HomeMy WebLinkAbout995400031 Geotech Assessment
Geologic Report
for
Single Family Residential Property
7 Camp Talbot Road
Parcel No. 995400031
Jefferson County, Washington
October 12, 2021
Project #21268
Prepared For:
Richard & Tiffiney Pederson
7 Camp Talbot Road
Quilcene, Washington 98376
Prepared By:
Envirotech Engineering
PO Box 984
Belfair, Washington 98528
Phone: 360-275-9374
10/12/202
1
TABLE OF CONTENTS
1.0 INTRODUCTION ................................................................................................................................. 1
1.1 PROJECT INFORMATION .................................................................................................................... 1
1.2 PURPOSE OF INVESTIGATION AND SCOPE OF WORK ........................................................................ 1
2.0 SURFACE CONDITIONS.................................................................................................................... 3
2.1 GENERAL OBSERVATIONS.................................................................................................................. 3
2.2 TOPOGRAPHY ..................................................................................................................................... 3
2.3 SURFACE DRAINAGE .......................................................................................................................... 3
2.4 SLOPE AND EROSION OBSERVATIONS ............................................................................................... 3
3.0 SUBSURFACE INVESTIGATION ..................................................................................................... 5
3.1 FIELD METHODS, SAMPLING AND FIELD TESTING ........................................................................... 5
3.2 GEOLOGIC CONDITIONS .................................................................................................................... 5
3.3 SPECIFIC SUBSURFACE CONDITIONS ................................................................................................. 6
3.3.1 Groundwater............................................................................................................................... 7
4.0 ENGINEERING CONCLUSIONS ...................................................................................................... 8
4.1 LANDSLIDE HAZARDS......................................................................................................................... 8
4.2 EROSION HAZARDS AND CONTROL ................................................................................................... 9
4.3 SEISMIC HAZARDS .............................................................................................................................10
4.4 SLOPE IMPACTS .................................................................................................................................10
4.5 UPLAND/ SLOPE DRAINAGE CONTROL AND GROUNDWATER CONTROL ........................................10
4.6 VEGETATION MANAGEMENT ............................................................................................................11
4.7 SEPTIC SYSTEM IMPACTS ..................................................................................................................11
5.0 CLOSURE .............................................................................................................................................12
Appendix A - Site Plan
Appendix B - Soil Information
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Envirotech Engineering Geotechnical Report
PO Box 984 page 1 Parcel 995400031
Belfair, Washington 98528 Jefferson County, Washington
Ph. 360-275-9374 October 12, 2021
1.0 INTRODUCTION
Envirotech Engineering, PLLC (Envirotech) has completed this geologic report for a residential
property located at 7 Camp Talbot Road, identified as parcel number 995400031 in Jefferson
County, Washington. As presented herein, this assessment includes information pertaining to the
project in this Introduction Section; observations of the property and surrounding terrain in the
Surface Conditions Section; field methods and soils descriptions in the Subsurface Investigation
Section; and assessments for landslides, erosion, seismic hazards, and other considerations in the
Conclusions Section.
An initial geological/ geotechnical evaluation of the project was conducted by Envirotech on
September 17, 2021. It was determined that development activity is within an area of geologic
hazard, but does not require geotechnical recommendations. Subsequently, the project requires a
Geologic Report pursuant to Geologically Hazardous Areas of the Jefferson County Ordinance
(JCO) 18.22.
During the evaluation and site visit by Envirotech, surface and subsurface conditions were
assessed in order to determine if further geotechnical studies are required. After completion of the
field work and applicable Project research, Envirotech prepared this geological assessment.
1.1 Project Information
Information pertaining to the project was provided by the proponent of the property, and
observations from a field visit by Envirotech. The proposed development is expected to consist of
a new single-family residence, on-site septic, and other ancillary features typical of this type of
development. Approximate site features are illustrated in the Site Map in Appendix A.
