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HomeMy WebLinkAbout935100008 Geotech Assessment (1997)_ 111 SHANNON &WlLSON, INC. GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS SEATTLE RICHLAND FAIRBANKS ANCHORAGE SAINT LOUIS BOSTON February 6, 1997 Mr. Rus Ferguson c/o Kitsap Tire Center, Inc. 20245 Viking Way Poulsbo, Washington 98370 FEB i 0 t997 JEFFERSON 00UNTY i~!JBLIC WORKS DEPT. RE: EVALUATION OF RECENT SLIDING AT 851 THORNDYKE ROAD, SOUTHPOINT, WASHINGTON Dear Mr. Ferguson: This letter summarizes our observations, conclusions, and recommendations regarding the sliding that occurred at the above residence on January 1, 1997. The purpose of our work was to evaluate the risk of damage to the residence as a result of the sliding, evaluate the potential cause(s) of sliding, and develop recommendations to reduce the risk of additional movement. Preliminary observations, conclusions, and recommendations were provided to you at the time of our site visit on January 21, 1997. Our work has been conducted in accor~ce with our written agreement dated January 20, 1997. SITE AND SLIDE DESCRIPTION The referenced property is located in Jefferson County, approximately two miles south of State Route 104 near Southpoint on the east side of Thorndyke Road. The property is roughly 90 feet wide (north-south) by 275 feet long (east-wes0 and straddles the crest of a steep slope that extends down to the east toward Hood Canal. The west half of the site slopes gently down to the east at about 15 degrees and is located above the crest of the steep slope. The residence, drive, lawns, and septic drain field are located on the upper portion of the site. The east half of the site is located below the crest and slopes steeply down to the east between 30 and 40 degrees and is approximately 100 feet high. The property extends approximately half way down the steep slope and is undeveloped. The sliding at the site occurred within the soils on the steep slope. Prior to the slide, it appeared that this slope was timbered with alder, maple, fir and ced~s no bigger than 1 to 2 feet in diameter. From the base of the steep slope, the ground surface continues to slope 400 NORTH 34TH STREET' SUITE 100 P.O. BOX 300303 SEATTLE, WASHINGTON 98103 206,632,8020 FAX 206-633,6777 TDD: 1,600-833-6368 W-7578-01 Mr. Rus Ferguson e/o Kitsap Tire Center, Inc. February 6, 1997 Page 2 SHANNON 6VVILSON, INC. down to the east at about 15 degrees to Hood Canal approximately 800 feet farther east, with various roads and residences situated thereon. The residence is a two-story structure that includes a basement that daylights out to the east. The foundation along the east side of the residence is located within 10 feet of the crest of the steep slope; footings for the deck and green house that extend farther east are situated at the crest of the slope. No signs of significant foundation settlement or movements were observed. Roof drains from this residence and the house on the lot to the north join at the crest of the slope and were reportedly tightlined to the base of the steep slope. The tightline apparently broke at the time of the slide and has been temporarily repaired and routed down the slope. As previously indicated, the recent sliding on January 1, 1997, occurred on the steep slope. Sliding also reportedly occurred on this slope approximately 15 years previously. The width of the recent slide is approximately 70 feet (north-south) and extends to within 10 feet of the crest of the slope and house, to the base of the steep slope and onto the lower 15- degree slope where the slide debris was deposited. Based on the depth of the side scarps, the upper 3 to 5 feet of soil on the slope slid down the hill. The neighbor to the north also reported some movement of a concrete crib wall built within the last one or two years on the steep slope near the crest. At the time of our site visit, nearly three weeks after the slide, groundwater seepage was observed near the center of the slide scarp, approximately 20 to 30 feet below the crest of the slope. GEOLOGIC CONDITIONS m'qI) CAUSE OF SLIDING Geologic maps of the area indicate that the site is underlain by very dense/hard Pleistocene deposits. These deposits are typically very dense/hard because they have been overridden and consequently compacted by the weight of thousands of feet of the last glacial ice sheet that covered the area until approximately 13,000 years before present. Exposures of these soils in the slide scarp confirmed the presence of these very dense/h .ard, glacially overridden~. deposits. These deposits appeared to be chiefly fine sand at the top of the scarp, grading to silt farther down the slope. The observed groundwater seepage occurred at the contacts between the fine sand and underlying lenses/layers of less pervious fine sandy silt and silt. W-7578-01 Mr. Rus Ferguson c/o Kitsap Tire Center, Inc. February 6, 1997 Page 3 SHANNON 6WILSON, INC. Along the sides of the slide scarp, 3 to 5 feet of loose topsoil and/or colluvium was observed. Topsoil develops from the underlying very dense/hard Pleistocene .deposits as they weather and loosen. Colluvium is also weathered material but has reached its present location due to the forces of water and gravity. The soils on the slope that slid appear to be confined primarily to the upper 3 to 5 feet of topsoil and/or colluvium. The topsoil