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HomeMy WebLinkAbout954600116 Geotech AssessmentAugust 31, 1999 RECEtVED Project File 99-624 Ms. Cherie Howry 113 Gilbert Yakima, WA 98902 J[[E SO CQUNfl' DCO RE: Geotechnical assessment of slope stability conditions on Lot 25, Block 1, Goodfellows Manhattan Beach Tract, located in NW ¼, Sec. 20, T. 27 N., R. 1 E.W.M., Jefferson County, WA. Dear Ms. Howry: At the request of Ms. Barbara Blowers and with your authorization, a geotechnical assessment was made of slope stability conditions on your lot. Since slopes failures have occurred in the immediate area and certain county and state personnel have grouped a small landslide zone near your lot with a large surface area landslide located about 400 feet to the south, you are concerned about both the stability of your lot and what effect this will have on the value of your lot. While emphases will be directed toward discussing slope stability, the scope of this report will be expanded to that of a typical Hazardous Area Ordinance type report required, for permit purposes. The field reconnaissance phase, accompanied by Ms. Blowers, was made on June 23, 1999. Photographs were taken to document field conditions on your lot and adjacent lots in the area. The subject evaluation consisted of the review of (1) Coastal Zone Atlas Of Washington, volume 11, Jefferson County, Washington Department of Ecology, 1978; (2) Soil Survey of Jefferson County, Washington, USDA Soil Conservation Service, 1975; (3) "Geotechnical report on Lot 23, Block 1, Goodfellows Manhattan Beach Tract" by J. B. Scott & Associates, June 21, 1990; (4) "Chemical Stabilization of Landslides by Ion Exchange", California Geology, vol. 27, California Division of Mines and Geology, 1974; (5) a series of DNR Aerial Photographs of the general area, dated 1990 through 1998 and (6) the field reconnaissance which consisted of walking over the subject lot and property both to the north and south to inspect surface conditions, slope stability, attitude of conifer trees, formation-soil exposures, and surface drainage. Also a very cursory examination was made of surface conditions along Manhattan Avenue to observe landslide conditiOns that offset the road. Ms. Cherie Howry August 31, 1999 SUMMARY- The Coastal Zone Atlas classes the entire area, which includes your lot, as being Unstable. Recent slope failure movement has occurred to the south of your lot. On your lot there are no surface indications of slope failure movement, past or present. The attitude of all conifer trees on your lot confirms that stable conditions exist. In regard to other features such as bearing capacity values, after stripping off the thin topsoil zone down to glacial outwash "hardpan", the bearing capacity values will exceed 2.0 TSF. Also, because of good bearing capacity values, since the underlying outwash is rather dense, the potential for damage on your lot as generated by a seismic event is considered as low. Since the surface soil is well armored with gravel, erosion should be low. A medium sized drainage course exists to the south and enters the Hood Canal about 1 i0 feet south of your lot. Then a very minor drainage course crosses through the lot along the north side and then crosses the lot and enters Hood Canal just south of your lot (see Figure 1). I LOT 25 LOT 24 LOT 23 GOODFELLOW LOT 22 Scale 1" = 200' Top of Bluff -'~" --' '~'° '.-,,. 'Setback -' MinorCreck ..--~.---4~"'-..~. t.__~.~ E~mg Access Road /~ ~"~! ~ Major Creek -~ ~-, ~-' Existing Access Road ~ I~. ~' ~' '/ - Erosion & Failure Zone Northern Limit of Major Slide FIGURE 1 - PLOT MAP OF LOT 25 AND ADJACENT LOTS TO THE SOUTH The bluff slope down to the beach is standing at an average slope of 1.5:1 or 34 degrees and is about 25 feet in height. Toe erosion, as a result of wave action, has resulted in a near vertical t~tce about 4 to 6 feet in height. The beach zone has stone armor ha the form of gravel and coarse sand. A beach drift dime parallel to the shoreline appears to be the result of coastal drift moving uplift toe slide material on to your lot from the south. This drift dune fades out within another 300 feet to the north. Because of the dense and slightly cemented glacial outwash material,_ perched groundwater conditions exist during the winter months, and this results in near surface water and seasonal wetland conditions. In the parcels located to south, many areas showing typical wetland conditions were noted. DISCUSSION OF CONDITIONS - Based on the Soil Survey Map, the soil-type present on your lot is classed as Cassolary sandy loam,_ 0 to 15% slopes. This soil is shown to have a thickness in excess of 5 feet and ranges from sandy loam near surface to silt loam. Ms. Cherie Howry August 31, 1999 and silty clay loam at depth. Based on the Unified Soil Classification, the soil is SM (silty sand) to CL (silty clay) at a depth of about 3 feet. Because of the clay, perched groundwater conditions exist during the winter months. This soil was derived from the underlying Vashon advance outwash. According the Coastal Zone Atlas, this outwash mostly consists of sand and gravel. However, some clayey sand horizons are present and probably accounts for the clayey soil. This clay would result in perched groundwater conditions above each clayey sand horizon. While slope stability problems exist in the vicinity, none are present on your lot. There