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HomeMy WebLinkAbout992600045 Geotech Assessment1451 June 15, 2007 Dick Stockmat 7515 W Port Madison Road Bainbridge Island, WA 98110 Re: Geology Hazard Evaluation Tax Parcel 992600045 Jefferson County Dear Mr. Stockmat: Stratum Group is pleased to present the referenced property. The purpose of this failures impacting development on the s maintenance recommendations. This ev, and vicinity, a visual inspection of the s: geologic mapping in the area, and inspe~ tATUM GROUP Street, Bellingham, WA 98225 hone (360) 714-9409 RECENED j~~ ~ ~ ~~~ ~rwironm~ntal 4ieat~~ results of our geology hazard evaluation of the above geology evaluation was to evaluate the risk of slope ubject property and provide general site development and iluation was limited to a visual inspection of the property opes on the property and vicinity, review of available ;tion of the shoreline at the base of the bluff. The property is located east of Sea Ho Road between the road and the shoreline of Hood Canal. The location of the property is in icated on Figure 1. It is my understanding that the preferre building location is located on the western portion of the property. Suitable building sites are loc ted on the western portion of the property that not at risk from lanslides or other slope movement OF SERVICES The scope of our services included the 1) Conducted a site visit to isually inspect the subject property including the slope conditions, shoreline co itions, and relevant conditions in the vicinity of the properly. 2) Observed surface soil co ditions on the slope and on the uplands above the slopes. 3) Prepared this reports arizing our findings, including a qualitative evaluation of the slope stability in r gards to the proposed residence on the property, recommendations for sit development, and recommendations for further investigation, if necess RECEIVED June 15, 2007 Parcel 992600045, Jefferson County, WA Geologic Hazard Assessment ~~ ~ ~ ~~~~ d~g~ofi ~p1~11'ty' 1=nvironme~alHea GEiNERAL GEOLOGY Northwestern Washington has been occ pied by continental glaciers at least four times during the Pleistocene Epoch (1.6 million to 1 ,000 years ago). During these glacial and accompanying interglacial periods, the underlying bedr ck was eroded and a relatively thick layer of glacial related and interglacial fluvial sediment were deposited over the underlying bedrock in the vicinity of the subject property. The Geolo ig c Map of Surficial Deposit (Yount, Minard, and Dembroff, 1.993) a County, Washin on (Birdseye, 1976) ii and Vashon advance outwash deposits. underlain by Pre-Vashon stratified. sedir bluff. Yount, Minard, and Dembroff (1 S consists of nonsorted, nonstratified clay deposited directly by glacial ice. The ad deposits deposited by meltwater from th located topographically above the advar Fraser deposits consist of stratified silt, d the Geolo imp of the East-Central, Jefferson iicate the property is underlain by Vashon glacial till. ~irdseye (1976) indicates the advance outwash is ents at the east end of the property along the shoreline ~3) refer to this unit as Pre-Fraser sediments: The till silt, sand and gravel up to boulder size. The till was ante outwash deposits consist of clay to gravel stratified ;advancing glaciers at the start of the ice age. The till is ;e outwash. The pre-Vashon stratified sediments or Pre- and gravel with peat layers. Observations on the subject property, above .the subject property are consist The Slope Stability Map of East-Cen identifies the subject property area as areas are the gentle upland slopes of 1 No recent landslides or areas of past ~ the subject property. LIDAR imagery indicates two landslide past. The north landslide complex appe~ from the shoreline. This landslide comp flowing from upland areas to the west a landslide complex to the south has imps Observations confirm that there are pots shoreline bluff erosion and areas associ; landslide movement was observed with. creep along portions of the steep slopes associated with the -steep slope above th Stratum Group pes in the vicinity of the property and upland areas with the mapping described above. fefferson County, Washington (Birdseye, 19?6) ing both unstable areas and. stable areas. The stable -roperty and the unstable areas are the steeper slopes. 1 movement are identified on the slope stability map on mplexes have impacted the subject property in the to have impacted upland areas as much as 700 feet has subsequently been deeply incised by a stream eroding down through the landslide feature. The ;d upland areas as much as 600 feet from the shoreline. tial unstable slopes on the subject property related to ~d with the landslide complexes. No evidence of recent the north landslide complex other than very minor soil f headwall areas. Likewise there is only minor soil creep incised stream to the north of the subject property. File: 4.24.07E 2 R~ct~v~~. DUL 2~ 2DOi June l5, 2007 Parcel 992600045, Jefferson County, WA Geologic Hazard Assessment Portions of the south landslide complex the landslide complex indicates that mi the past. A sketch map of the property with perti property is bounded on the west by Sea approximately 300 feet above sea level. Hood Canal. An access driveway winds end of the property. Most of the subject tree-covered with mature second growl] immediately above the-steep shoreline condition. The cabin is clearing