and colluvium are not as strong as the underlying, very dense, unweathered Pleistocene deposits. The slope may be locally stable for the unweathered material, but too steep for stability of the topsoil or colluvium, particularly at times when the topsoil and colluvium become saturated. Therefore, the topsoil and colluvium on the slope are inherently unstable and slump or slide from time to time. It is likely that the topsoil and/or colluvium on the slope were marginally stable under relatively dry, favorable conditions. However, the rapid snow melt and heavy rains immediately preening the slide saturated the topsoil and colluvium and thereby increased the Weight and the forces driving these soils down the slope and effectively reduced their strength to resist these forces. ~ It is also likely that the groundwater seepage observed in the slope also contributed to the saturation and instability of the soils on the slope. Water leaking from the broken tightline could have also contributed to the saturation of the soils on the slope; however, it is not known if the tightline broke prior to the slope movement or if the slope movement mused the tightline to break. CONCLUSIONS AND RECOMMENDATIONS At the time of our site visit, it did not appear that the recent sliding damaged the residence as no significant signs of foundation settlement or movement were observed. However, due to the proximity of the house to the steep slope and recent sliding, there is a significant potential that future sliding will extend back into the deck/house foundations, which could potentially damage the residence. Risk Reduction Measures For the short term, we recommend that the slope be kept as dry as possible. In this regard, the roof drain tight lines should be repaired and discharged at a suitable point at or below W-7578-01 Mr. Rus Ferguson c/o Kitsap Tire Center, Inc. February 6, 1997 Page 4 SHANNON 6WILSON, INC. the base of the steep slope (e.g., storm sewer, road ditch, existing drainage swale/ravine). The ground surface at the crest of the slope and the house/deck foundations should be monitored periodically for cracks or other signs of settlement or movement. Should signs of movement at or near the crest develop, we shoulct be contacted immediately to reevaluate the potential hazard to the residence posed by the movement. -Because of the potential consequences of fUture sliding on this slope, it would be prudent to reduce the risk of slope movement to the residence prior to permanent occupation. The risk can be reduced by either significantly improving the stability of the slope or relocating the residence so that fUture sliding will not damage the structure. To significantly improve the stability of the slope, the geometry of the slope wOuld require modification to a more stable configuration. This would require constructing walls or buttresses to retain the hillside and regrading the slope above the wall/buttress to a more stable (i.e., flatter) configuration. Drains would also need to be constructed to intercept groundwater flow observed in the slide scarp and improve the stability of the slope. Subsurface explorations and detailed engineering studies would be required to fUrther evaluate soil strength characteristic, stable slope and wall/buttress configurations, and develop the geotechnical parameters for the design of a wall. The costs associated with subsurface explorations and engineering studies would be on the order of $5,000 to $10,000. We would be pleased to develop a detailed scope of work and cost estimate for these services should you want to try to significantly improve the stability of the slope. Depending on the results of the subsurface explorations and engineering studies, the costs of constructing the recommended walls/buttress, installing the drainage, and regrading the slope will likely run well into the tens of thousands of dollars. It may be more cost effective for the amount of risk reduction achieved to simply relocate the residence away from the slope so that fUture movement of the soils on the slope would pose little potential to damage the residence. While the stability of the slope would not be significantly improved, the house could be located a sufficient distance back from the crest of the slope so that sliding of the topsoil and colluvium on the slope would pose little threat of damage to the residence. As a minimum, the residence should be located at least 50 feet back from the crest of the slope and preferably as far as possible from the crest toward Thorndyke Road. - W-7578-01 Mr. Rus Ferguson c/o Kitsap Tire Center, Inc. February 6, 1997 Page 5 SHANNON &WILSON, INC. Regardless of the risk reduction measures implemented, there is some risk of instability present on all hillsides, which the owner must be prepared to accept. Such instability could occur because of future water breaks/leaks, uncontrolled drainage, unwise development in adjacent areas, or other actions or events on a slope that may cause sliding. Erosion Control Measures The fine sand and silt exposed in the slide scarp will be susceptible to significant erosion until a vegetative cover can grow. To speed the re-vegetation, the slope may be hydro- seeded with an appropriate grass seed, mulch, and fertilizer mixture. To provide short term erosion control, a tacldfier can be added to the mixture and applied as soon as possible. At