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HomeMy WebLinkAbout998200305 Geotech AssessmentGeologic Slope Stability Evaluati Stair Access to Bea 340 Tala Shores ro Port Ludlow, Washingt t JEFFERSON COUNTY OEPT. OF COMMUNI~ DEVELOPMENT July 20 At Shannon & Wilson, our mission is to be a progressive, well- managed professional consulting finn in the fields of engineering and applied earth sciences. Our goal is to perform our se~n;ices ~.~,ith the highest degree of professionalism ~vith due consideration to the best interests of the public, our clients, and our employees. Submitted Ms. Cathy Gilm 340 Tala Shores R~ Port Ludlow, Washington 98; Shannon & Wilson, 400 N 34t" Street, Suite Seattle, Washington 98 21-1-09399-~ SHANNON &WILSON. INC. July 6, 2001 Ms. Cathy Gilmore 340 Tala Shores Road Port Ludlow, WA 98358 SEATTLE RICHLAND FAIRBANKS GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS ANCHORAGE DENVER SAINT LOUIS BOSTON JEFFERSON COUNTY l DEPT. OF COMMUNITY DEVELOPMENT GEOLOGIC SLOPE STABILITY EVALUATION FOR STAIR ACCESS TO BEACH, 340 TALA SHORES ROAD, PORT LUDLOW, WASHINGTON Dear Ms. Gilmore: This letter summarizes our observations, conclusions, and recommendations regarding the stability of the slope at the above referenced property for construction of a stairs to the beach. These conclusions are based on observations made during our visit to the site on May 5, 2001, and published area geologic, hazard, topographic, and soil maps. SITE DESCRIPTION The referenced property is located on the west side of Hood Canal near Tala Point, as shown on Figure 1. As indicated on Figure 2, the property is bounded by Tala Shores Road on the west and Hood Canal on the east. From Tala Shores Road on the west, the property extends to the east approximately 275 feet to Hood Canal; in the north-south direction, the property is approximately 77 to 80 feet wide. Topography across the site consists of the following (from east to west): · Beach · Steep waterfront slope, (approximately 30 to 35 feet high) that extends from the beach on the east to the upland portion of the site on the west. · Relatively gently-sloping upland that slopes up to the west at about 3 to 10 degrees on which the existing residence is located. A generalized profile that shows the approximate topography is sketched on Figure 3. 400 NORTH 34TH STREET · SUITE 100 P.O. BOX 300303 SEATTLE. WASHINGTON 98103 206'632-8020 FAX 206-695,6777 TDD: 1'800'833,6388 21-1-09399-001 Ms. Cathy Gilmore July 6, 2001 Page 2 i JEFFERSON COUN i'"Y ..... ',.,.,.., :.,~, v, DEVELOPMENT ._ SHANNON ,SWILSON, ~NC. The beach consists mostly of sand and gravel with cobbles and boulders and scattered driftwood along the toe of the slope. Also lying along the toe of the slope are scattered, toppled trees and root balls, which have apparently moved down the slope onto the beach. No backshore is present, and the foreshore extends up to the toe of the slope. The waterfront slope extends from the beach up to the west at about 48 to 50 degrees with near vertical soctions, particularly along the toe. Vegetation on the slope includes salmon berry and alder trees up to about 1 foot in diameter. Vegetation at the crest of the slope includes cedar trees (up to 2 feet in diameter) and fir trees (up to 1 foot in diameter). Most of the trees on the slope or at the crest of the slope have trunks that are bowed down-hill. This generally indicates that the soils in which the trees are located are moving down-slope. A small bench has been excavated into the slope near its crest for the existing path. The existing wood stairs extend from the path down to the beach. The existing stairs are approximately 28 feet high, slope down to the beach to the east at about 38 degrees, and extend about 11 feet beyond (east of) the toe of the slope. The upland portion of the site is landscaped around the existing residence and includes a lawn area between the house and the crest of the slope. We understand that the proposed stairs will consist of an aluminum frame located in approximately the same area as the existing wood stairs. However, the distance the existing stairs extend beyond the toe of the slope will be reduced by constructing a platform on which the new stairs will land. We understand that the foundations for the platform will include a monolithic slab constructed below the beach level. Four concrete posts (approximately 1 foot in diameter) will extend above the slab and beach to support the platform on which the lower end of the stairs will rest. We understand that the upper end of the stairs will be attached to a concrete anchor block that will extend about a 2- to 3-below the ground surface. 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. Lodgment till is typically an unsorted mixture of clay, silt, sand, and gravel with occasional cobbles and boulders, which is deposited directly 21-1-09399-001 -L1/WP/MG I 21-1-09399-001 Ms. Cathy Gilmore July 6, 2001 Page 3 ~' "" :-- ~ "' S~gg-NNON ~W~LSON, ~NC. JEFFERSON COUNTY DEPT. OF COt',,.MuN} ~Y DEVELOP~'.