HomeMy WebLinkAboutPettygroves Block 3 Lot 1 to 2 Geotechnical Report 1
Stratum Group File: 7.7.21
PO Box 2546, Bellingham, Washington 98227
Phone: (360) 714-9409
September 10, 2021
Holley Carlson
holleycarslon@gmail.com
Re: Geologic Hazard Assessment
Jefferson County Parcel 984600301
Dear Holley Carlson:
This geologic hazard assessment was conducted to assess the potential landslide and erosion
hazard risk to the property to inform future development of the site per the City of Port
Townsend’s Environmentally Sensitive Areas (ESA) Ordinance, Port Townsend Municipal Code
(PTMC) 19.05.
Based on our geologic hazard assessment, portions of the cut slopes on the subject property meet
the criteria for a landslide hazard area (LHA) per PTMC 19.05.100.C.2 e. Specifically, portions
of the cut slopes on the subject property are 40 percent or steeper and greater than 10 feet in
height. However, it is our opinion that under current and expected conditions, the slopes meeting
the criteria for a landslide hazard area per PTMC do not pose a risk of landslides or erosion to
the subject property or vicinity. Therefore, it is our opinion that no landslide hazard area buffer
is required on the site per PTMC 19.05.100.F.2. The home site is still subject to a 15-foot
landslide hazard area building setback per PTMC 19.05.100.F.4.
Due to large portions of the subject property being covered by the LHA per PTMC and 15-foot
building setback, elimination of the LHA per PTMC through mitigation measures is likely
required to construct a house on the site. This can be accomplished through the use of engineered
retaining walls that eliminate the cut slopes meeting the criteria for an LHA. The retaining walls
can either be standalone structures or incorporated into the design of the home. It also may be
possible to eliminate the LHA per PTMC through filling and grading efforts. However, this will
involve the use of large volumes of fill and careful slope angle calculations and measurements to
ensure that the regraded slopes do not meet the criteria for an LHA per PTMC 19.05.100.C.2 e.
This geologic hazard assessment included a field inspection of the subject property 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.
Srnlruu Gnoup
Re:
PO Box 2546, Bellingham, Washington 98227
Phone: (360)714-9409
June 5,2024
Steve Hacala
stevehacala@gmail. com
Geologic Hazzrd, Assessment Addendum
Jefferson County Parcel 984600301
Dear Steve Hacala:
This letter serves as an addendum to our previous geologic hazard assessment (Stratum Group
Flle 7 .7 .2I) completed in 202L It is our understanding that you are proposing to fill and regrade
the existing excavation on the subject property to eliminate the cut slopes meeting the criteria for
atandslidi hazard area and critical slopeper PTMC 19.05.100.C.2.e.The cut slopes meeting the
criteria per PTMC 19.05.100.C.2.e are shown in the attached site plan. Slope mitigation to
"eliminate the hazard by restoring or stabilizing the hazard through engineered or other
methods" is allowedper PTMC I9.05.060.A.d.
Following the proposed filling and grading, the slope will no longer meet the criteria for a
landslide hazard area anda new home and driveway willbe constructed on the site. Therefore,
the proposed home on the site will not be subject to the 15-foot building setback required per
PTMC 19.05.100.F.2.
Based on our geologic hazard,assessment, it is our opinion that the proposed filling and grading
can be completed so that the existing cut slopes on the site will no longer meet the criteria for a
lands;lide hazard area or critical slope per PTMC 19.05.100.C.2.e. Furthennore' the proposed
filling and grading as well as site development will not increase the risk of landslides or erosion
on or off the site as long as the following recommendations are followed:
a The cut slopes should be graded such that the new graded slopes are not greater than 10
feet in height and 40 percent (22 degrees) or steeper. The slopes should be graded to have
a similar grade as the adjacent natural slopes except for the home building pad and
driveway area.
New excavation should be limited to rounding out or laying back of the top of the
existing cut slopes and removal of any loose, organic material where necessary.
Excavation should be completed just before the filling and grading efforts and all work
should be conducted during the dry season between June l't and October 1't.
a
a
o
a
o
Fill material should be well drained, clean, structural hll that is properly machine
compacted. Fill should be placed on compact, native soils and not be placed over any
loose or organic soils.
