HomeMy WebLinkAboutWebster's Block 12 - Geologic Hazard Assessment - 2019.07.29 1
Stratum Group File: 6.20.19
PO Box 2546, Bellingham, Washington 98227
Phone: (360) 714-9409
July 29, 2019
Jean Walat
811 E Street
Port Townsend, WA 98368
Re: Geology Hazard Assessment
811 E Street
Port Townsend, Washington 98368
Dear Ms. Walat:
This geologic hazard assessment was conducted to assess the potential landslide and erosion
hazard risk to the property per the City of Port Townsend’s Environmentally Sensitive Areas
(ESA) Ordinance, PTMC 19.05. It is our understanding that you are planning to build an addition
to the east side of the existing home and/or an accessory dwelling unit (ADU) on the property.
Based on our geology hazard assessment, the slopes on the property do not meet the criteria for a
geologically hazardous area per PTMC 19.05.100.C. Furthermore, the proposed addition and/or
ADU 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 property and vicinity
including the slopes on the property. Our evaluation also included review of available geologic
mapping, lidar (light detecting and ranging) imagery and 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 Port Townsend South and Part of the Port Townsend North 7.5-minute
Quadrangles, Jefferson County, Washington (Schasse and Slaughter, 2005) indicates that the site
is underlain by Vashon till (Ogt) (Figure 1). Vashon till consists of an unsorted, unstratified
mixture of silt, sand, and gravel that has been over consolidated by overriding glacial ice,
resulting in a dense, well compacted unit. The till averages approximately 50 feet thick and was
deposited beneath the glacial ice during the Vashon stade of the Fraser glaciation, approximately
18,000 to 13,500 years ago (Schasse and Slaughter, 2005).
Our observations are consistent with the above described mapping. Hand dug test pits on the
upper portion of the property adjacent to the home and the lower, southern portion of the
property revealed compact, poorly sorted, sand and silt with gravel consistent with Vashon till
(Figure 2).
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July 29, 2019
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Geologic Hazard Assessment
Stratum Group File: 6.20.19
Figure 1. Clipped and annotated geologic map (Schasse and Slaughter, 2005) showing the subject property location.
Qgt = Vashon till, Qml = modified land, Qgdme = Everson glaciomarine drift.
Figure 2. Sand and silt rich Vashon till on the subject property.
Subject property
Qgt
Qgdme
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SITE SPECIFIC OBSERVATIONS
The 0.5-acre subject property is located on gently sloping terrain southwest of Morgan Hill in
Port Townsend. Access to the property is via E Street. Currently there is an existing home with a
daylight basement and paved driveway on the site. The area south of the house drops down to a
modified, terraced depression previously utilized as a sand and gravel pit. The property is planted
with juniper, grand fir, Douglas fir and various bushes, along with a small apple orchard in the
southern portion of the property. The rest of the property is covered in grass lawn.
The site topography is characterized by relatively flat land broken up by several moderate cut
slopes. One slope, measured at approximately 60 degrees (173 percent), exposes the house’s
daylight basement and drops down from the front yard to the back yard (Figure 3). A second cut
slope, measured at between 35 and 40 degrees (70 to 84 percent), divides the back yard of the
house from the terraced orchard and sand and gravel pit area on the southern portion of the
property (Figure 4).
Figure 3. Cut slope near the edge of the home that exposes the daylight basement.
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Figure 4. Cut slope between the back yard and the apple orchard area on the southern portion of the property.
PORT TOWNSEND GEOLOGICALLY HAZARDOUS AREAS EVALUATION
As per Port Townsend’s Code 19.05.100 C 1 through 4, the following classification criteria were
evaluated to determine if the subject property is located in a geologically hazardous area. Based
on this evaluation and the conditions described in the above sections, the subject property does
not meet the criteria for a geologically hazardous area.
19.05.100.C.1: Erosion Hazard Areas. Any area containing soil or soil complexes described or
mapped within the United States Department of Agriculture/Soil Conservation Service Soil
Survey for Jefferson County as having a severe to very severe erosion hazard potential.
Soils underlying the subject property are mapped as Townsend gravelly loam with slopes that are
0 to 15 percent. Townsend gravelly loam is classified as “moderately well drained” by the
USDA. These soils do not have a severe to very several erosion hazard potential. This is
consistent with the sandy and silty well-drained soils observed at the site.
19.05.100.C.2.a: Landslide Hazard Areas. Areas of historic failures, such as: i. Those areas
delineated by the USDA’s Natural Resources Conservation Service as having a “severe”
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limitation for building site development; ii. Those areas mapped by Ecology (Coastal Zone
Atlas) or Washington Department of Natural Resources (WDNR) (slope stability mapping) as
unstable (U or Class 3), unstable old slides (UOS or Class 4), or unstable recent slides (URS or
Class 5); iii. Areas designated as landslides on maps published by the USGS or WDNR; or
iv. Areas mapped in the Liquefaction Susceptibility Map of Jefferson County published by the
Washington Department of Natural Resources;
The subject property is not designated or mapped as an unstable area, landslide hazard area, or
liquefaction hazard area on any of the above referenced maps.
