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HomeMy WebLinkAbout701224012 Geotech Assessment STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 February 6, 2007 lib) '" "'i fP 'l~:',!, .n Ii; I' 1"- I,! ' r,' ~--_._--.:....... l!r , .\ " II FEB I L WJ ~:: n 1\ I. Greg Barron Seaborne Services, Inc. 7425 Thorndyke Road Quilcene, WA 98376 1 2 2007 I L:.J ~ .TV JEVELOPMENT Re: Geology Hazard Assessment Tax Parcel 701224012 Camp Discovery Road Quilcene, W A Dear Mr. Barron: 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 for a septic drain field above the slope on the east side of the property, hand dug test holes on the site and inspection of the shoreline on the west side of the property. The property is located at the end of Camp Discovery Road on the west side of the Toandos Peninsula along the shore ofDabob Bay. The location of the property is indicated on Figure 1. 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 for site development, and recommendations for further investigation, if necessary. February 6, 2007 Tax Parcel 701224012, Camp Discovery Road Geologic Hazard Assessment -. ""'" ... ,'.S q'., ri, e n, , ~f!' r' \, \ j" !' pC! H\!J G::. . It) I, r------' l, " <\ ~ ( .: ' \i ' I, Ii FEB 1 2 2DO~b .TV \ ., DEVELOPMENT "~,-."_.",_,,,._,,,,,~"d'" L- 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. After the glacial period, shoreline erosion has taken place along the west side of the property and vicinity. Sediment from this erosion as well as sediment carried down by streams to the shoreline has built up a shoreline beach including a barrier beach on west side of the property. The Geologic Map of Surficial Deposits in the Seattle 30' by 60' Quadrangle. Washington (Yount, Minard, and Dembroff, 1993) and the Geologic Map of East-Central Jefferson County. Washington (Birdseye, 1976) indicate the bluff on the subject property is underlain by undifferentiated Pleistocene deposits. These deposits are described as consisting of poorly exposed gravel, sand, silt, and clay of uncertain provenance. Both maps indicate that the land at the topmost part of the slope is underlain by glacial till. 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. Stratified sediments on the bluff slope at the subject property and in the vicinity consist primarily of stratified sand and gravel deposits in a compact condition. Till where present on the uppermost slopes above the property is compact silty sand with minor gravel. 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. A recent slide is present on the slope south of the subject property. Past shallow slope failures appear to have taken place on the slope on the property in the past. Shallow soil creep is evident throughout the slope. 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 0.8 acres in size. The property is bound on the west by Dabob Bay. A steep slope rises up from the east side of the property and continues up to a gentle Stratum Group File: 1.2.07 2 February 6, 2007 Tax Parcel 701224012, Camp Discovery Road Geologic Hazard Assessment r;:::::--....'..-:,".'---.-------..- "I \!.,\ i ! dt . . , ' f- j [ ',~' <'. if i ~ ! I' I ! ."__r.~.___..~--~-~-.--.--"'. ~,~ ------, --r--n he 1 2 2007 \ b \ ---3 i _ ,.ry \ DEVElOPMENTJ sloping area above the property to the east. A sketch map of the property with pertinent features is provided in Figure 2. The property consists of three terrain areas: I) beach area including the beach, storm berm and back beach, 2) bench area between the base of the steep slope and the beach area, and 3) the steep slope on the east side of the property. Beach Area The western portion of the property consists of a broad beach area. The beach is a sand and gravel beach derived from sediment from erosion of the bluff slopes to the south and from sediment entering the bay from a stream approximately 800 feet south of the property. The beach between the subject property and the stream to the south is fairly broad with a well developed storm berm and back slope beach area. The storm berm at the subject property and along adjoining properties is covered with large drift wood logs and is partially vegetated. Trees have become established within the back beach area. Based on my observations it appears that the topping of the natural beach berm by waves is a very rare event. At the present time it appears that the beach on the subject property is an accretionary shore form and is stable. A small cabin is present on the beach area in the southern portion of the property. Bench Area An approximately 40-foot wide bench area is located between the steep slope on the east side of the property and the beach area. The bench area appears to be primarily the result of the construction of a road along the base of the slope between the beach area and the toe of the steep slope. The bench area is an extension of Discovery Bay Road. Steep Slope A steep slope rises up from the bench area to the east. This slope is on the order of 40 degrees to 45 degrees. The slope is generally plainer; however, old shallow surface soil failures are evident on the slope. The old failures are approximately 3 to 4 feet in depth at maximum. The slope at the head of the old failures is somewhat steeper across the old failure scarps. The slope is partially tree-covered with primarily mature second growth Douglas fir and an understory of salal and other brush. Except for minor bends in a few trees, trees on the slope are straight. No recent slides are evident on the subject property; however a failure that appears to be a several years old is located on a property south of the subject property. That failure may be partially the result of slope disturbance from the road cutting into the hill slope. The slope is underlain by compact sand with minor gravel. The top soil and upper soil layers are looser and evidence of soil creep is present within the looser surface soils on the slope. Stratum Group File: 1.2.07 3 F :.. 