have attempts by state and county personnel to include the area where your lot is located into what they considered as a very deep rotational landslide located at the nearest point, over four hundred feet to the south. I can go into a long discussion of geology of the area and that of the large landslide and then a second small slide zone, located less than two eral hundred feet to the south, and how they do not have any relation to the stability of your lot, but that is not within the scope of this report. I will say, that the large landslide is certainly a rotational failure. However, I do not consider it as being a deep rotational slide. Then the second slide area, located just south of your lot, is not a portion of the rotational slide but instead the result of excessive stream erosion that unbalanced the creek bank slopes. This erosion resulted in a series of slumps along the creek. These slumps do not have continuity as would be expected if they were a portion of the big slide to the south. Then in addition to the excessive erosion, it appears that unbalanced conditions as a result of grading and placing fill on steep slopes are also involved with the failures. The subject of the two landslide areas will be discussed in detail in a separate report to issued in the future. In regard to the past stability history of an area, the attitudes of conifer trees are the best and quite frankly the only real reliable indicator of past slope stability. All Slopes are subject to annual wetting and drying of soils. This seasonal change of moisture conditions results in the soil "creeping" down slope. All vegetation, which includes trees, then tends to bend downslope because of this soil movement. Conifer trees, which are also affected, always grow vertically. As the trees bend, they constantly attempt to recover to a vertical position. This is why all conifer trees on slopes where soil creep occurs always show bending or curving of the tree trunks. The faster the soil movement, the greater the curve of the mink. In the case of a rotational slope failure, the lower portion of the arc results in vegetation leaning upslope. The conifer trees within portion of the arc will also lean upslope and recover to the vertical in a downslope direction. When a group of conifer trees show this characteristic mode, that indicates that a slope failure occurred during the life span of those trees. Based on the Coastal Atlas, your lot is within a very large area classed as Unstable. Because of the relatively gentle slopes in the area, avalanche or even slab failures would not be expected to occur. Therefore, rotational failures would be normal type slides to expect in the immediate area. These slides would be documented by the presence of adverse leaning trees. No such trees were on or even seen adjacent to your lot. In Ms. Cherie Howry August 31, 1999 addition to the use of trees as an indicator, surface expression is very important. Rotational failures always have a "rumpled" surface due to both material movement and "bulking". None of those surface conditions were seen on or adjacent to your lot. However, the most important factor in relation to slope stability is both surface and subsurface drainage. Except along drainage courses, your lot does have any areas showing wetland type growth, so perched groundwater conditions does not appear to be a problem as conditions now exist. However, you must realize that development along the Hood Canal with associated removal of vegetation, grading, and modification of surface drainage has been the cause of more slope failures than all other causes combined. It is for that reason that careful consideration should be given to controlling and collecting surface drainage. If a perched groundwater potential exists, then curtain drains must be installed. Proper placement of drainage systems will not only help in controlling erosion and lowering the buoyant effect of subsurface water, but in many cases can even increase the factor of safety to more than what exited prior to any development. Ignoring the effect of uncontrolled drainage and excessive erosion can and does result in landslides such as exists in the drainage course just south of your lot. Based on existing conditions and with the understanding that drainage (both surface and subsurface) be addressed, allowing for the height of the bluff slope of about 25 feet, along with a moderate earthquake, that a setback of 50 feet is reasonable. Also cutting of trees, especially on the bluff slope, should be kept to a minimum. Bearing capacity values after stripping off the forest floor material and topsoil will provide you with at least a 2.0 TSF value, which is more than ample for any type structure you may place. Both the setback and bearing capacity values are subject to placing your septic drainfield as far to the west as possible so as to reduce the groundwater mound (buoyant) effect that it would cause. The watershed for the surface flow is quite limited in area so normal drainage is not considered a problem except for concentrated flows from the structure roof and other imperious surfaces. The soil is armored, so normal sheet flow is of no concern. Also, your lot will not be subjected to the types of flows being generated from the clear-cut areas west of Manhattan Avenue, so bank erosion is not considered a problem. I was advised that I must provide data on my