~ENT beneath a glacier. The Vashon Lodgment Till was deposited directly beneath Vashon Stade ice sheet that covered this area approximately 13,500 to 17,000 years before present. The ice sheet that overrode the till and the underlying soils 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. Subsurface explorations were not performed at this site for this evaluation; however, soils exposed on the steep slope on and adjacent to the property confirm the presence of till beneath the site. The till observed at the site consists of very dense, non-sorted, trace to silty, gravellY sand with scattered cobbles. Since the retreat of the glaciers, the upper few feet of the very dense soil has loosened and weathered, and topsoil, colluvium and/or slide deposits have developed at the ground surface. Colluvium is weathered material that has reached its present location due to the forces of water and gravity and is typically found on, and at the base of steep slopes. The topsoil and colluvium that develop on the steep slopes typically have a lower shear strength (i.e., are not as strong) as the underlying, very dense glacially overridden sediment. Consequently, the topsoil and/or colluvium may move, resulting in slide deposits on and near the base of the steep slopes. Light seepage was observed on the slope on the property to the north, about 10 feet north of the property line and 8 feet above the toe of the slope. The seepage zone did not appear to be continuous along the face of the slope as no signs of springs, seeps, damp soils, or other indications of near-surface water were observed on the slope on the subject property (340 Tala Shores Road) or on the adjacent property to the south. CONCLUSIONS AND RECOMMENDATIONS Slope Stability Geologic hazard maps of the area classify the slope as unstable. During our site visit, we observed scattered, toppled trees and root balls at the base of the slope on the beach, which have apparently moved down the slope as a result of instability of the soils on the slope. 21-1-09399-001 -L 1/WP/MGI 21 - 1-09399-001 Ms. Cathy Gilmore July 6, 2001 Page 4 JEFFERSON COUNTY DEPT. OF COMMUNITY OEVELOPMENE SPLA,NNON &W~LSON, INC. Based on our observations of the site, it appears that the unstable soils consists primarily of the topsoil and colluvium on the slope and that the risk of deep-seated slope movement is relatively low, in our opinion. The very dense, glacially overridden lodgement till that presumably underlies the slope may be stable on relatively steep slopes (e.g., 50 degrees or more). However, the relatively loose topsoil and colluvium are not as strong, and they are susceptible to movement on slopes where the underlying glacially overridden soils are relatively stable. Recent slope movements of topsoil and Colluvium on the slope are indicated by the areas where vegetation is sparse or absent, and toppled trees and root balls exist at the base of the slope. Signs of deep- seated slope movement (i.e., failure through several tens of feet of the very dense glacial soils beneath the site) were not observed during our site visit. While there may be some risk of deep- seated slope movement at the site, it is our opinion that this risk is relatively low because of the lack of evidence of deep-seated movement observed during our site visit and the nature of the very dense soils observed along the slope. With enough time, movement of colluvium and topsoil toward the base of the slope and continued weathering and erosion of the glacially overridden soil up-slope would result in a flatter, more stable slope. However, wave erosion at the toe of the slope does not allow the colluvium, topsoil, and slide debris to accumulate at the toe of the slope and maintains the slope in an over-steepened condition. Consequently, continued movement of topsoil and colluvium on the slope should be expected in the future. Potential future movements could require repair or reconstruction of the proposed stairs. It is our opinion that the proposed stairs would not adversely impact the stability of the slope on this or adjacent properties. Provided that prudent construction practices are implemented, it is unlikely that the risk of slope movement would increase significantly during construction. 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 other actions or events on a slope that may cause sliding. 