HomeMy WebLinkAboutBLD2008-00077 Geotechnical Report • FILE
STRATUM GROUP
P.O.Box 2546,Bellingham,WA 98225
Phone(360)714-9409
April 7,2008 T
EDStan Hume a
180 Forest Drive APR 1 0 2008
Brinnon, WA 98555
Re: Geotechnical Report `,,':s %ii#� n(ftf
Case#BLD08-00077 '
180 Forest Drive
Brinnon, Washington
Dear Mr. Hume:
Stratum Group is pleased to present the results of our geology evaluation of the proposed gabion
wall work on the steep slope located north of your home at 180 Forest Drive south of Brinnon,
Washington. The slope failed in early December 2007. The cause of the failure was the
concentration of concentrated water flow over the slope due to a plugged or overwhelmed culvert
on Forest Drive. Overflow water flowed down Forest Drive and down the driveway of the
property and over the face of the steep slope on the north side of the property. The water
saturated soils on the slope and the slope failed.
The purpose of this evaluation is to determine if the proposed slope work will cause a greater
geologic risk of failure on the site and off the site and if the work will improve slope conditions.
This evaluation included a visual inspection of the property and vicinity and review of available
geologic mapping in the area and a review of the plans for repairing the slope.
The property is located on the east side of the Olympic Peninsula on a slope above Hood Canal.
The location of the property is indicated on Figure 1. The property is located in the southwest
quarter of Section 31,Township 25N,Range 2 West W.M.
SCOPE OF SERVICES
The scope of our services included the following:
1) Conducted a site visit to visually inspect the subject property including slope
conditions and relevant conditions in the vicinity of the property.
2) Observed surface soil conditions on the slopes of the subject property and vicinity.
3) Prepared this report summarizing our findings,including a qualitative evaluation
of the stability in regards to the proposed garage and residence on the property,
recommendations for site development, and recommendations for further
investigation, if necessary.
• .
April 7,2008
Case#BLD08-00077, 180 Forest Drive,Brinon,WA
Geology Hazard Assessment
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. Glacial related deposits were
deposited over the bedrock in various places on and in the vicinity of the subject property.
The Geologic Map of Surficial Deposits in the Seattle 30' by 60' Quadrangle, Washington
(Yount, Menard and Dembroff, 1993)indicates that the subject property and vicinity is underlain
by Vashon recessional outwash deposits. The recessional deposits were deposited by melt water
streams from the retreating glacial ice near the end of the last glacial period approximately
13,000 years ago. Observations on the subject property are generally consistent with this
mapping. The upper portion of the slope is underlain by layers of sand and gravel. However,
glacial till underlies the northern boundary of the property.The till is very hard and compact and
consists of silt and clay with rock fragments firmly embedded in the silt and clay. A stream
channel located on the property northwest of the subject property is incised down through the till
and into bedrock. The bedrock appears to be weathered basalt and basalt volcanic breccia.
SPECIFIC SITE OBSERVATIONS
The subject property is located on a slope above Hood Canal and U.S. Highway 101 on the west
side of Hood.Canal. The location of the subject property is indicated on Figure 1, Site Vicinity
Map. A site plan sketch showing pertinent features is provided on Figure 2.
The property is accessed via a driveway off of Forest Drive. A home is located on fairly level
ground on the site. The ground slopes gently upward towards Forest Drive. The property has two
steep slopes. One slope faces east and slopes down towards US Highway 101 and Hood Canal on
the east side of the property. The other slope slopes down to a stream valley located north of the
subject property.
The north facing slope is presently mostly bare exposed soil due to a landslide that took place on
the slope in December 2007. Riling from surface water flow is present along the drive way and at
locations near the top of the slope and on the slope itself. The unnamed stream that flows to the
north of the property at the base of the slope over flowed the culvert on Forest Drive upslope of
the property and a significant portion of the surface water flow flowed down the driveway and
onto the steep slope. The concentration of a large volume of flowing water caused the upper
portion of the slope to fail and the entire slope slid down to the base of the slope pushing a home
at the base of the slope off its foundation.
Stratum Group '' t' l
File:3.27.08
2
April 7,2008
Case#BLD08-00077, 180 Forest Drive,Britton,WA
Geology Hazard Assessment
The upper approximately 15 feet of the slope is underlain by sand and gravel fluvial deposits that
I interpret to be recessional outwash. The sand and grave may have been deposited along the
retreating ice margin or is a remnant terrace of fluvial sand and gravel deposited prior to isostatic
adjustment at the end of the last glacial period.
The sand and gravel is underlain by very hard silty to sandy glacial till. Below the till layer the
slope is underlain by loose coluvium associated with the failed soils from above. Bedrock is
exposed on the slope to the west within the incised stream ravine. The bedrock consists of basalt
and basalt volcanic breccia(fragments of broken basalt).
The upper 20 feet of the steep slope has an overall angle of approximately 60 degrees with a few
areas that are vertical or nearly vertical.The slope below the glacial till exposure is
approximately 40 degrees with a few areas that are somewhat steeper.
Except for very minor moist area at the till,no springs or seeps are present on the slope and there
is no evidence that groundwater hydrologic pressure has caused slope stability problems.
