HomeMy WebLinkAboutGeotech Report 821085001Geotechnical Slope Assessment Report
Project Information
Project Name: Walden Lane
Storage Location: Port Ludlow, Washington
Client: Mrs. Sandi Johnson
Project #: ESC 17-GO61
Date: September 26, 2017
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JEFFERSON COUNTY DCD
Company Information
P.O. Box 776
Tracyton, Washington 98393
Phone: 360-698-5950
Fax: 360-698-5929
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EnviroSound Consulting Geotechnical Consulting • Environmental Consulting
September 26, 2017
Mrs. Sandi Johnson
P. O. Box 65361
Port Ludlow, WA. 98365
GEOLOGIC SLOPE ASSESSMENT REPORT
WALDEN LANE
300 WALDEN LANE
PORT LUDLOW, WASHINGTON 98365
Tax Parcel # 821-08-5001
Dear Mrs. Johnson
Project No. ESC 17-GO61
TR
9
JOFSRSON COUNTY DCO
This geologic assessment report presents the results of a soil and slope assessment directed at evaluating
erosion, drainage, vegetation, building setback, and potential landslide hazards (excluding temporary
construction cuts) at the referenced parcel. This report is in response to a request from the Jefferson
County Department of Community Development. The scope of this study was developed during a
September 20, 2017 site visit and our experience with projects of a similar nature. The services
performed under this authorization were in general compliance with geological report requirements
outlined in the Jefferson County Critical Areas Ordinance, Article V: Geologically Hazardous Areas, and
section 18.22.420: Geotechnical Report and Geological Report.
Slopes in the west -central portion of the parcel are mapped as moderate landslide hazard areas. There is a
drainage channel that trends from north to south across the central portion of the parcel.
Site Conditions
The subject property consists of a rectangular shaped, approximately 8.98 acre parcel located in Section
08 Township 28N Range 01 E, at 300 Walden Lane in Port Ludlow, Washington, as shown on Figure 1.
The subject property is located in a wooded, residential area, and is bounded on the west by Todd Road,
and on the east by Osprey Ridge Drive. Walden Lane consists of a gravel road that extends west from the
north end of Osprey Ridge Drive to Todd Road, crossing the generally southern portion of the parcel.
P.O. Box 776 • Tracyton, Washington 98393 0 (36o) 698-5950 9 Fax: (36o) 698-5929
Project No. ESC 17-GO61
Walden Lane Slope Assessment
September 26, 2017
Page No. 2
A recently constructed, single-family residence (SFR) is located at the west end of the parcel. A large
water storage tank, and a well house are located on relatively flat terraces established on a slope east of
the residence, in the west -central part of the parcel. The parcel is generally vegetated with a mature
forest of coniferous and deciduous trees and secondary vegetation such as ferns and salal. Open areas are
vegetated with a thick underbrush of bushes, small trees and blackberry bushes.
The site topography consists of areas of ground that generally slopes gently to the east and southeast in
the eastern and western portions of the parcel, with generally southeast -facing, steeper sloping ground
between. The steeper sloping area was inclined at angles of between about 5 to 10 degrees, with locally
steeper sections at up to about 55 degrees. Higher ground is located in the western end of the parcel.
Based on topographic contours provided by the Jefferson County Parcel Viewer website, the parcel is at
about elevation 430 at the west end, and at about elevation 320 at the east end. The slopes on the parcel
appeared to be relatively stable with no erosion or significant sloughing noted at the time of the site visit.
A north -south trending drainage channel crosses the central portion of the subject property. A steel CMP
culvert crosses under Walden Lane in the center of the drainage channel. Side slopes along the channel
were at about 10 to 30 degrees. No water or evidence of stream flow was observed in the channel at the
time of our visit. No groundwater springs were observed at the site.
Prottw d I)VI CloprnE•nl
We were provided with an electronic copy of plat map titled, "Preliminary Johnson Short Plat", prepared
by Clark Surveying, undated, for our use in the preparation of this report, which is included as Figure 2.
It is our understanding that you plan on subdividing the parcel into an approximately 4.99 acre western
lot, and an approximately 3.98 eastern lot, and at sorne point in the future, constructing a single-family
residence on the eastern lot. This report provides a geologically hazardous area evaluation for the
proposed development of the planned eastern lot.
