HomeMy WebLinkAbout602292014 Geotech Assessmentr,
Geotechnical Assessment of Building Sites _ ,y ,, ;r ,
Prepared For Forrest Towne
May 16, 2001
For the Property Located at
4104 Dosewallips Road, Tax # 602292014
~~~;~~~v ~~i~y
~paa~
JEFfENSUNCUUNIV DCD
Section 29, Township 26 North, Range 2 West, W.M.
Jefferson County, Washington
Prepared by
NTI Engineering and Surveying
717 S. Peabody Street
Port Angeles, Washington 98362
Phone 360-452-8491 Fax 360-452-8498
Web Site www.nti4u.com
E-mail info@nti4u.com
Mr. Forrest Towne
P.O. Box 694
Brinnon, Washington 98230
May 16, 2007
Subject: Geotechnical Assessment of Building Sites at 4104 Dosewallips Road,
Tax Parcel # 602292014
1.0 Forrest Towne Parcel Geotechnical Assessment
In April of 2007, Bill Payton, a registered geologist, and Steve S. Luxton, a
registered engineer with significant experience in geotechnical engineering,
inspected a 20.35-acre tract of land owned by Forrest Towne. The property is
located at 4104 Dosewallips Road off of Highway 101, north of Brinnon (Figure 1).
r c ~~~~` ~ ``~'~ ~
1P ~ : ,L~ e iF+
•.c ~. I
`q
R
t~l,pc N#ianal F~r~~t ~~ '~ ~-~~
Property ,~ ~=
Vicinity ',~ `'
~ ~~t
~ ~A ,.
~' ~
i~~t
~`,°~
~~101~
un~_,
;::.
~:~
9i
Figure 1
The parcel lies on the northerly and south facing slope of the Dosewallips Valley in
the Northwest Quarter of Section 29 of Township 26 North Range 2 West WM, in
Jefferson County, Washington. On the north, the parcel is bounded by high
ground and the Olympic National Forest. The Dosewallips River lies below, south
of the Dosewallips Road, which lies along the south line of the parcel (Figures 2 &
3).
s~
Figure 2
3
1
3
°`~?zm. .. ~...
At the time of the inspection, Mr. Towne was planning the development of a house
site and a storagelgarage building on the parcel. This report provides specific
recommendations for the house site and the shop area development
2.0 Geology of the Site
The site lies on the north side of the Dosewallips Valley in Jefferson County,
Washington. The area is a recently glaciated valley in the Quilcene Range. The
parent rock, which rises to over 1500 feet above the property, is a dense basalt
mapped in that area as basalt flows and brecciated basalt. The consolidated rock
is a part of the Crescent Formation, a volcanic island that collided with the
continent and formed much of the Olympic Peninsula (Tabor and Cady, 1974).
The lower portion of the south-sloping property shows glacial features including a
thin colluvium of glacial drift.
At least two small, incised drainage channels cross the parcel from north to south.
Both are ephemeral and flow only during heavy rain or snowmelt.
Figure 3
3.0 Geotechnical Indicators At the Site
Most of the soil at the lower portion of the property is gravely silt and gravelly sand
that was probably deposited at the site as advance outwash and was over-ridden
and formed into glacial till. This material can be embanked and, if properly
compacted it can maintain a 1 to 1 slope.
The middle portion of the rocky slope where the owner showed us a prospective
house-site is overlain with a thin mantle of colluvial glacial sand and gravel. In
much of the central section, this slide colluvium soil is only a foot ortwo in
thickness. Inspection showed that the underlying basalt rock (Photo 1) is sound
and little affected by weathering, hence the site has little or no potential for deep-
seated sliding.
The upper portion of the slope may have some loose bounders that could slide or
roll down the slope in an earthquake. Thus, the house site should be protected
from rock falls.
Photo 1
4.0 Specific Observations and Recommendations for the Proposed House
Site
During the inspection Mr. Towne showed us the location of a prospective site for a
single family residence (Figure 3, Photo 2). The intended structure is a log house
about 36 feet in width and 50 to 60 feet in length.
We examined the site that lies on an old logging road that ascends the slope about
100 feet above Dosewallips Road and the valley floor below. The area above the
proposed house site has shallow outcrops of basalt rock and rock fragments
showing at the surface in the proposed house footprint suggesting that the
consolidated rock is just below the surface there.
