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HomeMy WebLinkAboutBLD2010-00338 Geotechnical Report • 440 Aspect piRmu # BL (CONSULTING February 22, 2011 FF lSON COUNTY BCD Ms. Laura Southmayd 702 Adelma Beach Road Port Townsend, Washington 98368 Re: Geotechnical Reconnaissance and Geologically Hazardous Area Considerations Southmayd Residence 702 Adelma Beach Road Port Townsend, Washington 98368 Project No. 070052-002-01 Dear Ms. Southmayd: This letter report summarizes our observations made during a geologic slope reconnaissance regarding the rebuild of the single-family residence located at 702 Adelma Beach Road, Bainbridge Island, Washington(Site).Aspect Consulting,LLC(Aspect)performed the slope reconnaissance in accordance with our agreed upon scope of work, detailed in our proposal dated February 9, 2011. Our field work was performed on February 11, 2011. We understand the proposed project will consist of a complete rebuild of the single-family residence located in the central portion of the Site. The new residence will be located approximately in the same footprint of the existing residence with improvements including a new deck on the western(downslope)side of the new residence and an upgraded septic system that will pump effluent water to the eastern(upslope)portion of the property. Based on available topographic data and a review of the Jefferson County Department of Community Development(County)maps,the Site is located partially within a mapped geologically hazardous critical area.As a result,the County Critical Area Ordinance(CAO)restrictions are being imposed on this project. A geotechnical report is necessary to determine the underlying stability of the property, address the County CAO restrictions,and to provide a design basis for the proposed improvements. Observations Site Conditions and Topography The Site is located on the west side of the Quimper Peninsula,southwest of Port Townsend, Washington. The Site location is shown on Figure 1. The Site was comprised of a west facing slope of varying steepness bordered by similarly developed residential properties to the north and south,by Adelma Beach Road to the east, and by Discovery Bay to the west.A steep beach slope existed along the western edge of the Site separating the upland and beach portions of the Site. The Site consisted of two parcels with the eastern parcel occupied by the existing residence and upland portion of the Site and the western parcel occupied by the beach cabin, beach slope, and beach portion of the Site. Aspect Consulting,LLC 179 Madrone Lane N. Bainbridge Island,WA 98110 206.780.9370 www.aspectconsulting.com • • Southmayd Residence February 22, 2011 Project No. 070052-002-01 The existing single-family residence and access driveway occupied the gently-sloping eastern portion of the Site,approximately halfway between the top of the beach slope and the edge of Adelma Beach Road.An older cabin structure occupied the relatively flat area adjacent to and east of the top of the beach slope. There was no formal toe protection at the base of the beach slope on Site.At the base of the beach slope on the neighboring property to the south, an approximately 5-to 6-foot tall wooden bulkhead was observed. Scattered driftwood logs that were restrained or anchored to the beach to provide 'soft' beach protection were observed at the base of the beach slope on the neighboring property to the north. Topographically,the upland(eastern)portion of the property included slope inclinations ranging from 15 to 20 percent with artificial landscape terraces between the existing residence and Adelma Beach Road.The areas immediately west of the existing residence and around the existing beach cabin were relatively flat. West of the cabin was a steep beach slope with inclinations ranging from 85 to 100 percent with flatter portions closer to the toe of the slope. The total elevation change across the Site was approximately 80 feet with the total height of the steep beach slope estimated at 45 feet. The beach slope and associated buffer area extending 30 feet east from the top of the slope on Site is mapped by the County Geographic Information Services(GIS)as a moderate landslide hazard area.The areas east of the beach slope buffer zone are mapped as a slight landslide hazard area. The Coastal Zone Atlas(1979, Washington Department of Ecology, Coastal Zone Atlas)maps the western portions of the Site as"Unstable"and the central and eastern portions of the Site are mapped as"Stable". Soils Open test pits for septic design were observed in the upland area east of the existing residence at the time of our reconnaissance. The test pits were typically 3.5 feet in depth and consisted primarily of dense, brown to gray, slightly moist gravelly sand with trace silt to slightly silty. Very dense, light brown to tan, slightly moist, silty sand with occasional seams of gravel was encountered in the uppermost face of the beach slope. This stratum formed a near-vertical face in the upper 6 to 12 feet of the slope 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 feet of the ground 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. Hand probing the beach soils revealed very dense or hard soils at less than 12 inches depth. Surficial sediments on the Site were mapped by Schasse and Slaughter(June 2005, Washington State Department of Natural Resources, Geologic Map of the Port Townsend Page 2 • • Southmayd Residence February 22,2011 Project