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HomeMy WebLinkAbout001302009 Geotech Assessment (2000)FEB 1 3 2.001 Geological Evaluation of Slide at 191 Lane DeChantai Port, Townsend, Washington June 2000 GEOTECHNICAL ANO ENVIRONMENTAL CONSULTANTS At Shannon & Wilson, ot~r mission is to be a progressive, well- managed professional consultbzg finn b, the fields of engineering and applied earth sciences. Our goal is to peo~orm our services with the highest degree of professionalism with &ce consideration to the best #,terests of the public, our clients, and our employees. Submitted To: Mr. and Mrs. Jerry and Martina Haskins 191 Lane DeChantal Port Townsend, Washington 98368 By: Shannon & Wilson, Inc. 400 N 34t" Street, Suite 100 Seattle, Washington 98103 21-1-08976-001 SHANNON WlLSON, INC. GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS June 22,2000 SEATTLE RICHLAND FAIRBA[.IKS ANCHORAGE DENVER SAINT LOUIS BOSTON Mr. and Mrs. Jerry and Martina Haskins 191 Lane DeChantal Port Townsend, WA 98368 RE: GEOLOGIC EVALUATION OF SLIDE AT 191 LANE DECHANTAL, PORT TOWNSEND, WASHINGTON Dear Mr. and Mrs. Haskins: This report summarizes our observations, conclusions, and recommendations regardi.ng the slope movement that occurred at the above site in February 2000. The conclusions and recommendations in this report are based on geologic reconnaissances of the site on May I0 and 31, 2000, areal photos, area geologic maps, and a topographic survey of the site by Wood Surveying, Inc., dated May 23, 2000. SITE DESCRIPTION The site is located on the east side of Discovery Bay on the Quimper Peninsula near Port Townsend, Washin¢on (Figure 1). As shown on Figure 2, the property on which the residence is located is a roughly rectangular lot, orientated northeast-southwest that is approximately 260 feet wide (northwest-southeast) in the vicinity of the residence, and narrows to about 140 feet on the southwest side along the beach. The residence (two and one-half-story wood-frame structure with a basement that daylights out to the southwest) is located at an elevation of about 340 feet above the beach. The horizontal distance between the beach and house is about 510 feet, resulting in an overall slope angle betweefi the beach and the house of about 33 ½ degrees. Locally, the slope is steeper or flatter. In particular, at the beach and beyond the limits of the recent slide, the ground slopes up at about 52 degrees for a vertical height of 100 feet. Immediately southwest of the residence, the ground slopes down to the southwest at about 20 degrees for a horizontal distance of about 110 feet. A topographic profile from the residence down t o the beach through the recent slide is shown on Figure 3. 400 NORTH 3.4TH STREET · SUITE 100 P.O. BOX 300303 SEATTLE. WASHINGTON 98103 208.632.8020 FAX 206'695.67'77 TDO: 1.800.833-6388 21-1-08976-001 Mr. and Mrs. Jerry and Martina Haskins June 22, 2000 Page 2 SHANNON &WILSON, INC. At the time of our reconnaissance, the head scarp from the recent slope movement was approximately 180 feet wide (northwest-southeast) near the top of the slide and was roughly centered on the southeast property line (see Figure 2). The head scarp appeared to be approximately 15 to 25 feet high. Two approxima[ely 2-foot-high tension cracks were observed extending from near the top of the head scarp, northwest across the property. The locations of the tension cracks are shown on Figure 2. The slide extended from the head scarp to bottom of the slope at the beach. The' areal extent of the slide is indicated on Figure 2. Vegetation on the site typically consists of Douglas fir, cedar, and madrona trees up to 3-feet in diameter. The trunks of many of the trees are bowed down slope, which is indicative of soil creep. Soil creep is the slow, gradual down slope movement of near surface soils under the effects of gravity and water and occurs on most slopes to some degree. Undergrowth includes salal and Oregon grape. The presence of madrona trees and salal and Oregon grape are indicative of relatively well-drained, near-surface soils. While the vegetation had largely been removed within the limits of the recent slide, the remaining vegetation and debris within the slide deposits indicate that the slope on which the slide occurred was similarly vegetated, including large-diameter trees. GEOLOGIC CONDITIONS Geologic maps of the area indicate that the site is underlain by Pleistocene-age (+17,000 years old) Vashon Advance Outwash, which is, in turn, underlain by older, undifferentiated, stratified, Pre-Vashon sediments. The undifferentiated, Pre-Vashon sediments may consist of both glacial and non-glacial deposits and may include stratified sand and gravel with lesser amounts of silt, clay, and peat. The Pre-Vashon sediments are not. lithified (are not rock). The overlying Vashon Advance Outwash typically consists of sand with lesser amounts of .