HomeMy WebLinkAbout502141018 Geotech Assessment
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STRATUM GROUP
1451 Grant Street, Bellingham, W A 98225
Phone (360) 714-9409
November 3, 2005
Chuck Robinson
250 Robinson Road
Brinnon, W A 98320
Re: Geology Assessment of Land Grading
Tax Parcel 502151018, Robinson Road
Jefferson County, W A
Dear Mr. Robinson:
I visited the Jefferson County Tax Parcel 502151018 in September 2005 to evaluate the geology
of the site and assess the potential risks to the site and nearby steep slopes from land grading .
activity on the subject property. This evaluation included a visual inspection of the property and
vicinity, an inspection of cut and fill slopes on the property and on the access easement, a review
of available geologic mapping in the area, and inspection of the other slopes in the area.
Two areas of grading have taken place on the site. An access road has been constructed along an
easement to the north end of the property. This work involved cutting a portion of the existing
easement road downward approximately 3 feet and placing removed soil in a fill area to build up
the access road route on the north end of the property. Based on field measurements after the
work was completed, the volume of soil moved was approximately 250 cubic yards.
The other area of grading work took place on the upland area of the property from the new access
road. This work involved cutting into a portion of the uppermost part of the west facing slope and
placement of the removed soil onto the lower slope. Based on filed measurements the volume of
soil moved was approximately 350 cubic yards.
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 glacial related and interglacial fluvial
sediments were deposited over the underlying bedrock in the vicinity of the subject property.
The Geolosric Map of Surficial Dq>osits in the Seattle 30' by 60' ~uadrangle. Washinsrton
(Y oun~ Minard and Dembroff, 1993) indicate glacial advance outwash deposits underlie the area
where the subject property is located. These deposits primarily consist of sand and gravel
deposited by glacial melt water streams and rivers as the Puget lobe of ~v~'-;---~'._-'--;=:--1
hsouthwhaard inbeeto the Puget Sounded area. The deposits have been over rid~P.... ,.'ti~~ ~~~ 4hd:t; . 1, '\ I
ence ve n very compact . i : ',. , : ~
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NOV 1 4 2005
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November 3, 2005
Tax Parcel 5021510 18, Jefferson County, W A
Geology Evaluation of Grading Work
Site observations are consistent with the mapping. The subject property is underlain by sandy
gravel with cobbles in a relatively compact condition consistent with the interpretation of
advance outwash deposits.
SPECIFIC SITE OBSERVATIONS
Subject Property
A site location map is provided on Figure 1 and a general site plan sketch illustrating pertinent
information is provided on Figure 2. The property located east of U.S. Highway 101 north of the
Duckabush River delta in southern Jefferson County. The property is located on a hill sloping up
above the north side of the Duckabush River delta and tidal flats. The site is accessed from
Highway 101 via Robinson Road and then a short gravel access road or via an easement off of
Daphne Lane.
The lower portions of the lot are located on a level area between Robinson Road and the tidal
flats. The property then slopes steeply up to the north to a west-sloping upland area. Much of this
west-facing upland area has recently been cleared and graded in preparation for possible fuwre
development as a home site.
The grading activity on the site has essentially involved cutting partially into the hill slope on the
eastern side of the property and placing excavated soil on the western portion of the property. In
addition grading has taken place in order to lower the grade angle between the northern portion
of the site and the area where development is proposed such that the grade will allow access to
the building site from the easement onto the south end of the property.
The cut slope on the east side of the property is a maximum of 6 feet high. The cut slope is nearly
vertical and consists of poorly sorted (well graded) very compact sandy silty gravel towards the
south end of the cut slope and layered well sorted (poorly graded) sand and gravel layers in a
compact condition on the south end of the cut slope. The slope above the cut slope to the east
slopes at an angle of less than 20 degrees and becomes less steep further up the slope.
The fill slope where cut sand and gravel soil has been placed on the west side of the property
slopes down to the west at an angle of approximately 30 degrees.
Property west of the subject property
The property to the west of the subject property continues to slope down tp:wanl.stb~_~~t t~ the
~tto~ o~ the hill. A building. site is loca~ on a le,,:eled area are on ~ str~l~e ~tp~.~~~- - ._~..
this bwlding area and the subject property IS approxImately 20 feet high ~~rrfOventlrSIOp&':::..-_._..;
Stratum Group ';' ... NOV FllAuiQQiA
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November 3, 2005
Tax Parcel 502151018, Jefferson County, W A
Geology Evaluation of Grading Work
angle of 45 degrees. Portions of the base of the slope have been cut into and are nearly vertical
for heights of 6 to 8 feet. These slope cuts on the adjoining property appear to have been
excavated sometime in the past in order to create a building site on the slope. The slope above
the vertical portions are on the order of 30 degrees or less. The slope is for the most part covered
with thick brush and a few trees and is underlain by compact sand and gravel.
