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HomeMy WebLinkAbout921043013 Geotech AssessmentGeologic Slope Stability Evaluation of Burke Property 5945 Flagler Road, Marrowstone Island Nordland, Washington November 2000 'GEOTECHNIcAI;. AND' ENVIRONMENTAL.CONSUI'TAN'T8 At Shannon & Wilson, our mission is to be a progressive, well- managed professionaI consulting firm in the fields of engineering and applied earth sciences. Our goal is to perform our services with the highest degree of professionalism with due consideration to the best interests of the public, our clients, and our employees. Submitted To: Ms. Stephanie Burke P.O. Box 191 Nordland, Washington 98358 By: Shannon & Wilson, Inc. 400 N 34th Street, Suite 100 Seattle, Washington 98103 21-1-09215-001 I I --IIISHANNONSWiLSON INC. ~ GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS SEATTLE RICHLAND FAIRBANKS ANCHORAGE SAINT LOUIS BOSTON November 14, 2000 Ms. Stephanie Burke P.O. Box 191 Nordland, WA 98358 GEOLOGIC SLOPE STABILITY EVALUATION OF BURKE PROPERTY, 5945 FLAGLER ROAD, MARROWSTONE ISLAND, NORDLAND, WASHINGTON Dear Ms. Burke: This letter summarizes our observations, conclusions, and recommendations regarding the stability and development of the Burke property at 5945 Flagler Road on Marrowstone Island for a single-family residence. These conclusions are based on observations made during our visit to the site on November 11, 2000, and published area geologic, hazard, topographic, and soil maps. SITE DESCRIPTION The referenced property is located on Scow Bay on the Southwest side of Marrowstone Island, as shown on Figure 1. As indicated on Figure 2, the property is bounded by Flagler Road on the north and Scow Bay on the south. The property is approximately 320 to 380 feet long (north- south) by about 56 feet wide (east-west). Topography across the site consists of the following (from south to noah): · A beach · A near-vertical waterfront bluff, (approximately 35 feet high) that extends from the beach on the south up to the upland portion of the site on the noah. · A relatively gently-sloping upland at the top of the bluff that slopes up to the northeast at about 9 degrees. A generalized profile that shows the approximate topography is- sketched on Figure 3. The beach consists mostly of sand and gravel, with scattered driftwood along the toe of the bluff. Also lying along the toe of the bluff are toppled trees with root balls still attached, Which have apparently toppled over the crest of the slope onto the beach. No backshore is present, and the foreshore extends up to the toe of the bluff. 400 NORTH 34TH STREET' SUITE 100 RO. BOX 300303 SEATTLE, WASHINGTON 98103 206.632.8020 FAX 206-633"6777 TDD: 1'800'833-6388 21-1-09215-001 Ms. Stephanie Burke November 14, 2000 Page 2 SHANNON &WILSON, INC. The bluff is typically near-vertical. A Small, steeply sloping bench located about 5 feet above the beach is present along portions of the bluff. There is no vegetation over most of the bluff face other than scattered grasses on the small bench near the base. At the crest of the bluff, fir and madrona trees up to 1 ½ feet in diameter extend roots and trunks part way down over the face of ' the bluff. The upland portion of the site is vegetated with native fir and madrona trees up to about 2 feet in diameter, with an undergrowth of salal and Oregon grape. This native vegetation is-irindicative of relatively well-drained near-surface soil conditions. The existing septic drainfield and sand filter and the proposed residence are located on the upland portion of the site, the approximate locations of which are indicated on Figure 2. A wooden stairway that extends from the beach to the upland portion of the site was under construction at the time of our site visit. GEOLOGIC CONDITIONS Published geologic maps of the area indicate that the site is underlain by Pleistocene-age (13,500 to 17,000 years old) Vashon Lodgment Till and older Vashon Advance Outwash. Vashon Advance OutWash typically consists of sand with lesser amounts of silt and gravel. The advance outwash was deposited on the pre-existing land surface, in front of the continental Vashon Stade ice sheet that advanced from Canada across the Puget Sound region approximately 17,000 years ' ago. Lodgment till is typically an unsorted mixture of clay, silt, sand, and gravel with occasional cobbles and boulders, which was deposited directly beneath the ice sheet as the glacier advanced over the area. The Vashon Lodgment Till was deposited directly beneath Vashon Stade ice sheet that covered this area appro_.ximately 13,500.to 17,000 years before present. The