1.2 Purpose of Investigation and Scope of Work
The purpose of this geologic report was to evaluate the project in order to confirm that the
proposed development is outside of any landslide hazard area and its associated buffers and
setbacks as determined in the JCO. The investigation included characterizing the general project
surface and subsurface conditions and evaluating the suitability of the soils to support the planned
site development. In order to fulfill the purpose of investigation, the geological/ geotechnical
program completed for the proposed improvements of the project include:
• Review project information provided by the project owner’s representative;
• Conduct a site visit to document the site conditions that may influence the construction
and performance of the proposed improvements;
• Define general subsurface conditions of the site by observing soils within test pit
excavations, review well logs from existing wells near the project, and evaluate
geological maps depicting the site geology for the vicinity of the project;
• Perform soils testing, such as visual classifications, to determine selected index properties
of the soils;
• Complete an engineering assessment supported by planned site alterations and the surface
and subsurface conditions that were identified by the field investigation, soil testing, and
applicable project research; and,
• Establish engineering conclusions based on findings and anticipated project.
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Envirotech Engineering Geotechnical Report
PO Box 984 page 2 Parcel 995400031
Belfair, Washington 98528 Jefferson County, Washington
Ph. 360-275-9374 October 12, 2021
Vicinity Map from Washington Department of Natural Resources Website
Project
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Envirotech Engineering Geotechnical Report
PO Box 984 page 3 Parcel 995400031
Belfair, Washington 98528 Jefferson County, Washington
Ph. 360-275-9374 October 12, 2021
2.0 SURFACE CONDITIONS
Information pertaining to the existing surface conditions for the project was gathered on
September 17, 2021 by a representative with Envirotech. During the site visit, site features were
documented that may influence construction or reveal potential geological hazards. This Surface
Conditions Section provides information on general observations, vegetation, topography,
drainage and slope/ erosion conditions for the project and surrounding areas.
2.1 General Observations
The property is accessed from Camp Talbot Road, an existing paved roadway in unincorporated
Quilcene. The roadway is adjacent to the west end of the property. The project is vacant land with
evidence of past clearing and an existing driveway. Beyond the property, rural residential
development exists. The lot currently consists mostly of firs and other typical vegetation common
to this area.
2.2 Topography
The property, including areas of development, has varying natural slopes descending to the east.
The entire property has grades averaging 15% with isolated areas of less than 5% and more than
40%.
2.3 Surface Drainage
The natural drainage from the site appears to sheet flow towards the east. Indications of
significant scour or erosion from surface drainage were not observed on the property where
development is expected to occur.
2.4 Slope and Erosion Observations
The existing moderate slopes near the project signal a potential landslide or erosion hazard area.
Some indicators that may suggest past slope movements include:
• Outwash of sediments near the bottom of the slope,
• Fissures, tension cracks or naturally stepped land masses on the face or top of the slope,
and parallel to the slope,
• Fine, saturated subsurface soils,
• Old landslide debris,
• Significant bowing or leaning trees, or,
• Slope sloughing or calving.
The above-mentioned indicators, or other signs of significant mass wasting on the property or
within the general vicinity of the project were not observed or discovered during research.
Indications of past landslides, current unstable slopes, deep-seated slope problems, or surficial
slope failures were not observed during the site visit.
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Envirotech Engineering Geotechnical Report
PO Box 984 page 4 Parcel 995400031
Belfair, Washington 98528 Jefferson County, Washington
Ph. 360-275-9374 October 12, 2021
Aerial Photo from Jefferson County Website
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Envirotech Engineering Geotechnical Report
PO Box 984 page 5 Parcel 995400031
Belfair, Washington 98528 Jefferson County, Washington
Ph. 360-275-9374 October 12, 2021
3.0 SUBSURFACE INVESTIGATION
Information on subsurface conditions pertaining to the project was gathered during research and a
site reconnaissance. The site visit was accomplished on September 17, 2021 by a representative
with Envirotech. Specific information on field methods, sampling, field testing, subsurface
conditions, and results from soil testing are presented in this section of the report. Appendix B has
pertinent information on subsurface conditions for the project, including test pit log(s)
representative of the site soils, and applicable water well report(s) originating from the subject
property and/ or nearby properties.
3.1 Field Methods, Sampling and Field Testing
Information on subsurface conditions for the project was accomplished by probing anticipated
foundation areas with hand tools and observing soils within test pit excavations and/ or earth cuts.
