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GEOTECHNICAL REPORT
Prepared For SeaHome Services, Inca -
PERMIT # BED March 28, 2006
• {
0 6 _ 0 3 7 7 For the Property Described as 2006 L:„l,
247 Franks Lane 1
JEFFERSON COUNTY DCD Tax # 9926000019 ---
Section 36, Township 27 North, Range 1, 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
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7/ NORTHWESTERN TERRITORIES, INC.
—17
A JLS GROUP COMPANY
717 SOUTH PEABODY STREET,PORT ANGELES,WA 98362
Engineers Land Surveyors Geologists
NTIConstruction Inspection Materials Testing
(360)452-8491 FAX 452-8498 www.nli4u.com E-Mail:infoc@nli4u.com
JLS GROUP,
INC.
Geotechnical Report
Tax# 992600001
March 28, 2006
SeaHome Services, Inc.
Attn: Greg Barron
7425 Thorndyke Rd.
Quilcene, WA 98376
Subject: Geotechnical Report for Tax# 992600001 located in Section 36, Township 27
North, Range 1, W.M., Jefferson County, WA
Dear Mr. Barron:
Background
At your request, Bill Payton, Engineering Geologist with NTI Engineering and Surveying (NTI)
conducted a bluff stability inspection at the above referenced location on January 24, 2006. The
purpose of this inspection was to examine the marine bluff at the subject site by visual means
and provide an opinion regarding the relative impact that a proposed repair/reconstruction of the
existing beach access stairway would have on the stability of the bluff. Concurrent with this
inspection is an engineered drainage, sediment and erosion control plan that is being performed
by David Hanna, P.E., also of NTI.
It is our understanding that you desire to repair/reconstruct the existing stairway (Photo 1) due
to it being unsafe and very steep (48°to 50°/111% to 119%), which includes constructing a
landing (in the area of the bottom of the existing stairs), with a change in direction in order to
lessen the steepness of the stairway (Figure 1). We understand that the new/rebuilt stairway will
be in approximately the same location as the existing stairway and be above the ordinary high
water mark. We further understand that the stair system will be founded on piers and that the
toe of the bluff will not be excavated out in order to construct the stairs. And further, that bluff
disturbance will be limited to the immediate vicinity of the stair location and will not encroach
onto adjacent properties, and that disturbed areas on the bluff will be restored upon completion
of the stairs.
Site Description
The subject waterfront property is located at 247 Franks Lane, off of Thorndyke Road on the
Toandos Peninsula. The property overlooks Hood Canal to the east and is bounded on the
north and south by residential property, and on the west by Thorndyke Rd. (Figures 2 and 3).
1
• •
Existing developments to the property include the main house, guesthouse, garage and other
buildings (Photos 2 and 3). There are existing tightline drains (Photo 4) on the bluff north of the
stairs that are causing erosion/sloughing of the bluff and it is understood that these drains will
be repaired as part of the proposal.
The upland portion of the property, in the area of development, is predominantly open and
vegetated with grass, with some trees and shrubs. The rest of the property towards the west is
wooded. The topography of the upland slopes gently eastward toward the bluff(Figure 4). The
property south of the garage was wet and had runoff flowing into a culvert under the driveway
and then into another culvert on the bluff edge south of the property.
The bluff at the property (Photo 5) is about 35 feet high with an average slope angle of about
55° to 60° with some short steeper sections at the top of the bluff that are 70°+. The lower bluff
angle is about 20° to 24°. The bluff is mostly vegetated with English ivy and berry bushes with
some young alders, cedars and horsetail ferns. There has been some recent sloughing north of
the stairs as mentioned above. The existing house is about 45' to 55' from the top edge of the
bluff. young alders and brush. Groundwater seeps were noticed on the bluff face.
Site Geology
The Washington State Department of Ecology's Coastal Zone Atlas maps the soils in the area
of the subject property as Vashon lodgment till (Qvt1) over Vashon advance outwash (Qva) over
Undifferentiated stratified sediments older than Vashon lodgement till (Qpf) over Possession(?)
Drift (Qpd) (Figure 5). The till soil is described as consisting mostly of a compact mixture of
boulder to sand size particles with some silt and clay. The Atlas lists this soil as excellent for
foundation stability, good for seismic stability, and says that that it stands in steep natural and/or
cut slopes for long periods. The advance outwash typically consists of well-sorted, well-stratified
sandy pebble to cobble size gravel. The undifferentiated soil is described as consisting mainly of
sand and gravel, but in some areas contains silt, clay, peat, and possibly till. Environments of
deposition of this unit include glacial outwash, glacial lake deposits, and interglacial alluvium
and lake deposits. The Possession Drift is described as a till deposit. The Atlas also maps the
stability of the bluff as Unstable (U) with small areas of Unstable recent slide (Urs) (Figure 6).
The Washington State Department of Ecology's "Geology and Ground-Water Resources of
Eastern Jefferson County, Washington", April 1981 gives essentially the same description of the
soils in the area of the subject property as mentioned above.
