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HomeMy WebLinkAbout601342015 Geotech Assessment (2005)xtif >~:~ ~~~~~~~ 1451 November 8, 2005 Larry Laporte 921 Daily Street Edmonds, WA 98020 S TUM GROUP nt Street, Bellingham, WA 98225 Phone (360) 714-9409 ocT - ~ z~Q~ JEf~~~~ ~~;~.°~~ s ~.:. Re: Geology Evaluation Tax Parce1601342015 Jefferson County, Washington Dear Mr. Laporte: Stratum Group herein presents the res~ properly on a shoreline bluff property. suitable locations on the property for t the home sites, 3) qualitatively evalua development and maintenance recomr potentially unstable slopes. This evalu vicinity, a visual inspection of the blu. geologic mapping in the area, and insl The subject property is approxi~ property that would be feasible building sites can readily be acc that will not be impacted by the 5 acres. Three home sites were identified on the subject sites from a geologic perspective.. Access routes to the ~d along routes across areas of good geologic stability ;tion of an access road. PE OF SERVICES The scope of our services included alts of our geology evaluation of the above referenced The purpose of this geology evaluation was to 1) locate ie siting of homes, 2) determine feasible access routes to e the risk of slope failures, and 4) provide general site iendations for development of the property adjacent to ttion included visual inspection of the property and f slopes and other steep slopes, review of available ection of the shoreline at the base of the bluff. following: 1) Conducted a site visits to visually inspect the subject property including the bluff slope conditions, shore ine conditions, and relevant conditions in the vicinity of the property. 2) Observed surface soil c shallow hand dug test i 3) Prepared this report st feasibility of building qualitative evaluation development, and rec~ and shallow soil conditions by excavating our findings, including an evaluation of the >idences and access roads on the subject property, a the shoreline bluff stability, recommendations for site mendations for further investigation, if necessary. ~ ,, November 8, 2005 Tax Parce1601342015, Jefferson County, WA Geology Evaluation ~~~~~~ CtilliYll U~U GEOLOGY Northwestern Washington has been o cupied by continental glaciers at least four times during the Pleistocene Epoch (1.6 million to 0,000 years ago). During these glacial and accompanying interglacial periods, the underlying be rock was eroded and a relatively thick layer of glacial related and interglacial fluvial sedime is were deposited over the underlying bedrock in the vicinity of the subject property. The (Yount, Minard and Dembroff 1993) d Geolo fig c 1VIap ofEast-Central Jefferson County, Washington (Birdseye, 1976) indicate the subject property is underlain by and pre-Fraser glaciation stratified sediments. Both aps indicate that Vashon Till is present on the upland area on the southwest uppermost portion o the property and to the west of the subject property. d ~~ _ ~ ~~~ in Observations on the subject property nd vicinity are consistent with the above-described mapping. Most of the property is and lain fluvial deposit deposited by streams and rivers during interglacial periods prior to the last gl cial period. These deposits have been highly compacted by the overriding glacial ice. The preglac al deposits are predominantly sand and gravel with a few lenses of silt and clay. The Relative Slope Stability in Ea slopes on the subject property and are unstable areas. (Birdseye, 1976) indicates the The site location map is provided on 2. The property is approximately 9.5 up into several terrains as indicated c within the following subsections. Ov two incised intermittent stream chats of approximately 180 feet and genera Canal. Level Upland Area C SITE OBSERVATIONS figure 1 and a general site plan sketch is provided on Figure ;res in size. For purposes of this report the site is divided the Figure 2 site plan sketch. The terrains are described all the property consists of a sloping shoreline bluff with ls. The uppermost portion of the property is at an elevation y slopes down to the northeast to the shoreline of Hood The level upland area on the property consists of a very gentle sloping area located between two steep sided stream ravines. The uplan area gradually tapers from a width of approximately 150 feet at the southeast side of the prope 0 feet wide approximately 230 feet to the northeast. The upland area is bounded on the north b a steep sided ravine and on the south by another steep Stratum Group File: 6.6.056 2 / .