ice sheet that ? . overrode the till and the underlying soils (including the advance outwash) is estimated to have been on the order of 3,000 to 4,000 feet thick in this area. Consequently, the till and the underlying soils have been compacted to a very dense or hard state. Since the retreat of the glaciers, the upper few feet of the very dense/hard soil has loosened and weathered, and topsoil has developed at the ground surface. Subsurface explorations' were not performed at this site for this evaluation; however, soils exposed on the bluff on the property confirm the presence of the till beneath the site. Advance outwash was observed on the adjacent property to the northwest in the bottom 3 feet of a portion 21-1-09215-001-L1 .DOCdWP/EET 21-1-09215-001 I m. I II II Ms. Stephanie Burke November 14, 2000 Page 3 SHANNON 6Wi~N, INC. of the bluff. The till consisted of very dense, non-sorted, slightly silty to silty, gravelly sand with scattered cobbles. The advance outwash appeared to be a very dense sand to sandy silt. .. In addition to the very dense till and advance outwash, it appears that slough from the face of the bluff has accumulated along the toe of the bluff, producing the small, steeply sloping bench along the toe. No signs of springs, seeps, damp soils, or Other indications of near-surface water-were observed on the property or on the bluff on the adjacent properties. CONCLUSIONS AND RECOMMENDATIONS Slope Stability Geologic hazard maps of the area classify the bluff as unstable. During our site visit, we observed blocks of hard/very dense till near the base of the bluff that have spalled off the face of the bluff. The trees that have fallen onto the beach were likely supported by some of these blocks. Spalling will likely continue in the future. Specifically, vertical cracks were observed in the till exposed on the bluff. It is likely that as water, roots, freeze/thaw and other forces extend and widen these cracks, portions of the bluff will spall off in the future. Based on our observations of the site, it appears that the unstable soils are primarily blocks that. spall off the near-vertical bluff face. While there may be some risk of deep-seated slope movement at the site (i.e., failure through Several tens of feet of the very dense glacial.soils beneath the site), it is our opinion that this risk is relatively low because evidence of deep-seated movement was not observed during our site visit. With enough time, the accumulation of spalled soils (or slough) at the base of the bluff and continued weathering and erosion of the glacially overridden soil beneath the bluff would result in a flatter, more stable slope. However, wave erosion at the toe of the bluff does not allow the Slough to accumulate at the toe of the slope, but maintains the slope in an over-steepened condition. Consequently, spalling of the bluff face should be expected to continue in the future. Please note that there is some risk Of future instability (shallow or deep-seated) present on' all hillsides, which the owner must be prepared to accept. Such instability could occur because of future water line breaks/leaks, uncontrolled drainage, unwise development in adjacent areas, or l! 21-1-09215-001-L1.DOC/WP/BET 21-1-09215-001 Ms. Stephanie Burke November 14, 2000 Page 4 SHANNON 6WILSON, INC. other actions or events on a slope that may cause sliding. The following section provides further discussion of risk reduction measures that may. be effective at this site. Provided that the risk reduction measures discussed in this letter are implemented, it is our opinion that the pioposed development will not adversely impact the stability of the adjacent properties. Measures to Reduce the Risk Posed by Slope Movement In general, the risk of soil movement on a slope can be reduced by not over-steepening the slope (e.g., avoiding excavation at the toe of the slope); not increasing the weight on the slope (e.g., avoiding placement of yard debris or fill at the crest of the slope); maintaining the slope as dry as possible (e.g., routing roof downspouts and yard drains to the base of the slope or storm drain system, minimizing the amount of surface water that could flow down the face of the slope); and maintaining a vegetative cover on or above the slope. In addition, measures that can be taken to reduce or minimize the rate of wave erosion at the toe of the slope (e.g., construction of a seawall, not removing large wood debris or driftwood near the top of the beach) will decrease the rate at which the slope erodes. Building Set Back The measures discussed above may reduce the risk of soil