~ilt and gravel. The advance outwash was deposited on the pre-existing land suffacel in front of the continental Vashon Stade ice sheet that advanced from Canada across the Puget Sound region approximately 17,000 years ago. The Vashon ice sheet that overrode the advance outwash and underlying sediments is estimated to have been up to 4,000. feet thick and, as a result, the advance outwash and underlying sediment were overconsolidated to a very dense or hard state due to the great weight of the ice. 2 i-1-08976-o01-LI .DOC/WP/SJM 21 - 1-08976~001 Mr. and Mrs. Jerry and Martina Haskins June 22, 2000 Page 3 SHANNON &WILSON, INC. ./ Geologic maps also indicate that below an elevation of about 200 feet, both ancient and recent landslides have occurred for several thousands of feet up and down the beach. Very steep to near-vertical slopes on the adjacent properties, approximately between elevations of 200 and 250 feet appear to form the head scarp of the ancient landslide' The presence of Vashon Advance Outwash and Pre-Vashon stratified sediments was confirmed in exposures in and along the margins of the slide scarp. Specifically, dense to very dense advance outwash was observed in the head scarp above an elevation of about 245 feet, consisting of interbedded, slightly gravelly silty, sand; fine sand; silty fine sand; and fine sandy silt. Approximately between elevations 230 and 245 feet, a silty clay with a blocky or fractured texture was observed within and along the margins of the slide scarp. The approximate location of the clay is shown on Figure 2. Measurements of strike and dip at the base of this clay indicate that it is relatively fiat-lying. It is our opinion that this clay represents the upper most unit of the Pre-Vashon stratified sediment. Along the margins of the slide, the fractured silty clay was dry and appeared to be hard silt. Within the slide scarp, damp so/1 conditions and slight groundwater seepage were observed within this unit (see Figure 2). At these locations where the clay was damp or wet, the upper portion of the unit was not as hard and appeared to consist of hard pieces in a softer matr/x resulting in an overall medium stiff to very stiff clay. Directly beneath the clay unit, stratified, very dense fine sand and hard silt was observed. A cross-section showing the relative location of the Vashon Advance Outwash and the Pre-Vashon fractured silty clay and the underlying stratified fine sand and silt are shown on Figure 3. CONCLUSIONS The recent slide does not appear to be a re-activation of the large ancient landslide (the larger ancient landslide probably developed under different geomorphologicaI and climatic conditions that are not currently present). Rather, the recent slope movement appears to be associated with a more local instability in the head scarp of the ancient Iandslide. The recent slope movement appears to be relatively deep seated, extending from head of the recent scarp and tension cracks, through the fractured silty clay as'shown on Figure 3. Based on 21 - 1-08976-00 I-L 1 .DOC/WP/SJM 21-1-08976-001 Mr. and Mrs. Jerry and Martina Haskins June 22, 2000 Page 4 SHANNON &WILSON, INC. our site observations and experience with similar sites, it is our opinion that the failure mechanism is most likely associated with wetting of the fractured silty clay and accompanying reduction in shear strength. Specifically, the silty clay appears to have a relatively high shear strength when dry, based on the near Vertical exposures on the southern margin of the slide. However, when water is introduced into the fractures, the shear strength is significantly reduced. It is likely that the groundwater observed within the fractured silty clay tn the slide scarp caused a reduction in the soil shear strength. With the reduced shear strength, the silty clay was no longer able to support the overlying advance outwash. The overlying advance outwash moved out over the silty clay and then cascaded down the face of the slope, removing vegetation as it moved. About 15 vertical feet of slide debris accumulated on the bench near an elevation of 100 feet on the slope, while the remainder of the slide debris moved down to the beach (see Figure 3). There are various possible sources for the groundwater in the fractured silty clay. The water could be from naturally-occurring precipitation infiltration. Heavy precipitation similar to the winter of 1996/1997 did not occur this last winter. However, depending on infiltration rates, it may have taken a few years for the groundwater to accumulate in the fractured silty clay. In addition, it is possible that man-made point sources (e.g., septic drainfields, roof down spouts) could contribute'to groundwater levels. The recent slope movement appears to be an ongoing adjustment of the ancient landslide head scarp that results in a flatter, more stable head scarp over geologic time. This process appears to occur not only on this property but up and down a large length of the ancient landslide head scarp. The rate at which this regression (i.e., landsliding) occurs is not known. However, based on the size of the trees on the slope (e.g., 3-lo.or-diameter Douglas Fir) it would seem reasonable that the last significant slope movement would be at least 75 to 100 years ago. In addition, it would be expected that the rate of regression would slow as the ancient landslide head scarp flattens. Please note, however, that it is our opinion that there is a relatively' high potential for the ground down slope of the observed tension cracks to continue to move in the near future (e.g., 1 to I0 years). 