19.05.100.C.2.b: Landslide Hazard Areas. Areas with all three of the following characteristics:
a. Slopes steeper than 15 percent; b. Hillsides intersecting geologic contacts with a relatively
permeable sediment overlying a relatively impermeable sediment or bedrock; and c. Springs or
ground water seepage;
Slopes on the subject property are steeper than 15 percent. However, no intersection of
permeable over impermeable sediment is present and no seeps, springs or perched water were
noted anywhere on the slopes. We observed moderately well drained sandy and silty Vashon till
on the property. No indications of potentially unstable stopes were observed on the site or
adjoining sites.
19.05.100.C.2.c: Landslide Hazard Areas. Any area potentially subject to mass movement due
to a combination of geologic, topographic, and hydrologic factors, but not limited to those areas
mapped or described by the Soil Conservation Service, the Washington State Department of
Ecology, Department of Natural Resources or U.S. Geologic Service. These classifications may
be based on performance standards rather than mapping;
We did not observe any indications of previous, ongoing, or incipient slope failure on the subject
property. We also did not observe any geologic contacts, perched water, seeps or springs
anywhere on the property. There are no geologic contacts underlying the site that could act as
potential failure surfaces for landslides.
19.05.100.C.2.d: Landslide Hazard Areas. Any area potentially unstable or subject to erosion
or sloughing as a result of rapid stormwater runoff, soil saturation or undercutting by wave
action;
The subject property is not in an erosion area for any streams and is not in a shoreline area. No
evidence of surface water flow or indications of erosion are present on the site. The soils
underlying the site are well drained and are not subject to saturation, perching, or seepage.
19.05.100.C.2.e: Landslide Hazard Areas. Any slope of 40 percent or steeper that exceeds a
vertical height of 10 feet over a 25-foot horizontal run;
Slopes on the subject property are steeper than 40 percent. The maximum slope measured on the
site was approximately 60 degrees (173 percent) for the cut slope that exposes the house’s
daylight basement, near the proposed addition to the house. The slope, however, is
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approximately 7 feet high, and therefore does not meet the criteria for a landslide hazard area
per this section.
The cut slope south of the house was measured to have a maximum slope of 35 to 40 degrees (70
to 84 percent). This southern cut slope is approximately 7 feet high on the east side, nearest the
location of the proposed ADU construction, rising to a maximum of approximately 9 feet high on
the west side. Due to the maximum height of the slope being below 10 feet, the southern cut
slope also does not meet the criteria for a landslide hazard area per this section.
The remainder of the property consists of flat land or gentle, short slopes that do not meet the
criteria for a landslide hazard area per this section.
19.05.100.C.3: Earthquake Hazard Areas. Areas subject to severe risk of damage as a result of
earthquakes, slope failure, settlement, soil liquefaction or faulting. These areas are identified by
the presence of poorly drained soils (greater than 50 percent silt and less than 35 percent coarse
material) loose sand or gravel, peat, artificial fill and landslide materials or soils with a high
organic content.
Soils on the site are predominantly compact, well drained, and are not susceptible to earthquake
induced mass movement, liquefaction, or faulting.
19.05.100.C.4: Tsunami Hazard Areas Tsunami hazard areas are coastal areas and large lake
areas susceptible to flooding and inundation as the result of excessive wave action derived from
seismic or other geologic events. These areas have been mapped by WDNR.
The subject property is located approximately ¾ mile away from any shoreline areas and is at an
elevation well above any potential tsunamis.
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CONCLUSION AND RECOMMENDATIONS
Based on our geology inspection of the property and vicinity, and a review of readily available
geologic information on the site, it is our opinion that the subject property is not located in a
geologically hazardous areas per PTMC 19.05.100.C. The subject property is not subject to
landsliding or rapid erosion. Furthermore, the proposed home addition and/or ADU construction
will not increase the risk of landslides or erosion on or off the site.
Due to the potential for differential settlement, the base of the foundation for an addition off the
east side of the existing house should be placed a minimum of 8 feet back horizontally from the
face of the cut slope in this area. If the addition is constructed less than 8 feet horizontally from
the slope face, we recommend that either an engineered wall be constructed to mitigate against
potential differential settlement or the base of the foundation may be set deeper such that the
base of the foundation is located at least 1:1 from the face of the cut slope.
Bearing for foundation loads can use 2,500 psf. Higher values are possible but likely not needed.
If a higher value is needed, 3,500 psf can be achieved, but foundation footing soils should be
inspected prior to installation.
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. Ben Carlson, G.I.T. M.Sc.
Licensed Engineering Geologist Geologist-in-Training