Loading...
HomeMy WebLinkAbout701304012 Geotech Assessment. s. .: STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 November 28, 2006 Martin DeBono 1104 8`h Avenue West Seattle, WA 98119 Re: Geology Hazard Evaluation Parcels 701304012 and 70130412 Jefferson County, WA Dear Mr. DeBono: Stratum Group is pleased to present the results of our geology hazard evaluation of the above referenced property located on East Quilcene Road in Jefferson County. The purpose of this geology evaluation was to qualitatively evaluate the risk of slope failures impacting development on the subject property and provide general site development and maintenance recommendations. No specific home development plans are proposed at this time. The purpose of this evaluation is to provide recommendations for future home siting and development. 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, and an inspection of the shoreline on the western side of the property. Shallow test pits were excavated on the property as well to assess the geology of the site and for purposes of evaluating storm water impacts on the site and vicinity. The property is located on the east and west sides of East Quilcene Road. Quilcene Bay is located on the west side of the property. The location of the property is indicated on Figure 1. SCOPE OF SERVICES The scope of`our services included the following: l) 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 slopes and on the uplands above the slopes on,the property and vicinity. 3) Prepared this report summarizing our findings, including a qualitative evaluation of the slope stability in regards to proposed development on the property, recommendations for site development, and recommendations for further investigation, if necessary. November 28, 2006 Parcel 701304012, East Quilcene Road, 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 Geologic Ma~of Surficial Deposits in the Seattle 30' by 60' Quadrangle, Washington (Yount, Minazd, and Dembroff, 1993) indicates the bluff is underlain by undifferentiated Pleistocene deposits. These deposits are described as consisting of poorly exposed gravel, sand, silt, and clay of uncertain provenance. The Geologic Map of East-Central Jefferson County, Washington (Birdseye, 1976) indicates the shoreline and bluff on the subject property and vicinity is underlain Pre-Vashon glaciation stratified deposits. 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. The subject property and the steep shoreline slopes on the subject property are primarily underlain by compact sand and gravel. The Relative Slope Stability in East-Central Jefferson County, Washington (Birdseye, 1976) map indicates the subject property and slopes in the vicinity of the subject property are unstable. However, Birdseye did not identify any areas of former or recent landslides on or in the vicinity of the subject property at';the time the map was produced. SPECIFIC SITE OBSERVATIONS A sketch map of the property with pertinent features indicated is provided in Figure 2. The property is bounded on the west by Quilcene Bay. East Quilcene Road bisects the property. The east end of the property is located at an elevation of approximately 300 feet. The property essentially straddles a west plunging ridge line that is truncated on the west by a west facing shoreline bluff slope. The primary building site is located on the west side of East Quilcene Road. The building site is accessed via a gravel driveway. The building area consists of a level, grass-covered area that is approximately 80 feet long and 42 feet wide. The land slopes up from the level building area to the east to East Quilcene,Road. This slope is relatively gentle and stable. The land slopes downward from the level building area to the north, west and south. The slopes to the north and south are stable; however are potentially unstable if disturbed. The slope down to the water on Stratum Group File: 4.15.06 2 ~ _~_ ~ ~; ~, c. ~ ~ ~ _. __ ~ ___ .._. ~~ ~ ~' .. November 28, 2006 ~ Parcel 701304012, East Quilcene Road, Jefferson County, WA Geology Assessment ! the west side of the building area is steep with slopes up to 45 degrees. The slope is generally stable and minimal erosion has taken place along the shoreline at the base of the slope. Minor soil creep is evident on