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Home My WebLink AboutBLD2010-00344 Geotechnical Report • • STRATUM GROUP P.O. Box 2546, Bellingham, WA 98227 Phone(360)714-9409 May 14, 2009 Becky Whit sA3 a,w 2 C Nit S� P.O. B 757 70 7 C• eo rsie.- R—o�d J son, WY 83002-9757 - W A B36 B Re: Geology Hazard Assessment Parcel 001305003, Cape George Road Port Townsend, WA Dear Ms White: Stratum Group is pleased to present the results of our geology hazard evaluation of the above referenced property. The purpose of this geology evaluation was to evaluate the risk of slope failures impacting the proposed residential development area on the subject-property and provide general site development and maintenance recommendations. This evaluation included a visual inspection of the property and vicinity, a visual inspection of the slopes on the property and vicinity, review of available geologic mapping in the area, and inspection of the shoreline conditions in the vicinity of the property. The property is located on an approximately 220-foot high bluff on the east side of Discovery Bay. The location of the property is indicated on Figure 1. The upland area of the property is located on the west side of Cape George Road and the steep west facing bluff. SEP 28 Z010 SCOPE OF SERVICES The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property including the slope conditions, shoreline conditions, and relevant conditions in the vicinity of the property. 2) Observed surface soil conditions on the slope and on the uplands above the slope. 3) Prepared this report summarizing our findings, including an evaluation of the slope stability in regards to potential development on the property, recommendations for site development, and recommendations for further investigation, if necessary. • . May 15,2009 Parcel 001305003,Cape George Road,Jefferson County, WA Geology Hazard Assessment 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, a relatively thick layer of glacial related and interglacial fluvial sediments were deposited in the vicinity of the subject property. The Geologic Map of the Port Townsend South and Part of the Port Townsend North 7.5-minute Quadrangles, Jefferson County, Washington (Schasse and Slaughter, 2005) indicates the bluff is underlain by Olympic age glacial and non glacial deposits with Vashon glacial advance outwash deposits overlying the upper part of the bluff and the upland area of the property. The Olympic age deposits consists of sands and gravels deposited by rivers and streams and silts deposited as overbank deposits along the rivers. A carbon date on the bluff face just north of the subject property indicates an age of approximately 34,000 years. These deposits were laid down prior to the last glacial period. The advance outwash deposits consist of sand and gravel deposited by melt water streams from the advancing glacial ice approximately 18,000 years ago. Observations on the subject property, slopes in the vicinity of the property and upland areas above the subject property are consistent with the mapping described above. However, glacial till is present on portions of the upland. The till consists of poorly sorted silt, sand and gravel with occasional boulders. The till was deposited directly by glacial ice during the last glacial period approximately 15,000 years ago. The steep slope on the subject property is underlain by very compact sand and gravel consistent with advance outwash on the upper portion of the bluff. The sediments on the mid section of the steep slope consist of very compact silts and the lower half of the bluff slope is underlain by very compact sand and gravel. Looser material raveled from the upper slope is present as a thin veneer of soil over the underlying geology. SPECIFIC SITE OBSERVATIONS The subject property is located on a high shoreline bluff on the northwest shore of Discovery Bay. The property is accessed via a gravel driveway from Cape George Road from the southeast • of the lot. The location of the property is shown on Figure 1. A sketch map of the property with pertinent features indicated is provided in Figure 2. The upland portion of the property slopes gently towards the west. A very old road cuts across the upper portion of the upland area and extends to the north of the property. Low cut slopes on this road show the uppermost portions of the property are underlain by sandy to gravely glacial till and outwash sand and gravel. The upland area between Cape George Road and the top of the steep bluff slope is mostly tree covered with a mix of Douglas fir and madrone. A grass-covered clearing is located on the gentle sloping area just above the bluff. Stratum Group File:4.22.09 2 • • May 15,2009 Parcel 001305003,Cape George Road,Jefferson County, WA Geology Hazard Assessment The steep bluff slope slopes down to the base'of the bluff at an overall slope of approximately 35 degrees. However, portions of the slope are steeper with sections of the upper bluff having slopes of up to 60 degrees but generally around 45 degrees. The bluff slope is tree-covered with straight second growth mature Douglas fir and madrone with a thick brush understory. Soils underlying the slope consist of a veneer of loose raveled sand and gravel overlying very compact sand and gravel with some silt layers. The colluvium becomes thicker towards the base of the bluff and the slope becomes less steep as well. The base of the bluff is located at a wide accretionary shore form that has built out over 100 feet from the base of the bluff The area is grass-covered with a few scattered trees. Scattered drift wood indicates that on a very rare basis water covers this area during a combination of very high tide and storm surge. As such there is no indication of erosion at the toe of the steep slope as it is well protected from shoreline processes. The beach conditions at the edge of the accretionary shore form suggest that accretion is continuing at this shoreline reach. Hence, no erosion should be expected for many years. The steep slope on the subject property is the result of the site being a former shoreline bluff The slope has eroded back to a fairly stable angle through root throw and raveling of soils on the slope. CONCLUSIONS AND RECOMMENDATIONS Based on my visual inspection of the subject property and vicinity, I conclude that the steep slope on the subject is a potential landslide hazard area. However, the primary source of slope movement on the slope appears to be root throw from occasional toppling trees and raveling associated with deer and thaw freeze. However, the upper slope is steep enough that shallow landslides should be expected on a periodic basis. If slides do take place, I anticipate that they would involve only a few feet at most of the upper soil horizon. S E?' 2 8 Lv() A residence can be located on the gentle sloping upland area without being at risk from landsliding as indicated in the site plan sketch. The building should be setback from the top of the steep west slope a minimum of 30 feet. A deck may be constructed within the setback from the top of the steep slope as long as it is not an integral part of the structure of the'residence. That is if a slope failure occurs, the deck will detach from the house. No debris should be placed on the top of the bluff or on the steep slope. Thinning and trimming of trees on the bluff slope and within the setback area should be limited to topping and limbing for view corridors. Thinning should leave at least one tree on a 30-foof center. Any debris from thinning on the steep slope itself should be removed from the slope. Stratum Group File:4.22.09 3 O May 15,2009 Parcel 001305003,Cape George Road,Jefferson County,WA Geology Hazard Assessment Soils on the upland portion of the property appear to be well drained. Storm water run off from the roof gutters should not be routed towards the bluff and should be infiltrated into the ground in a dispersed manner. I recommend that any storm water drainage be directed into a level spreader and site tree clearing of the upland area be limited to the area necessary for construction of the home site. The septic drain field should be located at least 30 feet back from the top edge of the bluff. The addition of water at this location does not pose a risk of causing slope stability problems. CLOSURE Please note that there are inherent risks associated with 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 please contact our office at (360) 714-9409. Sincerely yours, 8. Stratum Group %FY' Dan McShane, M.Sc., L.E.G. 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