Loading...
HomeMy WebLinkAbout501031015 Geotech Assessment ~~~~ ~ fh,~ II STRATUM GROUP t-, ~:. R 1451 Grant Street, Bellingham, WA 98225 ~~ ~ ~` ~^' `' Phone (360) 714-9409 pQ,~ i i March 26, 2007 ~L~! [A.1~i i-~t~~~~ ~~ Nancy Gereaux 26 Gereaux Lane Quilcene, WA 98376 Re: Geology Assessment 26 Gereaux Lane Quilcene, Washington Dear Ms Gereaux: Stratum Group is pleased to present the results of our geology assessment of the above referenced property adjacent to a shoreline bluff. The purpose of this geology assessment was to 1) determine -the suitability of the property for the siting of a residence, 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. Based on my investigation of the site and vicinity, it is my opinion that the site is suitable for development as a residential property provided the recommendations presented in this report are followed. This assessment included a visual inspection of the property, slopes on and in the vicinity of the property, available geologic mapping in the area, and investigation 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 and vicinity including the bluff slope conditions, slope conditions on and in the vicinity of the property and shoreline conditions. 2) Observed surface soil conditions on the shoreline slope and the slopes on the west side of the properly and at the top of the bluff by excavating. shallow hand dug. test pits. 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. ,,;~'~ ; ~ , A ~ March 26, 2007 ~ - L 26 Gereaux Lane, Jefferson County, WA Geology 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 Geolo is Map of Surficial Deposits in the Seattle 30' x 60' Quadrangle, Washington (Yount, Minard and Dembrof, 1993) and the Geolo ig c Map of West-Central Jefferson County, Washington (Birdseye, 1976) indicate the subject property and the bluff exposed on the subject property are 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. My observations on the bluff face and within road cuts and test pits on the upper portion of the property and on the bluff face in the vicinity of the property are consistent with the mapping described above. Most of the property and bluff face is underlain by coarse, well graded; rounded gravel with. sand and. silt. Several discontinuous one- to three-foot thick sand layers and silt layers on the bluff face. Very hard, varved silts are located near the base of the bluff at several locations. All of the silts and gravels were very compact and dense. No woody layers were present on the. bluff face on the subject property; however, woody layers are present at nearby areas on the. bluff face. I interpret the deposits to have been deposited in a fluvial environment prior to the last glacial advance as they are in a highly compact and dense condition and some organic material is present. Most of the mid section of the bluff is underlain by well rounded, well sorted sand and gravel with cobles in a very dense condition. The uppermost portion of the bluff is underlain by poorly sorted sand and gravel with some silt consistent with the upper bluff being. underlain by ice- contact deposits. SPECIFIC SITE OBSERVATIONS. The subject property consists of a sloping upland area bounded on the south by a steep shoreline bluff. The southern portion of this upland area is bounded and on the east by a ravine. The upland Stratum Group ~ File:2.15.07 2 March 26, 2007 26 Gereaux Lane, Jefferson County, WA Geology Assessment ~~ 1 ~~e11~tt `~'~~~ 1 UU~E portion slopes gently towards the south; and is covered primarily with second growth Douglas fir and madrone trees with an understory of salal, evergreen huckleberry, and ferns. The property is accessed via Gereaux Lane. An access drive off of Gereaux Lane accesses an existing home between Gereaux lane and the shoreline bluff. The overall slope of the steep shoreline bluff averages approximately 70 degrees; however, portions of the bluff are vertical. The total height of the steep portion of the bluff on the subject property is approximately 60 feet. Except for the nearly vertical to vertical sections of the steep shoreline bluff, the steep shoreline slope is well vegetated