Loading...
HomeMy WebLinkAbout701283006 Geotech Assessment (2006)~Cy~~ - c7c.72~jb STRATUM GR~JUP ]451 Grant Street, Bellingham, WA 98225 Phone (360) 7 ] 4-9409 August 21, 2006 Peggy Hanchett 8516 231st Street SW Edmonds, WA 98026 Re: Geology Hazard Assessment Tax Parcel 7012830006 Quilcene, WA Dear Ms Hanchett: 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 qualitatively evaluate the risk of slope failures impacting the proposed residence 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, inspection of test pits excavated on a portion of the property, hand dug test holes on the site and inspection of the shoreline at the base of the bluff on the site. The property is located on an approximately 500-foot high bluff on the east side of the Bolton Peninsula. The location of the property is indicated on Figure 1. The property is approximately S acres in size and is located in the southwest 1/4 of Section 28, Township 27, Range 1 W W.M. 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 a qualitative evaluation of the slope stability in regards to the proposed residence on the property, recommendations far site development, and recommendations for further investigation, if necessary. August 21, 2006 Tax Parce] 7012830006, Quilcene, WA Geologic Hacard 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, 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 Geologic Map of Surficial Deposits in the Seattle 30' b~60' Quadrangle Washin tg_on. (Yount, Minard, and Dembroff, 1993) indicates the bluff is underlain by undifferentiated Pleistocene deposits. These deposits are described as consisting of poorly exposed gravel, sand, silt, and clay of uncertain provenance. The Geologic Map of East-Central Jefferson County Washington (Birdseye, 1976) indicates the shoreline and bluff exposed on the subject property and vicinity is underlain glacial till and Pre-Vashon stratified deposits. The glacial till is described as consisting of a mixture of clay, silt, sand and gravel and is in a very compact condition. Birdseye indicated that there are three glacial till deposits on the bluff in the vicinity of the property, the Possession Till, the Double Bluff Till and the Vashon Till. The Pre-Vashon stratified deposits are described as consisting of sand, gravel, silt and clay deposited prior to the last ice age. Observations on the subject property, slopes in the vicinity of the property and upland areas above the subject property are generally consistent with the mapping described above. One exception described below is the presence of glacial ice contact/ice wasting deposits on a portion of the property. Glacial till is present at an exposure along the beach a short distance south of the subject property. Glacial till is also present on the property below the proposed building site and the beach. However, these two till exposures are not likely the same unit, as the thickness of the unit if it were a single unit would have to be on the order of 125 feet. In addition, stratified silts and sands are present below the till on the subject property and on the steep shoreline slope south of the property. The till exposed on the beach is probably the Possession Till and the till observed below the building site on the subject property is probably the Double Bluff Till. However, without age dates this correlation can not be confirmed. Vashon Glacial Till underlies the upper elevations of the Bolton Peninsula and is present on the top of the bluff west of the subject property. Stratified sediments on the bluff at the subject property and in the vicinity consist primarily of stratified sand and gravel deposits, fine sand and silt layers and peat layers. The silt, clay and peat deposits as well as sand and gravel with high silt or clay content are very compact condition due the fact that a thick layer of glacial ice has over ridden them. Most of the sands and gravel Stratum Group File: 8.7.06 August 21, 2006 Tax Parcel 7012830006, Quilcene, WA Geologic Hazard Assessment layers on the bluff contain enough silt that they are in a fairly compact condition. However, two areas of looser sand and gravel are present on the steep slope. An approximately 10-foot layer of loose to medium dense sand is located on the lower slope of the subject property. This layer appears to be discontinuous and does not define a distinct geomorphic feature across the slope, Loose to medium dense sand and gravel is located on the upper slopes of the bluff west of the subject property and below the Vashon Till at the top of the bluff. Glacial ice contact/ice wasting deposits are present on the topographic bench area on the subject property, These deposits consist of a very chaotic mix of sand and gravel and blocks and fragments of compact stratified deposits mixed together. Topographic features in this bench area are consistent with features found on glacial terrace benches and are not related to any current geomorphic processes. The Relative Slope Stability in East-Central Jefferson County Washington (Birdseye, 1976) map indicates slopes in the vicinity of the subject property are unstable and identifies areas of recent landsliding in the immediate vicinity of the subject property. Observations confirm that most of the slopes on the subject property and the vicinity are generally unstable or potentially unstable. Numerous recent small shallow soil slides are present on portions of the subject property and in the vicinity of the subject property. The implications and relative threat to the subject property and the siting of a residence on the subject property are discussed in detail in the following sections. SPECIFIC SITE OBSERVATIONS The subject property is approximately 5 acres in size. A sketch map of the property with pertinent features indicated is provided in Figure 2. Mast of the property slopes steeply towards the east. The east boundary of the property is Dabob Bay and the property slopes up from the water with an overall slope angle of approximately 38 degrees. The west boundary of the property is at an elevation of approximately 340 feet. The slope continues up above the property to an elevation on approximately 500 feet. The slope is mostly tree-covered with mature Douglas fir, western red cedar, alder and big leaf maple with an understory of bracken fern, salal and other brush. An access road crosses th.e property from the north to the south near the approximate mid slope of the bluff. This road accesses the property as well as properties south of the subject property. The