HomeMy WebLinkAbout902241020 Geotech Assessment (0002)Stratum Group File: 10.7.19
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PO Box 2546, Bellingham, Washington 98227
Phone: (360) 714-9409
November 5, 2019
Kavon Novak
Kavon.novak@olypen.com
Re: Geologic Hazard Assessment
Jefferson County Parcel 902241020
This geologic hazard evaluation was conducted to assess the risk of erosion and landslide
hazards on the subject properties. It is our understanding that you are planning to construct a new
home and septic drainfield on the site. The subject property is located on a gentle inland slope.
Based on our geologic hazard investigation, it is our opinion that the subject property is not
located in a geologically hazardous area. Furthermore, the proposed home and drainfield will not
increase the risk of landslides or erosion on or off the site.
This geologic hazard assessment included a field inspection of the subject properties and vicinity
including all slopes. Our evaluation also included review of available geologic mapping, lidar
(light detecting and ranging) imagery and our own notes and observations we have made in the
vicinity of the site and at locations with similar geologic conditions.
GENERAL GEOLOGY
The Geologic Map of the Center 7.5-minute Quadrangle, Jefferson County, Washington
indicates that the subject property is underlain by Vashon recessional delta deposits (Figure 1).
The unit is described as moderately sorted sand with silt and lenses of gravel.
Our observations are consistent with the above described mapping. Hand dug test pits on the site
revealed moderately dense, moderately sorted sand with silt gravel (Figure 2). In one test pit in
the central portion of the site, we encountered a massive thick medium stiff silt and clay layer
that is discontinuous in extent (Figure 3).
November 5, 2019
Jefferson County Parcel 902241020
Geologic Hazard Assessment
Stratum Group File: 10.7.19
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Figure 1. Clipped and annotated geologic map showing the subject property location. Qgod = Vashon recessional
delta deposits, Qaf = alluvial fan deposits, Qb = beach deposits.
Figure 2. Vashon recessional delta sand and gravel deposits on the subject property.
November 5, 2019
Jefferson County Parcel 902241020
Geologic Hazard Assessment
Stratum Group File: 10.7.19
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Figure 3. Discontinuous silt and clay layer on the subject property.
SITE SPECIFIC OBSERVATIONS
The 1.16-acre subject property includes three roughly triangular pieces of land on the east side of
Fairmount Hill Road in Port Townsend. Access to the two southern pieces of the property is via a
shared gravel driveway. The small northern piece is accessed directly from Fairmount Hill Road.
The property predominantrly consists of gentle planar slopes with slope angles between 0 and 15
degrees (Figure 4). The only moderately sloped portion of the property is a human made side
slope along Highway 20 on the southwest side of the property. This planar fill slope slope ranges
in height from 8 to 14 feet with measured slope angles between 15 and 25 degrees (Figure 5). A
small seasonal stream crosses the northeast corner of the property before entering a culvert under
the shared driveway. Vegetation on the site consists of mixed forest stands of western red cedar,
Douglas fir, red alder, and big leaf maple with a dense understory of sword fern, bracken fern,
salal, ocean spray, snowberry, vine maple, Himalayan blackberry and other brush.
November 5, 2019
Jefferson County Parcel 902241020
Geologic Hazard Assessment
Stratum Group File: 10.7.19
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Figure 4. Gentle slopes on the subejct property.
Figure 5. Moderate slope below Highway 20 on the southwest side of the property.
November 5, 2019
Jefferson County Parcel 902241020
Geologic Hazard Assessment
Stratum Group File: 10.7.19
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GEOLOGIC HAZARD ASSESSMENT
The slopes on the property are modest in height and steepness. During our site visit we did not
observe any evidence of previous, ongoing, or incipient slope failure on the site. We also did not
observe any evidence of groundwater perching, seepage, or springs. There are no conditions on
the site that portend future landslides. Surface water on the site is limited to a small seasonal
stream that flows across the northeast corner of the property that does not pose a rapid erosion
hazard.
CONCLUSIONS
Based on our geologic hazard assessment, the subject property is not at risk of landslides or
erosion. Furthermore, the proposed home and drainfield will not increase the risk of landslides or
erosion on or of the site.
Stratum Group appreciates the opportunity to be of service to you. Should you have any
questions regarding this assessment please contact our office at (360) 714-9409.
Sincerely yours,
Stratum Group
Dan McShane, L.E.G., M.Sc. Geoff Malick, G.I.T. M.Sc.
Licensed Engineering Geologist Geologist-in-Training