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HomeMy WebLinkAbout701344025 Geotech Assessment (2001)Myers Biodynamec~ inc. MB geotechnical and environmental science and engineering May 10, 2001 Mr. Lloyd Powell ' 725 Market Street i Kirkland, Washington 98M3 . " ~ ~ ;' Cr ~V 1 8 t~~3 ~ ~ -' ' Re: Geotechnical Slope Evaluation 751 Beach Drive ` Jefferson County, Washington ,l~- (l~.nT r z 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 Tf act 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 MERCANTILE BUILDii~TG 11254 SUNRISE DRIVE BAINBRIDGE ISLAND WA 93110 (206) 842-6073 _ Powell 01904-5 ~ __._. May 10, 2001 _ page 2 of 5 NQV ~ $ 2{03 `, fi _7 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 der;ved 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 cgmposition, 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 ~ i The site was evaluated 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. Topographv 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 located 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. - Powell 01904-5 ~ ^~uV ~ 1 ~3 ~t?3 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 axea, soil exposed along the stepped landslide scarps generally consisted of sli~'htly 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 . downslope as a result of the mass movement. SLOPE EVALUATION 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 when 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, Inc. Powell 01904-5 M 10 2001 ~; _ . .. ~ ~ N~V ~ 8 ~03 ay , page 4 of 5 - ' ' , ,.' ~ _... _ _ ~ a portion 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 RECOMMENDATIONS 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 seasor. 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. ~ , Pov-e1101904-5 ' ~. May 10, 2001 ~jQ~ ~ $ 203 ' page 5 of 5 .._. _.. boring explorations, laboratory_testing,'and other site investigations in order to perform comparative evaluations. Additionally, slope instrumentation could be installed to monitor groundwater levels and the depth and magnitude of soil mass movement. The proposed geotechnical investigations are performed over a period of time and do not guarantee that slope remediation measures will be a cost- effective option for the property. CLOSURE This. letter report was prepared for the exclusive use of i~Ir. Floyd Powell for specific application 'to'tlie :.7-51 Beach Drive. property in Jefferson County, Washington as described herein. The conclusions and interpretations within this letter report should not be construed as a warranty of subsurface conditions. Within the limits of scope, schedule, and budget .the evaluation `and recommendations presented in this letter report were prepared in general accordance with accepted professional .geotechnical engineering principles and practices in the area at the time this letter report was prepared. No other warranty, whether expressed or implied, is made. The evaluation presented herein was based on our observations of the subject property at the time of our site visits. ' If there is a substantial lapse of time, conditions have changed at the site, or if conditions appear different from those described in this letter report, we should be contacted and retained to review the changed conditions. The purpose of the review is to determine the applicability'of the considerations and recommendations presented in this letter report considering the time lapse and/or changed conditions. We appreciate the opportunity to provide you with geotechnical engineering, services. If you have any questions regarding the evaluation and recommendations presented herein or we may be of further assistance, please contact our office at your convenience. Sincerely Yours, MYERS BIODYNAMICS, INC. C~ <~ e N. Myers, P.E. Principal Geotechnical Engineer JNM/saf Myers Biodynamics, Inc.