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HomeMy WebLinkAbout901051004 Geotech AssessmentGEOTECH CONSULTANTS, INC. 13256 Northeast 20th Street, Suite 16 Bellevue, Washington 98005 (425) 747-5618 FAX (425) 747-8561 ~~q ~~ ~[~j~~ecember 10, 2004 ~, ~ ., ~.- JN 03035 John and Nancy Rowe ~ '` ~ 480 Adelma Beach Road Port Townsend, Washington 98368 ~~ .. Subject: Slope Setback and Buffer Recommendations Proposed Rowe Residence, Garage and Tram - 5523 State Route 20 Jefferson County, Washington Dear Mr. and Mrs. Rowe: This letter presents our observations and conclusions related to slope stability and an appropriate slope setback and buffer for the proposed residence. On January 20, 2003 and December 7, 2004, the undersigned principal geotechnical engineer met with you at the subject property. The purposes of these visits were to: • Observe the existing conditions on the site and the bluff. • Discuss the planned site development with Mrs. Rowe. Following the initial site visit, we contacted Michelle Farfan with the Jefferson County Department of Community Development regarding their knowledge of any previous landslides in the site vicinity. No subsurface exploration was performed as part of our scope of services. However, the subsurface soil conditions were readily evident on the westem bluff, and in septic test holes that had been conducted previously. The scope of our services is consistent with that outlined in our Contract for Professional Services, which you authorized. From our discussions with Mrs. Rowe and review of the provided Site Plan, we anticipate that a new single-family home will be constructed on the westem portion of the property. The house will be situated near the abandoned railroad bed and the old roadbed for Adelma Beach Road, which is now vacated and contains underground power lines. No deep cuts are anticipated for the house construction. A garagefaccessory dwelling unit will be located northeast of the planned house site. A septic drainfield will also be included in the development, east, upslope of the planned structures. The driveway will extend westward to the house and garage site from State Route- 20 (SR 20) along the alignment of the current access road. Acable-suspended tram is to be installed at one of the steeper places of the bluff, for access to the beach. This type of tram does not require intermediate supports, with the cables spanning between anchorage points at the top and bottom of the bluff. The provided plan also shows a dispersion area for downspout water south of the proposed house site. If the scope of the project changes from what we have described above, we should be provided with revised plans in order to determine if modifications to the recommendations and conclusions of this report are warranted. ~. John and Nancy Rowe December 10, 2004 SITE CONDITIONS JN 03035 Page 2 The site is a rectangular 9-acre lot located in the Four Corners area of unincorporated Jefferson County, south of Port Townsend. The property is bounded on the east and west sides by SR 20 and Discovery Bay, respectively. Adelma Beach Road extends to the north boundary of the property on the western one-third of the lot. There is a gate across the south end of the street at the north boundary of the site. In the past, Adelma Beach Road continued southward through the site and adjacent lots to meet SR 20. The old roadbed is now vacant, but contains power lines for Puget Sound Energy. An abandoned railroad bed roughly parallels the old roadbed. The majority of the site is heavily wooded with mature evergreen trees. An access road has been extended in to the property from SR 20, reaching as far west as the old roadbed for Adelma Beach Road. The ground surface over the majority of the property slopes moderately downward toward the west from SR 20. Generally, the ground surface inclination is less than 40 percent. Along the western, downslope edge of the abandoned railroad alignment and roadbed is a short fill slope that is steeper than 40 percent, but is only 5 to 6 feet in height. The only slope of geotechnical concern on the property is the tall, very steep bluff that drops to the beach along Discovery Bay on the western edge of the site. This slope has a height of approximately 60 feet. The upper portion is inclined at 70 to 80 percent, and is near 1:1 (Horizontal:Vertical) in many places on the lower portion of the bluff. At the proposed tram location, the bluff is inclined at least as steep as 1:1 (H:V) over its entire height. We traversed the entire height of the bluff. Upon walking the beach to the north and south of the site, it was obvious that the lower portion of the slope has experienced numerous episodes of shallow failures. This appears to be the result of undercutting of the unprotected slope toe from wave attack. Dense, glacially compressed sand and silt are exposed on the majority of the bluff and in these areas of slope movement. It appears that the upper section of the bluff is comprised of glacial till, a glacially compressed, gravelly, silty sand. Slope movement on the upper portion of the bluff appears less extensive, likely consisting of shallow mudflows as the near surface soils are loosened over time by weathering. We observed no indications of deep-seated movement, such as downsets or tension cracks on, or near, the site. Many of the evergreen trees on the moderately sloped area above the bluff have curved trunks. These curves bend to both the east and west, with no consistency. The tree trunks are not leaning, as they would if large-scale slope movement had occurred. It appears that these curves are