Photo 2
The road grade is, at present too narrow to site the house. The area must be
excavated to provide additional width for the house and toe of slope and slope rim
setbacks. The following is recommended:
4.1 House Site Recommendations
• Excavate the rock and slope colluvium so that a total width of 52 feet is
available from front to back to serve as a house site. If possible remove spoils
from the excavation and use the material to fill the low areas below near the
shop building. Such an excavation will require that the site be lowered by about
10 feet with a two foot cross slope on the house-site for good drainage.
• Steepen the slope above to not more than 1 on 1 so that the excavated slope
above the house is not more than 45 degrees from the horizontal in steepness.
• Site the house so that there is at least 10 feet from the toe of the slope on the
north to the foundation line of the house
• Provide at least 6 feet from the front of the house foundation (south side) to the
rim of the original slope. Site the house foundation on original rock so that the
vertical lines of the foundation and footing are at least 6 feet clear from any of
the side cast slope fill material.
• As side cast material is being placed, do as much as possible to compact it
against the original slope. The use of a track excavator is recommended.
Thus, the excavator could be used to "pound" the slope fill into place.
Complete the slope fill with a slope not steeper than 1 vertical to 1 horizontal.
• If a deck is to be constructed on the area south of the house, provide concrete
piers that penetrate into the original slope to support the outboard edge of the
deck. These may be built using concrete tube forms and concrete or short
steel pipe piles that are driven into the slope fill.
• The proposed location of the septic system drainfield (Figure 3) is acceptable
from a geotechnical perspective.
4.2 House Site Rock-Fall Barrier Recommendations
Due to the potential for rock-falls, the upslope side of the house should be
protected.
The following actions are recommended:
• Prior to the construction of the house, search the slope above the site for
loose rocks and roll them out or shoot them with explosives.
• Construct an "ecology block" barrier that is at least two courses high (5
feet) that protects the entire length of the house. Site the barrier as close to
the house as practical so that the space available to capture rocks is as
wide as possible.
• As an alternative to ecology blocks, gabion baskets may be placed to a
height of 5 feet and local rocks from the excavation may be used to fill the
baskets.
5.0 The Equipment StoragelShop Building Site
Mr. Towne was actively working on a site for apre-engineered steel equipment
storagelshop building 44 by 60 feet in length with the larger dimension oriented in
a north-south direction (Photo 3). The building will normally be unoccupied.
The site lies a few feet above Dosewallips Road and is being created by filling the
toe of the slope and steepening the grade of the native slope with rises at about 25
degrees from the horizontal (1 vertical to 2 horizontal). The site is suitable for the
proposed use. The gravelly sand and rock fragment mixture being used to make
the fill is good in quality and the side cast fill is unlikely to slide or slump into the
road below.
Photo 3
The following are recommended
• All materials placed in the fill should be compacted in lifts of not more than
one foot.
• Slope cuts and fills should be compacted as much as possible
• The slope above the shop should not be steepened to more than
approximately 1 foot vertical to 1.5 feet horizontal to prevent sloughing of
the granularfill forming that slope.
• Provide 10 feet clear between the toe of the slope and the back wall of the
building to provide some protection from rock falls.
• Ensure that no part of the building foundation lies on fill materials
..,~,~
~' 1
`~ .~
1 ; , ~,.
6.0 Limitations of This Report
,i~`,p
! . `'
JEFFE8S9t~~~~~~ BCD
The observations, conclusions and recommendations of this report were based
upon observations of the surface soils, shallow excavations and visible conditions
at and nearthe site. No borings ordeep test pits were completed, norwereany
longer-term measurements made at the site, nor any soil samples tested.
Soil conditions at depth could vary from the visible indicators in an unexpected
way, thus affecting the recommendations made. Those who require a higher
degree of reliability in findings are encouraged to obtain additional observations
and testing.
The investigation of the site and the recommendations of this report were prepared
in accordance to the standard of care of professional engineering and in
accordance with the principles and practice of geologic engineering. No other
warranty, neither written nor express, is provided herewith.
Sincerely yours
NTI ENGINEERING & SURVEYING
~~
Bill Payton, LEG
Engineering Geologist
Steve S. Luxton, MSc. PE
Professional Engineer
~s: ~ ~ ~
G:1Gen16i111ReportslTOWF0701.29(26-2).Dosewallips Rd.SL.doc