tackifier typically requires a dry period of 48 hours above freezing to cure. We recommend that the hydroseeding be done by a reputable and experienced contractor. Summer applications must be irrigated. Seeding should be accomplished by September 15 to provide a measure of erosion control for the following rainy season. A synthetic erosion control mat (e.g., Tensar Erosion Mat TM3000) also can be placed on the slope, in conjunction with the hydroseeding, to provide a higher degree of short and long term erosion control. The mat should be fastened to the slope using the materials and methods specified by the maker. Regardless of the erosion control measures used at the site, these measures should not be expected to significantly improve the overall stability of the slope. Consequently, these erosion control measures may be damaged during future slope movements. LIMITATIONS The conclusions and recommendations in this letter are based on site conditions observed during our site visit and assume that observed conditions are representative of the subsurface conditions throughout the site; i.e., the subsurface conditions are not significantly different from those observed during our site reconnaissance or indicated on geologic maps. If, during subsequent site activities (e.g., construction), subsurface conditions different from those inferred from this letter are observed or appear to be present, we should be advised at once so that we can review those conditions and reconsider our recommendations where necessary. W-7578-01 Mr. Rus Ferguson c/o Kitsap Tire Center, Inc. February 6, 1997 Page 6 SHANNON &WILSON, INC. This report was prepared for Mr. Ferguson for the evaluation of the slide at this site. With respect to possible future construction, it should be made available for information on factual data only and not as a warranty of subsurface conditions. Please not the scope or our services did not include any investigation for the presence or absence of hazardous or toxic material in the soil, surface water, groundwater, or air on or below or around this site. Shannon & Wilson has prepared the attached, "Important Information About Your Geotechnical Report," to assist you and others in understanding the use and limitations of our report. If you have any questions regarding the observations, conclusions, or recommendations, please call us. Sincerely, SItANNON & WILSON, INC. William J. Perkins, R.P.G. Senior Geologic Engineer W. Paul Grant, P.E. Vice President WJP:WPG/wjp Enclosure: Important Information About Your Geotechnical Report W7578-01 .LTR/W7578-1kd/dgw W-7578-01 SHANNON & WILSON, INC. Geotechnical and Environmental Consultants W-7578-01 Attachment to Report Page 1 of 2 Datr~h February 6 ~ 1997 To: Mr. Rus Ferguson c/o Kitsap Tire Center, Inc. Important Information About Your Geotechnical/Environmental Report CONSULTING SERVICES ARE PERFORMED FOR SPECIFIC PURPOSES AND FOR SPECWIC CLIENTS. Consultants prepare reports to meet the specifc needs of specific individual,~ -A report prepared for a civil engineer may not be adequate for a construction contractor or even another civil engineer Unless indicated otherwise, your consultant prepared your report expressly for you and expressly for the purposes you indicated. No one other than you should apply this report for its intended purpose without first conferring with the consultant. No party should' apply this report for any purpose other than that originally contemplated without first conferring with the consultant. THE CONSULTANT'S REPORT IS BASED ON PROJECT~C FACTORS. A geotechnlcaUemSronmental report is based on a subau~ace exploration plan designed to consider a unique set of project- specific factors. Depending on the project, these may include: the general nature of the stmchue and property involved; its size and configuration; its historical use and practice; the location of the structure on the site and its orientation; other improvements such as access roads, parking lots, and undergwund utilities; and the additional risk created by scope-of-service limitations impo. sed by the client. To help avoid costly pwblems, ask the consultant to evaluate how any factors that change subsequent to the date of the report may affect the recommendations. Unless your consultant indicates otherwise, your report should not be used: (1) when the namm of the proposed project is changed (for example, if an office building will be erected instead of a parking garage, or if a refrigerated warehouse will be built instead of an unm~gerated one, or chemicals are discov~ on or near the site); (2) when the size, elevation, or configuration of the proposed project is altered; (3) when the location or orienta- fion of the proposed project is modified; (4) when there is a change of ownership; or (5) for application to an adjacent site. Consultants cannot accept responsibility for problems that may occur if they are not consulted after factors which were considered in the development of the report have changed. SUBSURFACE CONDITIONS CAN .CHANGE. Substance conditions may be affected as a result of natural processes or human activin. Because a geotechnical/environmental report is based on conditions that existed at the tim~ of subsurface explolation, construction decisions should not be based on a lv4x)rt Whose adequacy may have been affected by time. Ask the consultant to