All cut slopes and fiIl slopes 4 feet or higher should be retained with engineered retaining
walls that include drainage.
Stormwater should not be discharged directly on or within the subsurface of the fill
slopes. It is our understanding that stormwater from the home will be routed to the
municipal stormwater system.
Construction should be completed using best management practices including temporary
erosion and sediment control (TESC). Soil and vegetation disturbance should be limited
to the proposed construction area and access.
Based on our geologic hazard assessment, it is our opinion that the proposed site development
will not be at risk of landslides or erosion and will not increase the risk of landslides or erosion
as long as thd recommendations of this letter and the original report are followed.
Shatum 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
Geoffrey Malick
PO Box 2546, Bellingham, Washington 98227
Phone: (360) 714-9409
July 26, 2024
Steve Hacala
stevehacala@gmail.com
Re: Geologic Hazard Assessment Addendum II
Jefferson County Parcel 984600301
Dear Steve Hacala:
This letter serves as a second addendum to our previous geologic hazard assessment (Stratum Group File
7.7.21) completed in 2021 and as a response to recent comments from the Port Townsend Department of
Community Development.
In our original geologic hazard report from 2021, we indicated that cut slopes in the central portion of the
property met the criteria for a landslide hazard area per PTMC 19.05.100.C.2.e. We also indicated that a
partial fill slope near the northwest corner of the property met the criteria for a landslide hazard area per
PTMC 19.05.100.C.2.e. Both of these determinations were made using the previous interpretation of the
code in which all slopes 40 percent or greater and 10 feet in height or greater were considered landslide
hazard areas.
However, under the current interpretation of the code, the cut slopes on the subject property and partial
fill slope near the subject property meet the criteria for critical slopes but do not meet the criteria for
landslide hazard areas. The subject property and vicinity are not geologic hazard areas per PTMC
19.05.100. This is consistent with our original geologic hazard report from 2021 in which we concluded
that there were no slopes on the subject property that were at risk of landslide or erosion hazards. The
proposed grading and development on the site will not increase the risk of landslides or erosion on or off
the site as long as the recommendations of the original report and first addendum letter (June 2024) are
followed.
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
Geoff Malick, L.E.G., M.Sc.
Licensed Engineering Geologist
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September 6, 2021
Jefferson County Parcel 984600301
Geologic Hazard Assessment
Stratum Group File: 7.7.21
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
subject property is underlain by Vashon till.
Figure 1. Clipped and annotated geologic mapped (Schasse and Slaughter, 2005) showing the subject property
location. Qgt = Vashon till, Qga = Vashon advance outwash, Qgos = Vashon recessional outwash sand, Qf = fill,
Qml = modified land, Qb = beach deposits.
Schasse and Slaughter (2005) describe Vashon till as a very poorly sorted mix of clay, silt, sand,
and gravel with scattered cobbles and boulders. The unit is highly compacted to the point of
resembling concrete and commonly forms steep, nearly vertical coastal bluffs along Puget Sound
and the Strait of Juan de Fuca. Vashon till was deposited directly by overlying glacial ice
between 18,000 and 14,000 years ago. The maximum ice thickness in the area has been
estimated to be approximately 4,000 feet thick during the last major regional glaciation (Schasse
and Slaughter, 2005; Thorsen, 1981).
Geologic observations made on the subject property are consistent with the above-described
mapping. Compact, unsorted clay, silt, sand, and gravel with scattered cobbles and boulders
consistent with Vashon till were observed in hand dug test pits and cut slope exposures on the
subject property (Figure 2).
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September 6, 2021
Jefferson County Parcel 984600301
Geologic Hazard Assessment
Stratum Group File: 7.7.21
Figure 2. Vashon till on the subject property.
SITE SPECIFIC OBSERVATIONS
The 0.28-acre subject property is located on the south side of Morgan Hill in Port Townsend.
Access to the property is via Roosevelt Street. The property is currently undeveloped. Relevant
topographic features on the site and vicinity are shown below in Figure 3.
Figure 3. Annotated lidar hillshade of the subject property and vicinity.
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September 6, 2021
Jefferson County Parcel 984600301
Geologic Hazard Assessment
Stratum Group File: 7.7.21
Natural slopes on the subject property (Figure 4) are predominantly between 5 and 15 degrees (9
to 27 percent). Limited steeper natural slopes areas up to 20 degrees (36 percent) are present on
the eastern third of the property, with very limited slopes up to 25 degrees (47 percnt) on the
northeast corner of the property.
