HomeMy WebLinkAbout921182036 Geotech Assessment~~i~~~~~~c~~~~
STRATUM GROUP ~ ,
1451 Grant Street, Bellingham, WA 98225 ~ ~
Phone (360) 714-9409
May 18, 2005 ,~i ~r~~~ ~~
Winnie Yabe rn ~~ ~~ -' S
121 Randolph Street
Port Hadlock, WA 98339
Re: Geology Assessment
Tax Parcel 921182036
Jefferson County, Washington
Dear Ms Yabe:
I visited the above-described parcel on Oak Bay Road on May 9, 2005 to qualitatively evaluate
the slope stability of the slopes on the subject property and in the vicinity of the subject property.
Based on my assessment of geologic conditions, the slopes on the subject property are stable and
the risk of the .property being impacted by off site slope ha~.ards is very low.
This letter provides some background on the general geology of the site, site specific
observations made, an assessment of the geologic risk, and recommendations for siting a home
on the properly.
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 deeply eroded. The Surficial Geologic Map of
the Port Townsend 30- by 60-Minute Quadrangle, Puget Sound Region, Washington (Pessl,
Dethier, Booth and Minard, 1989) and the Geologic Map of Northeastern Jefferson County
(Gayer, 1976) indicate that the subject property is underlain by glacial till. The Vashon Till
consists of silt and clay with sand, cobbles and occasional boulders embedded in the finer matrix
of silt and clay. The Vashon Till was deposited directly by glacial ice during the last glacial
period, which ended approximately 13,000 years ago. 'The till is very compact and has very low
permeability.
Pessl and others (1989) as well as Gayer (1976) indicate that the advance outwash deposits are
located a short distance north of the subject property. Advance outwash deposits were deposited
by melt water streams as the glacial ice advanced over the area. The outwash exposed on the
slopes north of the subject property consists primarily of sand units but limited lenses of gravel,
silt and clay are present as well. This unit was also very compacted by the weight of glacial ice
and is in a very dense compact condition.
My observations of the subject property and vicinity are consistent with the above-described
mapping; however, limited areas of sand and gravel are present on the upper portions of the
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_ ~ ;~:~
May 18, 2005 _
Tax Parce1921182036, Old Oak Bay Road, Jefferson County, WA ~~~~~~I ~j~11~1~~(~
Geology Assessment ( ~ t i(~
subject property. These deposits are not highly compacted, and I interpret them to be either
recessional outwash from the retreating glaciers or reworked glacial till.
SPECIFIC SITE OBSERVATIONS
The property slopes gently up from Old Oak Bay Road, which bounds the east side of the subject
property. The slope gradually becomes steeper to the west, but the slopes are all less than 25
degrees. No indications of slope movement or potential slope movement are present on the site.
The property is tree and brush covered with a mix of alder, maple, fir and cedar trees and a thick
brush except for a small grass covered area in the central portion of the site.
A small drainage that drains an area southwest of the subject property passes through a culvert
under Oak Bay Road southwest of the subject property and flows as a diffuse wet area along the
south portion of the lot. A smaller drainage flows across the area north of the subject property.
Beyond the subject property to the east the ground slopes gently for another approximately 300
feet before sloping steeply down to Oak Bay. Approximately 150 feet north of the subject
property there is adeep-seated landslide. Old Oak Bay Road crosses the southern limb of the
slide and joins Oak Bay Road near the headwall scarp just north of the centerline of the slide
axis. The head wall scarp of the slide can be seen immediately west of the intersection of Old
Oak Bay Road and Oak Bay Road. The slide is approximately 1,000 feet wide. Topography
within the landslide is hummocky in places, but no obvious indications of recent deep-seated
movement are present. The toe area of the slide does have some secondary slides along the
steeper portions of the shoreline. Bedrock is exposed along the shoreline both to the south and
north of the landslide area, but not within the slide area itself. However, toe movement that
includes deeply weathered bedrock overlain by glacial till is evident in the central portion of the
slide area along the shore and it is my understanding that at least one home has been damaged on
the slide by differential ground movement on the slide surface.
ASSESSMENT OF GEOLOGIC RISK
Based on my observations on the subject property and in the vicinity of the property, it is my
opinion that there is a very low risk of landsliding impacting the subject property. No unstable
slopes are present on the site.
Because of the presence of glacial till soils on the site foundation drainage will need to be
provided if the foundation is an excavated stem wall or slab type foundation. Storm water run off
and foundation drainage should be dispersed into a native vegetation area in separate level
spreaders.
Stratum Group File: 4.29.05
2
F y dFR®~~
May 18, 2005
Tax Parcel 921182036, Old Oak Bay Road, Jefferson County, WA ~~~ ~~~~~i
Geology Assessment tf
The conditions that triggered the deep-seated landslide to the north of the subject property are not
present at the subject properly. A thick unit of advance outwash sand underlain by compact silts
and clays that lie below the advance glacial outwash and deeply weathered bedrock underlie the
lower slopes of the slide area. The shoreline slope below the subject property has more
competent bedrock that rises further up the slope than in the area of the slide.
It is my opinion that the risk of the deep-seated slide expanding towards the south towards the
subject property is very low and that the subject property is far enough from the outer edge of the
landslide to be at very low risk of being impacted by the slide.
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, L.E.G., M.Sc. ~~ '~
Licensed Engineering Geologist
~.
(~~tit@~ ~C~~4;~PtL'
Stratum Group
File: 4.29.05
3
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