According to the Soil Survey of Jefferson County Area, Washington (United States Department
of Agriculture, 1975), the subject property is in an area mapped as Cassolary sandy loam (CfC).
This soil formed in reworked glacial and marine sediments and is typically composed of
stratified silty sand with a layer of clay from 23" to 38".
Visual observations of the bluff face soils are consistent with the above soil descriptions.
Conclusions and Recommendations
Based upon our understanding of the project and our site investigation, we conclude that the
beach access stairs will have minimal impact on the stability of the bluff. However, the natural
processes of bluff retreat are occurring at the subject property and periodic sloughing of the bluff
should be expected to continue which may necessitate future maintenance/repair of the stair
system.
2
• •
•
Drainage is an issue at the property and is likely contributing to the bluff retreat process. Thus,
anything that can be done to reduce the amount of water reaching the bluff should be beneficial
to bluff stability. Controlling surface water and directing it away from the bluff as well as curtain
drains to intercept shallow groundwater are two of possibly many ways to achieve this goal. A
more involved approach that could deal with deeper groundwater would be to install a series of
wells across the yard at some distance back from the edge of the bluff at a depth that would
intercept groundwater and to continuously pump the water out. We recommend that the existing
tightline drain located north of the stairs be repaired and that the tightline be continued to the
base of the bluff. Specific drainage recommendations will be in the drainage, sediment and
erosion control plan being completed for this property, also by NTI.
The following recommendations should also be considered with regards to the proposal:
1. It will be necessary to maintain ground cover in order to reduce erosion from surface
runoff. Any bare areas that develop, on the bluff or on the upland, should be
revegetated. Native deep-rooted vegetation that requires little or no irrigation would be
the most beneficial. Please consult the enclosed publications for further information.
2. Vegetation on the bluff face provides stabilization to the bluff face soils and helps
remove water from the soil. Existing established vegetation should be left in as natural
state as possible. Once construction of the stair system is completed, any areas that
were disturbed should be revegetated as needed. As far as pruning for a better view,
minor thinning and pruning should be done in such a way that minimizes disturbance
to the soil and root zone and that insures the continued health of the vegetation.
3. Tree limbs, lawn clippings, etc. should not be thrown over the bluff because these piles
have a tendency to damage/kill the underlying vegetation which in turn increases the
erosion potential of the bluff soils.
4. Heavy irrigation or other activities that would contribute large quantities of water to the
soil should be avoided.
5. If new surface runoff from hard surfaces such as roofs, driveways, walkways and
patios will be created as part of the proposal, it should be controlled and routed to a
drainage control device such that surface water discharge to adjacent properties does
not significantly exceed predevelopment conditions. We recommend that on site
infiltration of this water not be considered for this site. This issue, as well as on site
surface and groundwater derived from off site sources (i.e. not from roof's, etc.) will be
addressed in the above mentioned drainage, sediment and erosion control plan being
completed for this property.
6. Silt fences or other sediment control devices may be needed during construction such
that sedimentation to adjacent properties does not significantly exceed
predevelopment conditions.
7. Drainage control devices should be maintained in good working order and inspected at
least once a year.
For further information please review the three publications (included with the original of this
report) published by the Washington State Department of Ecology (DOE) entitled: "Slope
Stabilization and Erosion Control Using Vegetation", "Vegetation Management: A Guide for
3
• •
Appendix
• •
Puget Sound Bluff Property Owners" and "Surface Water and Groundwater on Coastal Bluffs".
These publications can also be viewed on the DOE website at:
http://www.ecy.wa.gov/biblio/sea.html under the 1993 and 1994 year heading. The DOE website
also contains additional useful information regarding slope stability and site development; this
reference is highly recommended.
Limitations
This report has been prepared for your exclusive use in conjunction with the above referenced
project. The report has not been prepared for use by others or for other locations. It may be
used for other purposes only with the expressed written permission of the Engineer.
Within the limits of scope, schedule and budget, this report was prepared in general accordance
with accepted professional engineering and geological principles and practices in this or similar
localities at the time the report was prepared. No other warranty, expressed or implied, is made
as to the conclusions and professional advice included in this report.
The observations, conclusions and recommendations presented in this report were based on
our visual observations of the subject property at the time of our site visit; no laboratory tests
were performed. Soil and geologic conditions can vary significantly between test holes and/or
surface outcrops. If there is a substantial lapse of time, conditions at the site have changed or
appear different than those described in this report, we should be contacted and retained to
evaluate the changed conditions and make modifications to our report if necessary.
Sincerely,
_oki A. z Fq
NORTHWESTERN TERRITORIES, INC. •<<, of® sy�yc�y
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Robert A. Leach, P.E., MBA ,PF13772 to
Principal Engineer ' 4:9 % '4`
/1c EXPIRES 12/30/2006
Bill Payton, L.E.G. f ..
Engineering Geologistecp
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