HomeMy WebLinkAbout601342028 Geotech Assessment (2005)~. ,..
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1451
November 8, 2005
Larry Laporte
921 Daily Street
Edmonds, WA 98020
Re: Geology Evaluation
Tax Parce1601342015
S
Jefferson County, Washington
Dear Mr. Laporte:
Stratum Group herein presents the res
property on a shoreline bluff property
suitable locations on the property for
the home sites, 3) qualitatively evalua
development and maintenance recom~
potentially unstable slopes. This evah
vicinity, a visual inspection of the blu
geologic mapping in the area, and ins:
tATUM GROUP
Street, Bellingham, WA 98225
hone (360) 714-9409
OCT - 3 2~~7
is of our geology evaluation of the above referenced
'he purpose of this geology evaluation was to 1) locate
e siting of homes, 2) determine feasible access routes to
the risk of slope failures, and 4) provide general site
endations for development of the property adjacent to
lion included visual inspection of the property and
'slopes and other steep slopes, review of available
-ction of the shoreline at the base of the bluff.
The subject property is approximately .5 acres. Three home sites were identified on the subject
property that would be feasible buildin sites from a geologic perspective.. Access routes to the
building sites can readily be accomplis ed along routes across areas of good geologic stability
that will not be impacted by the const ction of an access road.
OF SERVICES
The scope of our services included the
1) Conducted a site visits visually inspect the subject property including the bluff
slope conditions, shorel ne conditions, and relevant conditions in the vicinity of
the property.
2) Observed surface soil c nditions and shallow soil conditions by excavating
shallow hand dug test p ts.
3) Prepared this reports marizing our findings, including an evaluation of the
feasibility of building r sidences and access roads on the subject property, a
qualitative evaluation o the shoreline bluff stability, recommendations for site
development, and reco mendations for further investigation, if necessary.
,. +,
~~`~~~'~
November 8, 2005
Tax Parce1601342015, Jefferson County, WA
Geology Evaluation
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 bed ock was eroded and a relatively thick layer of glacial
related and interglacial fluvial sedimen were deposited over the underlying bedrock in the
vicinity of the subject property.
The
(Yount, Minard and Dembroff 1993) d Geologic M~ ofEast-Central Jefferson County,
Washing-ton (Birdseye, 1976) indicate t e subject property is underlain by and pre-Fraser
glaciation stratified sediments. Both m ps indicate that Vashon Till is present on the upland area
on the southwest uppermost portion of he property and to the west of the subject properly.
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J~F~E~ C~tl~ DCa
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Observations on the subject property d vicinity are consistent with the above-described
mapping. Most of the property is under ain fluvial deposit deposited by streams and rivers during
interglacial periods prior to the last gla ial period. These deposits have been highly compacted by
the overriding glacial ice. The preglaci 1 deposits are predominantly sand and gravel with a few
lenses of silt and clay.
The Relative Slope Stabilit~in East-Ce
slopes on the subject property and area
unstable areas.
(Birdseye, 1976) indicates the
The site location map is provided on F
2. The property is approximately 9.5 a~
up into several terrains as indicated on
within the following subsections. Over
two incised intermittent stream channe
of approximately 180 feet and general]
Canal.
Level Upland Area
SITE OBSERVATIONS
ure 1 and a general site plan sketch is provided on Figure
s in size. For purposes of this report the site is divided
ie Figure 2 site plan sketch. The terrains are described
1 the property consists of a sloping shoreline bluff with
. The uppermost portion of the property is at an elevation
slopes down to the northeast to the shoreline of Hood
The level upland area on the property ~ onsists of a very gentle sloping area located between two
steep sided stream ravines. The uplan area gradually tapers from a width of approximately 150
feet at the southeast side of the prope 0 feet wide approximately 230 feet to the northeast. The
upland area is bounded on the north b a steep sided ravine and on the south by another steep
Stratum Group
File: 6.6.OSB
2
OAT - 3 2007
November 8, 2005
Tax Parcel 601342015, Jefferson County, WA
Geology Evaluation
sided ravine. A northeast plunging
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extends from the upland area between the two ravines.
