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HomeMy WebLinkAbout935100034 Geotech Assessment~arce.F ~(3~0003~ S~~Pt~5=3~ GARY A. FLOWERS, PLLC .~__ :GEOLOGICAL & GEOTECHNICAL CONSULTING ~ ~ `~~ft ~, ~. _f Transmi~t~l Form ~~~ ~ ~ ,,,~a Date: October 4, 2005 ;: _ : ` ~.JPtaEPd7 To: Steve Hargrove Project Geotechnical Report Name: From: Gary Flowers Project No: GF05085 Subject: Report Enclosed is a revised copy of the report. It is unbound so you can make whatever additional copies you need for submittal. Please dispose of the copies currently in your possession so there is no mix up. I made a couple of minor changes to make the intent of the 15 foot actual setback/30 foot effective setback clearer for the county. Please call if you have any questions. Phone: 206-417-7640 19532 12`" Avenue NE, Shoreline, WA 98155-1106 F:206-68b-3834 Gary A. Flowers, PLLC Geological & Geotechnical Consulting 1953212 Avenge NE Shoreline, WA 98155-110b September 19, 2005 , , Pmject No. GF05076 Mr. Steve Hargrove `` E 1 `"''" ~ ., ' P.O. Box 2715 Poulsbo, WA 98370 _-~ __._._-~JpfV,EraT Subject: GeologicaUGeotechnical Report Proposed New Hargrove Residence Lot 7, Bridgehaven Division 5 Thorndyke Road Port Ludlow, Jefferson County, Washington Deaz Steve: This report presents the results of our geologicaUgeotechnical assessment for Lot 7, Bridgehaven, Division 5 on the east side of Thorndyke Road. The subject pazcel is located in the SW1/4 Section 9, T27N, R1E in Jefferson County, Washington. The purpose. of this study was to provide an assessment of the property and steep slope area in order to meet the requirements of Jefferson County and to provide design recommendations for placement and construction of the new residence. We understand that you intend to build a two story house with daylight basement on the relatively flat bench area near the top of a steep slope. Jefferson County has mapped the slope area as both a landslide hazard azea and a seismic hazard area. As such, a geotechnical report addressing these issues is required. This report summarizes our fieldwork and offers recommendations based on our present understanding of the project. SITE CONDITIONS The subject parcel is rectangulaz is shape and measures approximately 75 feet wide by 240 feet long. The parcel contains both a relatively level bench area (west portion of the property) where the new house will be situated and a steep slope area (east portion of the property) that descends towazds the east. Beginning from the west boundary and going east, the property is relatively level for about 90 feet and then slopes gently down to the east for about 30 feet with an elevation loss of about 6 feet. At this point the ground surface descends steeply to the east. The slope height is on the order of 160 feet although the subject property only extends about 70 vertical feet down the hillside. Based on field measurements the slope is relatively constant at about 43 degrees or about 93 percent slope gradient. Gary A. Fbwus, PLLC. 19532 l2'~ Avemu NE Shoreline, WashingFon 98155-1106 206-417-7640 !. tt Hargrove Residence Geateclatical Services Report Jefferson County, Waslfngton The steep slope is heavily vegetated with scrub brush, small trees, nettles and bramble bushes. We understand from anecdotal evidence that the slope has been logged in the past by local residents for firewood and to preserve a view corridor so no large trees were observed. During our slope reconnaissance we did not observe any areas of ground water seepage, hydrophilic vegetation, erosion or historic surficial slope movement. The top of the steep slope in this area is almost entirely developed with single family houses with setbacks from the top of the steep slope that appear to range from almost zero to about 30 feet. A foundation excavation for a new house is located on Lot 8, immediately to the north of the subject property. Surficial Soil Conditions Soil conditions at the project site were inferred from field observations, visual reconnaissance of the site, and a review of applicable Soil Conservation Service literature. Surface soils on the steep slope area are reported by the Soil Conservation Service (SCS) to consist of Casollary sandy loam, 30 to 50 percent slopes. These soils are found on steep canyon slopes and ocean bluffs. They formed in reworked glacial and marine sediments. Runoff is rapid and the hazard of water erasion is severe. Soils on the upland part of the property, where the home will be located, are mapped as Everett gravelly sandy loam. This soil formed on glacial outwash terraces. Runoff is slow and the hazard ofwater-erosion is slight to moderate. Subsurface Soil and Ground Water Conditions Subsurface soil and ground water conditions for the steep slope are inferred from subsurface explorations performed for a geotechnical evaluation