21-1-09399-001-Ll/WP/MGI 21-1-09399-001 Ms. Cathy Gilmore July 6, 2001 Page 5 ,..,~ ,iON &WILSON, INC. Foundations and Alternate Platform Location To reduce the potential for future slope movements to impact the stairs, we recommend that all stair foundations (anchor and platform) penetrate at least 2 feet into very dense, undisturbed till. Foundations should not be located in topsoil and/or colluvium as these are more susceptible to movement if they area founded in these materials. At the request of the stair designer, Tuscan Windorn, we reviewed alternative platform locations that would not be located on the beach. We understand that the purpose of this request was to evaluate alternatives that may have less impact on the beach and wildlife. As previously indicated, much of the lower portion of the slope is near vertical, and as such, is not practical for foundation construction. Other portions of the toe of the slope that are less steep appear to consist of recent slide deposits and as such, are expected to erode relatively quickly to the existing beach level. Platform foundations constructed in these slide deposits would have to penetrate below the slide debris into the underlying very dense till. As the slide deposit is eroded from around the platform, the platform would effectively be located on the beach beyond (east of) the toe of the slope. In addition, as the slide deposit around the platform is eroded by wave action, the slope would become oversteepened, and additional slope movements could occur that could damage the platform. Erosion Hazard We note that according to published USDA soil maps, surficial soils on the upland portion of the site are classified as Cassalory-Kitsap series on 0 to 30 percent slopes, while the soils at the toe of the slope are indicated to be Coastal Beach. The soil maps do not classify the soils on the slope, but based on the observed naturally occurring erosion on this steep slope, it is expected that the erosion hazard associated with the topsoil, colluvium, and slide deposits would be relatively high. The erosion hazard associated with the underlying till is expected to be relatively low. It is anticipated that the stair replacement will not significantly affect soil erosion and associated hazard on the site provided that prudent construction practices with respect to erosion are used, and that foundation construction for the anchor is accomplished during dry weather. 21-1-09399-001-L1/WP/MGI 21-1-09399-001 Ms. Cathy Gilmore July 6, 2001 Page 6 -S~NNON 8WILSON, INC. LIMITATIONS The conclusions and recommendations presented 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. This letter was prepared for the use of Ms. Gilmore in the evaluation of the stability of this slope. 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 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. 21-1-09399-001-L1/WP/MGI 21-1-09399-001 Ms. Cathy Gilmore July 6, 2001 Page 7 JUL JEFFERSON COUNTY DEPT. OF COMMUNITY DEVELOPMENT -- , SHANNON ~WILSON, INC. 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, SHANNON & WILSON, INC. William J. Perkins, R.G. Senior 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: Tuscan Windom 21-1-09399-001-L 1/WP/MGI 21 - 1-093 99-001 ' \ ,, i:~ , ' '" ' "" ' ' : ' ' ' \ ' " / /; ' '~ ~' ._.: ',,. ! ( ~ ", ~ : ::.: s · . ' /' ..... -.. . \ ! ' \ · WI~ *!! ............................... / ........ "~'X(~'}!::' :}~-)-ji ~ i~ i:x~,ii!!21;ii-i.,iil.--; --:-; --" .............................' .:.- ....: ,.-?."..."'t'"'~'' -}~" . -. '4,,kkx\\ / ~. ' : Id'::':" .... '?;; ...... +,4: '": : ~ ' i ' ' ,~ ,0~- - '-.'., zz,.?:,,,~ ? 1 .,,,., ,,:.,.,-,:.,, ..,.,, ,. , , '' ~.',-.: .-'' .". '~.~ :?.' ~- ,~"-"";' ~.;5;:/:/ ~ i~' '. '-' ' · .-...~ :,' :..'. , ..~.'.--' ,~:G.:-~' ,/ ~'l .,- ×-~:~', s t . . ~ ","':?" o //.,:;' ,.,, . /';>}:, t, ,' i . ',, . .. 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' , ~': "i , =~'~..?:.-:,;~ '-"i i'~"', ' ', - 0 1/2 I i i i I I Gilmore Beach Access Scale in Miles 340 Tala Shores Road Port Ludlow, Washington NOTE VICINITY MAP Map adapted from 1:24,000 USGS topographic map of Port Ludlow, WA quandrangle, dated 1953, July 2001 21-1-09399-001 photorevised 1973. SHANNON & WILSON, INC. I FIG. Geotachnical and Environmental Consultants I Z' // ~Z xci.E FIG. 2 . DEP~ ~,-~E~.~RSON COUNTY ,. ur UUMMUNITY DEVELOPMENT 50 40 A West Existing Residence I Topsoil A~ East 50 40 30 20 10 Very dense, trace to slightly silty, gravelly SAND (Vashon Lodgement Till) Colluvium/Slide Deposit (typical) 30 Existing/Proposed 20 Stairs 10 -10 0 -10 10 20 30 40 50 60 70 80 90 100 0 20 I I I I I Scale in Feet (horizontal: vertical) 40 NOTES The topographic profile and geologic conditions are based on field observations. Variations between the profile and actual topography and geologic conditions are likely. The residence and stair dimensions are for illustrative purposes only. The actual sizes may vary. Gilmore Beach Access 340 Tala Shores Road Port Ludlow, Washington GENERALIZED PROFILE July 2001 21-1-09399-001 SHANNON & WILSON, INC. Geotechnicol and Environmental Consultants FIG. 3 Attachment to and part of Report 21-1-09399-001 Date: July 6, 2001 To: Ms. Cathy Gilmore Port Ludlow, Washington JEF~!:.R$ON COUNTY ~}EPT 5;F OOL'~'.?~_~r-i!"~-Y OEVELOPMENT Important Information About Your Geotechnical/Environmental 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 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-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 geotechnical/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 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 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 1/2001