21-1-08976-001-LI.DOC/WP/SJM 21-1-08976~001 Mr. and Mrs. Jerry and Martina Haskins June 22, 2000 Page 5 SHANNON ~WIESON. IN(;, RECOMMENDATIONS As a general rule, the closer a structure is to a slope or slide, the greater the risk and hazard to the structure posed by potential future slope movements. Based on our observations, the residence is not located on soils that are moving and is located 60 feet from the nearest tension crack, which is the likely location of the head scarp of the next deep-seated slope movement on the property. Potential failure surfaces would have to develop beyond the existing tension cracks to a point near the residence for the structure to be in eminent danger and unsafe, in our opinion. We recommend that the property between the house and the existing tension cracks be monitored for signs of movement. If movements are detected between the house and the existing tension cracks, the safety of the structure should be re-evaluated. Movement across the existing cracks should also be monitored with respect to potential risk to the residence. Various risk-reduction measures can be taken to reduce the potential for additional slope movement to affect the residence. These measures include reducing the amount of water entering the soils on the slope, construction of soil retaining Structures (vertical walls or slope anchors), or removal of a portion of the fractured silty clay and construction of a high strength buttress. Different costs and levels of risk reduction are associated with each of these options. Construction of soil retaining structures or a high strength buttress are more expensive, but because their effectiveness is much more quantifiable, the associated reduction in risk is typically much greater. A subsurface exploration program and geotechnical engineering studies would be required to further evaluate and design a soil retaining structure or buttress. The following provides a more detailed description of these measures. Control of Surface Water Reducing the amount of water entering the slope is probably one of'~he simplest and least costly risk reduction measures that can be made. This can be accomplished by conveying water collected in roof drains, yard drains, deck drains, driveways, or from other hard or impermeable surfaces around the residence in a tightline to the base of the slope near the beach. Relocation of the septic drainfield may also be beheficial depending on its existing location. The overall effectiveness is difficult to quantify and may not be significant, depending on how much groundwater in the silty clay comes from these sources. - 21-I-08976-001-L1 .DOC/WWSJM 2 1-1-08976-001 Mr. and Mrs. Jerry and Martina Haskins June 22, 2000 Page 6 SHANNON ~WlLSON. INC. Soldier Pile and Tieback Wall and Soil Anchors outwash The cost Depending on the subsurface conditions encountered, a soldier pile wall with tiebacks or tied-back soil anchors may be cost-effective methods to compensate for the reduced shear strength of the fractured silty clay. The purpose of the wall or soil anchors would be to retain the advance outwash overlying the silty clay by penetrating below the clay and holding the advance in place above the silty clay with anchors into the underlying stratified sand and silt. for this type of soil retai ,ning system could be o~n the order of $i00,000 to $150,000. Buttress Removal of the fractured silty clay and replacement with a high-strength material would effectively buttress the silty clay and overlying advance outwash, and would be used to regrade the slope to a more stable configuration. The buttress could be constructed of free-draining, . angular material (e.g., shot-rock, quarry spalls). The angular material is typically placed in sections so that the entire slope is not excavated at once, and compacted in 1- to 2-foot horizontal lifts by running back forth with heavy tracked excavating equipment (e.g., excavator or dozer). Drain pipes could also be placed at the base of the blanket along the face of the slope to collect water from the face of the native, in-place soils. Compacted angular material is typically stable on slopes as steep as 1.5 Horizontal to 1 Vertical (1.5H: iV) with an adequate factor of safety. Therefore, the face of the slope could be