report, by AGRA Earth & Environmental, dated December 6, 1995 for the Ciani Residence Yard Repair and Retaining Wall Design at 831 Thorndyke Road, located about 41ots to the north and somewhat downslope of the subject site, subsurface explorations for the proposed new Ciani residence to be located at the toe of the steep slope, observation of the building pad excavation located on Lot 8, adjacent to and north of the subject property and review of test pits dug for percolation tests for the septic design on the subject property. All of the above explorations and observations indicate that the hillside is comprised of glacially consolidated sands with varying amounts of silt and gravel. We interpret these sediments to be correlative with Vashon age advance outwash deposits. In some areas the upper few feet is loose to medium dense and is likely correlative to Vashon age recessional outwash deposits. Ground water was not encountered in the borings, test pits or foundation excavation evaluated for this study. The AGRA report for Peter Ciani determined that due to the density of the core sediments the risk of deep seated,. rotational landslide failure of the slope was minimal. Their report w-~s, .__ ~, . ', Gary A. Flowers, PLLC. i 19532 12'" Avenue NE Shoreline, Washington 98155-1106 206-417-7640 `rv` 2 JQP:SEPi7 Hargrove Residence Ceotechnica! Services Report Jefferson County, Wirehington generated in response to a slope failure determined to have occurred wholly within man-placed fill soils on the steep slope. Slope movement is known to have occurred adjacent to the Ciani residence at 851 Thorndyke Road during the winter of 1996-97. This parcel is located 3 lots too the north of the subject property. This slope movement was classified as a debris flow of sunficial colluvial sediments triggered by excess storm water runoff and ground water emanating on the slope due to poor storm water control from the developed azea to the west, across Tt-orndyke Road from the slide. Since that time, the drainage ditch above both the 851 and 831 Thorndyke Road properties has been asphalted to prevent storm water infiltration into the sand soils in that area. The failure azea on the 851 Thorndyke Road property appears to have been successfully stabilized by the placement of several rockeries and additional control of storm water runoff. Based on anecdotal evidence we anticipate that there are silty/clayey layers within the generally sandy hillside, where emergent water may occur during very wet periods of the year. These layers are anticipated to be quite thin and likely are discontinuous throughout the slope azea. CONCLUSIONS AND RECOMMENDATIONS On the basis of our field investigation and engineering stability analysis, it is our opinion that the property is grossly stable from a potential rotational type of landslide failure. However, steep slopes are naturally active and bluff retreat affects all steep slopes to some extent and can impact rear yard areas due to soil weathering and saturation of loose, surficial soils. Due to the very dense nature of the over-consolidated glacial deposits that foam the hillside, and the heavy vegetative cover, the potential for even surFcial slides is low under natural conditions. Based on the proposed location of the planned residence, with an actual setback of 15 feet and an effective setback from the top of the steep slope of 30 feet, with no development or alteration of the steep slope area, the proposal will not decrease slope stability on adjacent properties and the newly created building site will be stable under normal geologic conditions. Slope Stability Conclusions Based on our geological reconnaissance and literature search, it is our opinion that the existing steep slope area is currently stable for large rotational type of landslides. This is indicated as follows: • No seepage of ground water was observed on the steep slope although seepage is believed to occur within the heavily vegetated tce azea of the slope. • The core of the slope is comprised of dense to very dense, glacially consolidated, granular sediments with some interbeds of fine grained sediments. • There are no current indications of slope movement on the site. Adjacent sites to the north have had failures in the recent past. One of the failures~poxtedlys~___ ~; Gary A. Flowers, PLLC. 19532 12'~ Aven~u NE Shoreline, Washington 98155-1106 206-417-7640 i`('~. Ut/ I ll ~~ r 3 Hargrove Residence Geo~technical Services Repart Jefferson County, .Washington entirely within man-placed fill materials. The other failure occurred during an extreme storm event and was a surficial debris flow, not a lazge rotational failure. • The presence of large trees elsewhere on the steep slope area indicate that the natural wasting cycle for the slope in this area is long enough to allow mature trees to