No. 070052-002-01 South and Part of the Port Townsend North 7.5-minute Quadrangles,Jefferson County, Washington)as glaciomarine outwash of the Everson Interstade of the Fraser glaciation. The published mapped unit of Everson Interstade glaciomarine outwash is consistent with the results of our reconnaissance Site.Hand probing with a steel t-probe typically revealed dense to very dense soils across the Site, indicating the soils appear to have been glacially consolidated. Drainage No distinct natural surface water drainage features were observed on the Site, and the Site vegetation did not suggest perennially wet areas. Observation of the neighboring beach slopes did not reveal any uncontrolled runoff or seepage within 1,000 feet of the Site at the time of our reconnaissance. Observation of the upland areas surrounding the Site, constrained by limited access, did not readily reveal any significant groundwater springs or seepage within 1,000 feet of the Site. We did not observe any Site drainage extending down the beach slope,but scattered 4-inch- diameter corrugated drain lines were observed descending the neighboring beach 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 reconnaissance. 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 moderate groundcover of salal and black berry bushes.The beach slope was vegetated with small (less than 6-inch- diameter)alder and cedar trees,blackberries, ivy, salal,and other bushes. Mature fir and madrona trees were present along the top of the bluff at the subject property and neighboring properties. The mature vegetation near the top of the beach slope typically exhibited little to no trunk curvature while vegetation on the face of the beach slope exhibited moderate trunk curvature.No hydrophilic(water-loving)vegetation was observed on the beach slope. Landslide Activity A small surficial landslide occurred on the beach slope approximately 5 years ago and damaged the existing beach stairs. The Coastal Zone Atlas indicates isolated small areas mapped as"Urs"or Unstable Recent Slides are located along the beach slope approximately 1/4 mile south of the Site. The results of our reconnaissance indicate the Site beach slope has isolated areas which lack established vegetation and is susceptible to shallow, surficial slope instabilities and an increased rate of erosion. Seismic Design Considerations The Site is located in a seismically active area and is prone to seismic hazards such as amplified seismic response and seismically-induced landslides. The Site lies approximately 10 miles southwest of the Whidbey Island Fault Zone, a shallow crustal tectonic structure that is considered active(meaning it has the potential to cause earthquakes in the future)and is capable of producing earthquakes of magnitude 7.0 or greater.The recurrence interval of earthquakes on this fault zone is believed to be on the order of several thousand years. The most recent large earthquake on this fault is believed to have occurred about 3,000 years ago. Page 3 • • Southmayd Residence February 22, 2011 Project No. 070052-002-01 The Site also lies within the zone of strong shaking from subduction zone earthquakes. The recurrence interval of these earthquakes is thought to be on the order of about 500 years. The most recent subduction zone earthquake occurred 300 years ago. Deep intra-slab earthquakes also occur in the region every decade or two, including the 2001 Nisqually earthquake. These earthquakes are generally less severe than the shallow crustal and subduction zone earthquakes but have the potential to cause damage to older structures built before modern seismic codes were enacted, and those in liquefaction-sensitive areas. Based on the stratigraphy and visual reconnaissance of the Site, it is our opinion that earthquake damage to any proposed structures founded on a suitable bearing stratum would likely be caused by the intensity and horizontal ground acceleration associated with the event. Per Chapter 18 of the 2006 International Building Code (IBC),maximum peak ground acceleration without a specific Site study is 0.32g. Surficial Ground Rupture The Site is located southwest of the Whidbey Island Fault Zone. The recurrence interval of movement along this fault system is still unknown, although it is hypothesized to be in excess of several thousand years. Due to the suspected long recurrence interval and offset of the Site from the previous rupture surface, surficial ground rupture is considered to be low during the expected life of the Site improvements. Liquefaction Due to the inherent density and grain size distribution of the geologic strata mapped and observed at the Site combined with the absence of continuous groundwater conditions,the liquefaction potential for the soils at the Site is considered low. Ground Motion Using the 2006 IBC criteria, in lieu of deep Site-specific subsurface data,the Site would be characterized by a Seismic Site Class D. The mapped, maximum considered earthquake spectral response accelerations for short period(SS)= 1.175g; and for 1-second period(Si)= 0.431 g. Site coefficients for this Site are Fa= 1.030, F„= 1.569. The maximum considered earthquake spectral response accelerations adjusted for Site class effects are Sds=0.807g, Sd1 = 0.451 g. Erosion Hazard The soils encountered have a slight to moderate erosion potential when exposed during construction. The erosion risk increases on sloped areas,whether natural or excavated during