Mutiludolo.-,
Tile subject property soil and slope conditions were visually examined and evaluated using site slope soil
exposures. This information provided a basis for a general understanding of the local soil and geologic
P.O. Box 776 ■ Tracyton, Washington 98393 0 (36o) 698-5950 • Fax: (36o) 698-5929
Project No. ESC 17-GO61
Walden Lane Slope Assessment
September 26, 2017
Page No. 3
conditions as discussed below. In addition to the observation of available soil exposures and
reconnaissance of the area, the following readily available resources were reviewed:
Resources used:
• "Washington State Coastal Zone Atlas", Washington State Department of Ecology and
Jefferson County, Volume 11.
• "Soil Survey of Jefferson County Washington", United States Department of Agriculture,
1975.
• USDA Online Web Soil Survey—Imp:
U.S.G.S. 7.5 minute series topographic map, "Port Ludlow, Washington Quadrangle".
• Slope Stability Map of the Uncas-Port Ludlow Area, Jefferson County, Washington by
Kathryn Hanson, 1976.
• "Geologic Map of Washington — Northwest Quadrant", Washington State Department of
Natural Resources, Division of Geology and Earth Resources, GM — 50, 2002.
• Washington State Division of Geology and Earth Resources (WDGER) Online Interactive
Geologic Map — hitp,C/ti4►r;N. nr.r py It� portal,
• "Geologic Map of the Port Ludlow and Southern Half of the Hansville 7.5 -minute
Quadrangles, Kitsap and Jefferson Counties, Washington", Washington State Division of
Geology and Earth Resources, Map Series 2015-02, Polenz et al., 2015.
• Washington State Department of Ecology Water Well Logs —
hitps: I� rtrc,,.%%:I i--toN iiiap/W('I.SWchrvil�il)/detitili.asp .
• Jefferson County Online Parcel Information —
ht ps: j f c�»�.I.maps..rrcl i ,�t,rn al,13, ��thapp, ie►4err,
Geologic Setting
The subject site lies within the central Puget Lowland. The lowland is part of a regional north -south
trending trough that extends from southwestern British Columbia to near Eugene, Oregon. North of
Olympia, Washington, this lowland is glacially carved with a depositional and erosional history including
at least four separate glacial advance/retreats. The Puget Lowland is bounded on the west by the
Olympic Mountains and on the east by the Cascade Range. The lowland is filled with glacial and
nonglacial sediments consisting of interbedded gravel, clay, sand, silt, till, and peat lenses.
P.O. Box 776 • Tracyton, Washington 98393 0 (36o) 698-5950 • Fax: (36o) 698-5929
Project No. ESC 17-6061
Walden Lane Slope Assessment
September 26, 2017
Page No. 4
Surface and Subsurface Conditions
Geologic mapping of the area indicates that the site is located in an area mapped as Vashon age glacial
till (Qvt). Glacial till is a gray, compact -to concrete -like poorly sorted mixture of clay, silt, sand, and
gravel. The till is impermeable and often responsible for a perched water table in gentle -to flat -lying
topography. Thickness ranges from 0.5 meter to 20 meters.
During our site visit, we excavated several shallow hand -dug test holes up to about 1 foot deep at
locations across the subject property. Very dense glacial till soil was encountered near the ground
surface in the western part of the subject property. Loose to medium dense glacial soils were observed in
the eastern part of the subject property.
The United States Department of Agriculture -Natural Resource Conservation Service, Web Soil Survey
for Jefferson County indicates that the following soil types are present on the site:
• Alderwood gravelly sandy loam, 0 to 15 percent slopes
• Cassolary-Everett Complex, 0 to 15 percent slopes
'The soil survey description of this soil type is summarized in the following table.
USDA Soil Survey Name
USDA Textural
Classification & USCS Soil
Type
Parent Material
Permeability
Drainage
Erosion Hazard _
Runoff
AIC-Alderwood gravelly sandy ChC-Cassolary-Everett
loam, 0 to 15 percent slopes
Complex, 0 to 15 percent slopes
0-7 inches, Gravelly sandy loam
0-23 inches, Sandy loam
7-59 inches, Very gravelly sandy
23-49 inches, Stratified fine sandy
loam
loam to silty clay loam
49-60 inches, sand 1
Glacial drift and/or glacial outwash
Glacial drift and/or marine
over dense glaciomarine deposits
deposits/Glacial outwash
Moderately rapid above the hardpan
Moderately slow
and ver% slow in the pan.