with a mix of brush and trees. The brush consists of a mix of salal, ferns, salmon berry, and poison oak. A number of mature madrone and Douglas fir trees are growing on the steep bluff face: The steep bluff slope above the shoreline on the subject property consists primarily of very compact sand and gravel with silt and clay that I interpret to be pre-Fraser fluvial deposits. A few discontinuous silt/clay layers less than one-foot thick-are present on the bluff face. A silt/clay layer extends across a portion of the bottom two to six feet of the bluff. The silticlay layers are very haxd and compact. Small seeps of water were observed within a thin sand layer approximately 20 feet above the, beach level. The sand layer from which-water was seeping appeared to contain less silt and clay than the sand and gravel layers above and below and, hence is more permeable. Except for this. area of seepage the remaining portions of the bluff face and the upland portion of the subject property were dry at the time of my site visit in late February 2007. 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 material eroded from the bluff face is removed from the- base of the bluff and transported along the shore towards the west and east by wave action. Very little landslide debris has accumulated at the base of the bluff. I did not observe any evidence (tension cracks or trees rotated inward away from the top of the bluff) indicating an incipient global-type or deep-seated failure on the subject property. The presence of straight mature Douglas firs on the bluff face indicates that the erosion rate on the steep slope must be relatively slow. The areas of seeping water did not appear to be eroding at a faster rate than the layers above or below the seeps. A stream channel is present on the east side of the property and this stream has incised downward to form a ravine along the east side of the south half of the property. The.property slopes down towards a ravine at a maximum angle of approximately 45 degrees. Downward erosion from flowing stream water is apparent in the upper portion of the ravine. The lower portion of the Stratum Group File:2. F5.07 3 ~, ~ ,~ ` ;~ ~ ` ~ _, ,~. March 26, 2007 26 Gereaux Lane, Jefferson County, WA - ~ ,~ ,~,-4, A; , Geology Assessment .. ~ ~ ~ ~ ~~; ~ ~ ~ ~ ~ ~ ~ ~ 4, _; ravine is relatively stable with little erosion as the base of stream channel is annealed with cobbles. Seeps and springs are present on the lower slopes of the ravine and are likely associated with perched water above the very impermeable silts observed near the base of the shoreline bluff. Except for shallow minor soil creep, there was no evidence of slope instability on the slopes of the lower portion of the ravine. Some shallow soil slumps and potential shallow soils slumps are present in the upper part of the ravine where stream incision and greater presence of springs and seeps are located. Stratum Group File:2. 15.07 4 ~ ~ ;~ ~} II~~; II March 26, 2007 ~`~~ ~ ~ ~ f 26 Gereaux Lane, Jefferson County, WA Geology 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 upland portion of the property south of the existing home site such that bluff erosion or landsliding will not threaten the structure within the expected -life of the structure (100 years) as long as my recommendations are followed.. Building Setbacks I recommend a set back distance of 40 feet from the top of the. steep portion of the bluff. I recommend a set back distance from the top of the steep slope of the ravine on the east side of this upland area of 25 feet. It is my opinion from an engineering geology standpoint that these distances will be adequate to allow for expected bluff face failures that will result from the continued erosion of the base of the bluff by wave action and ravelling of the exposed soils on the bluff face as well as any possible shallow soil failures on the ravine slopes east of the proposed building area. Shoreline Because the bluff is an eroding bluff, slope failures should be expected to occur on a periodic basis. The only way to prevent continued erosion of the base of the bluff is to construct hard armoring at the