advise if additional tests are desirable before construction starts; for example, groundwater conditions commonly vary seasonally. Construction operations at or adjacent to the site and natural events such as floods, earthquakes, or groundwater fluctuations may also affect subsurface conditions and, thus, the continuing .adequacy of a geotechnical/environmental report. The consultant should be kept apprised of any such events, and should be consulted to determine if additional tests are necessary. MOST RECOMMENDATIONS ARE PROFESSIONAL JUDGMENt. Site exploration and testing identifies actual surface and sub,trace conditions only at those points Where samples are taken. The data were extrapolated by your consultant, who then apphed judgment to render an opinion about ovexall subsurface conditions The actual intedace betv, een materials may be far more gradual or abrupt than your report indicates, Actual conditions in. areas not sampled may differ from those predicted in your report. While .nothln.~ can be done to prevent such situations, you-and your consultant can work together to help reduce their impacta Retaining your consultant to observe subsurface construction opera- tions can be particularly beneficial in this respect. A REPORT'S CONCLUSIONS ARE PI~'~MINARIL Th~ conclusions contained in your consultant's report are pre~ becamse they must be based on the assumption that condi- lions revealed through selective exploratory sampling are indicati~ of actual conditions th~ughout a site, Actual subsm'itce conditions can be discerned only during earflnvork; thexefo~ you should retain your consultant to observe actual conditions and to provide concludons. Only the consultant who prepared the xeport is fidly familiar with the background information needed to determine whether or not the report's recommendations based on those conclusions are valid and whether or not the contractor is abiding by applicable recommendations. The consultant who developed your report cannot assume responMbility or liability for the ~ of the l'eport'$ recomm6ndations if anoth~ party is le.J~ained to obs6Tve construction. THE CONSULTANT'S REPORT IS SUBJECT TO ATION. Costly problems can occur when other design professionals develop their plans based on midntertnv~fion of a geotechnical/envir- -onmental report. To help avoid these problems, the consultant should be retained to work with other project design professiouals to explain relvant geotechuical, geological, hydrogcological, and environmental fmding~ and to revi~ the adequacy of their BORING LOGS AND/OR MONITORING WELL DATA SHOULD NOT BE SEPARATED FROM ~ REPORT. Final boring logs d~eloped by the consultant are based upon interpretation of field logs (assembled by site personnel), field test results, and laboratory and/or office evaluation of field samples and data. Only final boring logs and data are customarily included in geotechnical/elNi~nmental xepo~ The.se final logs should not, under any ci_rcumst~ces, be redrawn for inclusion- in arc, hiL~_~ml or other design drawings, because drafto~ may commit errors or omissions in tho transfer proco~ To redu~ the htelihood of boring log or monitoring veil n~int~p~tion, contmctom should be given ready access to the comple.~ geotechnical engineering/environmental report prepanxl or authorized for their use. If access is provided only to the report prepared for you, you should advise contractors of the report's limitation.5 assuming that a contractor was not one of the specific persons for whom the report was prepare, and that developing construction cost estimates was not one of the specific .purposes for which it was prepared. While a contractor may gain i .mportant knowledge from a report prepared for another party, tbe contractor should discuss the report with your consultant and perform the additional or alternati~ work believed necessary to obtain the data specifically al~ropdate for construction cost estimat~ purposes. Some chents hold the miaaken impression that simply disclaiming responsibility for the accuracy of subsurface information always insulates them from attendant liability. Provi~ the best available information to contractors helps prevent costly construction problems and the adversarial.attitudes that aggravate them to a dispwportionate scale, READ ~NSIBILITY CLAUSES CLOSELY. Because geotechnicaUenvironmental engineering is based extensively on judgment and opinion, it is far less exact than other design discipline~ This situation has resulted in wholly unwarranted clalrn~ being lodged against consult/mix To help prevent this problem, consultants have developed a number of clauses for use in their contracts, reports and other documents. The~ responsl'bility clauses are not exculpatory clauses designed to transfer the consultant's liabilities to other parties; rather, they are · definitive chuses that identify where the consultant's respondbilities .b~ and end. Their use helps all parties involved recog- -aim their individual responsibilities and ~ appropriate action. Some of these definitive clauses are likely to appear in your report, and you are encouraged to read them closely. Your consultant will be pleased to give full and frank answers to your question~ The preceding paragraphs are based on information provided by the ASFE/Association of Engineering Firms Practicing in the Oeoscieaees, Silver Spring, Maryland 1/97