movement on a slope. One of the most cost-effective measures to reduce the impact of slope movement is to provide an adequate building setback. An appropriate building setback is a function of the rate of slope' .. regression, the design life of the structure, and the risk the owner of the structure is willing to assume. The regression rate for this specific slope is unknown; regression rates measured elsewhere in the Puget Sound area are on the order of a few inches to one foot per year. The size (up to 1 ¥2 feet in diameter) of roots and tree-trunks that extend part way down over the face of the bluff would tend to indicate a relatively slow rate of regression, probably on the order of a few inches per year. In our opinion, a minimum building set-back equal to the height of the slope (i.e., 35 feet) would be adequate for this site. Greater risk reduction can be achieved with larger building set-backs, such as 1.5 or 2 times the height of the slope. The actual rate of slope regression will likely vary from-year to year (e.g., some years no noticeable regression may occur, while in other years the slope may regress by several feet due to slope movements). By implementing the measures outlined in this letter for reducing the risk of slope movement, the rate of slope regression may also be reduced. 21-1-09215-001-L1.DOC/WP/EET 21-1-09215-001 I i I I I Ms. Stephanie Burke November 14, 2000 Page 5 SHANNON&WILSON, INC. Drainage In general, reducing the amount of water entering and discharging onto the bluff can reduce the risk of slope movement. Drains should be constmcted and maintained to collect water from impermeable surfaces on the property (e.g., roof, decks, patios, and driveways) and directed to a suitable discharge point (e.g., bottom of the bluff). If water is discharged to the toe of the bluff,, we recommend that it be conveyed in a flexible tightline on the surface of the bluff and periodically inspected and maintained. The tightline can be supported by steel stakes driven into the soils on the crest of the bluff. Stainlegs or galvanized steel cable and clamps can be used to attach the tightline to stakes. Alternatively, the tightline could be supported on the wooden stairway under construction. Splices in the tightline on the bluff should be avoided, ff tightline splices occur on the bluff, the splice should be supported and reinforced using stainless or galvanized steel cable attached to the tightline both abo3e and below the splice. The water collected in the tightline should be discharged as close to the beach elevation as allowable. This is typically at or immediatelY above the high water elevation, and could be on top of the small steep slope at the bottom of the bluff, in an area behind some of the larger driftwood. An energy dissipater should be located at the discharge point. The energy dissipater may be constructed by connecting 3 feet (total width) of perforated drain pipe to form a '°I"' at the end of the tight line. The '°I"' should be located on a minimum 12-inch-thick layer of 6-inch or larger diameter cobbles or quarry spalls that extend a minimum of 3 feet beyond the ends of the perforated pipe that forms the '°I'." The '°-I"' may be covered with cobbles or quarry spalls to- help secure and hide the '~r." It may be necessary to add rock to the energy dissipater from time to time due to erosion of the small slope. Based on our understanding of the limited, single-residence development of this property and the relatively well-drained nature of the soils that underlie the upland portion of the .site, it is our opinion that the anticipated discharge of roof and footing drains as recommended above will not significantly affect the drainage conditions on the adjacent properties from pre-development conditions. Impermeable surfaces surrounding the residence (e.g., paved drives) should be minimized to reduce potential changes in the existing site drainage characteristics and impacts on adjacent sites. 