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HomeMy WebLinkAbout701344028 Geotech AssessmentSTRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 October 27, 2003 Greg Bjarko Bjarko/Serra Architects 1050 N 34th St Seattle, WA 98103 Re; Geology Evaluation, Shoreline Bluff Parcel 8, Dabob Cove Section 34, Township 27 N, Range 1 W Jefferson County, Washington Dear Mr. Bjarko: We are pleased to present the results of our engineering geology reconnaissance of the above referenced property adjacent to a shoreline bluff. The purpose of this geology evaluation was to 1) determine the suitability of the property for the siting of a residence as presented in your site plans, 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 and on other slopes on the subject property and vicinity. 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. October 27, 2003 Lot 8, Dabob Cove Geology Evaluation 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' x 60' Quadrangle, Washington (Yount, Minard and Dembrof, 1993) and the Geologic Map of West-Central Jefferson Coun _ty, Washington (Birdseye, 1976) indicate the steep shoreline bluff exposed on the subject property is underlain undifferentiated pre-Fraser glaciation deposits. These sediments are described as consisting of interbedded oxidized brown, red-brown, and gray gravel, sand, silt, and clay. The layers are moderately to well bedded and the unit contains minor amounts of ice-contact deposits and outwash gravel and sand. Generally the unit is nonglacial and has abundant peat and woody debris. These sediments were deposited sometime prior to the last glacial event and include deposits of the Whidbey and Olympia nonglacial periods. The sediments have been overridden and consolidated by glacial ice. Our observations of the bluff face on the subject property and upland areas on the subject property and vicinity of the property are consistent with the mapping described above. Very compact sand, gravel, and silt layers are exposed on the bluff face. The lower half of the bluff is predominantly underlain by very compact silty fine to medium sand (SM), and the upper part of the bluff is underlain predominantly by compact gravel. Several silt layers on the order of 2 to 3 feet were observed on the bluff face as well and in the vicinity of the subject property in the upland areas. We observed compact silt, and sandy gravel with silt soils on the upland portion of the property in the vicinity of the proposed building site. SPECIFIC SITE OBSERVATIONS The subject property is approximately 5 acres in size, and consists of an east-west ridge that is abruptly truncated on the west by a steep shoreline bluff. The ridgeline itself slopes gently towards the west. The proposed building site is located on the on the eastern portion of the ridge, away from the steep shoreline bluff. The proposed building site on the ridge is nearly level. The north side of the ridge slopes at an angle of approximately 30 degrees down towards a stream drainage approximately 75 feet below the ridge. This slope is well vegetated with trees Stratum Group File: 10.15.03B October 27, 2003 Lot 8, Dabob Cove Geology Evaluation consisting of western red cedar, big leaf maple, alder, and Douglas fir and an understory of salal, fern, and other brush. Except for minor shallow surface soil creep on the slope, we did not observe any evidence that the north slope of the ridge is unstable. The drainage channel at the base of the slope was not deeply incised into the slope. The south slope of the ridge slopes at an angle of approximately 18 degrees down towards a series of stream drainages approximately 75 feet below the ridge. The south slope of the ridge is tree-covered primarily Douglas fir with an understory of salal and ferns. The west end of the ridge is truncated by the shoreline bluff. The shoreline bluff slopes to the west at an overall angle of approximately 50 degrees, and is approximately 175 feet high. However, portions of the steep shoreline bluff are vertical. The base of the steep bluff consists of a relatively gentle slopping low bank above the beach. This bench area is approximately 100 feet wide and slopes gently towards the beach. The bench area is protected from shoreline erosion by rock riprap and is used by the local homeowners' association for accessing the water. It appears that several shallow surface soil failures have occurred within the past couple of years on the steep bluff face. Landslide debris consisting of failed soil and trees has accumulated on the bench above the tidelands to the west of the property. The steep bluff slope above the shoreline on the subject property consists primarily of very compact sand and gravel with silt and clay that we interpret to be pre-Fraser fluvial deposits. A few discontinuous silt/clay layers less than one-foot thick are present on the bluff face. Small seeps of water were observed approximately half way up the bluff face. The seeps of water appear to be associated with a less stable soil horizon, as the bluff face is less steep in this area than above and below. However, access to this area of the bluff was not possible due to the steepness of the slope. The bluff is eroded by a combination of wave action that undermines the base of the bluff, ravelling of material off the exposed soils on the unvegetated portions of the bluff, and periodic shallow topsoil failures. The rock riprap along the shoreline appears to be providing protection from wave action at the base of the bluff. 