Moderatel% well drained Well drained
ModerateModerate
Medium Medium _
Based on the observed soils, it is our opinion that the site soils are generally consistent with those
indicated on the geologic and soil maps of the area. Differences between the mapped and observed soil
conditions are generally minor and not unexpected, and appear primarily as a result of outcrop
availability and mapping scale.
P.O. Box 776 • Tracyton, Washington 98393 0 (36o) 698-5950 • Fax: (36o) 698-5929
Project No. ESC 17-GO61
Walden Lane Slope Assessment
September 26, 2017
Pa -c No. 5
The subject property is comprised of gently sloping ground in the western and eastern portions, with
steeper, eastern sloping ground between in the west -central portion of the parcel. The steep slopes have
been mapped on the Jefferson County Critical Areas Map as an area of Moderate Landslide Hazard. The
light detection and ranging (LIDAR) map for the site shows the slope area more clearly and is included
as Figure 3.
The Washington State Department of Ecology Coastal Zone Atlas has areas near the subject property
mapped as Stable slopes (S), and areas mapped as Intermediate slopes (1). Stable slopes are generally less
than 15 percent but can include areas of steeper slopes that are stable due to low groundwater
concentration or competent bedrock. The stable slope designation also includes areas underlain by weak
material such as peat, which are stable because they have no significant slope. Intermediate slopes are
generally greater than 15 percent but do not exceed 30 percent. Note that the mapping was performed in
the 1970's and does not reflect more recent activity. Although the mapping does not extend to the
subject property the general topography of the site would indicate the majority of the subject parcel
would be mapped as stable. There was no visible evidence of sloughing, erosion, or deep-seated
rotational slides on the subject property at the time of the site visit.
The Slope Stability Map of Jefferson County (Hanson 1976) shows the majority of the site mapped as
Class I (Areas believed to be stable) with the channel area on the site mapped as Class 2 (Areas believed
to be stable under normal conditions, but may become unstable if modified by man's activities.)
There was no visible evidence of ground water, springs or seeps observed on the slope faces at the time
of the September 20, 2017 site visit. There are no mapped springs without 1,000 feet of the subject site.
A north -south trending drainage channel crosses the central portion of the parcel.
CONCLUSIONS
We understand that the parcel is planned to be subdivided into a western lot and an eastern lot. The
western lot is currently developed with a SFR, and a SFR is proposed to be constructed on the eastern
parcel on a future date. No building location had been established at the time of this report. The western
portion of the planned eastern lot consists of steep side slopes of a drainage channel that trends north -
south across the site. The remainder of the site slopes gently to the east.
P.O. Box 776 + Tracyton, Washington 98393 0 (36o) 698-5950 • Fax: (36o) 698-5929
Project No. CSC 17-GO61
Walden Lane Slope Assessment
September 26, 2017
Page No. 6
Jefferson County requires a 30 -foot buffer and a 5 -foot building setback from an Area of Moderate
Geologic ILazard. In addition, a channel migration zone buffer may be enforced in areas of high channel
migration. It is our opinion that a 30 -foot buffer and a 5 -foot building setback from the top of the
drainage channel side slopes be maintained. If the buffer is maintained any development should not have
a significant impact on the overall stability of the slope due to the geology of the site and the relative
stable nature of the existing slopes.
Sitr Preparation
Clearing of any areas for construction should be limited to the greatest extent possible and should not be
performed on steep slope areas until necessary erosion control facilities are in place. Clearing in building
areas should include removal of vegetation, trees and associated root systems, wood, pavement, retaining
walls, rubble, and rubbish. No clearing should occur on the slope face and minimal clearing should take
place in the vegetation buffer. Site stripping should extend to a minimum depth of 6 inches, or until all
organics in excess of 3 percent by volume or other unsuitable soils are removed. These materials will not
be suitable for use as fill for roadway or building areas. However, stripped topsoil may be stockpiled and
reused in landscape or non-structural areas.
Any buried structures encountered during construction should be properly removed and backfilled.
Excavation, depressions, or soft and pliant areas extending below planned finish subgrade level should be
cleaned to firm, undisturbed soil and backfilled with structural fill to planned finish subgrade.
Erosion Concerns
We do not anticipate any unusual erosion concerns for this site, if the distance to the slope areas is
maintained. There were no visible signs of previous significant erosion at the time of the site visit.
However, best management practices for erosion control including silt fences, hay bales, etc. should be
used to prevent sediment from leaving the site and entering storm water sewer systems or surface waters.