grow. No lazge trees were observed on the steep slope area in the immediate vicinity of the subject property. However, we understand the lack of trees is due to logging operations, not landslide activity. In addition, a stability analysis of the steep slope was performed to verify gross stability of the hillside. Analysis was performed on three potential failure planes, shallow (beginning in front of the proposed house), medium (beginning just inside the bottom of the proposed foundation bearing) and deep (also beginning just inside the bottom of the proposed foundation bearing but at a steeper angle than the medium depth slide plane). The minimum factor of safety against sliding was determined to be 1.55 for a deep slide, 1.93 for the medium depth slide and 1.5 for a shallow slide. The minimum factor of safety for dynamic loading (seismic) was 1.1. Even though the steep slope area above the proposed project area is generally stable at this time, it is mapped as a landslide hazard zone (moderate) and a seismic hazard zone. According to Jefferson County Code, Title 18 Unified Development Code, Chapter 18.15 Land Use Districts, 18.15.165 Environmentally Sensitive Areas, Article VI-G, Geologically Hazardous Areas, the standard setback from the top, toe and all edges of landslide hazazd areas is 30 feet. It is our opinion that a 30 foot setback is sufficient for the subject property. If site conditions allow, the setback can either be a physical 30 foot setback or can be an effective or structural setback measured from the edge of the foundation to the slope face such as proposed herein. Erosion Hazard Conclusions The steep slope area is not mapped as an erosion hazard area but the SCS soil classification is Casollary sandy loam with a severe erosion hazard. However, no development or alteration is planned on the steep slope portion of the property. As such, no special erosion mitigation, beyond normal best management practices for construction as per Jefferson County Code is recommended for steep area of the site. The soils on the upper portion of the parcel, where the house will be located, are mapped as Everett gravelly sandy loam with a slight to moderate hazazd of erosion. Standard construction practices should suffice for this azea. This would include re-vegetating stripped areas as soon as possible, covering soil stockpiles with plastic, use of silt fences, channeling storm water runoff away from the steep slope area and constructing a suitable construction entrance to keep sediment off of the street. Seismic Hazard Conclusions The steep slope is mapped as a seismic hazard azea. Since the residence will have an effective setback of 30 feet and the core of the slope is comprised of dense to very "d"ense, glacially-~~ r! Gary A. Flowers, PLLC. 19532 12~' Avenue NE Shoreline, Washington 98155-1106 4 i ..~F'tvSCfj; ~. Hargrove Residence Geotechnical Services Report Jefferson County, R'asl~tgton consolidated sediments with low risk of large scale rotational failure, the risk to the home from seismically induced slope movement is low and no further mitigation is required. The potential risk from liquefaction induced settlement is negligible. It should be understood that all of the above geologically hazardous areas are located on the steep slope portion of the site. None of these hazardous areas are located on the bench area where the new residence is planned. Drainage Conclusions Provided all storm water is properly disposed of either on-site or into an approved storm water conveyance system, the proposed project will not increase surface water discharge or sedimentation to adjacent property beyond predevelopment conditions. It is our opinion that storm water should not be disposed on-site adjacent to steep slope areas. We understand that storm water discharge for this project will be piped to the tce of the slope and either infiltrated or spread horizontally at that location. This will involve obtaining an easement from the downslope property owner which we understand has been arranged. The storm water conveyance pipeline down the slope should be an HDPE (high density polyethylene), butt welded pipe that is anchored at the top and bottom of the slope only. An infiltration or spreader system should be designed by a drainage (civil) engineer for the site at the toe of the slope. Geotechnical Recommendations The new residence should generally be supported on continuous bearing wall footings that extend a minimum depth of 18 inches below final grade and bear on undisturbed or recompacted natural glacially consolidated soils that underlie the site. It is recommended that the bearing for footings along the east side of the residence be located no closer than 30 feet from the face of the slope. Since the house will be physically located 15 feet from the top of the slope, the 30 foot