regraded to 1.5H: IV. Based on the dimensions of the slope, the cost for a buttress could be on the order of $200,000 to $300,000. Regardless of the risk reduction measures taken or not taken, there is present on all slopes, such as the one at this site, some risk of future instability that you or future owners must be willing to accept. Such instability may occur due to natural processes or future water breaks/leaks, uncontrolled drainage, unwise development in adjhcent areas, or other actions or events on a slope that may cause sliding. LIM/TATIONS The conclusions and recommendations in this letter are based on site conditions observed during .our site visits and assume that observed conditions are representative of the subsurface 21-1-08976-001-LI.DOC/WP/SJM 21-1-08976-001 Mr. and Mrs. Jerry and Martina Haskins June 22, 2000 Page 7 SHANNON &WILSON, INC, conditions throughout the site; i.e., the subsurface conditions are not significantly different from those observed during our site visit or indicated on geologic maps. If, during subsequent site activities (e.g., construction), subsurface conditions different from those inferred in 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. Within the limitations of scope, schedule, and budget, the analyses, conclusions, and recommendations presented in this letter/vere prepared in accordance with generally accepted professional geotechnical engineering principles and practices in this area at the time this letter was prepared. We make no other warranty, either expressed or implied. The conclusions and recommendations were based on our understanding of the project as described in this letter and the site conditions as observed at the time of our site visit. This letter was prepared for the exclusive use of Mr. and Mrs. Haskins in the evaluation of the recent slope movement. 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, such as those interpreted from the site visit and discussion of geologic conditions included in this letter. Please note that the scope of our services did not include any assessment or evaluation for the presence or absence of wetlands hazardous or toxic, mater/al in the soil, surface water, groundwater, or air On or below or around this site. We are able to provide these services and would be happy to discuss these with you if the need arises. Shannon & Wilson has prepared the attached, ".Important Information About Your Geotechnical Report," to assist you in understanding the use and limitations of our report. 21-1-08976-001 -LI.DOC/WP/SJM 21-1-08976-001 Mr. and Mrs. Jerry and Martina Haskins June 22, 2000 Page 8 SHANNON ~WILSON, INC. If you have any questions regarding our observations, conclusions or recommendations contained in this letter, please call me. Sincerely, SHANNON & WILSON, INC. NO. t William J. Perkins, R.P.G. Principal Geologic Engineer WJP:TCK/wjp Enclosures: Figure 1 - Vicinity Map Figure 2 - Site Plan Figure 3 - Generalized Profile A-A' Thomas C. Kinney, Ph.D., P.E. Vice President Important Information About Your Geotechnical Report 2 I-I -08976-001-LI .DOC/WP/SJM 21-1-08976-001 PROJECT LOCATION 0 1/2 1 Scale in Miles NOTE Map adapted from 1:24,000 USGS topographic map of Port Townsend South, WA quadrangle, dated 1953, photorevised 1981. 191 Lane DeChantal Port Townsend, Washington VICINITY MAP June 2000 21-1-08976-001 SHANNON & WILSON, INC, FIG. 1 Ge~technical and Environmenta[ Consullants Fife:l:'~:x~men[sl~.t-l'~08976~.l-1..08976-O01 Fig 2.dwg Dale: 06-0~000 Au~m~. LR · z Elevalion in Feet 1 Elevation in Feet Il SHANNON & WILSON, INC. Geotechnical and Environmental Consultants Attachment to and part of Report 21-1-08976-001 Date: June 22, 2000 To: Mr. and Mrs. Haskins Port Townsend, Washinston Important Information About Your GeotechnicaFEnvironmental Report CONSULTING SERVICES ARE PERFORMED FOR SPECIFIC PURPOSES AND FOR SPECIFIC CLIENTS. Consultants prepare reports to meet the specific needs of specific individuals. A report prepared for a civil engineer may not be adequate for a construction contractor or even another civil engineen 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-SPECIFIC FACTORS. A geotechnical/environmental report is based on a subsurface exploration plan designed to consider a unique set of project-specific factors. Depending on the project, these may include: the general nature of the structure 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 underground utilities; and the additional risk created by scope-of-service limitations imposed by the client. To help avoid costly problems, 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 nature 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 unrefrigerated one, or chemicals are discovered on or near the site); (2) when the size, elevation, or configuration of the proposed project is altered; (3) when the location or orientation 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. Subsurface