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HomeMy WebLinkAboutIsland Vista Lot 4 - Geotechnical Report - 2005.07.20FHtffi ffi63hlY Subject Property t { Ft/) F-{. rfd( 5 t 15T ST F(o E{ to rylllI Ltl H;r::. :r'ir,:rkrl ,r:iil l<rt CtrL-t;t'.ir:'r:'.r .3.t';itt:r {l;l! 3 lLE---'i--, I I li Prepared For Linda Hodson July 20, 2005 For the Pro.p9ly_D^g1c1ibed As fax # 963700004 Lot 4, lsland Vista, City of Port Townsend Section 15, Township 30 North, Range 1 West, W.M Jefferson County, Washington Prepared by NTI Engineering and Surveying 717 S. Peabody Street Port Angeles, Washington 98362 Phone 360-452-8491 Fax360-452-8498 Web Site www.nti4u.com E-mail info@nti4u.com GEOTECHNICAL REPORT Tax # 963700004 July 20, 2005 Linda Hodson 51 Vista Blvd. Port Townsend, WA 98368 Subject: Geotechnical Report for Tax # 963700004, City of Port Townsend Dear Mrs. Hodson: lntroduction At your request, Bill Payton, Engineering Geologist, with NTI Engineering and Surveying (NTl) conducted'a geotechniial inspection of the above-mentioned property on July 12, 2005. fhe purpose of this inspection was to examine the marine bluff at the subject property by visual means and report our findings in regards to the proposed construction of a new single-family residence on the proPertY. Site Description The subject property is located on Vista Boulevard, in Port Townsend, Washington' The property is undeveloped high bank waterfront overlooking Port Townsend Bay (Figures 1 inrougn 4). The lot is geneially flat, level and vegetated in grass with some mature trees (Photo 1). The bluff is about 160 feet in elevation. The average slope angle of the bluff is about 34 degrees. The upper -15 feet has a slope angle of about 60 degrees, while lower sections of the blciff slope have slope angles of about 36 to 38 degrees. The bluff face is predominantly covered with grass and weeds (Photo 2). No springs or seeps were noticed on the bluff face. At the base of the bluff, there is a waterfront trail that is protected from wave erosion by a rock bulkhead. This bulkhead also protects the toe of the bluff below the subject property from wave erosion. Geologic Gonditions The Coast al Zone Atlas of Washington describes the soil on the upland portion of the subject property as the Vashon lodgement till (Qvt1), a compact mixture of boulders, cobbles, pebbles, sand, sitt and clay, generally overlain by 1 to 5 feet of ablation till. The Atlas also describes this soil as being excellent for foundation stability and good for seismic stability. The Atlas maps the 1 slope stability of the bluff at the property as Unstable old slide (Uos), and the upland portion of the property as Stable (S). The Atlas states that this soil stands in steep natural and cut slopes for long peiiods but may ravel and spall by wetting and drying and freezing and thawing. The Alas describes the soils found on the face bluff at the property as the Vashon advance outwash (Qva), a member of the Vashon Drift. This soil is composed of well-sorted, well- stratified sandy gravel. The Atlas describes the foundation stability of this soil as good to excellent but may be poor on slopes that approach the angle of repose (30 to 39 degrees). The slope stability of itris soil is described as generally stable in slopes up to the angle of repose and may stand in steeper slopes for short periods. Seismic stability is described as good' The USDA Soil Survey of Jefferson County Washington describes the upland soil in the area of the subject property as the Dick loamy sand (DcC), the Townsend fine sandy loam (TlC) and the Cassoliry sandy loam (CfC). The Survey maps the soil along the bluff face as Rough Broken Land (Ro). The Dick soilformed in glacial outwash and is classified as silty sand. This soil is somewhai excessively drained with rapid permeability. Runoff is slow and the hazard of water erosion is slight. The Townsend soil formed in glacial till and is classified as a silty sand or silty gravel. This ioil is moderately well drained with a cemented layer at abgu! 3' below the surface. Fermeability is moderate above the cemented layer. Thehazard of bluff slippage or slough-off is moderate to severe. The Cassolary soil formed in reworked glacial and marine sediments and is classified as a silty sand with a layer of clay from -23 inches to 38 inches below the surface. This soil is well drained with moderately slow permeability. Runoff is slow to medium and the hazard of water erosion is slight to moderate' Visual observations of the bluff face are generally consistent with the above descriptions. The upper -15 feet of the bluff was compact silty sand and gravel with cobbles. The majority of the rest of the bluff was covered with vegetation and eroded material from the bluff. Gonclusions and Recommendations As with most of the shoreline of the Puget Sound area, the bluff at the subject property is undergoing bluff recession, albeit at a slow rate due to the protection of the