located approximately 10 years ago as the bloc] ~n~~Nea~ E~ ~ mew have moved in the past few years and the topography on or slope .movement has taken place on a periodic basis in SITE OBSERVATIONS rient features indicated is provided in Figure 2. The Home Road. The property along Sea Home Road is The property generally slopes down to the east towards down through the property to a small clearing on the east property except for the clearing and driveway route is ~ fir. An A-frame cabin is located on a bench area lope. The cabin has been abandoned and is poor on a landslide block that dropped a few feet . slid towards the top of the bluff. The geomorphology of the eastern port n of the subject properly is very complex as the property traverses the intersection of two landsli a complexes and. a seasonal stream has incised a valley through this area as well. The proposed building area is located on very gentle sloping ground on the western portion of the property wes of the landslide complexes. An upland area with very gentle slopes is located on the. western portion of the property. This upland area is part of a large broad upl d that extends to the south and west of the subject property. Slopes are stable and slope gles are less than 10 degrees. No hazardous slopes are present within this area. The area is un erlain by sandy to gravelly very compact glacial till. The till is fairly sandy due to the underlyin sand and dominated preglacial deposits. The upland are is bounded on the no by a seasonal stream valley. The top of the stream valley slope is located approximately along th north property boundary. Slopes down to the stream channel bottom are on the order of 40 egrees. This area is underlain by till on the upper slopes and sand and gravel stratified sediment on the lower slopes near the stream. Slopes are generally stable but soil creep is evident and the lope is potentially unstable due to its steepness. A similar stream valley bounds the continues down the center of the pr Both of these streams are seasonal stre to the upland are on the subject proper events or rain on snow events. The ste stream flow has cut down through the Stratum Group ast portion of the upland area. This stream valley to the top of the steep shoreline bluff. ns with very little surface water flow in the area adjacent . Stream flow likely only occurs during very wet weather 1 sided valleys have formed at the nick point where the ;ry compact till to the more readily erodable sand units File: 4.24.07E 3 RECEIVED June 15, 2007 Parcel 992600045, Jefferson County, WA Geologic Hazard Assessment underlying the till. Further to the east stream bed elevation reaches to the rr intersects the perched ground water t. A large ancient landslide complex boun~ this landslide complex has a slope of ap gravel. Mature trees are growing on the has been deeply incised by two streams. trees throughout the landslide complex fractures, it appears that no significant n over the past 100 plus years. Another deep-seated large landslide con eastern third of the subject property and of this landslide complex extends inlanc subject property. ,The area covered by tY LIDAR imagery of the area. The headw fractures and hummocky areas are prese during past field investigations of the aY some movement has taken place. within past 10 years. Some of this movement t .property near the top of the steep shore] The failure mechanism of the slide app resting on top of impermeable clay. As calves off the top of the bluff to the bey provided in Figure 3. Based on observe uniform mass but as broken fragments showing no movement for many years. The shoreline bluff on the east end of overall slope of the bluff is approxim~ portion of the bluff is underlain by saa by very compact silt and clay with lay The beach at the base of the bluff is a s; Bluff failures are the result of toe erosii over steep. Additionally soil from land: face to the beach below. Stratum Group ~~~~ GOUT 1=rlrlironme'~1 Healt'~ ings and more frequent stream flow is apparent as the impermeable silt units underlying the area and thus above the silt units. the east side of the upland area. The headwall scarp of ~ximately 45 degrees and is underlain by sand and Grp and within the old landslide area. The slide area ased on straight tree trunk stumps and straight mature a as well as lack of any indications of recent ground vement has taken place on the slide or slide scarp area Alex is located along the top of the shoreline bluff on the ;xtends over 1,000 feet to the south. The upper headwall approximately 700 feet on properties to the south of the s landslide complex is readily apparent within the ll area is not well defined on the ground, but old ground. ~t within the landslide complex and have been observed a. Areas of recent movement are present as well and he western portion of the landslide complex within the ~k place on the far eastern portion of the subject ie slope. to be caused by periodic saturation of sand units slide moves towards the steep shoreline bluff the soil below. A schematic cross-section of the slide is Zs of the slide area, the slide has not moved as a h some areas having recently moved and other areas subject property is approximately 45 feet high. The ly 65 degrees, but portions are vertical. The upper units and the lower two-thirds of the bluff is underlain of peat. 1 and gravel beach with layers of silt and peat exposed. at the base of