1 2 2007 l L':.J February 6, 2007 Tax Parcel 701224012, Camp Discovery Road Geologic Hazard Assessment __.-J L_ .TY JEVtLOFMcNT The slope is a former shoreline bluff likely formed from toe erosion prior to the development of the wide beach area at the base of the slope. The slope appears to be generally stable, but is likely subject to shallow failures as the upper soil layer becomes loosened from weathering, tree falls or seismic events. CONCLUSIONS AND RECOMMENDATIONS Based on my visual inspection of the subject property and vicinity, I conclude that a residence can be located on the subject property as long as the home is sited at least 30 feet from the toe of the steep slope that is located on the east side of the property. It is my opinion that the risk of slope failures on this slope is relatively low and the size of slides is likely small; however, the consequences of a small slide with a large tree hitting the back side of the home warrants a setback of at least 30 feet. A setback of 30 feet will require building at least partially within the back beach area. With the possible low risk of flooding from extremely high waves overtopping the beach berm, the back beach area is not at risk under current and foreseeable shoreline processes at the site. If water does get into the back beach area, it will be in the form of low energy sheet flow and does not pose an erosion hazard and will cause shoreline accretion in the area. Because the home will be constructed at least 30 feet from the toe of the steep slope, at least a portion of the home will be constructed on the back beach area. In addition a portion of the bench area above the back beach is likely fill or landslide deposits over back beach land. There is a possibility that organic or soft soils are present. If this is conformed prior to construction, COJlstruction of the home should take into account the risk of differential settlement due to soft soils. The risk of differential settlement from poor bearing soils or the break down of organic material can be addressed in several ways. One approach would be to support the home on an engineered designed pile foundation. Another approach would be to build an engineered reinforced structural mat foundation. A third approach would be to over excavate the building area and remove poor soil and organic material and back fill with structural fill. Any of these approaches should involve consultation between the home designer structural engineer and a geotechnical engineer. The construction of a septic drain field above the steep slope as proposed will not cause slope stability problems on the slope as long as the drain field is located at least 30 feet back from the top of the steep slope. Except for hazard trees no trees should be removed from the steep slope on the east side of the property. Stratum Group File: 1.2.07 4 February 6, 2007 Tax Parcel 701224012, Camp Discovery Road Geologic Hazard Assessment m=~\ ~ -r '-'---:j--~---~:--'-~ ~ \ i ; f ~'--"--:--l i I'.' I ~ i ..."', Ii I' I'- I L_ ,_. r " I- yo. H i ;..,u \ 2 200'7 \ D \ .~ J' J[VELOt~'f..\ENT Storm water should be dispersed on the site and will readily infiltrate into the coarse grained sand and gravel of the beach area. No grading or clearing should take place on the beach area beyond the minimum necessary for construction of the home site. Under no circumstances should vegetation, logs or other material be removed from the natural beach berm. At the present time there are no shoreline defense works along the shoreline south of the subject property. If any shoreline work or alterations take place along the shoreline south of the subject property, Jefferson County officials should be contacted in order to ensure that sediment inputs and shoreline processes are not altered. 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 cOQ.cerns 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, ll~ Dan McShane, L.E.G., M.Sc. Licensed Engineering Geologist stratum Group File: 1.2.07 5 122049.000' W TOPO! map printed on 01/31/07 from "Untitled.tpo" 122048.000' W WGS84 122046.000' W z o o o ~ o " V z o o o o lJ) o ~ z o o o ~ o ~ TNt lMN VIS" 122048.000' W o .5 lMllE . L .Jim FEET 9 _ _ SQ!L _ ~003 METERS Map created with TOPOI18 @2OO3 NatioD8l Geographic (www.natioMl&eogmphic.eomJtopo) I WGS84 122o,46~QOO~. W .:J "- J i'--.. / '" ~ ~'q, ~ '-1 '-\= ~~ u~ ~~ l~ , \I o 0 M-f. 1\.1\ \, ~ ~ <l) ......... ~ ----\- ~ \b ...c <.) ......... N <l) ..::::t:. (])(/) L C :J 0 (J)- .- CL LL ..... ,.....- I r 1'l!)fJ J t cr.- ,~-.!.f. ~tj4.2I()>1.o / I I I f r_ .\ I I ,- I IT if) I I II ..I 6""V t~ ~ 1''- ~ VI \ 0 e ID V!' ,~-te { ~ ~lo~e III S -i .8 ~ j~! <0 ~ i gJ. f...t 0 dofi,f:: ~ f...t !1~'0' (/)d _dl! -, -,-,-- ~' -~-( I ( \ . 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