qualifications. Since I have never prepared a brochure, I am attaching a summary of my qualifications as prepared last year for a joint venture proposal. That should be sufficient for the county. We appreciate this opportunity to be of service to you. regarding this report, please contact the undersigned. Sincerely, James B. Scott, P.E. Attachment If you have any questions Page 6 - Huang & Scott Proposal STATEMENT OF QUALIFICATIONS J.B. SCOTT & ASSOCIATES KEY PERSONNEL November 20, 1998 James B. Scott, Principal Engineer-Geologist Education: BS (1951), Geological Engineer, Univ. of Nevada-Reno; PE (1959), Engineer of Geology, Univ. of Nevada- Reno Registration: Professional Engineer in Washington State and Oregon Summary of Experience: Mr. Scott's professional background embraces the fields of geotechnical engineering, engineering geology, environmental geology, economic geology, and groundwater geology Professional Societies: American Society of Civil Engineers Mr. Scott has been the principal of the firm J. B. Scott & Associates for 21 years, during which time over 750 consulting investigations were conducted that generated gross earnings exceeding $1,000,000.00. He directed engineering and geologic input into siting twenty small hydro facilities for a Northwest Utility; made a feasibility study for a series of flood control dams for a Northwest Indian Nation; made reservoir site studies for PUD's and a major Northwest Utility; involved in "joint venture" contracts to study and correct groundwater problems; design of pipe pile foundations and tie-back systems for bluff slope and structure sites; pipe pile jacking of multi-story buildings which had undergone differential settlement; foundation investigations for multi-story commercial buildings, bridges, and pipeline crossings. Made many slope stability analyses using WINSTABL computer program of existing and potential stability problem areas and designed slide correction measures. Evaluated and designed grouting procedures; conducted refraction seismOgraph surveys to establish rock lines and to determine excavation character of rock; surface and subsurface drainage analysis and design; erosion analysis of both upland and shoreline areas; design_ of shoreline erosion protection facilities; groundwater development and conservation; real estate development feasibility; seismic response evaluation; wetland delineation's; evaluation of gravel and rock sources for construction material; and expert witness or gave depositions in nine litigation's. Consulted to sixteen engineering and/or geotechnical firms and served on government advisory boards. Earlier work in the fields of geotechnical engineering and engineering geology consisted of participating or directing engineering-geologic input into exploration, design, construction, and the operation-maintenance phases of the following: (1) As Division Soils Engineer-Geologist for the Oregon Highway Department of over nine hundred miles of primary, secondary, and county highways. (2) On the California State Water Project, first as Chief of the North San Joaquin Design Exploration Unit and then as Chief of the Construction Geology Section of eight hundred miles of canals and pipelines, eight pumping plants, two power plants, ten dams, four tunnels, many bridges, two shallow subsidence areas, and many landslides. (3) While Chief Engineer of Ion Tech, Inc., directed development, made analysis, and directed landslide and expansive soil correction by chemical modification of engineering properties of clay. (4) While Staff Engineering Geologist at Santa Clara Valley Water District, inspected or detected unstable areas, made stability analysis of the slides and designed correction procedures, and conducted stability analysis on over twenty earth fill dams. His work in environmental geology includes input into numerous projects and developments in California, which included staff review and approval of many EIR's. Experience in groundwater includes two major groundwater-dewatering investigations in relation to construction projects, in general groundwater development and conservation while with the Santa Clara Valley Water District, he made input into methods of groundwater recharge and mitigating of salt-water intrusion. Was associated with study using chemical ion exchange to create impervious lining for landfill purposes and also treating wastewater. In Hawaii, he developed a program by which water and hydroelectric power could be developed fi.om high elevation groundwater sources. Mr. Scott is the author or co-author of six publications, 750 consulting reports, and about six hundred "in house" technical reports. In his work in economic geology, he wrote a paper on a concept he developed for the structural control of a major Mexican mineral deposit and based on the concept, discovered a major "new" deposit. He co-authored a paper on tunnel rock mechanics and was granted a gold medal from the American Institute of Mining Engineers for Outstanding Paper in Rock Mechanics for 1969. Another paper on ion exchange helped advance that method of modifying the engineering properties of clay fi.om a curiosity to a proven technique. PUBLICATIONS (author or co-author) · "Development Drilling, San Miguel County, CO." TEMR 454-5, U.S. Geological Survey, 1952. · "Helicopdon from Elko County, NV", Journal of Paleontology, v. 29, No. 5, 1955. · "Structure of Ore Deposits, Santa