toe of the bluff provided Oy tne rock bulkhead. Occasional sloughing of the bluff soils, predominantly due to weathering and erosion, should be expected. Typically, this type of bluff recession removes a foot or so of material at a time. A common average rate of bluff recession in the Puget Sound area is about an inch or two per year. However, the average rate of recession at the subject property is probably slower than this average due to the protection of the toe of the bluff. Factors that are contributing to the stability of the bluff include the protection of the toe from wave erosion due to the roCk bulkhead, the compactness of the bluff soils, and the majority of the bluff slope is close to or at the "angle of repose" which is defined as the maximum slope or angle at which loose, cohesionless material remains stable and commonly ranges between 33 and 37 degrees on natural sloPes. The Port Townsend Municipal Code mandates a minimum 2S-foot setback from the edge of the marine bluff. However, the lnternational Building Code (lBC) also regulates setback distances from steep slopes. ln this case, the IBC requires that the face of the footing of the house be at least 40 feet from the face of the slope (Figure 5). The house can be closer to the slope by means of extending the depth of the foundation as shown in Figure 1805.3.1 . Thus, with a building setback oi25 feet, the footings (or pilings in this case) would have to be about 10 feet deep, wtricn may not be practical from a cost perspective. However, if the house were set back 30 fbet from the bluff, then the footings (or pilings) would only need to be about 7 feet deep, and 2 at 35 feet back, the footings would only need to be 3 feet deep. Figure 5 can be used to determine the approximate depth of footing (or pilings) required for various building setback distances. Based upon our geotechnical review of the subject property we recommend that the proposed house be no clos-er than the 2s-foot setback mandated by the City of Port Townsend and that the IBC guidelines are followed with regard to footing depths and setbacks. ln consideration of the poteitial for future slides on the blulf and the longterm value of the property, we would. encourage a farther setback distance. The house to the south sits about 28 feet from the bluff, not incluling the deck, and the house to the north sits about 37 feet from the bluff. While standing 35-feet back irom the bluff on the subject property, there was still quite a commanding view to ihe north and south, with the added benefit that the view of the paper mill was blocked. Also, at this distance, the footings would only need to be three feet deep. Decks and patios, etc. would still be allowed up to the 25-foot setback line. The following recommendations should also be considered with regards to development of the property: 1. During construction, we recommend that heavy construction equipment and/or materials be kept away from the edge of the bluff. During any excavation work, say for a basement which would be acceptable from a geotechnical perspective), it would be better to remove the soil from the site rather than stockpiling it on site. 2. Maintaining ground cover will help reduce erosion from surface runoff. Any bare areas that develop-shoutd be revegetated. Native vegetation that requires little or no irrigation would be the most beneficial. 3. Heavy irrigation or other activities that would contribute large quantities of water to the soil should be avoided. One cause of landslides is the presence of excessive groundwater in the soil near the bluff. 4. Surface water should not be allowed to flow over the face of the bluff and cause erosion of the bluff face. This can be controlled with vegetation and using berms or swales to direct runoff to a drainage system. 5. Surface runoff from hard surfaces such as roofs, driveways, walkways and patios should be controlled and routed to the city storm sewer or to a drainage system if the city storm sewer is not available such that surface water discharge to adjacent properties does not significantly exceed predevelopment conditions. 6. Silt fences or other sediment control devices may be needed during construction such that sedimentation to adjacent properties does not significantly exceed predevelopment conditions. 7. All drainage control devices should be maintained in good working order and inspected at least once a Year. An engineered drainage control plan should be developed for this property to address items 4 through 7 above. NTI can provide this service if requested. A contractor knowledgeable in the construction of these types of features should be retained for this work. 8. 