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HomeMy WebLinkAbout998200323 Geotech AssessmentDraft Geologic Slope Stability Evaluation Residential Development of 650 Tala Shores Road Port Ludlow, Washington 42O03 Draft Geologic Slope Stability Evaluation Residential Development of 650 Tala Shores Road Port Ludlow, Washington -'3PM£Nr ~,pril 2003 At Shannon & Wilson, our mission is to be a progressive, weli- ma~kaged professional 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: Mr. Edward Foster c/o Ms. Catherine Hendy John L Scott Realty 40 Teal Lake Road Port Ludlow, Washington 98365 By: Shannon & Wilson, Inc. 400 N 34th Street, Suite 100 Seattle, Washington 98103 21-1-09874-001 SEATTLE RICHLAND FAIRBANKS ANCHORAGE DENVER SAINT LOUIS BOSTON April 3, 2003 Mr. Edward Foster c/o Ms. Catherine Hendy John L. Scott Realty 40 Teal Lake Road Port Ludlow, WA 98365 DRAFT GEOLOGIC SLOPE STABILITY EVALUATION FOR RESIDENTIAL DEVELOPMENT OF 650 TALA SHORES ROAD, PORT LUDLOW, WASHINGTON Dear Mr. Foster: This letter report summarizes our observations, conclusions, and recommendations regarding slope stability and development of the property referenced above for a single-family residence. Our review of the Jefferson County Geologic Hazard Area Maps indicates that the slopes on the site may be susceptible to movement. Therefore, we have prepared this report in accordance with the Unified Development Code for Jefferson County to evaluate the potential for slope movement and provide recommendations for development of the site with respect to slope stability. These conclusions and recommendations are based on observations made during our reconnaissance of the site on February 27, 2003, available published geologic, topographic, and soil maps, and an undated, hand drawn site plan prepared for the SPAAD application. SITE DESCRIPTION The referenced property is located on the west side of Hood Canal near Tala Point, as shown on Figure 1. Figure 2 indicates that 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 385 feet to Hood Canal; in the north-south direction, the property is approximately 80 feet wide. Topography across the site consists of the following (from east to west): 400 NORTH 34TH STREET ' SUITE 100 RO. BOX 300303 SEATTLE, WASHINGTON 98103 206.632.8020 FAX 206-695.6777 TDD: 1.800.833.6388 21-1-09874-001 Mr. Edward Foster c/o Ms. Catherine Hendy April 3, 2003 Page 2 SHANNON&WILSON, INC. · Beach. · Steep waterfront slope, (approximately 45 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 6 to 8 degrees. A generalized geologic profile that shows the approximate topography is sketched on Figure 3. 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 45 to 50 degrees with near vertical sections, particularly along the toe. Vegetation on the slope includes horse tails and other hydrophilic species, salmon berry and small alder trees (up to about 1 foot in diameter). At the crest of the slope, an approximately 20-foot long (east-west) by 20 foot wide (north- south) by about 15 to 20-foot deep scallop or'bowl is present near the north property line, that extends west, into the upland portion of the site (see Figure 2). The shape and location of this feature at the crest of the slope suggest that it is a relatively recent slide scarp. The eastern two-thirds of the upland portion of the site has been cleared of trees some time in the past and is mostly vegetated with salmon berries and grass. The western part of the upland portion of the site includes cedar and maple trees, up to about 2 feet in diameter. We understand that the proposed development of the site includes construction of a single family residence and on-site sewage disposal. The approximate location of the existing wells on and adjacent to the site and the location and dimensions of the proposed residence and septic drain fields for the sewage disposal system are shown on Figure 2. 21-1-09874-001-LRl.doc/wp/eet 21-1-09874-001 Mr. Edward Foster c/o Ms. Catherine Hendy April 3, 2003 Page 3 SHANNON ~,WILSON, INC. GEOLOGIC CONDITIONS Published geologic maps of the area indicate that the site is underlain by Pleistocene-age 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 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, gravelly silty sand with scattered cobbles and boulders that extends from the beach level at the toe of the slope up to within about 15 to 20 feet of the top of the slope. While not indicated on geologic maps, the upper 15 to 25 feet of the site appears to be mantled with Vashon recessional outwash. Recessional outwash is typically a sand and gravel deposit with lesser amounts of silt that was deposited from meltwater emanating from a receding glacier. The recessional outwash observed on scattered exposures on the slope and in the septic drain field test pits appeared to be a dense to very dense, slightly gravelly to gravelly, slightly silty to silty sand. Since the retreat of the glaciers, the upper few feet of the dense soils have 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, dense to very dense glacial sediment. Consequently, the topsoil and/or colluvium may move, resulting in slide deposits on and near the base of the steep slopes. 