Stratum Group File:3.27.08
3
• •
April 7,2008
Case#BLD08-00077, 180 Forest Drive,Brinon,WA
Geology Hazard Assessment
CONCLUSIONS AND RECOMMENDATIONS
I have reviewed and discussed the plans for stabilizing the upper slope.It is my understanding •
that gabion baskets will be installed along the northern property boundary and the upper slope
will be terraced from the northern property boundary to the top of the slope using the gabion
baskets.
It is my opinion that this approach will stabilize the upper slope and will not be at risk of failure.
The northern property boundary coincides with the exposure of the glacial till unit. Cutting a
level platform on top of till unit will provide a very stable base for the gabion baskets. If the
slope is not repaired additional upper slope failures should be expected as the upper sand and
gravel is not well compacted and will be subject to raveling and slumping due to the very steep
unsupported slope conditions at present. The potential for more failures will likely put the
existing house on the subject property at risk and does pose a risk to the property below. The
construction of the gabion basket supported terraces as proposed will greatly reduce the risk of
slope failures and does not add to the risk of slope failures on or off the site.
Stratum Group appreciates the opportunity to be of service to you. Should you have any
questions regarding please contact our office at(360) 714-9409.
Sincerely yours, ,n as
Str m Group
O 4'`
S ' .ir; o
S
McShane,L. .G., M.Sc. , j� : ,
Licensed Engineering Geologist , ,,, .. ;:y,.; '
ot kJ.. ai.:4.4,-..;-;,,,,,,,.% ,.4,
Daniel McShane
Stratum Group
RE C ET
File:3.27.08
4
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ALLEN L. HART ENGINEERING GEOLOGIST
1720 NORTH OAKES,TACOMA,WASHINGTON 98406
(253)752-8963
January 22,2008 Project No. 0801006
Stan Hume
180 Forest Drive
Brinnon, Washington 98329
Review Letter
Stan Hume Slide Repair
Brinnon,Jefferson County,Washington
Parcel Number: 932900105
This letter is drafted at the request of Herb Armstrong, of ADA Engineering, and follows my discussions
with Mr.Armstrong and review of the proposed slide repair drawings he has provided.The purpose of
these discussions and review was to provide a basis for commenting on the feasibility of the proposed
slope repair plan. This review is not intended to be a design level review and the provision of design
recommendations is beyond the scope of this review.It should be anticipated that a site specific
geotechnical investigation will be necessary to more fully evaluate the slope, subsurface soil conditions
and proposed retaining structure design as a basis for providing specific design recommendations for the
proposed structure.
From Mr.Armstrong it is understood that the down slope property owner will not allow any work on his
property. Therefore all work, staging and material storage must occur on the up slope property with no
down slope encroachments. It is also understood that as exposed in the slope face the site soils are
comprised of sand and gravel and a tie back sheet pile wall is proposed.
Based on my discussions with Mr.Armstrong and review of the drawings provided,it is my opinion that
4, conceptually the proposed tie back sheet pile wall is a feasible means of slope repair and stabilization of
the retained portion of the slope.However, it is my recommendation that if the project goes forward a
site specific geotechnical investigation be undertaken to more fully address siting and design
considerations including subsurface soil and ground water conditions,pile embedment depth,tie back
location(s)and depths,and design wall loadings.
GEOLOGIC AND GEOTECHNICAL CONSULTING
•
Project No. 0801006 S
Review Letter
Hume Slide Repair,Brinnon,WA
January 22,2008
Page No.2
Drawings Reviewed
Drawings prepared by ADA Engineering,LLC,Job Number: 08-E6056,Dated January 2008.
"Slide Repair Plan,Portion of Lots 6, 7 and 12, Plat of Williams Addition,Block 1"
"Slide Repair Cross Sections and Details,Portion of Lots 6, 7 and 12,Plat of Williams Addition,Block
1"
Letter Limitations
This letter has been prepared for the exclusive use of Stan Hume and his agents for use in considerations
regarding the subject property and proposed slide repair. The forgoing comments and conclusions are
based on verbal information provided by Mr.Herb Armstrong and review of the drawings noted above
and may be subject to modification following additional studies.The conditions described in this letter
and the conclusions and recommendations contained in this letter are provided for this specific project
only and should not be expanded for use on adjacent sites or properties or projects without additional
exploration and review of those sites by this firm. The results of this review are intended for the use of
the original client only.Use of this letter in whole or part by third parties will require a written agreement
be in place between the consultant and the third-party.
Within the limitations of scope, schedule, and budget for this work, it is warranted that the work has been
done in accordance with generally accepted practices followed in this area at the time this letter was
made.No other warranty,expressed or implied is made.
Should you have any questions or if I may be of additional assistance,please call my office at(253) 752-
8963.
Sincerely, �/,;:".,,� ��. 4
x(
"sy
Engineering Geologist
Allen L. Hart, CPG,RG/CEG `. .�;.;. Asa 5`..
Licensed Engineering Geologist '� Ytl -O 0,- i ,t-94--
.. '3.k: L. MART
CC: Herb Armstrong,ADA Engineering
ALLEN L.HART ENGINEERING GEOLOGIST 1720 North Oakes,Tacoma,Washington 98406