HomeMy WebLinkAbout601061003 Geotech Assessment (2003)Ut}/1}Ii U4 Uy:ii t11d i JUU V4t J«-4 31t~nie.~~s s,nu~r
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1451 t~rartt street, ~eliingham, WA 962?5
Phone (~~a) 7 i 4-94a9
August 7, 2404
Ivlichele Farfan
Jefferson County Building Permits
621 Sheridan Street
Port Tawnsenal, V4'A
lte° Geoteclanical deport, Shoreline Blu#li
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Jet'ferson Cc~t~rity, Va'ashrngtor~
Dear I'vls Farfan.:
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Lynn Olsen contacted me an Friday ,August f, 2004 regarding atcarrrt water frc~rn the proposed
development ~~n T?arcel £~(!i-4~i-403. Tn ir~y initial Geotechnical Report to T,~ynn t7lsen dated
March, 20G1 I recommended that storm water from the rac~f sheuld be collected and fautldation-
footing drains should be collected and discharged via tight line to the base of the bl~atT on the
PmPerh`. [still recamrriend this approach in order tc~ prevent str~rm water run off from being
concentrated at a location near the top afthe shoreline bluff. Alternatively, the storm water could
be infiltrated into the ground cast of the home site immediately above the drainage Swale.
Apparently a site de~~elapztaent plan was submitted that was inaansistent ~~ith this
recornrr3endatiQn. It is city utYderstand:ng that the ~lsens intend to continue to fallow my initial
recammendatiort vtid~ the exceptiota that the discharge of the tight line t~f storm water v~311 be ax a
Iacatior. witl`iiza lc~ayc, gravel and debris approxirnateTy 50 i'eet above the base of the bluff
Tn addition to the abv~~e recomiriendation, I recammendthat water direoted onto #T'ie suhj~t
prdpe~, r~ia a C.c~unty drainage ditch along G~,tfstavsan Rand be er~llected. irc a tight line pipe and
routed t® the Lase of the bluff. -Chia discharge of water onto the subject property flows into a
drainage sr~+ale that, when r~rater #laws through it, discharger water over the tap oftha bluff. At
the Larne cif.r~y initial 4it° visit, 1 observed that shal.law landslides and del5ris flows had recently
token place nn the slope below the area where the Swale discharged water on the bluff slope.
Sinee nay initial site visit in 204.1, I leave ret~.uned to the site on nurtieraus occasions. There was
at least oae additional slide on this area ~~t alnpeared to be a watery debris flaw. In addition, the
base of the Swale a,hove ttie Lluff f~as been eroded downward -arid obviously leas had water
flowing through it size a periodie basis. This was r!ot the ease at the time afmv initial site visit;,
The d;7~m~•ard. erasion and incision into the base Qf the sv~alc since my initial site visit is t~aical
of the impacts cif road drainage systerrts causing increased peals flvi~vs. jriost of the time the water
fmrri the ditch infiltrates into the ground prior to reaching the tnp afthe bluff:
Tl~e Olsens have fc~llawed my recarnrnendati.on and installed a tigEit line pipe that intercepts the
1.1 ry/l}(i V3 +l'J.1! L':Ss: 1 JVl~ vYr yr YY vaa«aa +.+.a ,..a...y. -.-~--~
excess wdt~r dYreeted onto alte property fraln the County drainage ditch before it rBaCh~S the top
of the blut~: 7~e tight line directs the water to an area of accumulated gavel at~d debris
approxirrsa±ely a~ feet alcove the base of the bluff where the water is inFttrated into the loose
debris.
Directing the storm wa+~er into this system or into an additional pips diseharging at the same
location. would be an appropriate solution. The Dose debris on the lower portions of the shoreline
bluff have bee„ d4posited in a steep debris fan at the base of the slope from the recent slope
failures caused by the previous excess water being discharged via the Co~.inty drainage ditch into
the drainage Swale above. The discharge of water at this lvcativi: will greatly decrease tl-ae
potential far future shallow rapid debris r"t+~ws and slides on the slope below the drainage Swale.
Should you h~.ve any questions regarding out' recormaissance please contact our office at (3t5D)
71 ~-~Q~S~.
Sittcerely yours,
Str um ~r4up .
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Din McShane, M.Ss:., L.I.G.
l~icen~ed Engineering tseologise
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J~~FERSON COUNTY DOD
STRATUM GROUP
1451 Grant Street, Bellingham, WA 98225
Phone (360) 714-9409
June 25, 2003
Lyrui Olsen
20017 SE 290' Place
Kent, WA 98042
Re: Geology Report, Shoreline Bluff
Parcel # 601061003
Township 26N, Range 1 W, Section 6
Jefferson County, Washington
Dear Ms Olsen:
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We are pleased to present the results of our geology evaluation of the above referenced property
located on a shoreline bluff. The purpose of this geology evaluation 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 steep slope. This evaluation was
limited to a visual inspection of the property, the steep slope on the property, other steep slopes
in the vicinity of the property, available geologic mapping in the area, and 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 including the steep
slope conditions and shoreline conditions in the vicinity of the property.