shoreline at the base of the bluff. However, the eroding bluff acts as a feeder bluff (erosion of the bluff provides sediment) for the beaches east. of the subject property, and any shoreline armoring will have a negative impact on properties down drift from the bluff. The construction of shoreline armoring on .other properties may cause an increase of erosion of the subject property. The owner of the subject property should contact Jefferson County officials regarding any proposed shoreline protection projects along the bluff or any shoreline protection construction. There are currently no shoreline-armored areas along this stretch of coastline. Storm and Septic Drainage Soils on the upland portion of the property appear to be relatively well drained. The soils appear to consist primarily of coarse grained ice-contact deposits. Roof drains and any other drainage. should be discharged. into a level dispersion/infiltration trench excavated perpendicular to the slope. Alternatively this water could be discharged via tightline to the base of the ravine east of the building site. Dispersion trenches should be setback at least 30 feet from the top of the shoreline bluff and 25 feet from the steep ravine slope. The septic drain field should not cause any problems for slope stability ~s long as the. drain field: Stratum Group File:2.15.07 5 ~~~\ ~ \ March 26, 2007 26 Gereaux Lane, Jefferson County, WA Geolo Assessment ~~ '~ ~''q kf::r ~ e is located at least 40 feet back from the top of the steep shoreline slope and 25 feet from the ravine slope. Vegetation 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 bluff or placed on the steep ravine slopes. Over time debris will break down and form an unstable mass that will slide damaging the slopes below. Native vegetation, particularly tress and low native brush within 30 feet of the top of the bluff should be disturbed as little as possible. No trails or grading be performed on the shoreline bluff face. If trees are thinned or limbed for view purposes on the bluff face, all debris should be removed from the bluff face. Some thinning of small Douglas fir 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. At least one tree per 30-foot center should be left within the buffer area. 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. Stratum Group appreciates 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. Licensed Engineering Geologist Stratum Group File:2. 15.07 6 Fgure 1. Site Location 47°41.274' N, 122°47.341' W WGS84 122°49.000' W 122°48.000' W 122°47.000' W WGS84 122°46.000' W ' ~ //f // ~ / j I / 1f ~_ / / ~ ~ ,.'. j ~ r ~ ~ ~/ /} ~ ~ ~~_ f~ "~ f^ {Rfr? i -~i-~-~ tr l ~~~ / ~ R f. \j . ~~~` •,f ~ E I ~ 4 t 61 ii ( r _ ~ f. ~ r ~r~__ mil- L\ 4 ~9 ~ - ~ i t \ }~. ~ b . ( ~ ~ ' 4 ~' 4 ~P \ , I i . ~ ,, ~ ~ ~_ ~ ~~ ~ -\ ~ ` p/ `~ , , \ ~, 4 ~+~ ~~ ~ ~ / \ _ ~ _4 l t \` ~ ~ ~~ ~ ~ ~ .` 1 ~~ \ J + . ~ ,./ + 1. Y `~ \ t v ~ ~ 1 ~ ~ ~ ( ` ~ ~}~ .lam 's ~ _/ ~~ ~ ~~~ ~ ~ F ~ ~ ~ ~-- ~``~^ a ~l `~ _~._ `t. `Jfi~ ~ ~ T C6 ~ ! v ~~ i~ ~ ~`. __. , i1 ~ ~,_ { - ~ L~ o~-`A ~ t i.f~ • '~.~`. ` ~~ `' , 4 1/~ i ~ ~r ~ '~".. ` SDB7EC'I' PROPERTY 4 t. c -- 1bQ ~'~ ! ~ -,.~.- , ~ ~ / ~...: r ~~-- ~_-- ~ fl ,/ ^3an` ~. r +I I' /~ i %~ , , • ~~ ~_ Wy'., 1 ~~ ry :7:'' ' ~ ~, if ~~ r ~r ~ ;~ ,.~ iif+ ,~, a ,~. . .' ~ .D'~'~.~~' ~ <~~~, ~~ ~ v, ' i~r ~ ' ~~ `' ~' C C CC V d Z O O O d' 2 0 d' Z O O O .-~ d' Z D .'7. d' 122°49.000' W 122°48.000' W 122°47.000' W WG584122°46.000' W ~* /~ D ~ 5 1 4AL6 . !~YJ/Ig° (~t0~0 Fktl p :Op ~p00 MTTEHS Map created with 1'OPO!®®2003 Nati°ml Geographic (www mtioraphic oomftopo) z ~ ,_,~. ~r ~ ~ ~ ~~~ S1 ~e~~~y ,-,-.. ~. ~ ~ ' ~ ~~ ,~: ~ •' f . ;~` } _ _ ~ ~~ ~ r ~` .~ 0 ~ i` ~ ~ . ~ ~~ ~ ~ 1- ~ ~ ~ ~,. ; , ~ ~ t f i '"~ ~ -- / t 1 r • .,Q~ ~i, ~' ~ / ~ .~ ~ {w ~ ~ ~1 C~ ~1$ r ``~ ~ ~ ~'~ (i S~ is . ,t ~ a ~ ~ ~•~ ~ _ ~ ` i ~ ~ `' ~~~ 9 X76 ~ ~ ~ ~ .? C~ . •~ _ = , . ~ ~ ~ t~ ~ ~ y ~ ~ ~.~,N,r.~,,,""". (-7 C3 O C~ i. G4 ~'1 G~ ~ sCct ~.2 GtZG~ (a~'~r77~>,s- a~1~Clioe~.s~,~r ~"'°° ~~::~:: Stratu Figure 2 ~: ~, :_:'. Group ~,,,,,,~ ~ ~`~ ~~~ ~ ~ Site Plan Sketch a~ sr. ~ -- - - BaNhgham. wA 98225 o~~e ~2/i0/bs ~ -u k ~ (3so ~t~-e4os