An old excavation is located on the central portion of the property and consists of cut slopes on
three sides (Figures 3, 5 and 6). Cut slopes forming the northern wall of the excavation are
between 6 and 16 feet high with slope angles between 40 and 70 degrees (84 to 173 perent). Cut
slopes forming the eastern wall of the excavation are beteen 3 and 12 feet high with slope angles
between 35 and 70 degrees (58 to 173 percent). Cut slopes forming the western wall of the
excavation are between 3 and 9 feet high with slope angles between 35 and 70 degrees (58 to 173
percent). Minor raveling was observed on portions of the cut slopes steeper than 50 degrees (119
percent).
A concrete retaining wall is located just north of the northern property boundary on parcel
984600304 (Figures 3 and 7). The wall is approximately 125 feet long. The wall ranges in height
from approximately 3 feet high at the east-northeast end to approximately 11 feet high at the
west-southwest end.
A partial fill slope is located near the northwest corner of the property (Figures 3 and 8). The
slope ranges in height from 4 to 11 feet. Slope angles on the fill slope are between 25 and 40
degrees (47 to 84 percent) with gentler areas at the bottom of the slope. Raveling was observed
on slopes steeper than 35 degrees (70 percent).
A moderately steep slope is located between the southern property boundary and Roosevelt
Street (Figures 3 and 9). The slope is between 5 and 11 feet high with slope angles between 20
and 38 degrees (36 to 53 percent).
Vegetation on the site consists of scattered Douglas fir, madrone, red alder, and other trees with a
brushy understory that includes English ivy, Himalayan blackberry, Scotch broom, holly, sword
fern, Nootka rose, grass, and weeds. The cut slopes are primarily covered in Himalayan
blackberry.
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Jefferson County Parcel 984600301
Geologic Hazard Assessment
Stratum Group File: 7.7.21
Figure 4. Slopes on the subject property.
Figure 5. The old gravel pit with steep cut slopes on the subject property.
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September 6, 2021
Jefferson County Parcel 984600301
Geologic Hazard Assessment
Stratum Group File: 7.7.21
Figure 6. Steep cut slope on the subject property.
Figure 7. Retaining wall just north of the northern property boundary.
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Jefferson County Parcel 984600301
Geologic Hazard Assessment
Stratum Group File: 7.7.21
Figure 8. Partial fill slope near the northwest corner of the subject property.
Figure 9. Moderately steep slope between the southern property boundary and Roosevelt Street.
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September 6, 2021
Jefferson County Parcel 984600301
Geologic Hazard Assessment
Stratum Group File: 7.7.21
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 or vicinity are located in a Geologically Hazardous
Area. Based on this evaluation and the conditions described in the above sections, portions of the
cut slopes on the subject property meet the criteria for a landslide hazard area per PTMC
19.05.100.C.2.e. A fill slope on the adjacent parcel 984600304 also meets the criteria for a
landslide hazard area per PTMC 19.05.100.C.2.e.
19.05.100.C.1: Erosion Hazard Areas. 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 property are mapped as Clallam gravelly sandy loam. This soil is classified
as well drained and does not have a severe to very severe erosion hazard potential.
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”
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;
Large portions of both natural slopes and cut slopes on the subject property are steeper than 15
percent. However, no intersections of permeable over impermeable sediment are present and no
seeps, springs or perched water were noted anywhere on the slope. We observed moderately well
drained to well-drained soils underlying all slopes on and in the vicinity of site.
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;
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September 6, 2021
Jefferson County Parcel 984600301
Geologic Hazard Assessment
Stratum Group File: 7.7.21
We did not observe any indications of previous, ongoing, or incipient landsliding on slopes on
the subject property or vicinity. We also did not observe any other conditions that indicate the
potential for landslides on or in the vicinity of the subject property. Slope activity is limited to
minor raveling on cut slopes steeper than 50 degrees (119 percent).
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 and vicinity are not in an erosion area for any streams and are not in a
shoreline area. The soils underlying the subject property and vicinity are moderately well drained
and not subject to soil saturation, groundwater perching, seepage, or erosion from stormwater
runoff.
