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HomeMy WebLinkAbout901051011 Geotech Assessment S c®nsulting p IN-DEPTH PERSPECTIVE Apri19, 2007 Ms. Laura Southmayd 702 Adelma Beach Road Port Townsend, Washington 98368 Re: Geological Slope and Beach Staircase Reconnaissance Southmayd Cabin 702 Adelrna Beach Road Port Townsend, Washington 98368 Project No. 070052-001-01 Dear Ms. Southmayd: ~~~-~'~ 1~ ~,. j f I (:il .. C~1L~uu~uu~~~~ y~..~+: W ~ . Thank you for the opportunity to perform a geologic slope reconnaissance of the residential property located at 702 Adelma Beach Road, in Port Townsend, Washington. We performed the reconnaissance at your request on March 27, 2007. This letter-report summarizes our field notes and conclusions regarding the nature of the site slope, our conclusions regarding the risk of damage to the slope and the proposed stairway to the beach, and our recommendations far minimizing the risk of future erosion and slope damage. These notes are the results of a reconnaissance-level investigation only and are intended only for developing general design criteria. No subsurface explorations were conducted. _ We understand that you wish to access the beach by building a staircase from the top of the steep waterfront slope. The stairs vcrill be built across the steep slope that falls within Jefferson County's Critical Areas Ordinances (CAO). The purpose of this reconnaissance is to specifically address the slope conditions with respect to the feasibility of construction of the stairs. Observations Site Conditions and Topography 'The waterfront site was located on the Quimper Peninsula, southwest of Port Townsend, Washington, on awest-facing slope on Discovery Bay. The site location is shown in Figure 1, Site Location Map. Site geography was dominated by the westward-facing steep slope that occupied the western portion of the property. A single-family house and access driveway occupied a flat to gently-sloping area upland in the western half of the property, approximately halfway between the top of slope and the Adelma Beach Road alignment. A cabin occupied a flat to gently sloping upland area adjacentto and east of the top of the slope. The cabin foundations were approximately 6 feet from the top of the slope at the time of our site visit. Similar residential developments bounded the site to the north and south. A timber bulkhead approximately 6 feet tall protected the toe of the slope at the neighboring property to the south; the slope at the subject property and the neighboring property to the north was unprotected. Laura Southmayd Apri19, 2007 Project No. 070052-001-01 Total elevation change across the slope, from the top of the slope to the high tide line, was estimated at 40 feet. The overall slope angle from top of slope to the beach below was on the order' of 90 percent, with flatter portions in the lower half of the slope and steeper to vertical portions iri the upper 8 to 10 feet of the slope. Soils Very dense, silty sand with occasional seams of gravel was encountered in the uppermost portion of the site slope. This stratum formed anear-vertical bluff in the upper 6 to 12 feet of the slopes on the subject property and adjacent slopes. Probing with hand tools on the upper flat area near the top of slope revealed very dense soils within approximately 2 to 3 feet of the surface. In-situ soils in the lower slope could not be directly observed on the subject property, due to the colluvial soils and landslide debris cover; however, intact soils were observed on nearby neighboring slopes. These soils consisted of dense, interbedded sand, and gravel layers with minor quantities of silt, at bedding angles ranging from horizontal to 35 degrees. Sediments on the beach face consisted of gravel, cobbles, sand, and scattered boulders (one 6- foot-diameter gneissic boulder was noted on the property). Hand probing the beach soils revealed very dense or hard soils at less than 12 inches depth. Surficial sediments on the slope in the vicinity of the project were mapped by Schasse and Slaughter (June 2005, Washington State Department of Natural Resources, Geologic Map of the Post Townsend South and Past of the Post Townsend NoNth 7. S-minute Quadrangles, JeffeNSOn County, Washington) as glaciomarine outwash of the Everson Interstade of the Fraser glaciation. However, the subject area is in close proximity to a change of geologic units in the published map, and the area to the immediate south is mapped as Vashon lodgement till overlying Vashon advance outwash soils. The published mapped unit of Everson Interstade glaciomarine outwash is consistent with our observations of the soils on the slope and in exposures off-site to the north and south; however, the site soils appear to have been glacially consolidated, thus are dense to very dense. Drainage No distinct natural surface water drainage features were observed on the site, and site vegetation did not suggest perennially wet areas. We observed 4-inch corrugated plastic drain lines on the subject slope and on neighboring slopes. The drains appeared to originate at the houses located on the upland bench and extended to the toe of the slope. Water was not flowing through the pipes at the time of our site visit. Vegetation Vegetation around the residence area consisted primarily of grass lawn and landscape plants. Vegetation around the cabin area consisted primarily of fir trees and a heavy groundcover of salal and other berry bushes. One mature madrona tree was overhanging the top of slope near the cabin. The steep slope was vegetated with small (less than 6-inch-diameter) alder and cedar Page 2 Laura Southmayd Apri19, 2007 Project No. 070052-001-01 trees, blackberries, ivy, salal, and other bushes. Mature fir and rnadrona trees were present along the top of the bluff at the subject property and neighboring properties. No hydrophilic (water-loving) vegetation was observed on the subject slope. Slope Stability Three types of slope failures are prevalent on slopes of this kind in the Puget Sound area. Shallow debris or colluvial landslides consist of failure of a thin skin of loose and weathered soil that mantles steep slopes. Failure usually consists of the colluvial layers sliding or flowing over the dense or hard, undisturbed bluff soils. colluvial landslides are common throughout steep slope areas in the Puget Sound region. Another common type of landslide failure is a rotational or deep-seated landslide failure. This type of failure involves rotation of a mass of soils along abowl-shaped or curved failure plane. This type of failure often results in move- ment of large blocks of relatively intact soils. The third common type of failure consists of toppling or spalling of blocks or slabs of relatively intact soils that have been fractured or undermined by erosion. This occurs where soil stress has been released on the face of the bluff due to erosion or past landslides, and tension fractures develop parallel to the bluff face. Roots and groundwater penetration loosen the blocks until the toe