Information on subsurface conditions also included reviewing geological maps within the project
vicinity, and water well reports originating from nearby properties. No soil samples were
collected for this project. Envirotech measured the relative density of the in-situ soils by gauging
the resistance of hand tools.
3.2 Geologic Conditions
In general, soils at the project are composed of materials from glacial advances. The geologic
conditions as presented in the “Geologic Map of Washington,” compiled by J. Eric Schuster,
2002 indicates Quaternary sediments, Qg. Quaternary sediments are generally unconsolidated
deposits, and dominantly deposited from glacial drift, including alluvium deposits. This project is
located within the Puget Lowland. Typically, “lower tertiary sedimentary rocks unconformably
overlie the Crescent Formation.” as revealed in the Geologic Map. Initial sedimentary rocks were
formed from shales, sandstones and coal deposits from rivers. During the Quaternary period, the
Puget Lowland was covered by numerous ice sheets, with the most recent being the Fraser glacier
with a peak of approximately 14,000 years ago. Upon the glacial retreat, the landscape was
formed by glacial erosion glacial drift deposits.
The “Lidar-Revised Geologic Map of the Uncas 7.5’ Quadrangle, Clallam and Jefferson
Counties, Washington” by Rowland W. Tabor, Peter J. Haeussler, Ralph A. Haugerud, and Ray
E. Wells, June 2011, provides the following caption(s) for the project area:
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Envirotech Engineering Geotechnical Report
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Belfair, Washington 98528 Jefferson County, Washington
Ph. 360-275-9374 October 12, 2021
Washington State Department of Natural Resources
3.3 Specific Subsurface Conditions
The following subsurface conditions are estimated descriptions of the project subgrade utilizing
information from the depth of penetration at all testing, sampling, observed and investigated
locations. Soils for this project were described utilizing the Unified Soil Classification System
(USCS). Using the USCS in conjunction with estimated relative densities and other anticipated
engineering properties of the soil, susceptibility for potential landslides, erosion and seismic
hazards may be assessed.
Soils within the upper 2 feet of natural ground were observed to be moist, brown silty sand with
gravel and cobbles (SM). Soils below the upper SM layer were observed to be .
According to the “Soil Survey of Jefferson County,” by the United States Department of
Agriculture, Soil Conservation Service, the site soils are described as Belfast silt loam, heavy
variant, Bh, and Carlsborg gravelly loamy sand, CaC, with 0 to 15 percent slopes.
Expanded and specific subsurface descriptions, other than what is provided in this section, are
provided in the soil logs located in Appendix B of this report.
Project
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Envirotech Engineering Geotechnical Report
PO Box 984 page 7 Parcel 995400031
Belfair, Washington 98528 Jefferson County, Washington
Ph. 360-275-9374 October 12, 2021
Soil Survey from USDA Natural Resources Conservation Service
3.3.1 Groundwater
From the water well report(s) and knowledge of the general area, permanent groundwater
is at least 20 feet directly below the property at the building pad location. Surface seepage
or perched groundwater at shallow depths was not observed on-site, nor indicated on the
well reports.
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Envirotech Engineering Geotechnical Report
PO Box 984 page 8 Parcel 995400031
Belfair, Washington 98528 Jefferson County, Washington
Ph. 360-275-9374 October 12, 2021
4.0 ENGINEERING CONCLUSIONS
The following sections present engineering assessments and conclusions concerning the project.
These conclusions have been made available based on the planned construction activities as
outlined in the Introduction Section of this report; general observations of drainage and
topography as summarized in the Surface Conditions Section; and soil conditions that were
identified by the field investigation and soils testing as outlined in the Subsurface Investigation
Section. Conclusions for the project that is provided herein, includes pertinent information for
landslide, erosion and seismic hazards.
4.1 Landslide Hazards
For the planned development, as provided in the Introduction Section, surface and subsurface
conditions as presented in this report, it is Envirotech’s opinion that the proposed development is
not subjected to or cause adverse impacts to a landslide hazard area or its associated buffer or
setbacks.
