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HomeMy WebLinkAbout001324039 Geotech AssessmentGEOTECH CONSULTANTS, INC. 13256 Northeast 20th Street, Suite 16 Bellevue, Washington 98005 (425) 747-5618 FAX (425) 747-8561 April 3, 2003 Bob Lockhart and John Buhler 1535 Garfield Street Port Townsend, Washington 98368-8026 Subject: Slope Considerations Existing Beachfront Cabin 5262 Adelma Beach Road Jefferson County, Washington Dear Messrs. Lockhart and Buhler: JN 03111 This letter presents our geotechnical observations and conclusions regarding stability and mitigation measures for the steep slope located behind the existing cabin. On March 18, 2003, the undersigned principal geotechnicai engineer met with both of you at the subject property. The purpose of this visit was to observe the existing surface and geologic conditions on the site and the steep slope. The scope of our services is consistent with that outlined in our Contract for Professional Services, which you authorized. From our discussions, we understand that you are pursuing a permit to remodel the interior of the existing cabin. The 'structure would not be enlarged and no grading is planned as part of this remodel. Jefferson County Department of Community Development issued a letter dated February 7, 2003 that requested a geotechnical report addressing site geology, an evaluation of the stability of the existing slope, and appropriate recommendations for mitigation of any potential hazard posed by the steep slope. SITE CONDITIONS The cabin is situated on the western side of the subject site, which extends between Adelma Beach Road and Discovery Bay in the Four Corners area of unincorporated Jefferson County. The eastern approximately 60 percent of the lot is currently undeveloped and slopes down to the southwest at only a moderate inclination. A residence may be constructed in this area in the future. On the western portion of the lot is a short, steeply inclined slope extending down to the existing cabin. This slope has a height of 30 to 35 feet and an inclination of 70 to 80 percent. An elevated timber walkway provides foot access down the slope. At the base of the slope is a flat area that was originally created by filling behind a concrete bulkhead on the site and the adjoining north and south properties. A small, one-car garage/shed is situated near the crest of the slope. The steep slope is covered with brush and ivy. A mature fir tree is located at the crest of the. slope, with several of its downslope roots exposed. We observed no indications of recent slope instability or significant soil erosion on the steep slope. It is our understanding that you are very familiar with the history of this property and that no slope instability has occurred since the cabin was originally built. Based on our observations, it is possible that some minor surficial sloughing has occurred, which may be the cause for the exposed roots on the large fir tree. On the adjacent north lot, there is an area of recently disturbed soil at the base of the steep slope. This disturbed soil is obviously fill that was placed following the neighbor's construction of a retaining wall behind their cabin. Bob Lockhart and John Buhler April 3, 2003 JN 03111 Page 2 The soil conditions exposed on the slope consist entirely of gravelly sands, which are likely glacial outwash deposits. These conditions were observed over the entire height of the slope, and were encountered in the septic test holes conducted previously on the eastern portion of the site. No groundwater seepage was visible on the slope, even following several months of rainy weather. CONCLUSIONS AND RECOIVlMENDA TIONS Based 'on the observations made during our site visit, and on information provided by you, the steep slope is comprised of competent granular soils that are not susceptible to deep instability. The toe of the slope is protected from wave attack by the existing concrete bulkhead, which will not be modified during the remodel. In our opinion, the planned remodel of the existing cabin will have no adverse impacts on the stability of the slope, provided the steep slope and the vegetation on it are not disturbed. It is possible that future shallow slope movement could result in soil traveling to the base of the slope. Such movement is not expected to be large in size. However, it is still appropriate to provide a protection measure for people that may be occupying or using the cabin. We recommend that a short, slide catchment wall be constructed at the base of the slope, on the east edge of the cabin's front porch. This wall should extend at least 4 feet above the existing ground surface to catch or slow soil that may move down the slope. Any accumulated soil and debris would need to be removed from behind the wall to maintain the slide protection. Considering the limited access to the base of the slope for equipment, use of hand-carried materials and tools will likely be necessary to construct the wall. One potential wall type would be constructed of two-inch- diameter pipe piles spanned by timber lagging. The piles can be installed using a 90-pound jackhammer and would be driven to a final penetration rate of one inch, or less, per minute of continuous driving. Attached to this letter is a typical sketch for such a wall. Typically, mature trees that exist on slopes should remain wherever possible, as an extensive root system does provide some localized slope stabilization. However, trees that are sickly or in danger of toppling due to undermining should be cut down before they can fall. Trees often pull up their root balls when they topple, potentially disturbing a large area of a slope. LllVlITA TION$ The conclusions and recommendations contained in this report are based on site conditions as they existed at the time of our site visit. If the subsurface conditions encountered during construction are significantly different from those anticipated, we should be advised at once so that we can review these conditions and reconsider our recommendations where necessary. Unanticipated soil conditions are commonly encountered on construction sites. Such unexpected conditions frequently require making additional expenditures to attain a properly constructed project. The recommendations presented in this report are directed toward protection of only the building from damage due to slope movement. Landslides and soil movement can occur on steep slopes before, during, or after construction. The existing and future property owners must accept the possibility that some of the ground on, and above, the slope could be lost to slope movement over time. - ~:~,, MAY -8 2003 GEOTECItCONSULTANTS, INC. ~ "'~;:'~ :: {; ~'.: Bob Lockhart and John Buhler April 3, 2003 JN 03111 Page 3 This report has been prepared for the exclusive use of Bob Lockhart and John Buhler and their representatives for specific application to this project and site. Our recommendations and conclusions are based on the site materials observed and on previous experience with sites that have similar observed conditions. The conclusions and recommendations are professional opinions derived in accordance with current standards of practice within the limited scope of our services. No warranty is expressed or implied. We trust that this report meets your immediate needs for the proposed development. contact us if we can be of further service. Please Respectfully submitted, GEOTECH CONSULTANTS, INC. Attachment - Sketch of Catchment Wall MRM: esm Marc R. McGinnis, P.E. Principal GEOTECI4 CONStJI.TANT.S, INC. Lil GEOTECH CONSULTANTS, INC. L 1 3256 NE 20th Street, Suite 16 Bellevue, WA 98005 (425) 747-5618 · FA~ (425) 747-8561 Job No. o3/~/ Project L,,~k/~,,~//~,~,/~,,- / - Subject ~k~]zJ,, ~,~ ~/~m~ff Wx// Made By ~ Checked By Date ~/~3 Page ! Of /