HomeMy WebLinkAbout602352009 Geotech Assessment STRATUM GROUP '
1451 Gran! Slreet, Bellingham, WA 98225
Phone (360) 714-9409
M~/y 7, 2001
Stan Johnston
Johnston Realty
P.O. Box 88
Brinnon, WA 98320 '
Re:
Geotechnlcnl Report
Proposed GeOrge Gruel Short Plat
.Township 26N, Range 2W; Section 35
Jefferson County, Washington
Dea~ Mr. Johnston:
We are.pleased to present the results ofour.geology evaluation of the above referenced property
located In a potenfiaHandslide hazard area. The purpose of this geology evaluation was to
determine the suitability ofth~' propo~ecl 'short plat for siting residence, __ each oftbe two,
proposed lots. This evaluation was limited to a visual inspection of the property, the slopes on the
property, other steep-slopes in the vicinity of the property, and available geologic mapping in the
SCOPE OF SERVICES
The scope of our services included'the following:
.1)
2)
Conducted a site visit to visually inspect the subject prope~rty inclUding the steep
slope conditions in the vicinity' of the property.
Observed surface soil conditions on the steep slope and at the top of the slope.
Prepared thisrepor~ summarizing'our findings,.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,'i 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 Coas~ Range Mountains &Canada and the mountains of Vancouver
Island. During these glacial and accompanying interglacial periods; the underlyin8 bedrock was
eroded. Glacial related sediments were deposited over the underlying bedrock in the vicinity of
the subject property.
May 7, 2001
Proposed C_~org¢ Greul Short Plat, lefferson County, WA
Geotechnicai Report
MAY 1 5 2002 i!~--".:''
The Geologic Map of Surficial Deposits in the Seattle 30' x 60' Quadrangle, ..Washington (Yount,
Minard and Dembrof, 1993) indicates the subject property and the vicinity &the subject ProPerty.
is underlain by the Vashon recessional outwash deposits and Vashon till. The Vashon glacial till is -
deseribedas consisting ora mixture &clay, silt, sand and gravel and is in a very compact
condition. These sediments were deposited directly by glacial ice. The Vashon recessional
' outwash deposits consist ora wide variety of glacial outwash sediments deposited while the
VaShon ice sheet retreated from the area. Carson (1976, unpublished Department of Natural
'Resources map) noted delta deposits in the Brinnon area deposited by the Duekabush River into
an ice margin lake created by the retreating Vashon ice sheet.
We observed glacial till and ice contact deposits on portions &the proposed Lot 1; however,
most of the property appears ~o be underlain by ice margin terrace deposits. The proposed short
plat appears, to be located on the edge of a glacial terraCe with the uppermost portion of Lot 1
u.nderlain by glacial till. The hummocky topography ofthe~proposed short plat and the area in the
Vicinity is typical of kame terrace deposits. That is depressions and low spots may have been
.........
created by blocks of wasting me along the glamal margins and vathin the outwash, sedunents.
spEcIFIC SITE OBSERVATIONS . "
The sub'ect roe is located a roximately one half mile north'ofBrinnon at the base ofthe
.,~ .p p ny pp .
south side of Oreen Hill.' The location of'the property is indicated on the attached Vicinity Map.
The subject property ,s approximately 10.5 acres. The proPerty generally slopes towards the
southeast. However, because ofhummoeky topography, and a drainage on the northern portion of
the property there are a variet~y ofslope directions. The maximum slopes are lO0.pereent, bUt the
overall sloPe ofthe property io approximately 12 percent. : -'
- .. .
comers of the property. A gravel-covered road crosses the soUthwest pOrtion of the property.
This road accesses residential properties and timber properties to the west and north of the subject'
property, i' : : ' ' .
The proPerty within the BPA easement is brash-covered primarily with h~ckberries and scotch
broom. The northwest and southeast portions of the property are tree- and brash-cover, ed.
A drainage swale is located on the northeasternmost portion of the property. A small trickle of
water was observed in the lower portion &this swale at the time &our site visit on APril 23,
200~ .' No hazards associated with erosion'or potential fl°oding appear to be posed by this stream.
May 7, 200 i
Proposed George Grcul Short Plat, Jefferson County. WA
Geotcchnicai Report
·
CONCLUSIONS AND ~COMMENDATIO~
Based on our visual inspection of the subject property and vicinity, we conclude that residences
can readily be located on the two proposed lots in proposed short plat. In addition the location of
drain fields on the lots should not cause any problems with instability as long as the drain fields
are not located on any ofthe slopes Steeper than 30 degrees.
We have reviewed tile proposed short plat map that shows the proposed building sites, well sites
and drain fields. Based on our understanding o£the geology of the site the proposed locations of
buildings, drain fields and wells, development on the subject property will not be at any risk fi.om
geologic hazards posed by erosion or landslides. We do not anticipate that development on the
proposed lots will cause any negative impacts on the stability of the slopes in the vicinity of the
subject prop~n-ty.
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,
3o..~tum' Group
Dan McShane, M.S.
Senior Geologist
File: 4,16.01