construction. Areas outside of the proposed construction area have low erosion potential due to the well-developed vegetative cover. Landslide Hazards and Mitigation Three types of landslides are common to the area, rotational landslides, surficial or debris landslides, and toppling or spalling of intact soil blocks from steep bluff faces. These slides may be triggered by natural events such as, extended heavy precipitation or an earthquake, or by manmade features such as broken water pipes or improperly managed stormwater flow. Page 4 r • • Southmayd Residence February 22,2011 Project No. 070052-002-01 Rotational Type Landslides Rotational landslides consist of deep-seated failures that typically involve slip along a curved shear plane. Rotational landslides may transport large masses of semi-intact soil downslope, resulting in alternating steep headscarps along the upper portion of the failure plane,with more gently-sloping benches composed of displaced soil. We did not observe any signs of recent, deep-seated rotational type landslides during our Site reconnaissance. Surficial or Debris Landslides Surficial or debris landslides consist of sliding of the weathered colluvial soil layer and overlying vegetation that typically mantles steep slopes. Surficial slides commonly result from a significant increase in the moisture content within the upper weathered soil layer on slopes. Increased moisture typically results from periods of extended,heavy precipitation, groundwater seepage, or concentrated surface water discharge onto a slope. Slides that occur within the upper several feet of weathered soils typically do not extensively impact the underlying, parent soils. The steep beach slope exhibited potential for surficial instabilities due to areas devoid of vegetation, isolated areas of near-vertical grades,the lack of formal toe protection, and known landslide history on Site. Any surficial failure would likely be limited to the upper colluvium soils and not affect the overall slope stability or the proposed improvements. Toppling/Spalling 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. Minor spalling may occur near the top of the beach slope where the slope exhibited near vertical inclinations. Slope Stability We observed evidence of shallow colluvial landslides on the Site beach slope and on properties to the north and south of the Site.The last slide on the Site destroyed the existing beach stairs and knocked down several trees on the beach slope.Debris from the old stairs,tree trunks and loose sand and gravel soils were observed on the beach slope at the time of our reconnaissance. An infinite slope analysis was prepared for a line from the edge of the proposed residence and the base of the beach slope. Using conservative strength values for the underlying strata of a 35 degree effective friction angle and 50 pounds per square foot effective cohesion. The factors of safety against sliding that would affect the proposed residence were 1.65 and 1.18 for the static and seismic cases,respectively.A seismic coefficient of 0.16g derived by methods presented in Chapter 18 of the 2006 IBC was used for analysis of the seismic case.These results indicate the areas around the proposed residence are stable under both static and seismic conditions. Page 5 • • Southmayd Residence February 22, 2011 Project No. 070052-002-01 Conclusions and Recommendations Our reconnaissance, observations, slope stability analyses, and understanding of the proposed work indicate that,from a geotechnical standpoint,the project Site is feasible for the proposed development. We did not observe evidence of slope instability which would affect the proposed residence or evidence that the proposed residence would negatively affect the nearby slope stability. Based on conditions present at the time of our reconnaissance,we consider the beach slope to 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 beach slope(the rate at which the slope face is moving landward)is estimated at 4 to 6 inches per year, averaged over many years.The proposed residence location currently has acceptable factors of safety against landsliding using very conservative soil strength parameters and an estimated 160 years before the slope retreat would impact the proposed structure such that mitigation would be needed. 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. Surficial Landslides The most likely impact to the Site from a slope stability perspective would be shallow debris landslides as a result of saturation of the near-surface,weathered, and colluvial soils on the beach slope. Slides of this type are typically limited to the weathered soil unit within the outer 2 to 3 feet of the slope and do not have significant effect on the underlying geologic units. The effects of a surficial event may result in the loss of portions of the yard, but it is unlikely to affect the proposed residence. To help mitigate these possible surficial events,the following recommendations apply to the mitigation of shallow debris landslides that are a result of saturation of the near-surface and colluvial soils on the slope.Factors that affect slope instability within the near-surface, weathered soil layer include the following(D.H. Gray and A.T. Leiser,Biotechnical Slope Protection and Erosion Control: Van Nostrand Reinhold,New York, 1982): Root Reinforcement: Roots mechanically reinforce a soil by transfer of shear stresses in the soil to tensile resistance in the