The upland area is underlain by silty an clayey sand and gravel that I interpret to be glacial till.
The area is tree covered with mature D uglas .fir, western red cedar and big leaf maple with an
understory of ferns and other brush.
Steep Ravine Slopes
Both ravines on the subject property ha
south of the ravines down to the interm
on the areas on the designated on the si
than 30 degrees with most of the slope:
evident on most of the steeper slopes. ~
observed appeared to consist of a few f
across.
e steep slopes from the upland areas north, west and
tent stream courses at the bottoms of the ravines. Slopes
sketch map as steep ravine slopes have slopes greater
Being on the order of 40 degrees or more. Soil creep is
few shallow soil landslides are present as well. Slides
~t thick soil slumps on the order of approximately 20 feet
Soils on the ravine slopes generally co ist ofwell-drained compact sand and gravel. However,
compact silt and clay layers are present as well. Perched ground water and associated seeps and
springs are located above these more i permeable units. These wetter areas are associated with
most of the slope failures seen within t e ravine slopes. Most of the seeps are located on the
southeast-facing slope of the north ravi e and at the lower end south ravine.
Except for areas of recent soil failures,
fir, cedar, maple and alder. No deep-se
ravines.
Slopes <30 degrees above the north
Slopes of 30 degrees or less are locate
These slopes are stable slopes underlai
with a mix of fir, cedar and maple and
present on the slopes and the slopes be
Intermittent Streams
Two intermittent streams are located o
sloping upland areas that underlain by
July 2005 both streams were mostly dY
length does not take place very often,
channel routes along most of the streat
Stratum Group
ravine slopes are tree-covered with a mix of mature
rotational failures are present on the side slopes of the
Stream
above the lower end of the north intermittent stream.
by compact sand and gravel. The slopes are tree-covered
n understory of salal and ferns. No seeps or springs are
ome less steep further up and further down the slope.
~ the property. The two streams drain from the gentle
;lacial till. At the time of my two site visits in June and
~. It appeared that stream flow along most of the stream
there was vegetation and leaf litter within the stream
i length. The lower most end of the south stream
File: 6.6.OSB
~~~°~~~I.L'e~Jl
OCT - 3 20Q7
November 8, 2005
Tax Parce1601342015, Jefferson County, WA
Geology Evaluation
contained a trickle of water at the time
over a fairly level area near the lower c
It appears that occasional stream flow
heavy rainfall or rain on snow events.
resistant to erosion. As the streams flc
till to the underlying sand and gravel s
and are susceptible to erosion. Hence,
sided ravines.
Gentle Slope Adjacent to Intermittent
The lower reaches of the ravines near
gradient becomes much less steep and
and magnitude of the stream flooding
However, the surface soil and channel
periodically take place on the lower pc
Gentle Slope on North Corner of
Ji~~~~ C~~191 f ~~~
my site visits. Gravel deposits cover the ground surface
of the streams.
ces place from the upland area during periods of very
ie upland areas are underlain by glacial till which is very
towards Hood canal they have eroded down through the
posits. The sand and gravel deposits have low cohesion
e streams incised rapidly downward to form the steep
shoreline are subject to periodic flooding as the stream
.vel is deposited in the stream channels. The frequency
stream flows is beyond the scope of this investigation.
~rphology indicates that flooding from the streams does
ins of the streams.
The upland area north of the north ravi eon the subject property slopes gently towards the
northeast towards the steep shoreline b uff above the shore of Hood Canal. The slope gradually
becomes steeper towards the top of the bluff; the maximum steepness is less than 25 degrees.
The gentle slope is underlain by well-d ained compact silty sandy gravel. The slope is tree
covered predominantly with very strai ht mature Douglas fir. The understory consists primarily
of salal. No evidence of soil movemen is present and there is no evidence of potential deep-
seated rotational failures on the slope.