- ~~c~~~(~~~~ OCT ° 3 2007 November 8, 2005 Tax Parce1601342015, Jefferson County, WA Geology Evaluation sided ravine. A northeast plunging The upland area is underlain by silty The area is tree covered with mature understory of ferns and other brush. Steep Ravine Slopes Both ravines on the subject property h south of the ravines down to the inters on the areas on the designated on the : than 30 degrees with most of the s1opE evident on most of the steeper slopes. observed appeared to consist of a few across. Soils on the ravine slopes generally c~ compact silt and clay layers are preset springs are located above these more most of the slope failures seen within southeast-facing slope of the north ra~ Except for areas of recent soil fail fir, cedar, maple and alder. No deg ravines. Slopes <30 degrees above the north ~~~~C CoU~V i l ~~~ extends from the upland area between the two ravines. clayey sand and gravel that I interpret to be glacial till. glas fir, western red cedar and big leaf maple with an ~e steep slopes from the upland areas north, west and ttent stream courses at the bottoms of the ravines. Slopes e sketch map as steep ravine slopes have slopes greater being on the order of 40 degrees or more. Soil creep is few shallow soil landslides are present as well. Slides et thick soil slumps on the order of approximately 20 feet silt ofwell-drained compact sand and gravel. However, as well. Perched ground water and associated seeps and lpermeable units. These wetter areas are associated with ~e ravine slopes. Most of the seeps are located on the ie and at the lower end south ravine. the ravine slopes are tree-covered with a mix of mature ~ted rotational failures are present on the side slopes of the Stream Slopes of 30 degrees or less are locate above the lower end of the north intermittent stream. These slopes are stable slopes underla n by compact sand and gravel. The slopes are tree-covered with a mix of fir, cedar and maple an an understory of salal and ferns. No seeps or springs are present on the slopes and the slopes b come less steep further up and further down the slope. Intermittent Streams Two intermittent streams are located ~ sloping upland areas that underlain by July 2005 both streams were mostly d length does not take place very often, channel routes along most of the strea Stratum Group n the property. The two streams drain from the gentle glacial till. At the time of my two site visits in June and . It appeared that stream flow along most of the stream s there was vegetation and leaf litter within the stream length. The lower most end of the south stream File: 6.6.OSB 3 acs - ~ z~o7 November 8, 2005 Tax Parce1601342015, Jefferson County, WA Geology Evaluation ~ii~~~i~ ~~ll~~ i)~~ contained a trickle of water at the time~of my site visits. Gravel deposits cover the ground surface over a fairly level area near the lower nd of the streams. It appears that occasional stream flow heavy rainfall or rain on snow events. resistant to erosion. As the streams flc till to the underlying sand and gravel < and are susceptible to erosion. Hence, sided ravines. Gentle Slope Adjacent to Intermittent ces place from the upland area during periods of very ~e upland areas are underlain by glacial till which is very towards Hood canal they have eroded down through the posits. The sand and gravel deposits have low cohesion streams incised rapidly downward to form the steep The lower reaches of the ravines near he shoreline are subject to periodic flooding as the stream gradient becomes much less steep and gravel is deposited in the stream channels. The frequency and magnitude of the stream flooding d stream flows is beyond the scope of this investigation. However, the surface soil and channel morphology indicates that flooding from the streams does periodically take place on the lower p rtions of the streams. Gentle Slope on North Corner of The upland area north of the north rav ne on the subject property slopes gently towards the northeast towards the steep shoreline Tuff above the shore of Hood Canal. The slope gradually becomes steeper towards the top of th bluff; the maximum steepness is less than 25 degrees. The gentle slope is underlain by well- rained compact silty sandy gravel. The slope is tree covered predominantly with very strai ht mature Douglas fir. The understory consists primarily of salal. No evidence of soil moveme t is present and there is no evidence of potential deep- seated rotational failures on the slope. Shoreline bluff The northern corner and eastern come The steep shoreline bluff is the result slope. The steep shoreline bluff at bol sandy gravel. Except for the erosion a to be subject to periodic shallow slab bluff is undermined. Stratum Group of the property are located along steep shoreline bluffs. -f wave erosion at the base of the slope undermining the i of these locations is underlain by very compact silty the toe of the slope, the slopes are fairly stable and appear ailures of material collapsing from the bluff face after the File: 6.6.OSB 4 ~~~~.