roots. Soil Moisture Modification: Evapotranspiration and interception in the foliage limit build-up of soil moisture. Buttressing and Arching: Anchored and embedded stems can act as buttress piles or arch abutments in a slope, counteracting shear stresses. Page 6 ` • • Southmayd Residence February 22, 2011 Project No. 070052-002-01 Surcharge: Weight of vegetation on a slope exerts both a downslope(destabilizing)stress and a stress component perpendicular to the slope,which tends to increase resistance to sliding. Root Wedging: Alleged tendency of roots to invade cracks, fissures, and channels in a soil or rock mass and thereby cause local instability by a wedging or prying action. Windthrowing: Destabilizing influences from an over-turning moment exerted on a slope as a result of strong winds blowing downslope through trees. Root reinforcement, soil moisture modification(reduction), and buttressing and arching are enhancing slope stability at the Site. Surcharge,root wedging and windthrowing will have a net destabilizing effect. Other sources of surficial slope instability include improperly managed storm and surface water runoff flowing near or over the top of the beach slope. Soil or landscaping debris should not be deposited on the beach 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. Water Management We recommend implementing good surface water management practices.This includes ensuring that all stormwater from hard surfaces such as roofs or the driveway is collected and transmitted to a County approved location. Stormwater management through infiltration into the sandy soil strata on Site would be acceptable provided the zone of infiltration is located a minimum of 50 feet from the top of the beach slope.A geotechnically preferred discharge point for stormwater would be at the base of the slope via hardpipe tightline if allowed by County code. At no time should water be allowed to flow over the top of the beach slope. All drain lines should permit water to flow from top to the approved discharge point without collecting in low points in the line.The outlet,if at the base of the slope, should be located where it will not cause erosion of the slope and meet Department of Natural Resources permit regulations. 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 outlet. 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. When it is time to replace any existing drain lines, consideration should be given to using fuse- welded high-density polyethylene(HDPE)instead of ABS. The HDPE lines are much more durable and UV-resistant than corrugated ABS. Page 7 • • Southmayd Residence February 22, 2011 Project No. 070052-002-01 Site Preparation Site preparation within the proposed construction area footprint should include removal of all debris and any other deleterious material. Additionally, all topsoil within the proposed footing areas should be removed.The on-site soils contain a slight amount of fine-grained material, which makes them potentially moisture sensitive and subject to disturbance when wet.The contractor must use care during site preparation and excavation operations so that any bearing surfaces are not disturbed. If this occurs,the disturbed material should be removed to expose undisturbed material. All footing excavations should be trimmed neat and the bottom of the excavation should be carefully prepared.All loose or softened soil should be removed from the footing excavation prior to placing reinforcing steel bars. We recommend that footing excavations be observed by the geotechnical engineer prior to placing steel and concrete,to verify that the recommendations of this letter report have been followed. If footing excavations are open during the winter season or periods of wet weather, it may be helpful to provide a layer of crushed rock or gravel to help preserve the subgrade until the placement of concrete. If gravel is used to protect the bearing surfaces, it should meet the gradation requirements for Class A Gravel Backfill for Foundations,as described in Section 9-03.12(1)A of the 2008 WSDOT Standard Specifications. Foundations Although current plans are conceptual,the following general foundation criteria are provided. Conventional shallow contact foundations such as spread footings may be used for building support of the proposed residence. Bearing surfaces for the footings should be prepared as described above. Based on our reconnaissance, we estimate the adequate bearing strata typically consisting of dense, brown to gray, slightly moist, gravelly sand with trace silt to be roughly 2 feet below the existing ground surface in the vicinity of the proposed residence footprint. We recommend an allowable foundation bearing pressure of 2,000 pounds per square foot (psf)be utilized for design purposes, including both dead and live loads for the proposed house. An increase in the above-mentioned bearing pressure of one-third may be used for short-term wind or seismic loading.Perimeter footings should be buried at least 18 inches into the surrounding soil for frost protection; interior footings require only 12 inches burial below outside grade. However, all footings must penetrate to the aforementioned unyielding bearing stratum, and no footing should be founded in or above yielding/loose or organic soils. We estimate the total settlement, including both elastic settlements and long term consolidation, of the foundation designed in accordance with our recommendations will