setback must be accomplished with an effective or structural setback whereby the foundation loads are transferred vertically downward to a point that is 30 feet from the slope face. This can be accomplished either by driving small diameter pipe piles to the effective depth and constructing a grade beam footing to connect the piles together or by excavating a foundation trench to the required depth and backfilling it with lean mix concrete. Either method must wrap around or return at each end a distance of 10 feet back to the west. If pipe piles are used we recommend that 3 inch diameter piles that are installed with a minimum 450 pound hammer. These piles will be capable of supporting loads of 12 kips per pile when installed with a driving resistance of 1 inch per 30 seconds of driving. However, due to the density of the underlying soils it may be necessary to overdrive the piles in order to achieve the proper depth of penetration. The tip of the piles must be located a minimum of 30 feet from the face of the slope. In general about 10 to 12 feet of penetration will be necessary to achieve the proper depth based on a daylight basement type of house that is proposed.r.~ ._ -_----- - ----- ~ ~ ~_;+ ~. Gary A. Flowers, PLLC. t9532t21°AvenueN6 Shordiae,Wasbington98155-1106 }„'.,r 2Q6-A17-7¢4¢ .ui .. 5 ~.JPt.°ENr Hargrove Residence Geootechnical Services Report Jefferson County,. Wir~ton An allowable soil bearing value of 3000 psf may be used for the lightly loaded house footings founded on undisturbed native soils. If any footings will be placed on properly compacted and approved structural fill an allowable soil bearing value of 2,000 psf should be used. Resistance to lateral loads on the structure can be provided by passive earth pressure on footings and stem walls. We recommend the use of 300 pcf for passive pressure and a coefficient of sliding friction of 0.5. Retaining Walls Retaining walls should be designed for an active earth pressure of 30 pcf if free to rotate and 50 pcf if fixed. Retaining walls should have a wall drain that extends to within 2 feet of ground surface comprised of an approved drainage mat or minimum 12 inch wide washed rock. The wail drain must connect to a footing drain to eliminate hydrostatic pressure buildup. All retaining walls should be backfilled with soil compacted to 90 percent of the maximum density to reduce the potential for future settlement under concrete walkways, driveways and patio slabs. Concrete Slabs Concrete slabs on grade may be placed directly on the undisturbed or recompacted natural sediments or on properly compacted and approved structural fill placed atop these sediments. Where moisture intrusion is a concern a heavy duty (minimum 10 mil) plastic vapor barrier should be placed between the concrete and soil subgrade. If the vapor barrier is compromised in any way during construction it should be replaced or an additional layer added. Any penetrations through the vapor barrier should be taped and sealed. Drainage In addition to foundation drains azound the perimeter of the basement walls footings, there will be area drains and roof drains that will collect storm water runoff. All of this water should be tightlined to an approved drainage system or to the bottom of the slope as discussed above. Every effort should be made to prevent surface water runoff over the top of the rear yazd slope and to prevent excessive infiltration into the upper soils near the steep slope. SUMMARY Construction monitoring and consultation services should be provided to verify that subsurface conditions aze similaz to those described in our report. Should conditions be revealed during construction that differs from the anticipated subsurface profile, we will evaluate those conditions and provide alternative recommendations where appropriate. Cary a Flowers, r~LC. 19532 12~' Avenuc NE Shoroline, Washington 98155-1106 -,206-~17-7640. ~; 6 Hargrove Residence Geotechnicarl Services Report Jefferson County, Washington Our findings and recommendations provided in this report were prepared in accordance with generally accepted principles of engineering geology and geotechnical engineering as practiced in the Puget Sound area at the time this report was submitted. We make no other warranty, either expressed or implied. Thank you for the opportunity to be of service on this project. Should you have any questions regarding this matter, please do not hesitate to contact our office. Sincerely, ~•of Wash;' • 4`' ~ .~ v....,/ 4 'fed dim ~ Gary A. Ftvw~ers Gary A. Flowers, P.G., P.E.G. Engineering Geologist r r:..-"~ ~, F``~ c~^~ ~~ QF W_ ~~ ,.~:,- ~. e ~ C~ ~/f, ~; o ,1 y t-- x ~~~ [~ zt,., o Robert M. Pride, P.E. Geotechnical Engineer lr ~--- ~, Gary A. Flowers, PL[.C. 19532 !2"'AvenueNE Shoreline, Washington 98155-1106 L~1b~-41~-T840= - - ~! I 7 ~ 1 _.i`~:9~t~1 f