Loading...
HomeMy WebLinkAbout977700058 Geotech Assessment~I L~ STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 November 5, 2006 Jan Garing 375 Merry Road Norland, WA 98358 Re: Geology Evaluation, Shoreline Bluff 375 Merry Road Norland, Washington Dear Ms Garing: ~iFa'~Q~~~~ d~~~- ~aIIV ~~~~ Stratum Group is pleased to present the results of our geology evaluation of the above referenced property on a shoreline bluff. The purpose of this geology evaluation was to 1) determine the suitability of adding to the existing residence, 2) qualitatively evaluate the risk of slope failures, and 3) provide general site development and maintenance recommendations for development of the property adjacent to a potentially unstable bluff. This evaluation included a visual inspection of the property and vicinity, a visual inspection of the bluff face, review of available geologic mapping in the area, and inspection of the shoreline at the base of the bluff. It is my opinion that the proposed addition to the north side of the existing home and the existing home on the site are will not be at risk from slope failures over at least the next 100 years. 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. 3) Prepared this report summarizing our findings, including an evaluation of the feasibility of building a residence on the subject property, a qualitative evaluation of the shoreline bluff stability, recommendations for site development, and recommendations for further investigation, if necessary. GENERAL GEOLOGY Northwestern Washington has been occupied by continental glaciers at least four times during November 5, 2006 375 Merry Road Norland, WA Geology Evaluation, Shoreline Bluff the Pleistocene Epoch (1.6 million to 10,000 years ago). During these glacial and accompanying interglacial periods, the underlying bedrock was eroded and a relatively thick layer of glacial related and interglacial fluvial sediments were deposited over the underlying bedrock in the vicinity of the subject property. The Surficial Geolo ig c Map of the Port Townsend 30 by 60 Quadrangle, Puget Sound Re ion, Washington (Pessl, Dethier, Booth and Minard, 1989) indicates the steep shoreline bluff exposed on the subject property is underlain glacial till and advance glacial deposits. Observations on the bluff face on the subject property and on the bluff face in the vicinity of the property are consistent with the mapping described above. The upper 20 feet of the bluff face and the entire upland area on the property are underlain by very compact glacial till. The lower 40 feet of the bluff consists of compact advance glacial outwash sand and silt. SPECIFIC SITE OBSERVATIONS The property consists of a level to very gently sloping upland area bounded on the east by a very steep east facing shoreline bluff. The upland portion slopes very gently towards the east. A home is present on the upland area. The east side of the home is located approximately 65 feet from the top edge of the steep shoreline bluff slope. The area around the home and between the home and the top of steep shoreline bluff consists primarily of lawn. The upper 20 feet of the bluff is very nearly vertical and is underlain by very compact glacial till. The till consists of silty sand with gravel and a few cobbles. Below the vertical till section of the bluff, the slope is less steep with an overall slope angle of 65 degrees. The lower 40 feet of the slope is underlain primarily with layers of very compact sand with occasional layers of silt. Where the slope is not vertical or nearly vertical the grasses and low growing brush have become established. However, bare slope areas are present where recent slope failures have taken place. Slope failures of the bluff have occurred within the past few years on portions of the bluff face on the subject property. Routine slope failures are apparent all along the bluff to the north and south of the property. There is very little bluff vegetation and the bluff is generally very steep all along this section of shoreline. Very little landslide debris is present at the base of the bluff. The bluff is eroded primarily by wave action that undermines the base of the bluff. The shoreline at the site is exposed to relatively open waters of the Puget Sound. Wave action during high tide from south, east or north winds will erode the base of the bluff. The cohesiveless sand unit at the Stratum Group File: l 1.1.06 2 November 5, 2006 375 Merry Road Norland, WA Geology Evaluation, Shoreline Bluff ~R~~~~e ~~~~- ~ ~ ~~~ I~~~~~ ~~ base of the bluff is readily erodable. At the time of my site visit on November 3, 2006 erosion of past failed sandy soil was taking place due to a high tide coinciding with high wind from the south. Because of the very compact nature of the bluff soils, the slope failures appear to consist of relatively narrow slab type failures. Ravelling of material off the exposed soils on the unvegetated portions of the bluff, and periodic shallow topsoil failures also play a minor role. The material eroded from the bluff face is removed from the base of the bluff and transported along the shore towards the north by wave action. I did not observe any evidence indicating an incipient global-type or deep-seated failure on the subject property or elsewhere in the vicinity. The layered nature of the bluff face likely would preclude deep-seated rotational type failures. No seeps or seasonal wet areas were observed on the bluff face. CONCLUSIONS AND RECOMMENDATIONS Based on my visual inspection of the subject property and vicinity, I conclude that the existing residence is not at risk of being impacted by slope failures over at least the next 100 years. It is my understanding that an addition is proposed for the north side of the existing home. The addition will be at least as far back as the existing portions of the home. Hence the addition will not be at risk over at least the next 100 years. Furthermore it is my opinion that the addition will not have an impact on slope stability. The existing residence is located at 65 feet from the top of the shoreline bluff. This distance is adequate to provide protection for a residence for at least 100 years even if an erosion rate of 6 inches per year is used. It is my opinion that this rate of erosion is very conservative and is likely much less rapid. I do not anticipate that the addition to the north. side of the existing home will cause any negative impacts on the stability of the slopes or the shoreline bluff as long as our recommendations are followed. Because the bluff is an eroding bluff, slope failures should be expected to occur on a periodic basis. The. only way to prevent continued erosion of the base of the bluff is to construct hard armoring at the shoreline at the base of the bluff. However, the eroding bluff acts as a feeder bluff (erosion of the bluff provides sediment) for the beaches north of the subject property, and Stratum Group File: 11.1.06 l I ~~ ~ I November 5, 2006 375 Merry Road Norland, WA Geology Evaluation, Shoreline Bluff 9 a~~ ~_ v y~ any shoreline armoring will have a negative impact on properties down drift from the bluff. The construction of shoreline armoring on other properties may cause an increase of erosion of the subject property. The owner of the subject property should contact Jefferson County officials regarding any proposed shoreline protection projects along the bluff or any shoreline protection construction. There are currently no shoreline-armored areas along this stretch of coastline. Soils on the upland portion of the property appear to drain well due to the coarse grained nature of the glacial till at this location. I recommend that roof run off from the addition be either allowed to drip directly onto the ground surface or be directed into a dispersion trench or French drain. The dispersion trench or French drain should be located at least 50 feet from the top of the steep shoreline bluff. Site grading soils or debris, landscape debris; or any other material should not be disposed of over the bluff face or placed at the top of steep bluff. Please note that there are inherent risks associated with building on lots near or adjacent to steep slopes. These are risks that the building owner should recognize and be willing to accept. If conditions appear different than those described in this. report, or other concerns arise, we request that we be notified so we can review those areas and modify our recommendations as required. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions please contact our office at (360) 714-9409. Sincerely yours, Stratum Group Dan McShane, M.Sc., L.E.G. Licensed Engineering Geologist Stratum Group r ~ -. ~} ~~~ .~. ~~~. t ;, File: 11.1.06 4