~~t ~~~~~ November 8, 2005 Tax Parce1601342015, Jefferson County, WA Geology Evaluation Storm Berm A high beach area underlain by gravel of the beach between the two steep she area along the lower reaches of the twc gravel deposits from very water wave , have begun growing on the berm. One intermittent streams ever reach the bea reaches of the streams are underlain b} from the streams likely flows through Beach ocr - ~ zaa7 ~~~F~ca~~~acD end logs with trees growing on it is located along the top reline bluffs and between the beach and the gentle slope intermittent streams. The berm appears to be the result of action that takes place on rare enough occasions that trees effect of the berm is that there is no evidence that the ;,h itself as the berm blocks the stream paths. As the lower gravel and the berm consists of gravel and cobbles, water he coarse material within the storm berm. The beach along the northeastern side f the property consists of a sloping gravel and cobble beach. The sediment on the beach is d rived from erosion of steep shoreline bluffs north and south of the subject property and from inputs from several stream courses located north and south of the subject property similar to the s reams on the subject property. The shoreline along the subject prop very few shoreline armoring sites ak on the property and vicinity takes ply particularly wind from the northeast. indicates that the rate of erosion and very slow. CONCLUSI Building Sites appears to be in a fairly natural condition as there are this section of shoreline. Erosion of the shoreline bluff during periods of very high tide combined with storms e presence of mature trees on the shoreline slopes ~sequent slope failures on the steep shoreline slopes is AND RECOMMENDATIONS Based on my inspection of the subject property and vicinity, I conclude that there are three suitable areas for building a home on he property. The three areas are: The level upland area between the two steep ravines on the outhwest portion of the property, 2) the gentle slope area on the northern portion of the prope ,and 3) the area of slopes less than 30 degrees south of the north intermittent stream. All of building area 1 is suitable fort e construction of a home as long as the home is setback at least 15 feet from the top of the steep lopes to the north and south. Building area 2 is located on the the site can be obtained via an e~ northeast slope above the steep shoreline bluff. Access to t from the upland property located northwest of the Stratum Group File: 6.6.OSB ~ ~ ~~~ November 8, 2005 Tax Parce1601342015, Jefferson County, WA Geology Evaluation ~1~~~Cf~aB ~iD~~~~ UG~ subject property. The home should be~etback at least 30 feet from the top of the steep shoreline bluff slope and at least 15 feet back fr m the steep ravine slope. Building area 3 is located south of the north intermittent stream. Slopes in this area are generally 20 to 23 degrees. Building is this area hould be setback at least 15 feet from the top edge of the steep ravine slope down to the south i termittent stream and at least 15 feet from the steep ravine slope above the north intermittent stre Due to the slope of the site the home uld be placed either on a pile foundation or could be constructed with a daylight basement oundation. Because of the ravine slopes on either side of the building area and the well-drained sand and gravel underlying the area, drainage for a daylight type basement will not be pr lematic. Access Route Building area 1 can readily be accesse~ via an easement from Johnson Road to the level building area 1. Based on the topography of the area d my inspection of proposed routes, the most feasible access route to building area 2 and bu lding area 3 is indicated on the site plan sketch. It may be possible to construct an access road t these sites solely on the subject property, but alternative access routes would entail significant ngineering design, maintenance and expense. The proposed route to building area 2 is across the upland area on the gentle slopes on the north side of the property. This access route will require minimal grading to access a building site within building area 2. In order to access building area 3 the the building 2 area then switchback t~ from 25 to 30 