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HomeMy WebLinkAbout002352005 Geotech AssessmentSTRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 November 25, 2002 Larry McMurray 18134 6t~ Place SW Normandy Park, WA 98166 Geology Evaluation, Shoreline Bluff 591 Batchelor Road Section 35, Township 30 N, Range 2 W Jefferson County, Washington Dear Mr. McMurray: We are 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 ofs the property for the siting of a 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 was limited to 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 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 the Pleistocene Epoch (1.6 million to 10,000 years ago). During these glacial and accompanying November 25, 2002 591 Gardner Road, Jefferson County, WA Geology Evaluation, Shoreline Bluff interglacial periods, the underlying bedrock was eroded and glacial related and interglacial fluvial sediments were deposited over the underlying bedrock in the vicinity of the subject property. The Surficial, Geologic Map of the Port Townsend 30 by 60 Quadrangle, Puget Sound Region_. Washington (Pessl, Dethier, Booth and Minard, 1989) indicate the steep shoreline bluff exposed on the subject property is underlain glacial till and bedrock. The Bedrock Geologic.Map of the Port Townsend 30 bv 60 Quadrangle, Puget Sound Region, Washington (Whetten, Carroll, Gower, and Brown, 1988) indicate the bedrock underlying the lower bluff consists of Marrowstone Shale of Durham (1944). Observations on the bluffface on the subject property and on the bluffface in the vicinity of the property are consistent with the mapping described above. However, the upland portion of the subject property is at least in part underlain by sand and gravel that is either reworked weathered glacial till or a thin veneer of recessional outwash sand and gravel. The steep bluffface is underlain by very compact glacial till on the upper slope and bedrock on the lower two-thirds of the slope. SPECIFIC SITE OBSERVATIONS The site location map is provided on Figure 1 and a general site plan sketch is provided on Figure 2. The property consists of a very gently sloping upland area bounded on the north by a steep north facing shoreline bluff The upland portion slopes very gently towards the north, and is covered primarily with second growth Douglas fir, western red cedar and brash. The overall slope of the steep shoreline bluff averages approximately 50 degrees; however, portions of the slope are steeper. The total height of the bluff is approximately 50 feet. The bluff slope is well vegetated with western red cedar, big leaf maple and alder as well as thick brush. The cedar trees are mature and appear to be 100 years or older. The uppermost portion of the bluff is underlain by very compact glacial till. The lower half of the slope is underlain by shale bedrock. The very base of the bluff at the ordinary high water mark is underlain by slope colluvium and landslide material. The bluff appears to be eroded primarily by wave action that undermines the base of the bluff The shoreline at the site is exposed to open waters to the northeast. Hence north to northeast winds coinciding with high tides will generate large waves that may erode the base of the bluff However, because the base of the bluff is underlain by bedrock the apparent erosion rate is very low and few recent landslides are evident along the shore in the vicinity of the property. No beach armoring is present in the vicinity of the property and the beach appears to be in good condition. il [,!i NOV 2 6 JF_FFEI~,S Or'4 OOUN[Y DEPT. OF COMMUNITY DEVELOPMENT November 25, 2002 591 Gardner Road, Jefferson County, WA Geology Evaluation, Shoreline Bluff That is, the level of the beach appears to be such that downward erosion is not taking place at this time. We did not observe any evidence (tension cracks or trees rotated inward away from the top of the bluff or indications of past uplift of the beach area) indicating an incipient global-type or deep- seated failure on the subject property. Slope failures appear to be infrequent and limited to slab type failures of topsoil as the base of the bluff is eroded by wave action. CONCLUSIONS AND RECOMMENDATIONS Based on our visual inspection of the subject property and vicinity, we conclude that a residence can be located on the upland portion of the property such that bluff erosion or landsliding would not threaten the structure within the expected life of the structure (75 years) as long as our recommendations are followed. The residence should be located at least 50 feet from the top of the shoreline bluffas indicated in the Site Sketch Map (Figure 2). We do not anticipate that the development of the subject property will cause any negative impacts on the stability of the slopes or the shoreline bluff as long as our recommendations are followed. Because the bluffis 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 northeast of the subject property, and 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 bluffor 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 be relatively well drained, and therefore we do not anticipate discharging of footing drains will be necessary. Roof drainage and any storm water catch basins must not be introduced into the perimeter footing drain. We recommend that roof drains and any other drainage be discharged to a water infiltration system that will distribute water into the ground at the site. 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 Stratum Group File:lO.30.02 November 25, 2002 591 Gardner Road. Jefferson County, WA Geology Evaluation, Shoreline Bluff Trees on the bluffface and within the setback buffer may be limbed or trimmed for view purposes as long a s limbs and debris generated is removed from the bluff face and the top of the bluff. The septic drain field should not cause problems for slope stability and may be located within 50 feet of the top of the bluff. It is possible to construct a trail down the bluff slope to the beach. However the trail alignment should follow the contours of the slope such that cutting into the slope is minimized. A game trail already exists on the property and can be used to access the beach. 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. We appreciate 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, Stratum Group Dan McShane, M.S., P.E.G. Professional Engineering Geologis Stratum File: 10.30.0 NAD27122~5226" 7; 1~2~5'54" W' TOPOi ©1998 Wikitlowex Pmd. u~Y~o~ (ww~v.topo~om) NADC; 121mST26" W