Shoreline bluff
The northern corner and eastern corner
The steep shoreline bluff is the result c
slope. The steep shoreline bluff at botY
sandy gravel. Except for the erosion at
to be subject to periodic shallow slab f
bluff is undermined.
Stratum Group
of the property are located along steep shoreline bluffs.
f wave erosion at the base of the slope undermining the
of these locations is underlain by very compact silty
the toe of the slope, the slopes are fairly stable and appear
Silures of material collapsing from the bluff face after the
File: 6.6.056
4
November 8, 2005
Tax Parce1601342015, Jefferson County, WA
Geology Evaluation
Storm Berm
A high beach area underlain by gravel
of the beach between the two steep sho
area along the lower reaches of the two
gravel deposits. from very water wave a
have begun growing on the berm. One
intermittent streams ever reach the beat
reaches of the streams are underlain by
from the streams likely flows through t
Beach
ACT - 3 2007
~~~f~D DD~l~~Y DCD
i logs with trees growing on it is located along the top
ine bluffs and between the beach and the gentle slope
termittent streams. The berm appears to be the result of
on that takes place on rare enough occasions that trees
ect of the berm is that there is no evidence that the
itself as the berm blocks the stream paths. As the lower
avel and the berm consists of gravel and cobbles, water
coarse material within the storm berm.
The beach along the northeastern side f the property consists of a sloping gravel and cobble
beach. The sediment on the beach is de ived from erosion of steep shoreline bluffs north and
south of the subject property and from ' puts from several stream courses located north and south
of the subject property similar to the st ams on the subject property.
The shoreline along the subject grope appears to be in a fairly natural condition as there are
very few shoreline armoring sites alon this section of shoreline. Erosion of the shoreline bluff
on the property and vicinity takes place during periods of very high tide combined with storms
particularly wind from the northeast. T e presence of mature trees on the shoreline slopes
indicates that the rate of erosion and su sequent slope failures on the steep shoreline slopes is
very slow.
CONCLUSIO1~fS AND RECOMMENDATIONS
Building Sites
Based on my inspection of the subject roperty and vicinity, I conclude that there are three
suitable areas for building a home on t e property. The three areas are: The level upland area
between the two steep ravines on the s uthwest portion of the property, 2) the gentle slope area
on the northern portion of the property and 3) the area of slopes less than 30 degrees south of the
north intermittent stream.
All of building area 1 is suitable for t
least 15 feet from the top of the steep
Building area 2 is .located on the
the site can be obtained via an e~
Stratum Group
construction of a home as long as the home is setback at
to the north and south.
northeast slope above the steep shoreline bluff. Access to
~t from the upland property located northwest of the
File: 6.6.OSB
5
OCT. - 3 ~Q~?
November 8, 2005
Tax Parce1601342015, Jefferson County, WA
Geology Evaluation
subject property. The home should be
bluff slope and at least 15 feet back fr
JE~~E C0~1~ DCD
at least 30 feet from the top of the steep shoreline
the steep ravine slope.
Building area 3 is located south of the orth intermittent stream. Slopes in this area are generally
20 to 23 degrees. Building is this areas ould be setback at least 15 feet from the top edge of the
steep ravine slope down to the south in rmittent stream and at least 15 feet from the steep ravine
slope above the north intermittent stre
Due to the slope of the site the home c ld be placed either on a pile foundation or could be
constructed with a daylight basement f undation. Because of the ravine slopes on either side of
the building area and the well-drained d and gravel underlying the area, drainage for a
daylight type basement will not be pro lematic.
Access Route
Building area 1 can readily be accessed via an easement from Johnson Road to the level building
area 1.
Based on the topography of the area an my inspection of proposed routes, the most feasible
access route to building area 2 and bull ing area 3 is indicated on the site plan sketch. It may be
possible to construct an access road to hese sites solely on the subject property, but alternative
access routes would entail significant a gineering design, maintenance and expense.
The proposed route to building area 2 i across the upland area on the gentle slopes on the north
side of the property. This access route ill require minimal grading to access a building site
within building area 2.