Barbara, Chih., Mexico," ECONOMIC GEOLOGY, v. 53, 1958. · "Influence of Engineering Geology on Design & Construction of Delta Pumping Plant," GSA (Abstract), 1965. · "Approach to Classifying Rock for Tunnel Liner Design", 11a` Symposium of Rock Mechanics, Chap I0, UC (Berkeley), 1969. · "Chemicai Stabilization of Landslides by Ion Exchange", California Geology, v. 27, 1974. ----~ _ ~ . Page 7 - Huang & Scott Proposal November 20, 1998 KEY PROJECT EXPERIENCE 1977 to Present J.B. Scott & Associates, Anacortes, WA Principal of Firm- Responsible for administration and technical supervision of Work undertaken as program manager or engineering service subcontractor. Scope of work included siting 20 small hydro facilities for electric utility, siting reservoir tanks, dams, and making reservoir foundation evaluation for utilities & P.U.D's. For Indian Nations, selected flood control dam'sites and evaluated reservoir sites In all phases of work, made seismic response estimates, slope stability evaluations and analysis using WINSTABL 95-computer program. Other work consists of uPland and shoreline erosion evaluation and design, of correction procedures, and established "tightline" installation standards. Provided sheet, pipe, and CIDH pile design. Designed grouting procedures and blasting programs for quarry and subdivision development. CondUcted pipelines, highway, and structure foundation exploration and design. Refraction seismic surveys to establish rock lines and excavation character of rock. Groundwater development, conservation, and permeability problems. Clay and waste water treatment using ion exchange. Conducted wetland delineations. In Hawaii, developed groundwater-hydro electric generation proposal for volcanic terrain. Gave depositions and appeared as expert witness in legal proceedings. Firm provided services as consultant to other consulting firms. ! 975 to 1977 Santa Clara Valley Water District, San Jose, CA Staff engineering geologist. Acted as "in house" consultant and supervised contract work of outside consultants. Reviewed and approved EIS's prepared by outside consultants. Conducted slope stability analysis (soil and rock) and designed correction methods. Prepared a regional inventory (map and text) of geological hazardous areas for government and private development planning purposes. Made dam site evaluations and earth fill dam stability analysis. Reviewed and approved urban development plans. Made seismic response estimates and mapped locations of active faults. Assisted in salt-water intrusion study and selected sites for groundwater recharge facilities. 1972 to 1975 Ion Tech Inc., South San Francisco, CA Chief Engineer. Geologic and engineering evaluations of clay related foundation, slope stability, and permeability problems. Use of ion exchange to increase or improve, factor of safety of landslides and to modify or improve conditions such as consolidation (settlement), soil creep, expansive clay, and permeability. Directed sales personnel. Assisted in development method and application of techniques. Wrote articles for publication. Supervised all geologic, engineering, chemical and soils laboratory functions. 1958 to 1972 California Depadment of Water Resources, Sacramento, CA Assistant through Senior Engineering Geologist. Held positions of up to Chief, Construction Geology Section and Acting Branch Chief. Supervised up to 40 technical and professional classification employees. Was responsible for and/or supervised geologic input into planning, primary and final design exploration. Also supervised geologic input into construction and operations of the project, which included "change of conditions" claims. Scope of work covered all exploration functions such as soil and rock sampling, testing, analysis of field and laboratory data, and selection of borrow and construction material sites. Soils and rock mechanics input regarding slope stability problems, tunnels, structure foundations, dewatering, grouting, and groundwater development. Made presentations to Consulting Boards. Designed and supervised preparation of technical manuals for the Branch and Design & Construction Division. Loaned out as consultant to other state and county agencies. Facilities includes canals, pipelines, power and pumping plants, tunnels, highways, and bridges. 1956 to 1958 CIA Minera Asarco, Santa Barbara, Chih., Mexi~ Mining Geologist. Was responsible for regional mineral evaluations in Central Northern Mexico. Was resident mining geologist at Santa Barbara Unit. Conducted and supervised drilling problems, sampling of mines and prospects for evaluation purposes, mapping (surface and underground), mineral evaluation of metallic, nonmetallic, and construction materials deposits. Developed new concept for structural control of Santa Barbara mining district deposits. 1952 to 1956 Oregon State Highway Department, Bend, OR Division Soils Engineer-Geologist. As soils engineer, was responsible for geologic and soils input into planning, pre- design, design, construction, and maintenance phases of state and county highways within the division. Supervised Division Soil Lab, drill crew, and inspectors. Made preliminary foundation designs and selected alignments based on geologic factors. Located and drilled water wells for State Roadside Parks. Was expert witness. Developed structural control concept for locations of cinder (scoria) deposits.