erodes enough that support is lost. Failure of this kind of landslide generally involves a slab several feet thick and these slabs typically fail along high angle (typically near-vertical) planes. We observed evidence of shallow colluvial landslides on the site slope and on properties to the north and south of the site. The last slide on the subject property destroyed the existing beach stairs and knocked down several trees. Debris from the old stairs, tree trunks and loose sand and gravel soils were observed on the slope. Conclusions and Recommendations The shape of the bluff is controlled by the type of sediments and shoreline conditions. The steep upper bluff is created by the very dense glacially-consolidated soils that maintain steep slope angles and tend to fail by shallow surficial sliding or toppling. The lower portion of the slope is mantled by colluvium and landslide debris that accumulate at the toe of the slope. Based on conditions present at the time of our site observations, we consider the beach bluffto possess a moderate risk of surficial landslides and toppling failures and a low risk of deep rotational landslide failures occurring during the life of the project. The rate of retreat of the bluff (the rate at which the bluff face is moving landward) is estimated at 3 to 4 inches per year, averaged over many years. The actual retreat rate will typically be episodic, with no observable movement for a number of years punctuated by loss of several feet or more in one season. If good surface water control, shoreline protection, and slope management practices are not followed, the rate of bluff retreat will increase. Beneficial vegetation on the slope will help reduce the year-to-year erosion of the slope soils, and will ultimately create fewer landslides. The mature madrona tree overhanging the slope near the cabin is now a net destabilizing influence and we recommend removing the tree but leaving the root ball in place. Based on our observations of conditions at the time of our site visit, we believe that construc- tion of the beach access staircase is feasible. A properly constructed stairway will not increase the rate of slope retreat. The owners must understand that the site slopes will move again, and Page 3 Laura Southmayd Apri19, 2007 Project No. 070052-001-01 any structures on the slopes should not be considered permanent. Future repair or replacement of the staircase should be anticipated. We believe that a properly constructed stairway will not pose a life safety risk during normal use of the stairway provided that the stairway is well maintained and repaired after any damaging slope movement. Water Management We recommend implementing good surface water management practices. This includes ensuring that all stormwater from hard surfaces such as roofs or driveway is collected and transmitted via tightline drainpipe to the base of the slope. At no time should water be allowed to flow onto the slope. All drain lines should permit water to flow from top to the bottom of the slope without collecting in low points in the line. The outlet should be located where it will not cause erosion of the slope. We recommend inspecting existing drain lines each fall and replacing damaged lines before the start of the wet season. Lines can be checked by adding water at the top and determining if the water makes it to the beach. If the full amount of water does not make it to the end of the line, a line leak or break is present and the line should be replaced. We recommend that all lines be suspended or restrained by a steel cable rather than allowed to hang from their own weight, or if feasible, the drain lines should be suspended from the staircase that is proposed for construc- tion. When it is time to replace any existing drain lines, consideration should be given to using fuse-welded high-density polyethylene (1-IDPE) instead of ABS. T`he I-IDPE lines are much more durable and UV-resistant than corrugated ABS. General Soil Management Soil or landscaping debris should not be deposited on the slope as this exacerbates conditions that lead to shallow landslides. Thick piles of debris prevent growth of plants that are beneficial to slope stability and allow build-up of perched groundwater. Foundation Criteria Foundations for the staircase should be constructed by excavating into the very dense, intact glacially-consolidated soils. Foundations placed on the steep slopes should be excavated a minimum of 2%z feet into the intact glacial soils. We do not recommend that any part of the staircase bear on the loose or soft colluvial soils or landslide debris. In areas where in-place soil is excavated, including around the foundation holes, any newly loosened soil should be replaced with structural fill or controlled-density fill (CDF) or concrete in order to minimize zones of potential stormwater concentration on the slope. The staircase should be designed to permit soil debris to move past the support structures or landings in the event that part of the slope above the staircase fails. The foundation areas should be inspected frequently to check for signs of movement or undermining of the soils that support the structure. Because the slope is susceptible to surficial soil movement, we recommend that the top landing bear on soils set back as far as practicable eastward from the top of the slope. The bottom of the staircase should be founded on intact, dense soils landward of the toe of slope. The posts or foundations should extend well below the depth of winter storm scour plus that amount of scour that will occur naturally over tens of years of normal ongoing shoreline retreat. We estimate that the beach surface near the high tide line could decrease by about 1 to 1 %z feet of elevation over a 30-year period due to normal shoreline and bluff retreat. Page 4 Laura Southmayd Apri19, 2007 Project No. 070052-001-01 Limitations This letter-report was prepared based on a limited field reconnaissance. Amore in-depth characterization of slope conditions and geotechnical analysis, including seismic response and a quantitative slope stability analysis or historic bluff retreat estimate was beyond the scope of work for this study. No amount of engineering or geologic analysis can yield a guarantee of stable slopes or more than an assessment of the type of geologic processes that are active in the area. The recommendations made herein go to reduce the risks to the client associated with slope movement. This letter-report was prepared for the exclusive use for Laura Southmayd and her agents with specific application to the project site. Thank you for the opportunity to provide these services to you. If you have any questions, please do not hesitate to call. Sincerely, ~~9/~'l James A. Peterson, PE John L. Peterson, PE Project Geotechnical Engineer Associate Geotechnical Engineer apeterson@aspectconsulting.com jpeterson@aspectconsulting.com Attachment: Figure 1 -Site Location Map cc: Pete Brockman, Brockman Builders, Inc. 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