conditions may be affected as a result of natural processes Or human activity. Because a geotechnicat/environmental report is based on conditions that existed at the time of subsurface exploration, construction decisions should not be based on a report 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 ora 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 JUDGMENTS. Site exploration and testing identifies actual surface and subsurface conditions only at those points where samples are taken. The data were extrapolated by your consultant, who then applied judgment to render an opinion about overall subsurface conditions. The actual interface between 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 nothing can be done to prevent such situations, you and your consultant can work together to help reduce their impacts. Retaining your consultant to observe subsurface construction operations can be particularly beneficial in this respect. A REPORT'S CONCLUSIONS ARE PRELIMINARY. The conclusions contained in your consultant's report are preliminary because they must be based on the assumption that conditions revealed through selective exploratory sampling are indicative of actual conditions throughout a site. Actual subsurface conditions can be discerned only during earthwork; therefore, you should retain your consultant to observe actual conditions and to provide conclusions. Only the consultant who prepared the report is fully familiar with the background information needed to determine whether or not the report's Page 1 of 2 1/2000 recommendations based on those conclusions are valid and whether or not the contractor is abiding by applicable recommendations. The consuItant who developed your report cannot assume responsibility or liability for the adequacy of the report's recommendations if another party is retained to observe construction. THE CONSULTANT'S REPORT IS SUBJECT TO MISINTERPRETATION. Costly problems can occur when other design professionals develop their plans based 6n misinterpretation of a geotechnical/environmental report. To help avoid these problems, the consultant should be retained to work with other project design professionals to explain relevant geote6hnical, geological, hydrogeological, and environmental findings, and to review the adequacy of their plans and specifications relative to these issues. BORING LOGS AND/OR MONITORING WELL DATA SHOULD NOT BE SEPARATED FROM TIlE REPORT. Final boring logs developed 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/environmental reports. These final logs should not, under any circumstances, be redrawn for inclusion in architectural or other design drawings, because drafters may commit errors or omissions in the transfer process. i To reduce the likelihood of boring log or monitoring well misinterpretation, contractors should be given ready access to the complete geotechnicaI engineerin~environmental report prepared or authorized for their use. If access is provided only to the report prepared for you, you should advise contractors of the report's limitations, assuming that a contractor was not one of the specific persons for whom the report was prepared, and that developing construction cost estimates was not one of the .specific purposes for which it was prepared. While a contractor may gain important knowledge from a report prepared for another party, the contractor should discuss the report with your consultant and perform the additional or alternative work believed necessary to obtain the data specifically appropriate for construction cost estimating purposes. Some clients hold the mistaken impression that simply disclaiming responsibility for the accuracy of subsurface information always insulates them from attendant liability. Providing the best available information to contractors helps prevent costly construction problems and the adversarial attitudes that ag~avate them to a disproportionate scale. READ RF_~PONSIBILITY CLAUSES CLOSELY. Because geotechnical/environmental engineering is based extensively on judgment and opinion, it is far less exact than other design disciplines. This situation has resulted in wholly unwarranted claims being lodged against consultants. To help prevent this problem, consultants have developed a number of clauses for use in their contracts, reports and other documents. These responsibility clauses are not exculpatory clauses designed to transfer the consultant's liabilities to other parties; rather, they are definitive clauses that identify where the consultant's responsibilities begin and end. Their use helps all parties involved recognize their individual responsibilities and take 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 questions. The preceding paragraphs are based on information provided by the ASFF_,/Association of Engineering Firms Practicing in the Geosciences, Silver Spring, Maryland II I! II II I! Page 2 of 2 1/2000 ~