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;
Portions of the cut slopes on the subject property are both steeper than 40 percent and exceed a
vertical height of 10 feet. These slopes are located on the north and east side of the old
excavation. Portions of the fill slope on adjacent parcels 984600304 and 984600302 also meet
the criteria for an LHA per this section. The slopes that meet the criteria for a landslide hazard
area (LHA) per this section are shown in Figure 10 below.
Although portions of the cut slopes meet the criteria for a landslide hazard area per PTMC
19.05.100.C, it is our opinion that the cut slopes are not at risk of landslides or erosion. The
slopes are underlain compact and moderately well-drained Vashon till. No indications of
previous, ongoing, or incipient failure were observed on the cut slopes. Furthermore, no
geologic, topographic, or hydrologic conditions that indicate the potential for future landslides
were observed on the slope. Observed activity was limited to minor soil raveling on slopes
exceeding 50 degrees (119 percent). Likewise, the fill slope on adjacent parcels 984600304 and
984600302 is also not at risk of landslides.
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Jefferson County Parcel 984600301
Geologic Hazard Assessment
Stratum Group File: 7.7.21
Figure 10. Annotated lidar hillshade image of the subject property.
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 compact, moderately 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 on Morgan Hill well above any inundation area and is not at risk
of tsunamis.
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September 6, 2021
Jefferson County Parcel 984600301
Geologic Hazard Assessment
Stratum Group File: 7.7.21
Port Townsend Requirements for Landslide Hazard Areas
PTMC 19.05.100F.3 states: Minimum Buffer for Existing Lots and Infill Subdivisions. For
existing lots, and infill subdivisions creating no additional waterfront lots, a landslide hazard
area buffer less than that required for new subdivisions under subsection (F)(2) of this section
may be permitted to allow development of a single-family residence; if the special study
concludes that doing so would not result in an increased risk to people or property or impacts to
environmental processes. Additionally, for proposals within the shoreline jurisdiction, the
reduced buffer width shall not be less than a distance equal to the sum of the bluff erosion rate
over at least 75 years plus 20 feet from the crest; or 10 feet from the sides and the toe of a
marine bluff (DR 6.8.1 of the shoreline master program). (Also see PTMC 19.05.060(D)(3),
Land Divisions – Building Pad.)
Portions of the cut slopes on the subject property (Figure 10) meet the criteria for a landslide
hazard area per PTMC 19.05.100.C.2.e. A fill slope on adjacent properties to the northwest
(parcels 984600304 and 984600302) also meets the criteria for a landslide hazard area per
PTMC 19.05.100.C.2.e. However, it is our opinion that both the cut slopes and fill slope meeting
the criteria are not at risk of landslides or erosion. Therefore, it is our opinion that no landslide
hazard area buffer is required from the landslide hazard area (LHA) on the property.
PTMC 19.05.100F.4 states: Building setback lines shall be measured from the outside edge of
required buffers and no setback shall be less than 15 feet from an established marine bluff or
inland bank buffer.
Structures on the subject property will be subject to a 15-foot building setback from the inland
bank buffer edge. This building setback is more than enough to protect structures on the site from
the effects of minor raveling and potential differential settlement near the top edge of the cut
slopes. Because the buffer will be zero, structures on the site must be located at least 15 feet from
all sides of the slope meeting the criteria for a landslide hazard area. The areas meeting the
criteria for a landslide hazard area (LHA) and the 15-foot building setback lines are shown in
Figure 10.
Figure 10 shows that the buildable area outside of the LHA and building setback lines is limited
to relatively narrow swaths of land between the building setback lines and the perimeter of the
property. Yard setbacks will further limit the buildable area on the site. Therefore, the LHA per
PTMC and the required 15-foot building setbacks likely preclude construction of a house on the
site unless the LHA per PTMC is mitigated.
MITIGATION MEASURES
Based on our geologic hazard assessment, it is our opinion that the cut slopes meeting the criteria
for a LHA per PTMC on the subject property can be mitigated so that they no longer meet the
criteria for a LHA per PTMC. Slope mitigation to “eliminate the hazard by restoring or
stabilizing the hazard through engineered or other methods” is allowed per PTMC
19.05.060.A.d. Two mitigation options are presented below.