be less than '/2-inch.Differential settlements can be expected to be less than half the total settlement. Our experience indicates the majority of these settlements will occur during construction. Wind, earthquakes, and unbalanced earth loads will subject the proposed structure to lateral forces. Lateral forces on a structure will be resisted by a combination of sliding resistance of its base or footing on the underlying soil and passive earth pressure against the buried portions of the structure. For use in design, an ultimate coefficient of friction of 0.45 may be assumed Page 8 • • • Southmayd Residence February 22,2011 Project No. 070052-002-01 along the interface between the base of the footing and subgrade soils. A passive earth pressure of 450 pounds per cubic foot(pcf)may be assumed for native soils adjacent to below-grade elements.The upper 1-foot of passive resistance should be neglected in design. The recommended coefficient of friction and passive pressure values are ultimate values that do not include a safety factor. We recommend applying a factor of safety of at least 1.5 in design for determining allowable values for coefficient of friction and passive pressure. Drainage Considerations All footings should be provided with a drain at the footing elevation. Drains should consist of rigid,perforated,polyvinyl chloride(PVC)pipe surrounded by washed drain gravel. The level of the perforations in the pipe should be set approximately 2 inches below the bottom of the footing and the drains should be constructed with sufficient gradient to allow gravity discharge away from the house. Roof and surface runoff should not discharge into the footing drain system but should be handled by a separate,rigid,tightline drain. In planning, exterior grades adjacent to walls should be sloped away from the structure to achieve proper surface drainage. Adequate seepage and surface water drainage control in the vicinity of the building excavations will reduce the amount of overexcavation of disturbed soils that may have to be removed from subgrade and foundation areas. Critical Areas Buffer Considerations Based upon our Site reconnaissance and the results from our slope stability analyses,the Site can be developed as proposed provided no alterations are made within the current steep slope buffer which extends 30 feet east of the top of the beach slope and any infiltration facility is located a minimum of 50 feet from the top of the beach slope. Additional Project Design and Construction Monitoring At the time of this letter report, site grading, structural plans, and construction methods have not been finalized and the recommendations presented herein are preliminary. We are available to provide additional geotechnical consultation as the project design develops and possibly changes from that upon which this letter report is based. We are also available to provide geotechnical engineering and monitoring services during construction. The integrity of the foundation depends on proper site preparation and construction procedures. In addition, engineering decisions may have to be made in the field in the event that variations in subsurface conditions become apparent. Limitations The conclusions and recommendations provided above are based on the information collected during our reconnaissance. Within the limitations of the scope, schedule,and budget; our services have been performed in accordance with generally accepted geotechnical engineering and engineering geology practices in effect in this area at the time our letter report was prepared.No other warranty, expressed or implied, is made. It must be understood that no recommendations or engineering design can yield a guarantee of stable slopes. Our observations, findings,and opinions are a means to identify and reduce the inherent risks to the owner. Page 9 • • Southmayd Residence February 22, 2011 Project No. 070052-002-01 This letter report was prepared based on a geologic reconnaissance of the project area and was prepared for the exclusive use of Ms. Laura Southmayd and her agents with specific application to the project Site. We are able to offer further assistance with this project, if desired. Please contact us to coordinate these activities. It has been a pleasure to provide these services to you. If you have any questions, please do not hesitate to call. Sincerely, Aspect consulting, LLC IN `11aN.AL I\ r � '✓� j John L.Peterson,PE Andrew J. Holmson Senior Associate Geotechnical Engineer Senior Staff Geotechnical Engineer jpeterson(u),aspectconsulting.com aholmsonLaspectconsulting.com Attachment: Figure 1 —Site Location Map Figure 2—Site Plan W:1 GEOTECH1070052 Southmayd ReconnaissancelLetter Reportlteb22.doc Page 10 • r '''. . , I J it , ' i, ' _ l4,\� ., r• / t, "",b rti' !; ! F `3 • \ ,. 4 29 . - IBM!2IT - Ir ' _ D�scoveiv Say _ r camp meeting o`y ,� - u_ a1 7. t r"n`���J t _ E 'AREA �'�' 32 / \\ \;` R Four Corners 3 Z r�o � � • \,e •J �✓ r % •L;ras&rP - �� Site Location li : 4 cri !iI:1 r i ` /^ o `J , ./- t I ' , O .t ,,•, i i `� t 30,, Y y �r 3' 4 . r T — 1 {1 i o . '11I ' '' r' _\ + l / �r ys(( 1 6/1) �/M • $ Y "k; r _ 'a�•</!aIN Point N +�C,i 1. Atti[3ER 0` LAKE m J E - 0 .. 2000 4000 i ,!. t Pil� ) L I s _ , - _ o rr Feet Wood .v; ( •\� S TA rE PAR CO Site Location Map wFebruary2011 PROJECT NO. S pect °.. 070052 4111iS rf AJH co SULTING Southmayd Residence °ww"pM6 FIGURE NO. 1-6702 Adelma Beach Road, Port Townsend,Washington ""°`°" _ 1 �? a