2) Observed surface soil conditions on the steep slope and at the top of the slope.
3) Prepared this report summarizing our fmdings, including an evaluation of the
feasibility of building a residence on the subject property, a qualitative evaluation
of the slope stability, recommendations for site development, and
recommendations for further investigation, if necessary.
GENERAL GEOLOGY
Northwestern Washington has been covered at least four times in the past two million years by
ice lobes advancing out of the Coast Range Mountains of Canada and the mountains of
Vancouver Island. During these glacial and accompanying interglacial periods, the underlying
bedrock was eroded. A relatively thin layer of glacial related and interglacial fluvial sediments
June 25, 2003
Tax Parcel # 601061003, Jefferson County, WA
Geology Report
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were deposited over the underlying bedrock in the vicinity of the subject property. However, a
short distance from the subject property, the thickness of these glacial and interglacial deposits is
significantly thicker.
The Geologic Map of Surficial Deposits in the Seattle 30' x 60' Quadrangle, Washin~,ton (Yount,
Minard and Dembrof, 1993) and the Geologic Map of West-Central Jefferson County,
Washington (Birdseye, 1976) indicate the subject property and the steep slopes in the vicinity of
the subject property are underlain by the Twin River Formation, Vashon glacial till and pre-
Fraser sediments.
The Twin River Formation consists of feldspathic, fine-grained, thin- to thick-bedded sandstone
and massive concretionary siltstone. The pre-Fraser 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 ofice-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 ovemdden
and consolidated by glacial ice. The Vashon glacial till is described as consisting of a mixture of
clay, silt, sand and gravel and is in a very compact condition. These sediments were deposited
directly by glacial ice.
Observations on the steep slope of the subject property and in hand dug test pits on the upland
portion of the property indicate that the property is underlain by Twin River Formation and
glacial till. The till was observed only in shallow hand dug test pits excavated on the upland
portion of the subject property. Essentially the entire bluff is underlain by bedrock of the Twin
Rivers Formation. Glacial till was observed near the top of the bluff to the west of the subject
property and very compact silt sandy gravel that are pre-Fraser sediments make up the bluff
several hundred feet east of the subject property.
The Twin Rivers Formation underlying the bluff face consists of approximately 90%
concretionary layered siltstone. Layers of fine-grained sandstone ranging from a few inches up to
several feet thick are present as well. The bedrock layers strike north 20 degrees west and dip 65
degrees to the northeast.
The Relative Slope Stability in West-Central Jefferson County, Washington (Birdseye, 1976)
indicates the subject property is located in an area of unstable slopes and identified active
landslides. The unstable slopes and active landsliding in the area is a result of the steepness of the
shoreline bluff and the eroding shoreline.
Stratum Group File: 2.21.01
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June 25, 2003 ' `'"
Tax Parcel # 601061003, Jefferson County, WA I _ __.
Geology Report
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SPECIFIC SITE OBSERVATIONS
The subject properly is located at the southern end of the Bolton Peninsula. The location of the
property is indicated on the attached Vicinity Map. The subject property consists of an
approximately 3.2-acre parcel. The parcel consists of a gently sloping upland area and a very
steep shoreline bluff slope. Th width of the upland portion of the property ranges is
approximately 160 feet on the east boundary of the parcel. The northwest corner of the lot is
located essentially on the slope break such that the entire western boundary of the parcel is on the
steep shoreline bluff slope.
The upland portion of the property slopes to the south at an angle of 10 degrees or less. The
upland portion of the property is covered with a mature stand of second growth Douglas fir with
a few alder and red cedar with an understory consisting primarily of salal and evergreen
huckleberry.
The shoreline bluff on the property is approximately 200 feet high. The bluff on the eastern half
of the property is a fairly consistent slope of approximately 55 degrees. The bluff on the western
half of the property is approximately 60 degrees for the first 30 feet down the bluff then slopes at
approximately 40 degrees for 1.00 feet, and the lowermost slope is approximately 65 degrees.
Most of the shoreline bluff on the property is covered with Douglas fir and madrone trees with a
thick brush understory. The lower 60 feet of the bluff on the west portion of the property has
little vegetation and the bedrock is exposed on the bluff face.
A drainage Swale is located on the easternmost portion of the property. It appears that surface
water may on rare occasions flow overland at this location. However, no water was or wet soils
in the drainage Swale at the time of our site visit on February 24, 2001 despite a recent heavy
snow and melt. The slope on the shoreline bluff below this drainage Swale appears to slightly less
stable and a debris fan approximately 20 feet across is located at the bottom of the bluff at this-
location.