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September 6, 2021
Jefferson County Parcel 984600301
Geologic Hazard Assessment
Stratum Group File: 7.7.21
Engineered Retaining Walls
Based on our geologic hazard assessment, it is our opinion that the cut slopes meeting the criteria
for an LHA per PTMC on the subject property can be eliminated through the use of engineered
retaining walls. Properly engineered retaining walls will not be at risk of landslides or erosion
and will not increase the risk of landslides or erosion on or off the site. The retaining walls can
either be standalone structures or can be used as part of the foundation for a home. The following
geotechnical and stormwater recommendations should be followed if this mitigation approach is
taken:
Undisturbed glacial till underlying the site is very compact and is capable of providing support
for bearing pressures of 2,500 psf. Higher loads up to 4,000 psf may be supported, but we
recommend that if higher loads are to be used that the foundation excavation be inspected by an
engineering geologist or geotechnical engineer prior to placement of the foundation to ensure
hard dense material is present at the base of the foundation.
Based on our test pits, foundation excavations from 18 to 20 inches will reach the top of the stiff
till deposits on the site. In some areas the depth of excavation may need to be slightly deeper as
we observed top soil and reworked till to a depth of 30 inches in one test pit. It is possible that
loose soils may be encountered at deeper depths due to past site disturbance or deep root zones
associated with past tree growth on the site. These areas should be over excavated down to hard
till soils and either the foundation footings can be deepened or structural backfill can be used to
raise the soil bearing level up to a minimum depth of 18 inches.
Stormwater should be routed to an approved stormwater system using methods consistent with
the Department of Ecology’s Stormwater Management Manual for Western Washington.
Retaining wall and footing drainage should be provided as shallow perched water is possible
within the glacial soils underlying the site. If backfill is required behind walls, well-drained fill
should be used. Due to the dense silt and clay rich soils, onsite stormwater infiltration is not
practical at the site.
The site is underlain by compact glacial till. An angle of internal friction of 40 degrees may be
used. For density 130 pcf can be used. A maximum pseudo-static earthquake load of 0.3 gravity
is recommended for soil seismic load.
Mitigation Measures-Filling and Grading
Based on slope angle measurements made on the site and slope profiles completed using lidar
digital elevation models, it may be possible to achieve slopes to just under 40 percent and less
than 10 feet in height with fill and grading efforts alone. However, this will likely involve the use
of large volumes of properly compacted structural fill to restore the slope to close to its original
grade. The volume may be reduced if a terracing approach is utilized. It will also require very
precise slope measurements to ensure that no slopes are 40 percent or greater and 10 feet or more
in height (or 10 or more feet in height over a 25-foot horizontal run). Filling and grading efforts
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September 6, 2021
Jefferson County Parcel 984600301
Geologic Hazard Assessment
Stratum Group File: 7.7.21
to eliminate the LHA per PTMC will not increase the risk of landslides or erosion on or off the
site as long as best management practices are followed.
CONCLUSIONS
Based on our geologic hazard assessment, portions of the cut slopes on the subject property meet
the criteria for a landslide hazard area (LHA) per PTMC 19.05.100.C.2 e. Specifically, portions
of the cut slopes on the subject property are 40 percent or steeper and greater than 10 feet in
height. However, it is our opinion that under current and expected conditions, the slopes meeting
the criteria for a landslide hazard area per PTMC do not pose a risk of landslides or erosion to
the subject property or vicinity. Therefore, it is our opinion that no landslide hazard area buffer
is required on the site per PTMC 19.05.100.F.2. Structures are is still subject to a 15-foot
landslide hazard area building setback per PTMC 19.05.100.F.4.
Due to large portions of the subject property being covered by the LHA per PTMC and 15-foot
building setback, elimination of the LHA per PTMC through mitigation measures is likely
required to construct a house on the site. This can be accomplished through the use of engineered
retaining walls that eliminate the cut slopes meeting the criteria for an LHA. The retaining walls
can either be standalone structures or incorporated into the design of the home. It is also may be
possible to eliminate the LHA per PTMC through filling and grading efforts. However, this will
involve the use of large volumes of structural fill and careful slope angle calculations and
measurements to ensure that the regraded slopes do not meet the criteria for an LHA per PTMC
19.05.100.C.2.e.
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, L.G., M.Sc.
Licensed Engineering Geologist Licensed Geologist