probably due to strong winds that occur in the area. They may also have resulted from damage resulting from logging operations many years ago. Some of the trees have been damaged or distressed by a fire in approximately 1995. We contacted Michelle Farfan of Jefferson County to determine the history of slope movement in the vicinity. She indicated that the County is not aware of any episodes of deep instability in the Adelma Beach Road area. The slope movement that they have documented nearby are the shallow failures at the base of the bluff resulting from wave attack. CONCLUSIONS AND RECOMMENDATIONS Based on the observations made during our site visit, and on the available information from Jefferson County, it does not appear that deep instability has occurred beneath the site or in the vicinity. The soils that underlie the property are competent and glacially compressed. However, the sandy soils that comprise the lower bluff face are more susceptible to erosion from wave action and weathering than silt or glacial till. Therefore, on-going recession of the bluff face can be expected as its toe is periodically undercut by wave attack. The effects of wave attack on the toe of GEOTECH CONSULTANTS, INC. John and Nancy Rowe JN 03035 December 10, 2004 Page 3 the bluff could only be slowed by constructing a bulkhead. However, the recommendations of this report are intended to protect the structures from damage due to bluff recession expected without a bulkhead. Based on our observations and information from Jefferson County, deep-seated slope movement is not expected. As a result, we anticipate only continued slow recession of the bluff face. The observed slides at the bluff's base appear to have involved the outer 2 to 4 feet of soil, and do not extend the full height of the bluff. Therefore, on average, we expect the bluff's crest to recede at a rate of less than 6 inches per year. Typically, the slope movement will only occur periodically, removing several feet of soil at a time when it does happen. In order to protect the planned house and garage structures and the septic drainfield from slope movement, we recommend that a buffer of no less than 65 feet be maintained behind the bluff's crest. No grading or significant clearing should occur in the buffer. Foundations for the house and garage should be set back from the buffer a minimum distance of 15 feet. Decks, patios, the roof downspout dispersion system, and septic drainfields can extend into the 15-foot setback, but must be outside the 65-foot buffer. The proposed residence and garage can be supported on conventional foundations that bear on medium-dense to dense, native soils. We recommend that an allowable bearing capacity of up to 2,500 pounds per square foot (psf) be used for the foundation design. Where overexcavation below planned footing grades is necessary to expose bearing soils, either the foundations can be extended downward, or the overexcavation can be backfilled with compacted crushed rock, ballast rock, or quarry spalls. The disturbance of trees and vegetation in the buffer zone and on the steep bluff should be minimized to reduce the erosion potential of the near-surface soil. If the vegetation is maintained, the impact of this project on the stability of the bluff face should be negligible. Careful limbing of the mature trees, and cutting down scattered immature trees (trunk diameter of 8 inches or less), can be accomplished in the buffer without adverse impacts to slope stability. It is important that healthy, mature trees remain in the buffer. However, sickly trees should be cut down to reduce the potential for substantial ground disturbance that results from a large root ball being pulled out of the ground when they topple. The use of acable-supported tram is preferable to prevent the slope disturbance that can result when tram supports or steps are installed on steep slopes. The cable anchors should be founded into dense soils, as this reduces the potential that the anchors will be undermined when soil movement occurs on the bluffs face. The few immature trees (alders less than 6 inches in diameter) within the alignment of the trams can be cut down without decreasing slope stability. The stumps of these cut trees should be left in the ground. No fill or debris should be placed in the buffer or on the slope. Water from storm and footing drains, and roof downspouts should not be discharged directly onto the bluff. Use of agravel-filled dispersion trench located upslope of the recommended buffer is acceptable. We recommend that such a dispersion trench extend in a north to south direction, across the fall of the ground, and have a minimum length of 50 feet. The septic designer should be contacted regarding the minimum separation distance allowed between the septic drainfields and a dispersion system. GEOTECH CONSULTANTS, INC. John and Nancy Rowe December 10, 2004 L/MITATIONS JN 03035 Page 4 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 buildings and septic drainfield from damage due to slope movement. Landslides and soil movement can occur on steep slopes before, during, or after construction. Therefore, the current and future property owners must accept the possibility that some of the ground on, and above, the steep western bluff could be lost to slope movement over time. This report has been prepared for the exclusive use of John and Nancy Rowe 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. Please contact us if we can be of further service. Respectfully submitted, GEOTECH CONSULTANTS, INC. ~CG' Marc R. McGinnis, P.E. Principal MRM: alt GEOTECH CONSULTANTS, INC.