21-1-09874-001-LRl.doc/wp/eet 21-1-09874-001 Mr. Edward Foster c/o Ms. Catherine Hendy April 3, 2003 Page 4 SHANNON &WILSON, INC. Moderate seepage was observed on the slope on the property and adjacent lots, approximately 25 to 30 feet above the toe of the slope at the contact between the recessional outwash and the underlying till. CONCLUSIONS AND RECOMMENDATIONS Slope Stability Geologic hazard maps of the area classify the slope as unstable with recent slope movements. Our observations of the site confirm that recent slope movements have likely occurred at the site and should be expected in the future. These slope' movements appear to include both shallow- and deep-seated instabilities. Specifically, 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 relatively recent, shallow instability of the soils on the slope. The bowl-shaped portion of the site appears to be an old scarp of a relatively deep-seated slide. Conditions that appear to affect the stability of the slope at this site include naturally oversteepened slopes caused by wave erosion at the base of the slope and the presence of perched groundwater at the contact between the recessional outwash and the till. The following provides a brief description of how these conditions lead to both shallow- and deep-seated slope instabilities. Based on our observations of the site, it appears that the shallow, unstable soils consist primarily of the topsoil and colluvium on the slope. The very dense, glacially overridden till that underlies a large portion of 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. The location of groundwater perched on relatively impervious soils in a steep slope has historically been the location of numerous slides and deep-seated slope movements in the 21-1-0987443o l-LRl.doc/wp/eet 21-1-09 874-001 Mr. Edward Foster c/o Ms. Catherine Hendy April 3, 2OO3 Page 5 SHANNON&WILSON, INC. region. At this site, it appears that groundwater perches within the recessional outwash, on top of the relatively impervious till. Where the recessional outwash/till contact and groundwater daylights on the face of a slope, springs and seeps develop. The springs and seeps saturate the soils on the slope and thereby decrease the shear strength of the soils and increase their susceptibility to movement, particularly of the shallow topsoil and colluvium on the slope. In addition, pressure gradients or build-up of hydrostatic pressures associated with this perched water may result in deep-seated slope instability. At this site, it appears that the deep seated instability occurs within the recessional outwash above the till. With enough time, movement of colluvium, slide debris and topsoil toward the base of the slope and continued weathering and erosion of the glacial soils 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 slope movement should be expected 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 other actions or events on a slope that may cause sliding. The following 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 proposed development will not adversely impact the stability of 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., do not excavate the toe of the slope), not increasing the weight on the slope (e.g., do not place yard debris or fill on or at the crest of the slope), maintaining the slope as dry as possible (e.g., locate septic drain fields a sufficient distance away from the slope, route roof downspouts and yard drains away from the slope, and minimize the amount of surface water that could flow down the face of the slope), and maintain a vegetative cover on the slope. 21-1-098744X)l-LR 1.doc/wp/eet 2 1-1-09 874-00 1 Mr. Edward Foster c/o Ms. Catherine Hendy April 3, 2OO3 Page 6 SHANNON &WILSON, INC. Building and Drain Field Setbacks The measures discussed above may reduce the risk of soil movement on a slope. One of the most cost-effective measures to reduce the potential impact of slope movement is to provide an adequate setback for buildings and drain fields. An appropriate setback is a function of the rate or risk of slope movement, the design life of the structure, and the risk the owner is willing to assume. Based on the overall height of the slope and the depth and potential for deep-seated slope movements within in recessional Outwash, we recommend a minimum building and drain field setback of 50 feet from the crest of the slope. Assuming that future deep-seated slope movements would be similar in size to the existing scarp on the site, a 50- foot setback would allow for at least two occurrences Of deep-seated failures in the recessional outwash and an additional few feet of erosion. Greater risk reduction can be achieved with larger setbacks. The approximate location of the recommended minimum 50-foot setback is shown on Figure 2. As can be seen on this figure, the distance between the crest of the slope and the proposed residence and septic drain fields is greater than 50 feet and consistent with the recommended setback. We note that proposed septic drain field is located east of the proposed residence and closer to the slope than the proposed reserve area. While the location of the proposed drain field is consistent with the recommend minimum 50-foot setback, additional risk reduction with respect to slope stability can be achieved by switching the location of the proposed septic drain field and reserve area; namely moving the septic drain field to the east of the proposed residence. Drainage In general, reducing the amount of water entering and discharging onto the slope can reduce the risk of slope movement. Drains should be constructed and maintained to collect water from impermeable surfaces on the property (e.g., roof, decks, patios, and driveways) and directed in a tightline to a suitable discharge point away from the crest and surface of the slope. Discharge from other drains, such as footing drains, should similarly be directed in a tightline to a suitable discharge point away from the slope. There may be a number of suitable discharge points that would provide adequate drainage away