Easement northeast of the subject property
An easement is located northeast of the subject property along Daphne Lane. Some grading work
has taken place along this easement as part of the grading work done on the subject property.
Essentially the east portion of the Daphne Road easement has been lowered and the sand gravel
soils from lowering the road have been placed within the easement to the west. This grading
work has lowered the grade along the easement route.
The west end of the easement route has a fill slope of approximately 35 degrees. The east end of
the easement route has been lowered such that a preexisting cut slope along the north side of the
easement has been increased from a maximum height of approximately 7 feet to a height of
approximately 10 feet. 1be cut slope consists of very compact silty sandy gravel. The slope
immediately above the cut slope slopes up at an initial maximum angle of approximately 30
degrees and then soon becomes much less steep.
CONCLUSIONS AND RECOMMENDATIONS
Based on my visual inspection of the subject property and vicinity, I conclude that the fill and
grading activity that has taken place on the site and along the easement route to the northeast of
the site has not created slope conditions that pose a threat to the subject property. I also conclude
that the grading on the subject property and within the easement route does not pose a risk to
other properties in the vicinity. Specific areas where grading has taken place are discussed below.
Cut slope along Daphne Road easement
The cut slope height has been increased from a maximum height of 7 feet to a maximum height
of 10 feet. Soils underlying the cut slope are very compact silty sandy gravel. Raveling of the
sand gravel should be expected on a periodic basis from wetting 8nd drying of the cut slope and
freeze thaw of the cut slope. The expected ravelling does not pose a threat of impacting any
properties. Over time the slope angle will be become less steep, but base on survey markers and
compass direction reading.Cl I took at the site, the eventual movement back of the top portion of
the cut slope will still be within the easement.
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NOV i 4
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stratum Group
File: 8.13104SA
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November 3, 2005
Tax Parcel 502] 51018, Jefferson County, W A
Geology Evaluation of Grading Work
FiB soils on the north side of the property
Soil from the lowering of the easement road described above was placed on the southern portion
of the property to extend the easement road onto the south end of the property. The fill slope
created is approximately a maximum of 15 feet high. The soil slopes at an angle of
approximately 30 degrees. Some soil minor movement may be expected initially before
vegetation becomes established on the slope. However, the fill soil slope poses no risk to any off
site locations.
Cut slope on the east portion of the subject property
The cut slope on the east side of the subject property is a maximum of approximately 6 feet high.
The cut slope consists of silty sand and gravel and layered sand and gravel. The silty sand and
gravel will stand up as a nearly vertical slope with minor periodic ravelling of material from
wetting and drying and freeze thaw. Because the layered sand and gravel lacks silt and has low
cohesion, a rock wall should be constructed in front of that portion of the cut slope to maintain
the slope angle.
FiB slope at the north end of the graded area
The fill slope at the north end of the graded area consists of soil removed from the cut slope area
above and placed on the slope to build up the slope to create a gentle sloping areas below the cut
slope area. The fill slope typically has a slope of 30 degrees and the slope below the area of the
fill slope is less than 30 degrees. No hazards at present from the placement of fill in this area.
Reeommendations
As noted above, the layered sand and gravel cut slope on the east side of the subject property
lacks silt and has low cohesion. A rock wall should be constructed in front of that portion of the
cut slope to maintain the slope angle or soil removed from that portion of the slope should be
replaced.
The cut slope on the property to west of the subject property does not pose a risk to the subject
property as long as no further cutting of that slope is done. No further slope cuts should take
place on the adjoining property unless an engineered design retaining structure is built at the cut
slope. Jefferson County should be contacted if any work involving the slope below the subject
property is conducted.
Because the level building area located to the west of the subject property is located at the base of
a steep cut slope, no fill soils should be placed on the slope a~; cWfs~~te
a fill slope of greater than 23 degrees. fIt.} lr r.!!:~~ ~~(:J~ lj ~J ~~ ,\\,!
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Stratum Group . .' ': NOV 1 4 ;!OOi~ Fee::~~ftl045A
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November 3, 2005
Tax ParceI5021510J8, Jefferson COWlty, WA
Geology Evaluation orGradiAg Work
Soils underlying the site consists of very well drained sand and gravel. The grading on the site
will not cause storm water run off problems, as rainwater will readily infiltrate into the ground.
Compaction of the easement route may lead minor run off from the road area, but run off will
readily be infiltrated along the sides of the easement road. If site development takes place, storm
water generated from impervious smfaces should be routed to level dispersion areas on the north
portion of the upland area. Water in the 'dispersion areas will readily infiltrate in the well drained
soils underlying the site.
The construction of a septic drain filed on the site will not pose a risk to slope stability on the site
or on adjoining sites due to the well drained nature of the soils at the site.
Stratum Group appreciates the opportunity to be of service to you. 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. 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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Stratum Group
File: 8. J3J04SA
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