Bedrock on the bluff consists primarily of siltstone with a lesser amount of fine sandstone. The
rock is deeply weathered and breaks into small angular gravel fragments. Where exposed the
slope below is covered with this gravel.
The bluff is eroded by a combination of wave action that undermines the base of the bluff and
raveling of material off the exposed bedrock and soils on bluff face. The material eroded from
the bluff face is removed from the base of the bluff and transported to the west along the shore by
wave action. Bluff erosion is episodic at this location. A combination of high tides with high
south winds will cause very high-energy wave action at the base of the bluff because of the long
Stratum Group File: 2.21.01
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June 25, 2003 ~ ,., i
Tax Parcel # 601061003, Jefferson County, WA ~-~'.%~ ~ ~ ~~~~~ _4
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Geology Report ~
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fetch of water towards the south down Hood Canal from the subject roe
The beach at the base of the bluff consists of a bedrock platform partially covered with gravel
from eroding bluffs to the east. Landslide debris covers the base of the bluff at several locations,
but at most locations in the immediate vicinity of the subject property the base of the bluff
consists of exposed bedrock.
No ground water seeps were observed during our site visit on February 24, 2001.
CONCLUSIONS AND RECONIlVIENDATIONS
Based on our visual inspection of the subject property and vicinity, we conclude that a residence
can be located on the property such that shoreline erosion and landsliding will not threaten the
proposed residence within the expected life of the structure (75 years) as long as our
recommendations are followed. We recommend a set back distance for the residence of 30 feet
from the top of the steep slope. It is our opinion from an engineering geology standpoint that this
distance will be adequate to allow for expected bluff failures. A deck maybe added to the house
within the 30-foot set back as long as the deck is not structurally attached to the house.
The presence of mature trees and the presence of bedrock underlying the bluff indicate that the
bluff is fairly stable despite its exposure to occasional high wave energy from the south. The
strike and dip angle of the bedrock is at an angle such that the slope will not be effected by
inherent weaknesses in the bedrock. Because of the thinly bedded nature of the sedimentary
layers in the bedrock, no large joint systems are apparent within the bedrock and the rock tends to
erode in small pieces instead of large blocks. Hence large rock falls or block failures are not a
risk at this site.
We recommend a 30-foot native vegetation buffer consisting of tress and low brush at the top of
the bluff. The top of the bluff should be disturbed as little as passible, and we recommend that no
grading be performed on the steep slopes of the bluff face. Trimming of tree branches for view
purposes will not have any negative impacts to the steep slope. However, no trimming debris
should be left on slopes steeper than 30 degrees as this material may become saturated and slide
down the slope.
Site grading soils or debris, landscape debris, or any other material should not be disposed of
over the steep slope on the property or placed at the top of the slope.
Stratum Group File: 2.21.01
June 25, 2003
Tax Parcel # 601061003, Jefferson County, WA
Geology Report
Wastewater System
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We have reviewed the proposed wastewater system for the proposed property designed by John
Scott Fleming Engineering, Inc. (JSF) for the site. JSF has indicated that the peak flow to the
wastewater system will be 240 gallons/day (gpd) with an average capacity of 160 gpd. The
application rate for the drainfield is 0.9 gpd per square foot. This is equivalent to approximately
1.3 inches of rain added to the area over the drain field. This additional water discharge
represents a very small percentage of the total water that infiltrates the ground surface on the
slope above the steep slope.
We do not anticipate that the septic drainfield will have any adverse affects on the stability of the
steep slopes on the subject property or elsewhere as long as our recommendations are followed.
We recommend that the drain field. is located at least 50 feet from the top of the steep slope and
that the following storm water control recommendations are followed.
Stormwater
Three areas of storm water will generated from development of the site: 1) roof runoff, 2) run off
from the access road construction and 3) intercepted perched ground water from the building
foundation and access road construction. Storm water from the roof should be collected and
foundation-footing drains should be collected and discharged via tightline to the base of the bluff
on the property. In addition, we recommend the construction of a drainage ditch on the uphill
side of the property access road, and water from this ditch be collected in a tightline pipe and
routed to the base of the bluff. The collection of water from the ditch should include the
collection of water from the existing ditch on Gufstavson Road. This ditch currently occasionally
discharges water to the steep slope of the subject property via a natural swale. It is our opinion
that the interception of drainage and runoff water will more than offset the water discharged to
the wastewater treatment system.
We do not anticipate that the development of the subject property will cause any negative
impacts on the stability of the slopes in the vicinity of the subject property as long as our
recommendations are followed.
Stratum Group File: 2.21.01
June 25, 2003
Tax Parcel # 601061003, Jefferson County, WA
Geology Report
CLOSURE
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.
We appreciate 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
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Dan McShane, M.Sc., L.E.G.
Licensed Engineering Geologist
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Daniel McShane
Stratum Group File: 2.21.01