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HomeMy WebLinkAbout002121007 Geotech Assessment ,'!,.. ~ . . . RECEIVED STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 JUL 2 7 2006 March 31, 2006 JEffERSON COUNlV DCD Mark Bolduc 2023 East Simms Way Box 325 Port Townsend, W A 98368 Re: Geology Hazard Assessment Tax Parcel 002121007 Jefferson County, Washington Dear Mr. Bulduc: I visited Jefferson County Tax Parcel 002121007 on Pond Road in March 2006 to evaluate the slope stability of the steep shoreline bluff on the northwest side of the subject property. The purPOse of the site visit was to assess the geology conditions on the site slopes slope and shoreline and determine an appropriate minimum setback distance for the construction of a home on the site as well as other development recommendations. Based on my assessment of the geology and geologic processes at the subject property a home sited on the property should be set back at least 100 feet from the top of the steep bluff slope. SCOPE OF SERVICES The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property including the bluff face slope conditions, shoreline conditions, and relevant conditions in the vicinity of the property. 2) Observed surface soil conditions on the bluff face and on the uplands above the bluff by excavating shallow hand dug test pits and observing test pits excavated for septic.drain field evaluation. 3) Prepared this report summarizing our findings, including an evaluation of the feasibility of building a residence on the subject property, an evaluation of the shoreline bluff stability, recommendations for site development, and recommendations for further investigation, if necessary. RECEIVED . March 31, 2006 Tax Parcel 00212007, Pond Road Geology Assessment JEffERSON COUNlY oeo GENERAL GEOLOGY Northwestern Washington has been occupied by continental glaciers at least four times during the Pleistocene Epoch (1.6 million to 10,000 years ago). During these glacial and accompanying interglacial periods, the underlying bedrock was deeply eroded. The Surficial Geologic Map of the Port Townsend 30- by 60-Minute Quadrangle, Puget Sound Region, Washington (Pessl, Dethier, Booth and Minard, 1989) indicates the upland portion of the subject property is underlain by glacial till from the Vashon-age ice sheet. The mapping indicates that the shoreline bluff slope on the subject property is underlain by undifferentiated non-glacial and glacial sediments. The Geologic Map of Northeastern Jefferson County (Gayer, 1976) indicates that the top of the shoreline bluff is underlain Vashon ablation till. The shoreline slopes are mapped as consisting of pre-V ash on stratified deposits. In addition, Gayer (1976) provides a profile of the stratigraphy of the bluff to the southeast of the subject property. The stratigraphic profile from the top of the bluff to the shoreline is Vashon till, Kitsap Formation, Possession Till, Whidbey Formation and Double Bluff Formation. . Site observations on the subject property, bluff face, and in the vicinity are generally consistent with the above-described mapping. However, the upland portion of the property is underlain by compact sand and gravel stratified sediments and as well as the glacial till indicated by the mapping. The shoreline bluff slope on the subject property is primarily underlain stratified sand and gravel with very minor silt. My site observations are consistent with these units being advance outwash near the upper part of the bluff and interglacial alluvial deposits in a river environment on the rest of the slope. A glacial till unit is located approximately two-thirds of the way down the slope. However, the unit is partially obscured by failed soils from the steep upper portions of the bluff. SPECIFIC SITE OBSERVATIONS The subject property consists of a gently sloping upland area bounded on the northwest by a very steep shoreline bluff that is approximately 160 feet high. The upland area of the property consists of nearly level area on the northeast half and an area that slopes towards the northeast on the southwest half of the upland area. The top edge of the bluff is very abrupt and very steep. The overall bluff slope on the subject property is approximately 55 degrees with the upper half of the bluff being nearly vertical. The lower part has slopes on the order of35 degrees. The upland area is underlain by silty sand and gravel with cobbles. Most of the upland is tree- covered with Douglas fir, cedar and alder with an understory of ferns and other brush. A grass- and brush-covered clearing is located in the mid portion of the upland portion of the lot. . File: 3.27.06C Stratum Group 2 RECEIVED . JUL 2720GB March 31, 2006 Tax Parcel 00212007, Pond Road Geology Assessment JEfFERSON COUNlY oCD The steep shoreline slope has substantial bare areas because of the steepness and due to loose recently moved material on the less steep area. Areas of the bluff are grass-covered with stands of fir and alder interspersed. The beach at the base of the bluff consists of a cobble and sand beach with a few scattered boulders. The beach material generally coincides with the source material from the bluff above. Waves reach the toe of the steep slope on a routine basis depending on the tide and the height of the waves. It has been my observation that waves reach the toe of the slope along this section of shoreline on a fairly regular basis during periods of higher tides. Wave action on the lower part of the bluff has been causing the bluff to become over steep triggering periodic landslides. Shallow soil landslides are evident all along this section of shoreline. Where the bluff is underlain by more cohesive sediments to the northeast, the bluff is very nearly vertical. The predominance sand and gravel units with little silt and clay with low cohesion has led to the lower bluff slopes near the subject property to slope less steeply than the vertical shore to the northeast. . The geologic units underlying the bluff at the subject property consist of very compact layered sand and gravel from the top of the bluff to the approximately half way down the bluff slope. A compact sand unit approximately 20 feet thick with very little silt underlies the stratified sand and gravel. This unit has a fairly low resistance to erosion. The lower slopes of the bluff are underlain by sand and landslide debris from the slopes above. No seeps or springs were observed anywhere on the bluff slope on the subject property. Due to the steepness of the slopes rotational failures may develop within thick uniform layers of sediment. Failures of this type have taken elsewhere along this section of bluff, but the units on the subject property are well layered and there is no evidence of rotational