the bluff causing the bluff to become le complex above the bluff cascades down the bluff File: 4.24.07E 4 June 15, 2007 Parcel 992b00045, Jefferson County, wA Geologic Hazard Assessment CONCLUSI Based on my visual inspection of the s~~ and in the vicinity of the subject proper gentle sloping upland portion of the we residence located within this area shou would threaten the structure within the The gentle sloping upland area is locate the stream channel to the north bounds stream channel bounds the southeast si with a large landslide complex bounds The slopes in down to the streams are The creek channel is lined with cobble way as to undermine the slope. The headwall scarp' of the large landsli covered with mature trees and old grog for minor soil creep and raveling for n is no longer active. No indications of r anywhere within the slide area that I it streams across the slide surface. The s~ are effectively draining the soils that fi it is my opinion that the landslide area proposed building area. Any building within the upland area avoid any impacts related to differen As noted above, the eastern portion of tl complex that extends to the south. The ~ is located approximately 270 feet away opinion that it is very unlikely that the s distance between the proposed building adequate. General Recommendations No timber harvest should take place Stratum Group RE~EIV~p 1UL 20 2001 ~efierson Court AND RECOMMENDATIONS ~nvir~r~me~~H ject property and vicinity and analysis of the terrains on I conclude that a residence can be located within the side of the property as indicated in Figure 2. A not be at risk from bluff erosion or landsliding that :nected life of the structure. outside of any landslide areas. A steep slope down to e north side of this area, a steep slope down to another of this area and a steep landslide head scarp associated east side of the area. ~, but are generally stable with only minor soil creep. and does not appear to be eroding downward in such a complex that bounds east side of the upland area is tree i stumps indicting that the slope has been stable except ~y years. It is my opinion that that this landslide complex vement impacting trees or the ground surface is present ected. The slide likely was active prior to the incision of nns have cut down through the former failure plain and nerly would have become periodically saturated. Hence, no longer expanding and does not pose a threat to the uld be setback at least 15 feet from the steep slopes to settlement and soil creep on the steep slopes. property is impacted by another large landslide -posed building area on the upland area of the property ~m the maximum extent of this landslide area. It my e area will expand in the foreseeable future and the ea and the large south landslide complex is more than of the upland area and timber removal should not File: 4.24.07E 5 RECEIVED June I5, 2007 Parcel 992600045, Jefferson County, WA Geologic Hazard Assessment exceed a total of 35 percent of the. p meeting storm water full dispersion reaching the base of the landslide cc Jan COl~rttyr ~nvironmerttal Health . The limitation of tree harvest is consistent with ;ments and will minimize the amount of water No debris should be placed on the steep~slopes on the property. Storm water run off from the roof guY~ and should be dispersed or infiltrated area appear to be generally well drain readily accommodated by the soils on The septic drain field should only be to of water from a drain field should have s should not be directed onto steep slopes routed towards o the ground in a dispersed manner. Soils on the upland and infiltration through dispersion trenches will be e site. within the gentle sloping upland area. The addition Bible impact to the landslide complexes. Construction of foot trails on the bluff f ce may cause slope problems on the bluff face. If a trail is to be constructed, a geology assessm t of the proposed trail route and impacts should be conducted prior to construction of the tr il. Access to the potentially building sites n the property can readily be achieved from an existing. driveway that is located on the property CLOSURE Please note that there are inherent risks ssociated with lots near or adjacent to -steep slopes. These are risks that the building owner hould recognize and be willing to accept. If conditions appear different than those described in is report, or other concerns arise, we request that we be notified so we can review those areas d modify our recommendations as required. Stratum Group appreciates the opporl questions regarding this report please Sincerely yours, St um Group Dan McShane, M.S., L.E.G. Licensed Engineering Geologist Stratum Group to be of service to you. Should you have ariy ~~ct our office at (360) 714-9409. File: 4.24.07E 6 Figure 1. Site 47°46.%4' M, 122°45.080' W iNC;S84 1.000' W 122°45.()00' W WCS84 122°44.000' 1N \. v\ \ + a-: 5~-: 'M~.~. :.a°~`1.cM_.v ,... 3~ :4!. ::; f/ -:~.~..,.a~a: 'l;: .:~ •d: ~ ' .~. 5v "i: +'~ j~ 's~"i + W : ~".1 ~ .,e:: y.~;N'a., t.-' _ '.:u; : ~..... ~ ' Y . . f . .x.36 r: ::~- .Yf, .A..ty .. ye. ee,~~~i. ~, 5" saS..~' l.t.r +f• Y;.. r.:.e^r+.+,~•.(:sC~ ..: ~_- .. •y h .~ "4; yH mo ~ A ~ ~ . s~ +.`f..",<yE. ..!~~ ~=. .:F '.~P•a': ~'• 1 ^~.~, A ' " ?' „ , ~.s• . .r2 ::es &~ .- .: w. . l ~ .. a: .. n .Y .. .. ... .. . f.. .fin.. ..t . ... , .y ,.f. . 5 ` ~ - F^ ~ ~ ' y.+ . 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