road crosses a ravine on the north side of the property and then climbs out of the ravine onto a topographic bench area that starts on the south side of the ravine and extends to the south approximately 700 feet. Stratum Group File: 8.7.Ofi August 21, 2006 Tate Parcel 7012830006, Quilcene, WA Geologic Hazard Assessment The proposed building site area is located on the topographic bench area to the west of the access road. The bench area on the subject property is approximately 200 feet wide. The topographic bench is bounded on the east by the steep shoreline bluff slope, an the north by the slope down to the ravine that cuts across the north side of the property, and on the west by the upper part of the steep bluff slope. The topographic bench area is underlain by a chaotic mix of sand gravel and fragments of stratified silt and clay. These deposits are consistent with the area being underlain by glacial ice contact lice wasting deposits. These deposits combined with topographic features that are unrelated to current geomorphic processes indicate the area is a glacial ice margin terrace. Along the beach a few slope failures are apparent along the shoreline south and north of the subject property. All of these slope failures appear to be relatively shallow soil failures due to the steepness of the slope. The primary cause of slope failures is from undermining of the slope by wave action at the beach. As the slope is undermined the lower portions of the slope fail first and the slope failures progress up the slope. Ultimately the slope failures will progress up to the top of the bench area on the bluff, However, no slides that involved the entire bluff slope were observed anywhere on the bluff in the vicinity of the subject property. The landslide debris from these slides acts as a natural protection for the base of the bluff until over time the debris is washed away and the process of progressive slope failures up the shoreline bluff repeats itself. Most of the soil exposed at the base of the bluff along the shoreline in the vicinity of the subject property appeared to consist of landslide failures. The time period between these failures has been generally sufficient to allow for the growth of mature trees on the landslide soils. Hence, most of the slopes above the beach in the vicinity of the subject property and on the property itself are heavily wooded. Based on the presence of 100 year old or alder Douglas firs and madrones on most of the bluff slope on this stretch. of coast line, and the consistent geology along this portion of the coast, it appears that the frequency of slope failures that remove all the trees has been less frequent than 100 years. As noted above in the geology section soils underlying the subject property consist of stratified silts, fine sand, peat, sand and gravel, and glacial till. The topographic bench area appears to be a glacial ice margin terrace. A schematic cross section sketch illustrates the approximate locations of the units observed. Several old slides are apparent in the vicinity of the subject property. One inactive slide is located on the lower slopes of shoreline bluff south of the subject property. A relatively recent failure took place near the top of the bluff above and down into the ravine on the north side of the property. Stratum Group File: 8.7.06 August 21, 2006 Tax Parcel 701283000b, Quilcene, WA Geologic Hazard Assessment CONCLUSIONS AND RECOMMENDATIONS Based on my visual inspection of the subject property and vicinity, I conclude that a residence can be located on the topographic bench area of the property as indicated in the site plan sketch. A residence located at this site should not be at risk from bluff erosion or landsliding that would threaten the structure within the expected life of the structure (100 years) as long as our recommendations are followed, The residence should be setback at least 15 feet from any slope greater than 25 degrees on the east or north side of the topographic bench. The residence should be setback at least 25 feet from the toe of the steep slope on the west side of the bench area. A mound that is approximately 20 feet high is located on the topographic bench. Grading this mound area to provide a larger level area is acceptable as long as soils from the grading are not placed on any slopes greater than 25 degrees. A deck may be constructed within the setback from the top of the steep slopes 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. Native vegetation, particularly tress and low native brush on the slope between the building site and the steep slopes below the building site should be disturbed as little as possible. No trails or grading should be performed on the steep slope below the building site except for that necessary for maintaining the existing access road across the property and the access road to the beach. If trees are thinned or limbed for view purposes, no debris should be placed on the top of the bluff. Some thinning of trees growing between the top of the bluff face and the home site should not cause any problems as long as the native brush understory is left in place. Thinning and trimming of trees on the bluff slope should be limited to topping of alders and maples and limbing for view corridors of Douglas fir and western red cedar. Any debris from thinning on the bluff slope itself should be removed from the slope. All storm water run off from the roof gutters should be routed away from the steep slopes below the bench area and should be dispersed on the forest floor to the west side of the topographic bench area. The septic drain. field may be located on the topographic bench area. The drain field should be located at least .l5 feet from any slope greater than. 25 degrees. I reviewed on site septic site plan prepared by Godsalve/Martin. The plan for the septic drain fields is consistent with my recommendations and will not cause slope stability problems. Stratum Group Filc: 8.7.Ob August 21, 2006 Tax Parce17012830006, Quilcene, WA Geologic Hazard Assessment Maintaining healthy native vegetation cover both on the uplands and on the slope will best control long term erosion. Yard waste and other deleterious material should not be placed on or over any of the steep slopes on the property. Such material smothers vegetation, retains water and acts to destabilize the slope. 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, Stratum Group Dan McShane, L.E.G., M.Sc. Licensed Engineering Geologist Stratum Graup Fite: 8.7.06