains on the slope above the residence and Beach Drive. Soil and Groundwater Conditions Exposed soil and daylighting groundwater conditions were observed on and above the subject property at the time of our site visit. At the top of the landslide scarp and adjacent to the debris flow area, .slightly silty sand soils were observed. Below the scarp, exposures of clay -were observed in the debris flow origin area and were heavily fractured and slickensided. In the deep-seated landslide area, soil exposed along the stepped landslide scarps generally consisted of slightly gravelly silty sand and fine to medium sand. Groundwater seepage and hydrophytic (water-loving) vegetation were noted on the lower portions of the deep-seated landslide mass area (above the residence). The daylighting groundwater suggests a relatively impermeable soil such as silt/clay underlies the exposed sand soils at the deep- seated landslide mass. Vegetation Vegetation was observed within the landslide area above the subject property. Along the debris flow, vegetation' was scoured as a result of the rapid soil movement.- Some herbaceous groundcover is establishing itself in the flow area. On the adjacent deep-seated landslide mass, mature trees, woody shrubs, and herbaceous groundcover remain on the majority of the slope face. However, the large soil displacements associated with the deep-seated movement and resulting scarps have reoriented most of the slope vegetation. The majority of the large vegetation angles back into the slope and several. mature trees were toppled.dorvnslope as a result of the mass movement. SLOPE EVALUATION r Observations of existing site conditions indicate that the landslide area above the residence is generally. composed of sand over clay soils v~ith significant groundwater seepage. This combination of soil and groundwater is a common geologic condition associated with deep- seated landslides and debris flows. Landsliding generally occurs wlsen heavy precipitation increases- groundwater levels and destabilizes some sand over silt/clay slope areas. `~ A strong correlation exists between landslide activity and precipitation, which is supported by the 1998-99 high precipitation levels and landsliding in the Dabob Cove area. In our opinion, the Beach Drive landsliding including the soil movement above the subject property occurred as a result of the cumulative winter rainfall and associated intense storm activity coinciding with the landslide events. Landslide risk on the subject property is further documented by Coastal Zone Atlas mapping which identifies the property and- local community as an "unstable recent slide." Site observations of the scarp heights and estimates of the potential deep-seated landslide mass indicate that at least 500 to 1,000 cubic yards of landslide material remain on the slope face above the subject property. Consequently, landslide risk assessment of the subject property must recognize the remaining soil material present on the slope above the residence. Although Myers Biodynamics, Ina . .. ~ ~ ~ ti .___ • ,; --- - ,, Powell 01904-5 ~ ~ ' ~l 1 May 10, 2001 ~ ! ~~V ~ $ Z~~3 ; ~ 1 _ ~~ page 4 of 5 ~ ~ , _._.. ` 4'_ ~~~._ ,, aportion of the landslide mass was removed from the slope face as a result of tlie,1998=99 debris' ~' flow, the soil material associated with the deep-seated landslide mass remains on the slope above the residence. Accordingly, soil on the slope above the residence may, more again as a deep-seated landslide and/or a debris flow. This will likely occur in association with future high cumulative precipitation, intense storm events, other significant sources of uncontrolled surface water, and/or seismic events. Based on the extent and magnitude of material remaining on the slope above the residence and the existing geologic, soil, and groundwater conditions, it is our opinion that the risk of future landslides effecting the subject property is high. In addition, the potential for significant volumes of rapidly moving. material (debris flow) indicate the landslide risk includes `not only potential economic loss of any future structure, but also life-safety risk to any occupants. The landslide risk associated with the property and local area is high during periods of prolonged, cumulative .precipitation and storm events. These conditions are generally associated with the wet weather season, typically extending from late October/November through late March/April. During drier periods of the year, the landslide risk is significantly reduced, such that an adequate factor of safety likely exists for seasonal use of the property assuming static (non-earthquake) conditions. Recommendations for potential future uses of the property given current landslide risk conditions are presented in the Considerations and Recommendations section below. CONSIDERATIONS AND RECOMPlIENDATIONS As described previously, there is a high risk of future landsliding on the property that could result in damage to the residence and/or create alife-safety risk to occupants. Because landslide risk is typically associated with periods of extended wet weather and intense storm events, we recommend the following options be considered for future use of the property: Remove or modify the existing structure and utilize the property for passive recreational purposes during the dry weather season only. Do not utilize the property during the typical wet weather season. or at any other time during and after periods of prolonged, intense precipitation. Recognize that the site can be impacted by landslide activity during the wet weather period including but not limited to debris flows, soil mass movement, and drainage route changes. - Contact Jefferson County Department of Community Development staff to discuss ' the potential for permitting a seasonal occupancy structure on the property. To help limit future risk associated with landsliding, do not utilize the property during the wet weather season or at any other time during and after periods of prolonged, intense precipitation. Recognize that the site and any limited occupancy structure can still be impacted, damaged, and/or destroyed by landslide activity or drainage changes. Investigate the feasibility of structural measures to reduce the landslide risk to any future residential structure on the site. If requested, an assessment of slope stability utilizing quantitative modeling techniques can be performed to provide you with an indication of the current slope stability factor of safety. Additionally, quantitative analyses could provide an indication of the increased factor of safety associated with various landslide mitigation alternatives such as retaining walls and/or landslide diversion structures. Quantitative analyses would require a topographic survey of both the subject property and the adjacent upslope property, soil. Myers Biodynamics, Inc.