slides on the bluff at the subject property. ASSESSMENT OF GEOLOGIC RISK Observations along the shore and the top of the bluff indicate that the primary cause of slope instability is the undermining of the base of the bluff by wave action. As wave action erodes the base of the bluff the lower slopes fail and the failed material with the exception of boulders is readily washed away by the large waves that periodically hit the shoreline. As the base of the bluff fails and the bluff becomes over steep the slope failures progressively move up the slope until the uppermost part of the slope fails as well and the process repeats itself. This stretch of shoreline bluff is in various stages of this process. Most sections of the shoreline in this area have very little or no landslide debris along the base of the bluff. Overall though the bluff is very steep and erosion at the toe of the bluff should be expected to continue. As the very steep slope weathers shallow slab type failures are likely to occur. Based on . File: 3.27.06C Stratum Group 3 . . . RECEIVED rpI 2 7 "H"lil5 ,.llh. lUU' March 31, 2006 Tax Parcel 00212007, Pond Road Geology Assessment JEffERSON COUNlY DCD my observations along the top of the bluff at the subject property and all along the top of the bluff northeast and southwest of the bluff, it is my opinion that these slab type failures will be on the order of a 5 to 10 feet. Erosion rates along the eastern end of the Strait of Juan de Fuca are estimated to be on the order of 6 inches per year. Because of the protection afforded by Protection Island and the height of the bluff (the higher bluff height means a greater volume of soil would need to be eroded), I estimate that erosion rates along this section of coast to be somewhat less - on the order of 4 inches per year. The top of the bluff will not systematically retreat 4 inches per year, but the bluff will progressively get steeper and mid slope failures will take place and eventually another failure will take place at the top of the bluff. As noted above the size of top of bluff failures should be expected to be on the order of 5 to 10 feet. The recurrence interval of top of bluff failures that send material to the base of the bluff appears to be on the order of 25 to 30 years based on the age of tree stands along the steep shoreline slopes. This estimate is complicated by a several factors: 1) the bluff height varies, 2) clearing of trees on the slope and at the top of the slope varies, 3) drainage and slope management on the developed properties and undeveloped properties likely has varied, and 4) variability in the underlying geologic units causes some slopes to have more frequent small slides and other slopes to have larger less frequent failures. Stratum Group File: 3.27.06C 4 ... . . . RE(~EIV1E1[) March 31, 2006 Tax Parcel 00212007, Pond Road Geology Assessment JEfHRSON COUNTY OCO CONCLUSION AND RECOMMENDATIONS I recommend that all occupancy structures sited on the property should be set back at least 100 feet from the top of the steep shoreline bluff. The septic tanks and drain field should be set back at least 100 feet from the top of the steep shoreline slope. Decks may be closer as long as they are not structurally attached to the home. The setback of 100 feet would allow for an erosion rate of 1 foot per year. This rate is substantially greater than the estimated rate of 4 inches per year. The setback of 100 feet will also provide protection of the building site in the remote event a sand on the bluff face rotationally fails in a similar manner to other rotational failures seen northeast and southwest of the subject property. I am of the opinion that due to the differences in the soil units on the slope this risk is very small. Although this setback distance is very conservative, it should be kept in mind that stabilizing erosion at the base of the bluff is not practical given the size of the waves, frequency of toe erosion and the need to. coordinate shoreline works with multiple property owners. As describe above most of the factors impacting the bluff stability over time are beyond the control of a property owner. However, some actions can be taken to reduce the frequency of slope failures. Based on site observations and my assessment of the geologic risk, I make the following recommendations: . Native vegetation, particularly tress and low native brush on the bluff slope should be disturbed as little as possible. No grading should be performed on the bluff face. If a trail is constructed, the trail layout and support structures should be approved by a qualified engineering geologist before construction. . Waste and other deleterious material should not be placed on or over the slope. Such material smothers vegetation, retains water and acts to destabilize the slope. . Storm water from roof run-off and driveway run-off should be dispersed as much as possible through allowing sheet run off and lateral dispersion spreaders such that storm water is not concentrated anywhere on the property and onto the bluff face. . Removal of alders on the upland portion of the bluff will not have a negative effect on slope File: 3.27,06C Stratum Group 5 . . . ., . RECCIV'ED JUl Z 7 20n6 March 31, 2006 Tax Parcel 00212007, Pond Road Geology Assessment conditions or increased infilttation of water, as alders are leafless in the windffEQSnM CnUMTi ncn The northeast-facing slope on the upland portion of the subject property is stable and development on this slope will not be at risk from landsliding or slope movement as long as the development is setback at least 100 feet from the top of the steep shoreline bluff. As noted above the stopping erosion and slope failures at the site is not feasible. Any shoreline works on this stretch of shoreline should be reported to Jefferson County and Ecology officials. Any reduction of erosion on properties to the southwest of the subject property would have a negative impact on the shoreline of the subject property. Except for a short section of rock rip rap at the low bluff section of Cape George, there are no armored sections along this section of shoreline. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions regarding our reconnaissance please contact our office at (360) 714-9409. Sincerely yours, ]Str m Group . -C>- #ill-- Dan McShane, L.E.G., M.Sc. Licensed Engineering Geologist File: 3.27.06C Stratum Group 6 )0 t. ,,&: z .~ o o ~ . ..r..... .......... ~ Figure 1. Site Vidnlty Map 122053'00" W 122054'00" W z <:> o 7D o o ~ z z <:> <:> 0 0 r... r... 0 0 0 0 OJ ~ V z (:) o ~ o o OJ V \ \ \ \: (;:y . 122054'00" W TNV~ 122053'00" W 122052'00" W WGS84 122051'00" W o .s 1 MILE o .JlXkI fEET 9 ~ _ ---2000 MmAS Map created with TOPO!@OOOO3NatilnalGeogtaphic(wwwmtionalgeogmphic.comIlopo)