HomeMy WebLinkAbout902243040 Geotech Assessment (2022) 1
Stratum Group File: 10.25.22
PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 November 21, 2022
Gerry Connolly
gjcon4@gmail.com 360-674-9081 Re: Geologic Hazard Assessment 162 Fairmount Road
Jefferson County Parcel 902243040 Port Townsend, WA 98368 Dear Gerry Connolly:
This geologic hazard assessment was conducted to assess the risk of landslide and erosion hazards to inform development on the subject property. It is our understanding that you intend to build a septic system on the subject property and later a home. The Jefferson County shoreline stability map indicates the subject property is located on area
mapped as ‘unstable’. No other geohazard areas are identified as being present on the subject property by Jefferson County. Based on our geologic hazard assessment, it is our opinion that no landslide hazard areas are present on the subject property and that the proposed septic system construction can be
accomplished without increasing the risk of landslides or erosion on or off the subject property. A home can be located on the site outside of any landslide or erosion hazard areas and 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.
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 Lidar-revised Geologic Map of the Uncas 7.5-minute Quadrangle, Clallam and Jefferson Counties, Washington (Tabor and others, 2011) indicates that the subject property is underlain by Vashon fine-grained recessional glaciomarine or glaciolacustrine deposits (Qvrf) (Figure 1). Tabor and others describe this unit as stratified fine silt with rounded pebble and cobble
dropstones interbedded with coarse silt and sand. This unit was deposited by glacial meltwater
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November 21, 2022 Jefferson County Parcel 902243040 Geologic Hazard Assessment
Stratum Group File: 10.25.22
channels where they flowed into Discovery Bay. Because the unit was not overridden by glacial ice and is therefore generally loose to medium dense and relatively well drained.
Figure 1. Clipped and annotated geologic map (Tabor and others, 2011) showing the approximate subject property location. Qvrf = Vashon fine-grained recessional glaciomarine/glaciolacustrine deposits, Qvt = Vashon till, Qb = beach deposits, Qa5 = Quaternary alluvium, af = artificial fill. Our observations of the subject property are generally consistent with the above-described mapping. Several cut slopes and machine-dug test pits excavated previously provided clear exposures of subsurface materials. Site soils consist primarily of either dense clayey silt with rounded pebbles (Figure 2), consistent with the mapped glaciomarine/glaciolacustrine deposits,
or dense silty sand with gravel (Figure 3). The latter deposit is more consistent with glacial till, which is mapped as underlying the glaciomarine/glaciolacustrine deposits in the vicinity. The spatial distribution of these units was not consistent across the property. Soils underlying the site generally appeared to be well-drained and no water was observed in test pits on site up to ~4 feet deep.
Subject Property
Qvt
af
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November 21, 2022 Jefferson County Parcel 902243040 Geologic Hazard Assessment
Stratum Group File: 10.25.22
Figure 2. Medium dense, clayey silt with pebble dropstones, observed in a machine-dug test pit on site.
Figure 3. Dense, silty sand with gravel, observed in a machine-dug test pit on site.
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November 21, 2022 Jefferson County Parcel 902243040 Geologic Hazard Assessment
Stratum Group File: 10.25.22
SITE SPECIFIC OBSERVATIONS
The subject property is a 2.01-acre parcel located near the head of Discovery Bay, approximately 200 feet southeast of the south shore of the bay. The property is located on a gentle upland slope between Fairmount Road to the north and SR-20 to the south. The property is accessed via a gravel driveway from SR-20 near the middle of the property edge.
Most of the subject property consists of a cleared grassy field on gentle slopes measuring 8 to 12 degrees with a more densely forested area by the southwest property tip that has similarly gentle slopes. Much of the grassy field consists of relatively flat terraces separated by 2- to 4-foot-tall cut slopes, each cut slope measuring 30 to 50 degrees (Figures 4 through 7). No significant fill placement appears to have occurred to create the flat terraces. Minor raveling was observed on
the steeper of the cut slope faces. The downslope edge of the property rolls over onto another short (~4 feet tall) steep slope into a roadside drainage ditch for Fairmount Road.
Figure 4. View of open grass areas of the subject property from the access driveway.
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November 21, 2022 Jefferson County Parcel 902243040 Geologic Hazard Assessment
Stratum Group File: 10.25.22
Figure 5. View of some of the terraces at cut slope from the northeast end of the property.
Figure 6. View of the gentle slopes and terraces on the subject property looking upslope to the southeast. One of the machine-dug test pits used to observed subsurface soils is located in the right-foreground of the photo.
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November 21, 2022 Jefferson County Parcel 902243040 Geologic Hazard Assessment
Stratum Group File: 10.25.22
Figure 7. View of additional terraces to the southwest of the main grassy area. No evidence of evidence of surface water flow, ponding, or surficial erosion was observed on the subject property.
Vegetation on the property outside of the grass field consists of mixed forest stand that includes
Douglas fir, big leaf maple and red alder trees with an understory that includes sword fern, snowberry, Oregon grape, and grasses. GEOLOGIC HAZARD ASSESSMENT
The subject property consists almost entirely of gentle slopes, portions of which have been modified to form relatively flat pads separated by short cut slopes. Neither the natural gentle slopes nor the short cut slopes on the subject property show evidence of previous, ongoing, or incipient slope failure. We did not observe any topographic, geologic, or hydrologic conditions
that indicate the potential for future slope failure on the site. Observed slope movement is limited
to minor raveling of the short cut slopes in the middle of the property. Based on our understanding of the geology of the site and our observations of the site, no landslide hazard areas are present on the subject property.
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November 21, 2022 Jefferson County Parcel 902243040 Geologic Hazard Assessment
Stratum Group File: 10.25.22
Soils observed in test pits on the property were well drained and capable of accepting additional water input contributed by a septic system and stormwater from roof run off from a home. We
did not observe drainage conditions upgradient of the subject property that will pose an erosion hazard to the site. CONCLUSIONS AND RECOMMENDATIONS
Based on our geologic hazard investigation, it is our opinion that there are no landslide hazard areas present on the subject property. Furthermore, installation of a septic system on the property will not increase the risk of landslides or erosion on or off the site. A home can be placed or built on the property and will not be at risk from landslides or erosion and will not increase the risk of landslides or erosion on or off the property.
Stormwater from a home can be readily dispersed consistent with the Department of Ecology’s Stormwater Manual for Western Washington. Stormwater will fully infiltrate if the dispersion areas are sized appropriately, and stormwater is dispersed and not concentrated on 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, Stratum Group
Ben Carlson, L.G., M.Sc. Dan McShane, L.E.G., M.Sc Licensed Geologist Licensed Geologist