In order to access building area 3 the a
the building 2 area then switchback to`
from 25 to 30 degrees. Slopes in this a
sand and gravel. Cutting into the slope
stability problems on the slope. I recor.
the slope to be use as fill across the bo
for the outer edge of the road. Hence tl
The intermittent stream channel at the
with moss and forest litter indicating t
occurs only during periods of very hey
approximately 6 feet wide and three ff
Stratum Group
;ss road will angle down across the gentle slope within
rds the south across a slope area with slopes ranging
1 are stable and are underlain by compact, well-drained
order to provide a stable roadbed will not cause any.
lend that cut soils be end hauled towards the bottom of
m of the ravine at this site versus being used as side cast
road should be a full bench cut.
~ttom of the ravine is well vegetated and was covered
stream flow at this location is uncommon and likely
rain fall or rain on snow events. The channel is
deep at the proposed crossing locations.
File: 6.6.OSB
6
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OCT-32007
November 8, 2005
Tax Parcel 601342015, Jefferson County, WA
Geology Evaluation
The slope south of the stream is betwe~
reach the building site in building area
Storm water
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10 and 20 degrees and will require minimal grading to
It is my understanding that at least 65% of the trees on the subject property will be retained. Soils
underling the areas of the property pro sed for development at building area 2 and building area
3 are well-drained and should readily i filtrate storm water from the home sites and access road.
As such this site should readily qualify or full dispersion of storm water per the Ecology storm
water manual
Storm water on the upper part of the ac ess road within the building area 2 should be handle by
grading the road in a manner that will low sheet flow of water off of the road surface onto the
forest floor adjoining the road. For the ortion of the access road that angles down across the 25-
to 30-degree slope, storm water should e directed into a ditch on the inside portion of the road
and directed to the bottom of the slope.
Storm water from the home site in buil ing area 2 should be directed to dispersion trench as
much away from the steep shoreline sl pe as practical. Because soils on the site consist of well-
drained sand and gravel, water from th dispersion trench should readily infiltrate into the
ground. No storm water should be disc arged within 30 feet of the top of the steep shoreline
bluff.
Storm water from the home site in buil ing area 3 should be directed into a dispersion trench
located on the lower portion of the slo e below the home site and the dispersion trench should be
excavated laterally across the slope. D e to the well-drained soils water should readily infiltrate
into the ground.
Septic Drain Fields
Septic drain fields for the home sites s]
drain fields are located within the built
be setback at least 30 feet from the top
least 15 feet back from the steep ravinf
setback at least 15 feet from the steep ~
Vegetation management
At building area 2 no trees should be
setback from the top of the steep sho.
Stratum Group
Auld not cause any slope stability problems as long as the
ng 2 and building 3 slope areas. The drain fields should
~f the steep shoreline bluff slope in building area 2 and at
slope. The septic drain field in building area 3 should be
wine slopes.
ved from the steep shoreline slope. Within the 30-foot
slope at least one tree per 30-foot center should be left
File: 6.6.OSB
7
.~ ~.
November 8, 2005
Tax Parce1601342015, Jefferson County, WA
Geology Evaluation
in place. Trees within the setback or on~the steep slope may be limbed or topped to create view
corridors. Any debris from limbing or t pping should be removed from the setback or steep
slope.
Trees may be removed and limbed for ~iew purposes for the building site at building area 3
without causing slope stability or erosi n problems.
Please note that there are inherent risks associated with building on lots near or adjacent to steep
slopes. These are risks that the buildin owner should recognize and be willing to accept. If
conditions appear different than those escribed in this report, or other concerns arise, we request
that we be notified so we can review th se areas and modify our recommendations as required.
Stratum Group appreciates the
questions regarding our reconr
Sincerely yours,
Stratum Group
`,f
~.
Dan McShane, M.Sc., L.E.G.
Licensed Engineering Geologist
iity to be of service to you. Should you have any
please contact our office at (360) 714-9409.
~ ~~~~~~ I~~ a
OAT - 3 2007
Stratum Group
File: 6.6.OSB
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