degrees. Slopes in this sand and gravel. Cutting into the s1ol stability problems on the slope. I recc the slope to be use as fill across the ~ for the outer edge of the road. Hence ;cess road will angle down across the gentle slope within yards the south across a slope area with slopes ranging 'ea are stable and are underlain by compact, well-drained in order to provide a stable roadbed will not cause any. upend that cut soils be end hauled towards the bottom of tom of the ravine at this site versus being used as side cast road should be a full bench cut. The intermittent stream channel at th bottom of the ravine is well vegetated and was covered with moss and forest litter indicating at stream flow at this location is uncommon and likely occurs only during periods of very he vy rain fall or rain on snow events. The channel is approximately 6 feet wide and three f et deep at the proposed crossing locations. Stratum Group File: 6.6.OSB 6 OCT - ~ 2007 November 8, 2005 Tax Parce1601342015, Jefferson County, WA Geology Evaluation The slope south of the stream is b reach the building site in building Storm water ~~~~~ ~~~~~' DCD 10 and 20 degrees and will require minimal grading to 3. It is my understanding that at least 65° o of the trees on the subject property will be retained. Soils underling the areas of the property pro osed for development at building area 2 and building area 3 are well-drained and should readily nfiltrate storm water from the home sites and access road. As such this site should readily quali for full dispersion of storm water per the Ecology storm water manual. Storm water on the upper part of the a cess road within the building area 2 should be handle by grading the road in a manner that will flow sheet flow of water off of the road surface onto the forest floor adjoining the road. For th portion of the access road that angles down across the 25- to 30-degree slope, storm water shout be directed into a ditch on the inside portion of the road and directed to the bottom of the slop . Storm water from the home site in bui ding area 2 should be directed to dispersion trench as much away from the steep shoreline s ope as practical. Because soils on the site consist of well- drained sand and gravel, water from t e dispersion trench should readily infiltrate into the ground. No storm water should be dis hanged within 30 feet of the top of the steep shoreline bluff. Storm water from the home site in bu' ding area 3 should be directed into a dispersion trench located on the lower portion of the sl e below the home site and the dispersion trench should be excavated laterally across the slope. ue to the well-drained soils water should readily infiltrate into the ground. Septic Drain Fields Septic drain fields for the home sites drain fields are located within the bui] be setback at least 30 feet from the toy least 15 feet back from the steep ravir setback at least 15 feet from the steep Vegetation management At building area 2 no trees should be setback from the top of the steep shor Stratum Group could not cause any slope stability problems as long as the ing 2 and building 3 slope areas. The drain fields should of the steep shoreline bluff slope in building area 2 and at slope. The septic drain field in building area 3 should be avine slopes. red from the steep shoreline slope. Within the 30-foot slope at least one tree per 30-foot center should be left File: 6.6.OSB 7 t November 8, 2005 Tax Parce1601342015, Jefferson County, WA Geology Evaluation in place. Trees within the setback or o the steep slope may be limbed or topped to create view corridors. Any debris from limbing or opping should be removed from the setback or steep slope. Trees may be removed and limbed for~view purposes for the building site at building area 3 without causing slope stability or eros on problems. Please note that there are inherent risk associated with building on lots near or adjacent to steep slopes. These are risks that the buildi owner should recognize and be willing to accept. If conditions appear different than those escribed in this report, or other concerns arise, we request that we be notified so we can review t ose areas and modify our recommendations as required. Stratum Group appreciates the o~ questions regarding our reconnai Sincerely yours, Stratum Group f/... ~'d~'{~ /y ,/ .:./ale.,.,. /II'~ ,f Dan McShane, M.Sc., L.E.G. Licensed Engineering Geologist pity to be of service to you. Should you have any please contact our office at (360) 714-9409. ~~~~~~~ tl~T - 3 2007 a~4 Stratum Group File: 6.6.OSB ~~ ~~ ~ .~~ r .. ~. 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