3 For further information please review the three attached copies of booklets published by the Washington State Department of Ecology (DOE) entitled: "Slope Stabilization and Erosion ControlL,lsing Vegetation", "Vegetation Management: A Guide for Puget Sound Bluff Property Owners" and"surface Water and Groundwater on Coastal Bluffs". These publications are now out of print but can be reviewed at the DOE website at: http:/lwww.ecy.wa"gov/biblio/sea'htfnl under ine t ggg and 19g4 year heading. The DOE website also contains more useful information regarding slope stability and site development and is highly recommended. Based on the findings, recommendations and limitations of this report, the proposal should pose no unreasonable threat to persons or property nor decrease slope stability. Limitations This report has been prepared for your exclusive use in conjunction with.the.above referenced prolect. The report nas nbt been piepared for use by. others or for other locations. Others may irsd it only with'the expressed written permission of the Engineer. within the limits of scope, schedule and budget, this report was prepared in general accordance rriir' l.""pteo professiirn|t engineering and leological principles and practices in this or similar toCjtitier bt tnei time the report-was prdpared. No other warranty, expressed or implied, is made as to the conclusions and irofessional advice included in this report. The observations, conclusions and recommendations presented in this report were based on our visual observations of the subject property at the time of our site visit; no laboratory tests were pefformed. Soil and geologic conditions can vary significantly between test holes and/or surface outcrops. lf there is a substantial lapse of time, conditions at the site have changed or appear different than those described in this report, we should be contacted and retained to evaluate the changed conditions and make modifications to our report if necessary. Sincerely, NTI ENGINEERING & SURVEYING, INC. Robert A. Leach, P.E., MBA Principal Engineer Bill Payton, L.E.G. Engineering Geologist G:\Gen\Bill\Reports.HODL0501.15(30-1).bluff stability.Vista Blvd PT.doc EXPIRES 12t30t2006 Expires 11106105 4 137720ETs otvAL I14. William C, Pa ton Jr, For further information please review the three attached copies of booklets published by the Washington State Department of Ecology (DOE) entitled: "Slope Stabilization and Erosion Control Using Vegetation", "Vegetation Management: A Guide for Puget Sound Bluff Property Owners" and "surface Water and Groundwater on Coastal Bluffs". These publications are now out of print but can be reviewed at the DOE website at: http://www.ecv.wa.qov/biblio/sea.html under the 1993 and 1994 year heading. The DOE website also contains more useful information regarding slope stability and site development and is highly recommended. Based on the findings, recommendations and limitations of this report, the proposal should pose no unreasonable threat to persons or property nor decrease slope stability. Limitations This report has been prepared for your exclusive use in conjunction with the above referenced project. The report has not been prepared for use by others or for other locations. Others may use it only with the expressed written permission of the Engineer. Within the limits of scope, schedule and budget, this report was prepared in general accordance with accepted professional engineering and geological principles and practices in this or similar localities at the time the report was prepared. No other warranty, expressed or implied, is made as to the conclusions and professional advice included in this report. The observations, conclusions and recommendations presented in this report were based on our visual observations of the subject property at the time of our site visit; no laboratory tests were performed. Soil and geologic conditions can vary significantly between test holes and/or sur-face outcrops. lf there is a substantial lapse of time, conditions at the site have changed or appear different than those described in this report, we should be contacted and retained to evaluate the changed conditions and make modifications to our report if necessary. Sincerely, NTI ENGINEERING & SURVEYING, INC. Robert A. Leach, P.E., MBA Principal Engineer 7-zo-o5 EXPIRES 12t30t2006 Bill Payton, L.E.G. Engineering Geologist G:\Gen\Bill\Reports.HODL0501.15(30-1).bluff stability.Vista Blvd PT.doc Expires 11/06/054 13772cgsT Wllllam C. Pa ton Jr, zf *afas Appendix Subject Property Figure 1 Jtmt Cd[E Siriocffi Subjecl Property Top of bluff Toe of bluff Figure 2 Jdrut Corf Ccrrd Ssriocc GG Subject Property Fiqure 3. Year 2000-02 Dept. of Ecoloqv Shore Subject Property Fiqure .Year 1992-97 Dept. of Ecoloqv Shore Photo Photo 1. View of subiect propertv lookinq east from street. Photo 2. View of bluff below subiect properW t 2003 lnternational B uilding Code FrcEOFl-,/ srwcruee IrctorIslopE FACEOF FOOflNA HBBUTNEEDNOT EXCEED4Ofr MAX lOPOF SLdFE For SI: I foot = 304.8 mm. SI.,TNEEDNOI E<CEEO 16 FE MAX FTGURE 1805.3.1 FOUNDATION CLEARANCES FROM SLOPES 1o -,135' 30', *5' t 6' T,B.L. F",ortvlg b€rorH CALAULATgA a 8' lcr' SCALE aecnox') 1 o *:\-7"\ Figure 5