the steep western facing slope. All the slopes around the level building area are tree-covered with a mix of trees with Douglas fir being the predominant tree. The shoreline at the base of the slope consists of a broad sand and gravel beach. Shoreline sediment drift is towards the north. Very little erosion at the toe of the bluff was present at the time of my site visit in April 2006. A high tide combined with strong south winds and tidal surge had impacted the area in' he winter of 2006; however, no erosion had taken place at the subject property. The septic drainfield area is located on a relatively level area west of East Quilcene Road and the slope down to the level building area. The portion of the property east of East Quilcene Road is a continuation of the west plunging ridge. Steep slopes up to', 45 degrees are located on the south and north sides of the ridge. However, the upper part of the ridge is gentle sloping with level areas. An old logging road angles up along the south side of the ridge from East Quilcene Road and switches back and forth up to the top of the ridge,. However, the road swings south of the subject property before the switch backs up the slope. The road is mostly a full bench cut and would not require significant upgrading to utilize as an access route to the top of the ridge. CONCLUSIONS AND RECOMMENDATIONS Based on my visual inspection of the subject property and vicinity the level building area is a suitable building site from a geologic perspective as long as the recommendations presented below are followed. The base of the foundation should be set back at least 20 from feet measured horizontally from the steep shoreline slope on the west side of the building area and at least I S feet from the slopes an the south and north side of the building area. Trees on the steep slope may be thinned as long as at least one tree per 30-foot center is left in place with'preference for mature trees. Mature trees may be limbed for view corridors. Any cut vegetation should be removed from the slope so that a thick mat of rotting vegetation does not develop on the slope. No debris or soil should be placed on the steep slopes north, west or south of the building area. Stratum Group File: 4.15.06 ~. i November 28, 2006 ~ ~ -- Parcel 701304012, East Quilcene Road, Jefferson County, WA ~ -r :a._ _-------. __~ . Geology Assessment k Storm water from the roof should either be allowed to drip directly onto the ground or should be directed into storm water dispersion trenches located to the north or south side of the building area. If dispersion trenches are utilized they should be located at least 15 feet from the top of the steep slopes. There are buildable areas located on the upper part of the ridge on the east side of the property. Access to these areas will require the reconstruction of the logging road with an easement across the property to the south or the construction of a new road via an easement from the east or the construction of a new road up the ridge. If an access road is required from East Quilcene Road a detailed geology assessment of the road route should be prepared and, if necessary, engineering design. 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 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 please contact our office at (360) 714-9409. Sincerely yours, Stratum Group ,~Wai 9 Dan McShane, M.S., L.E.G. Licensed Engineering Geologist ., Stratum Gmup File: 4.15.06 4 ~ ~ ~ ~ ~ .~ ~ ~ 4 ~~ ~ -~ ~ o 4 ~- ~ 4 y+ t ,.r„.r...., PP~~ r ~J~ ~ ; ~~-• `' i ~.r~ 9 ~ ~~ ~V~ 1 '{,J1 ~ t # t ~ ~ ! M ~y~`' ~, ,,,~ ~~ ..~.. ;~ ..,y .S r 1 ~ i 4 r 1 \ 1 ^1 ~/^~ ~ Y ~r f ~t ~ k-~`N, ~~ \~' `~ ~ •,,' ` k `'~-,. 1 1 ~ ~. ~ ,,~4 ~ + 1 ~ ~ r "` ` ~- J I 4~~ ' 61d ~ t ~~ ~ ~ ~~ ~ ~' ~, .~. ~ of ~ ~ { w ~. -~ ~ ~ ~ ~,~ ~ I ~ ~ ~ . ~ ~ '~1 ` - ~ ~..'`" ~ ` : ~~na '~ > >, ~ ~~ ~ ~, ~ `~ s ~~ ~_s~ '~.. ``~~ ~ III ~ \ ` ~. ~. a~ ~~ ~. ~,,,,,~.,.":~ i r~ V N ~ 4~ .rr~ ~ ~^ Q~ w ~ ~~ 4 '\ •~ ~ n~ ~ ~ ~ ~ ~ ~ 1~ 1 ~ ~ 1 ~ .y ~ 1 y ~, ~ ~ r 1 1 /~ ~ f~ / '~ i ~ ~ '~ ~ ~ r ~ ~ .,~~ v.. . ~y ! ,: ~~tirY r ~~~~/+.. rr. `• ~ V~ ~ ~~~ A~ ~~~ y Et ~ 0 ~ ~~ .- .~ o ~ ~ ~~~ U~U w~.. { ~ "~~~ 11 -~ J ~- y ~ +~ 41 o :, h ,r ~~- ~ ti S o ~. a ~ . ^1 ~ y ~ ,~ ~ ,~ ~ ` ~ ~ ~ ~ ~ 0 r ~„ V ,a ~ . ,~ ,, ~ ~~ ~ ~ ~ , ~ ~ '~ ~k t ~ _ .~~~~ Kµ . r ~~{ ~~