Water should not be allowed to flow over the slope in a concentrated manner. Stripping of vegetation on
the steep slopes should be limited to the extent possible for the proposed construction. We further
recommend that vegetation be replanted on the slopes as soon as practical following completion of
grading. Stripped slope areas should be protected from weather with a plastic visqueen cover when
construction will not be occurring on them for more than one to two days.
P.O. Box 776 9 Tracyton, Washington 98393 • (36o) 698-5950 9 Fax: (36o) 698-5929
Project No. ESC 17-6061
Walden Lane Slope Assessment
September 26, 2017
Page No. 7
The Washington State Department of Ecology (DOE) has three publications, which may be helpful in
developing long-term slope vegetation maintenance/protection and landscape plans:
• "Slope Stabilization and Erosion Control Using Vegetation: A Manual of Practice for Coastal
Property Owners", May 1993, Publication 93-30.
• "Vegetation Management: A Guide for Puget Sound Bluff Property Owners", May 1993,
Publication 93-31.
• "Surface Water and Ground Water on Coastal Bluffs: A Guide for Puget Sound Property
Owners", June 1995, Publication 95-107.
Seismic Concerns
EnviroSound has reviewed the 2015 International Building Code (IBC) for seismic design criteria for the
proposed construction. As the site and vicinity is in an area mapped as glacial till, the site may be
considered Site Class D, "stiff soil profile", in accordance with Table 1613.3.2 Site Class Definitions
from the above referenced standard.
Groundwater Considerations
At the time of our investigation, no springs or seeps were observed at the site. Groundwater fluctuates
seasonally and thus may increase during the wetter seasons, but it does not appear to be a concern for
construction purposes. No standing water was observed in a north -south trending drainage channel
crosses the central portion of the parcel.
REPORT LIMITATIONS
This report has been prepared for the exclusive use of Ms. Sandi Johnson and her agents for use in
planning of the referenced development. The conclusions and recommendations in this report are based
on our interpretation of site conditions, as they presently exist, anticipated future construction activities.
The soil conditions described in this report and the conclusions and recommendations contained in this
report are provided for this specific site only and should not be expanded for use on adjacent properties
without additional exploration and review of those sites by our firm. The data and report should be
provided to prospective contractors for their bidding or estimating purposes, but the report conclusions
and interpretations should not be construed as a warranty of the subsurface conditions. There are possible
variations in subsurface conditions. In the event that the scope or location of the project should change or
subsurface conditions different from those encountered during this study be observed or suspected, we
P.O. Box 776 • Tracyton, Washington 98393 0 (36o) 698-5950 9 Fax: (36o) 698-5929
Project No. ESC 17-GO61
Walden Lane Slope Assessment
September 26, 2017
Page No. 8
Should be advised. At that time a review of the changed conditions will be made, and alternative or
remedial recommendations given as required.
The owner and the contractor should make themselves aware of and become familiar with applicable
local, state, and federal safety regulations, including current OSHA excavation and trench safety
standards. Construction site safety generally is the sole responsibility of the contractor. The contractor
shall also be solely responsible for the means, method, techniques, sequences, and operations of
construction operations. The firm, EnviroSollnd Consulting Inc. is providing the preceding information
and recommendations solely as a service to Ms. Sandi Johnson. Under no circumstances should the
provision of this information or recommendations be construed to mean that the firm EnviroSound
Consulting, Inc. is assuming responsibility for construction site safety or the contractor's activities; such
responsibility is not implied and should not be inferred.
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 is report was
made. No other warranty, expressed or implied is made.
Should you have any questions or concerns, which have not been addressed, or if we may be of
additional assistance, please call our office.
Sincerely,
Shawn E. Williams L.E.G.
Senior Licensed Engineering Geologist
2208
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P.O. Box 776 • Tracyton, Washington 98393 0 (36o) 698-5950 • Fax: (36o) 698-5929
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FIGURE 1. Site Vicinity
/ Project Name: Walden Lane
Location: Port Ludlow, WA
Project: ESC17—G061
EnvinoSound Cansufting Client: Mrs. Sandi Johnson
Date: September, 2017 N.T.S.
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FIGURE 2. Site Plan
Project Name: Walden Lane
Location: Port Ludlow, WA
Project: ESC17—GO61
ErtvlroSound Cormultir� Client: Mrs. Sandi Johnson
Date: September, 2017 N.T.S.
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EnviroSound Consulting, INC.
P. O. Box 776
Tracyton, WA 98393
Phone: (360) 698-5950
Fax: (360) 698-5929