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. The presence of mature Douglas firs and madrones on the steep bluff face indicate that the erosion rate on the steep slope must be relatively slow. Large deep-seated landslides have impacted the area both north and south of the subject property. Stratum Group File: 10.15.03B October 27, 2003 Lot 8, Dabob Cove Geology Evaluation The property appears to be located on a ridge between these two large landslide areas. Unstable slopes are apparent on the slopes within the convergent topography along the axis of these slide areas and along the headwalls of the landslide areas, but the side slopes of theses slide areas in the vicinity of the subject property appear to be stable. The slopes on the subject property are such that the topography is divergent and ground water will not be concentrated on the slopes below the ridge. CONCLUSIONS AND RECOMMENDATIONS Based on our visual inspection of the subject property and vicinity, we conclude that a residence can be located as indicated on the site plans 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. Based on our observations of the subject property and vicinity, the property is located in an area of unstable slopes. Numerous recent slope failures have occurred in the past in the vicinity, and future slope failures should be expected in the vicinity of the property and on portions of the property itself. However, except for the west end of the ridge and the shoreline bluff below the west end of the ridge, slopes on the subject property appear to be stable. In addition, a residence located on the east end of the ridge will not be at risk from any potential landslides sliding onto the property from other areas due to the topography of the site. The proposed building site is located approximately 200 feet east of the steep slope break at the top of the shoreline bluff on the west end of the ridge. It is our opinion from an engineering geology standpoint that this distance will be adequate to allow for expected bluff face failures that will result from the continued erosion of the bluff. No setback is required on the slope on the north side of the ridge as long as the foundation support is located as indicated in the site plan and as indicated on the ground as inspected at the time of my site visit in 2003. The foundation should be set in on compact native soil underlying the site and no fill soils or other debris should be are placed on the slopes below the residence. If another site is located along the ridge, ! recommend that the building foundation support be set back a distance of at least 15 feet from the top of the slope on the north side of the ridge unless a subsequent site inspection indicates otherwise. It my opinion that no set back is necessary from the south slope of the ridge at the proposed building area. I do not anticipate that the development of the subject property will cause any negative impacts on the stability of the slope or shoreline bluff as long as our recommendations are folloWed. Because the bluff is very oversteepened, slope failures should be expected to occur on a periodic Stratum Group File: 10.15.03B October 27, 2003 Lot 8, Dabob Cove Geology Evaluation basis. Erosion at the base of the bluffhas been minimized by hard armoring at the shoreline. This armoring of the shoreline may have, over time, negative impacts on other properties to the north as the eroding bluff acts as a feeder bluff (erosion of the bluff provides sediment) for the beaches north of the subject property, and any shoreline armoring will have a negative impact on properties down drift from the bluff. Relatively little shoreline armoring has been constructed on the west shore of the Toandos Peninsula. Additional construction on other properties may cause an increase of erosion of the beach west of the subject property. The owner of the subject property (and the home owner's association) should contact Jefferson County officials regarding any proposed shoreline protection projects along the bluff or any shoreline protection construction including projects south of the property as shoreline armoring south of the subject property could lead to increased erosion rates and loss of beach area west of the subject property. 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 into a rigid perforated dispersion pipe. The dispersion pipe should be placed in a level infiltration trench excavated perpendicular to the slope on the south side of the ridge. 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 or on the slope on the north side of the ridge. Native vegetation, particularly tress and low native brush should be disturbed as little as possible. No trails or grading should be performed on the bluff face. If trees are thinned or limbed for view purposes, no debris should be placed within 30 feet of the bluff face. Some thinning of trees growing between the top of the bluff face and the proposed home site should not cause any problems as long as the native brush understory is left in place. The septic drain field is located on the south side of the ridge east of the proposed building site. It is our opinion that this location should not cause any slope instability for slopes on or adjacent to the subject property. Stratum Group File: 10.15.03B October 27, 2003 Lot 8, Dabob Cove Geology Evaluation 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.Sc., L.E.G. Engineering Geologist i uame cunane j Stratum Group File:10.15.03B 122%g'54" W · ? . Figm~ 1. Vici.tdty Map Subject Property 1/2 1 MILE 0 1000 FEET O~ 500m lO00rn P~ted F~m TOPOi ~199g W~r P~d~t~ · ..:?.'"/: ?::: =========================== :::': :: / ? :":: .: : : } ? :3 L* NAD27 122°45'26" W STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 April 28, 2004 Greg B j arko . B j arkolS erra Architects 1050 N 34t" St Seattle, WA 9$103 Re: Geology Evaluation, Shoreline Bluff Parcel $, Dabob Cove Section 34, Township 27 N, Range 1 W Jefferson County, Washington f ~~~.` `~ Dear Mr. Bj arko: Bolo re ort to ou dated October 27, 2003 I indicated that the proposed building site is In my g gy p y . located a roximately 200 feet east of the steep slope break at the top of the shoreline.bluff on pp . the west end of .the ridge. I did not provide a setback distance as the building site was ~n my inion ade uatel setback from the to of the steep slope. It is my understanding that actually op q Y p distance ou scaled off the map is 185 feet. Based on my site visit, this distance is more than y .. ade uate to protect the proposed budding. It is our op~nlon from an engineering geology q stand Dint that 150 feet will be adequate to allow for expected bluff face failures that will result p from the continued erosion of the bluff. We a reciate the opportunity to be of service to you. Should you have any questions regarding pp our evaluation please contact our office at X360} 714-94x9, Sincerely yours, Stratum Group ~~ an McShane, M.Sc., L.E.G. Engineering Geologist -. t Mw~r1nc~"yH7 ~:h E { r. ~w I ~' +1S` <'~ ~: APR 3 02004 ass ~~r~r~r~ ocn ~~El: =dam ~~~ ~ ~. pia ~ ~~ ~~~ ~ ~ ~-~~ ~~ ~~~~ ~~ ~,"" . ~" r. . ,` t , ... ,f ~ ~ ~ J 1 ,~ f L ~... t•. t`J y G. 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