21-1-0921S-O01-L1.DOC/WP/EET 21-1-09215-001 ! I I Ms. Stephanie Burke November 14, 2000 Page 6 ~NON &VVII_SON. INC. Vegetation Maintaining a healthy vegetative cover above the bluff can reduce erosion and the rate of slope regression. In general, native vegetation should be used on the top of the bluff t~) eliminate the need for irrigation and wetting the soils on or near the bluff. A healthy vegetative cover may include large, healthy trees. Unhealthy trees, snags or other trees with a weak root system could be removed or limbed to reduce the risk of potential erosion and slope movement caused by potential uprooting during heavy winds. If trees-are cut from the slope, the stumps-should not be removed and the area around the stump should be vegetated. A professional landscaper, landscape architect, arborist, or other qualified professional should be consulted in assessing the health of the trees and vegetation on the slope and vegetation that may be planted. Erosion Hazard We note that according to published USDA soil maps, surficial soils on the upland portion of the site are classified as Whidbey gravelly sandy loam D on 15 to 30 percent slopes. The USDA maps indicate that these soils have moderate erosion hazard. The soil maps do not classify the soils on the bluff. It is anticipated that the development on the'upland portion of the site will not significantly affect soil erosion and associated hazard on the site provided the recommendations in this letter are followed and prudent construction practices with respect to erosion are used. LIMITATIONS The conclusions in this letter are based on site conditions visually observed during our site reconnaissance and inferred from published geologic, topographic, and hazard maps, 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 inferred from the site reconnaissance or indicated on geologic maps. During subsequent site activities (e.g., construction), if 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 conclusions where necessary. Within the limitations of scope, schedule, and budget, the conclusions presented in this letter were prepared in accordance with generally accepted geologic engineering principles and practices in this area at the time this letter was prepared. We make no other warranty, either express or implied. 21-I-09215-001-L1.DOC/WP~ 21-1-09215-001 ! Ms. Stephanie Burke November 14, 2000 Page 7 SHANNON &WILSON, INC. This letter was prepared for the use of Ms. Burke in the evaluation of the stability of this site. With respect to possible furore 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 environmental assessment or evaluation regarding the presence or absence of wetlands or haZardous or toxic material 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 pleased 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. We appreciate the opportunity to provide geologic services to you, and are available to answer any questions regarding our observations and conclusions contained in this letter. Sincerely, ! William J. Perkins, R.P.G. Principal Engineering Geologist WJP:GJB:wjp Enclosures: Figure 1 - Vicinity Map Figure 2 - Site Plan Figure 3 - Generalized Profile Important Information About Your Geotechnical Report c: Michael Anderson, Tillman Engineering 21-1-o9215-001-L1 .~/EET 21-1-09215-001 5 \ 'x .'$ C'0 W' .T PROJECT LOCATION -% -. jle Point · · · /- ~,. ~:." , ¥i:'::"'.... .. '.2: .' .:, · .- . P~int 0 1/2 1 Scale in Miles NOTE Map adapted from 1:24,000 USGS topographic map of Nordland, WA quadrangle, dated 1953, photorevised 1973. 5945 Flagler Road Nordland, Washington VICINITY MAP November 2000 21-1-09215-001 SHANNONc. eot~,nd ~ & WlLSON,co~u~t~INC. I FIG. File: h~mffing~11~09215-001~21-1-09215-001 Fig 2,dwg Date: 11-13-2000 Author:. LR oo o . I N I I I i I I I I I I I ii lee_-I u! uo.qe^el~ eleLu.[xoJddv FIG. 3 SHANNON & WILSON, INC. Geotechnical and Environmental Consultants Attachment to and part of Report 21-1-09215-001 Date: November 14, 2000 To: Ms. Stephanie Burke Nordland, Washington Important Information About Your Geotechnical/Environmental Report :ONSULTING SERVICF~ ARE PERFORMED FOR SPECIFIC PURPOSE~ AND FOR SPECWIC CLIENTS. :onsultants prepare reports to meet the specific needs of Specific individuals. A report prepared for a civil engineer may not be adequate ~r a construction contractor or even another civil engineer. Unless indicated otherwise, your consultant prepared your report expressly >r you and expressly for the purposes you indicated. No one other than you should apply this report for its intended.p_urpose without first onferfing with the consultant. No party should apply this report for any purpose other than that originally contemplated without first onferring with the consultant CONSULTANT'S REPORT IS BASED ON PROJECT-SPECIFIC FACTORS. ~ geotectmical/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 stmcture and property involved; its size and configuration; its dstorical use and practice; the location of the structure On the-site and its orientation; other improvements such as access roads, parking ots, and underground utilities; and the additional risk created by scope-of-service limitations imposed by the client. To help avoid costly ~roblems, 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 :xample, if an office building will be erected instead of a parking garage, or if a refrigerated warehouse will be built instead of an mrefrigerated one, or chemicals are discovered on or near the site); (2) when the size, elevation, or configuration of the proposed project s altered; (3) when the location or orientation of the proposed project is modified; (4) when there is a change of ownership; or (5) for tpplication to an adjacent site. Consultants cannot accept responsibility for problems that may occur if they are not consulted after factors ~hich were considered in the development of the report have changed. ~UBSURFACE CONDITIONS CAN CHANGE. ~ubsurface conditions may be affected as a result of natural processes or human activity. Because a geotechnical/environmental report is >ased 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 stare; for ~xample, 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 geotechnicoJ/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 PROFESSION~ JUDGMENTS. Site exploration and testing identifies actual surface and subsurface conditions only at those points where sample~ 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. consultant who developed your report cannot assume responsibility or liability for the adequacy of the report's recommendations if party is retained to observe construction. THE CONSULTANT'S REPORT IS SUBJECT TO MISINTERPRETATION. Cosily problems can occur when other design professionals develop their plans based on misinterpretation of a geotechnical/environm~ report To help avoid these problems, the consulmt should be retained to work with other project design professionals to explain rele geotechnical, geological, hydrogeological, and environmental findings, and to review the adequacy of their plans and specifications rel~ to these issues. BORING LOGS AND/OR MONITORING WELL DATA SHOULD NOT BE SEPARATED FROM THE REPORT. Final boring logs developed by the consultant are based upon interpretation of field logs (assembled by site personnel), field test and laboratory and/or office evaluation of field samples and data. Only final boring logs and data are customarily include geotechnical/environmental reports. These final logs should not, under any circumstances, be redravmfor inclusion in architectural or design drawings, because drafters may commit errors or omissions in the transfer process. To ~educe the likelihood of boring log or monitoring well misinterpretation, contractors should be given ready accegs to the corn geotechnical engineering/environmental report prepared or authorized for their use. If access is provided only to the report prepare you, you should advise contractors of the report's limitations, assuming that a contractor was not one of the specific persons for whox report was prepared, and that developing construction cost estimates was not one of the specific proposes for which it was prepared. a contractor may gain important knowledge from a report prepared for another party, the contractor should discuss the report with consultant and perform the additional or alternative work believed necessary to obtain the data specifically appropriate for constm cost estimating purposes. Some clients hold the mistaken impression that simply disclaiming responsibility for the accuracy of subsu information always insulates them from attendant liability. Providing the best available information to contractors helps prevent c construction problems and the adversarial attitudes that aggravate them to a disproportionate scale. READ RESPONSIBILITY CLAUSES CLOSELY. Because .geotechnicaYenvironmental engineering is based extensively on judgment and opinion, it is far less exact than other d disciplines. This situation has resulted in wholly unwarranted claim~q being lodged against consultants. To help prevent this prol consultants have developed a number of clauses for use in their contracts, reports and other documents. These responsibility claus~ not exculpatory clauses designed to transfer the consultant's liabilities to other parties; rather, they are definitive clauses that identify x the consulmt's responsibilities begin and end. Their use helps all parties involved recognize their individual r ._.esponsibilities an~ appropriate action. Some of these definitive clauses are likely to appear in your report, and you are encouraged to read them closely. consultant will be pleased to give full and frank answers to your questions. The preceding paragraphs are based on information provided by the ASFE/Association of Engineering Firms Practicing in the Geosciences, Silver Spring, Maryland Page 2 of 2