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HomeMy WebLinkAboutBLD2000-00717 Geotechnical Report Ms. Stephanie Burke SHANNON&WILSON,INC. November 14, 2000 Page 3 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 2 1-1-0921 5-001-L 1.DOCIWP/EET 21-1-09215-001 Ms. Stephanie Burke SHANNON 6WILSON,INC. November 14, 2000 Page 4 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 proposed 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 ] 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. 2 1-1-0921 5-001-LI.DOC/W P/EET 21-1-09215-001 • Ms. Stephanie Burke November 14, 2000 SHANNON&WILSON,INC. Page 5 I Drainage 111 In general, reducing the amount of water entering and discharging onto the bluff 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 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. Stainless 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. If tightline splices occur on the bluff, the splice should be supported and reinforced using stainless or galvanized steel cable attached to the tightline both above 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 "T" at the end of the tight line. The "T" 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 "T." The "T" may be covered with cobbles or quarry spalls to help secure and hide the "T." 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 nt 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. Imperrrleable 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. I 2 1-1-092 1 5-001-L1.DOC/WP/EET 21-1-09215-001 I Ms. Stephanie Burke November 14, 2000 SHANNON&WILSON,INC. Page 6 I 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 to eliminate 1111 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, Ulandscape 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 1111 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 111 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 U practices in this area at the time this letter was prepared. We make no other warranty, either express or implied. 2 1-1-092 1 5-001-L1.DOC/WP/EET 21-1-09215-001 I Ms. Stephanie Burke SHANNON FiWILSON,INC. November 14, 2000 Page 7 111 This letter was prepared for the use of Ms. Burke in the evaluation of the stability of this site. With respect to possible future construction, it should be made available for information on factual data only and not as a warranty of subsurface conditions, 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, SHANNON WILSON, INC. NI D RoF\J c J OREGON � \ /I-I PrRKI" \ Ne. 65 ll A 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 21-1-09215-001-LI.DOC/WP/EET 21-1-09215-001