from the residence without significantly 21-1-09874-001-LR l.doc/wp/eet 21-1-09 874-001 Mr. Edward Foster c/o Ms. Catherine Hendy April 3, 2003 Page 7 SHANNON ~WiLSON, INC. impacting the stability of the slope or increase the surface water discharge or sedimentation to adjacent properties beyond pre-development conditions. These discharge points include the existing road ditch on the west side of Tala Shore Drive, an on-site drain field, or a cistern system that collects and stores the water for other uses at the site (e.g., landscape irrigation). If an on-site drain field is used for the discharge of water collected in the drains on the property, we recommend that it be located as far as practical from the crest of the slope and have a minimum setback of at least 50 feet. An on-site drain field should be designed to allow dispersion of water and dissipation of energy to reduce the potential for erosion. Conceptually, a perforated pipe buried in a shallow, gravel-filled trench could be one method to provide water dispersion and energy dissipation. If an on-site drain field is used at this site, it should :be designed by a civil engineer of other qualified design professional. Besides the recommended setback from the crest of the slope, the location of an on-site drain field should consider the location of the septic drain field and reserve area and should be reviewed by the septic system designer. Impermeable surface around the residence (e.g., paved drives) should be minimized to reduce potential changes in the existing site drainage characteristics and impacts on adjacent sites. Based on our understanding of the limited, single-residence development of this property, it is our opinion that the anticipated discharge of roof, footing and other drains in accordance with the recommendations outlined above will not significantly affect the pre- development drainage conditions on the adjacent properties. 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 21-1-098744)01-LRl.doc/wp/eet 21 - 1-09874-001 Mr. Edward Foster c/o Ms. Catherine Hendy April 3, 2003 Page 8 SHANNON ~WlLSON, INC. be relatively low. It is anticipated that the proposed residential development will not significantly affect soil erosion and associated hazard on the site provided that prudent construction practices with respect to erosion are implemented. 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 expressed or implied. This letter report was prepared for the use of Mr. Foster 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. 21-1-09874-001-LRl.doc/wp/eet 21-1-09874-001 Mr. Edward Foster c/o Ms. Catherine Hendy April 3, 2003 Page 9 SHANNON ~WILSON, INC. 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. 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 report. Sincerely, SHANNON & WILSON, INC. William J. Perkins, L.E.G. Senior Principal Engineering Geologist WJP:JW:wjp Enclosures: Figure 1 - Vicinity Map Figure 2 - Site Plan . Figure 3 -Generalized Subsurface Profile Important Information About Your Geotechnical Report 21-1-09874-O01-LRl.doc/wp/eet 21-1-09874-001 PROJECT '~ LOCATION -.. . \ 0 1/2 1 I I I I I I Scale in Miles NOTE Map adapted from 1:24,000 USGS topo~mphio map of Port Ludlow, WA quandrangle, dated 1953, photorevised 1973. 650 Tala Shores Road Port Ludlow, Washington VICINITY MAP March 2003 SHANNON & WILSON, INC. J 21-1-09874-001 FIG. 1 leUeO pooH 0 oouop!sa~! 6u!ls!x~t ~o uo.qeoo-I o)eLu!xo~ddv A West .c_ 40 O .LU x 20 I I Minimum 50 Ft. Building and Drain Field Setback Dense to very dense, slightly gravelly to gravelly, slightly silty to silty SAND (Vashon Recessional Outwash) A! East //-- Spnng / Seep Loose Colluvium / Slide Deposit (Typical) Very dense, gravelly, silty SAND with scattered cobbles and boulders (Vashon Lodgement Till) each -- 6O 40 .c_ 0 0 20 Scale in Feet Horizontal = Vertical 4O NOTE 1. This topographic profile and geological conditions are based on field observations. Variations between the profile and actual topography and geological conditions are likely. 2. Vertical Datum is arbitrary. 650 Tala Shores Road Port Ludlow, Washington GENERALIZED SUBSURFACE PROFILE March 2003 21-1-09874-001 SHANNON & WILSON, INC. Geotechnical and Environrnefltal Consultants FIG. 3 SHANNON & WILSON, INC. Geotechnical and Environmental Consultants Attachment to and part of Report 21-1-09874-001 Date: April 3, 2003 To: Mr. Edward Foster c/o Ms. Catherine Hendy, John L. Scott Realty 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. Areport 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, earth.quakes, 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. Page 1 of 2 1/2003 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 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 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 on 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 geotechnical, 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 THE 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 desig~ drawings, because drafters may commit errors or omissions in the transfer process. To reduce the likelihood of boring log or monitoring well misinterpretation, contractors should be given ready access to the complete geotech.nical engineering/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 aggravate them to a disproportionate scale. READ RESPONSIBILITY 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 pre.vent 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. Someof 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 ASFE/Association of Engineering Firms Practicing in the Geosciences, Silver Spring, Maryland Page 2 of 2 1/2003