Landslides are natural geologic processes, and structures near slopes possess an inherent risk of
adverse settlement, sliding or structural damage due to these processes. Geotechnical engineering
cannot eliminate these risks for any site with sloping grades because gravity is constantly
inducing strain on the sloping soil mass. Excessive wet weather and/ or earthquakes will
exacerbate these strains. Geotechnical engineering considers excessive wet weather and ‘design’
earthquakes in order to provide an acceptable factor of safety for developing on or near sloping
terrain. These factors of safeties are based on engineering standards such as defining engineering
properties of the soil, topography, water conditions, seismic acceleration, and surcharges.
Surface sloughing or other types of surficial slope movements usually do not affect the deep-
seated structural capability of the slope. However, excessive and/or repeated surficial slope
movements, if not repaired, may represent a threat to the structural integrity of the slope.
Maintenance of the slope should be completed if the situation does arise in order to prevent the
possibility of further surficial or deep-seated slope movements that may be damaging to life or
property.
According to the Department of Natural Resources (DNR) Interactive mapping, previous
landslide activity is recorded on or within the influence of the project. Per the Resource Map from
DNR, the project is not within terrain labeled ‘highly unstable’ or ‘highly erodible’ relating to
soils. DNR labeled portions of this project as medium slope instability with relation to slopes.
This delineation is primarily dependent upon slopes and convergence. Secondly, lithology and
precipitation are modeled within this delineation. In summary, this designation is based on
mapping without field observations or knowledge of the specific site geology or soils. A resource
map from DNR is provided below:
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Envirotech Engineering Geotechnical Report
PO Box 984 page 9 Parcel 995400031
Belfair, Washington 98528 Jefferson County, Washington
Ph. 360-275-9374 October 12, 2021
Map from Washington State Department of Natural Resources Website
Past landslide activity or high slope instability indicators near the proposed development was not
revealed during the project research. In addition, detrimental landslide activity or potential high
landslide indicators were not observed during the site visit as outlined in the Surface and
Subsurface Conditions Sections of this report.
4.2 Erosion Hazards and Control
Based on the USCS description of the project soils, the surface soils are considered moderately
erodible. According to the Resource Map from the Washington State DNR, as provided above,
the project is not within terrain labeled ‘highly erodible.’
Temporary and/ or permanent erosion control measures are required for any site when land
disturbance is involved. Erosion control will mostly depend on the timeliness of construction,
moisture content of the soil, and amount of rainfall during construction. Soil erosion typical to the
existing conditions and planned disturbance of this project include wind-borne silts during dry
weather, and sediment transport during prolonged wet weather. Sediment transport of disturbed
soils could be from stormwater runoff or tracking off-site with construction equipment. Although
an engineered erosion control plan is not warranted for this project, Best Management Practices
(BMP’s) should be employed during and after construction. Ordinary BMP’s includes silt
fencing, protection of drainage outlets and vegetating denuded areas.
Project
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Envirotech Engineering Geotechnical Report
PO Box 984 page 10 Parcel 995400031
Belfair, Washington 98528 Jefferson County, Washington
Ph. 360-275-9374 October 12, 2021
Erosion control information and specifications may be found in the applicable “Stormwater
Management Manual for Western Washington,” prepared by the Washington State Department of
Ecology Water Quality Program and its equivalent local jurisdiction drainage manual.
4.3 Seismic Hazards
Soils immediately below the expected foundation depth for this project are generally Type D,
corresponding to the International Building Code (IBC) soil profiles. According to the IBC, the
regional seismic zone is 3 for this project. The estimated peak ground acceleration ranges from
0.50g to 0.60g. This estimation is based on the United States Geological Survey (USGS) National
Seismic Hazard project in which there is an estimated 2% probability of exceedance within the
next 50 years.
There are no known faults beneath this project. The nearest Class ‘A’ or ‘B’ fault to this property
is the Hood Canal Fault Zone. This fault is a Class ‘B’ and is located less than 4 miles to the east
of the project. This information is supported by the USGS Quaternary Fault and Fold Database
for the United States.
The potential for liquefaction and other earthquake induced hazards are believed to be low for
this project. This is based on subsurface conditions such as soil characteristics and the lack of a
permanent and substantial shallow water table. Subgrade characteristics that particularly
contribute to problems caused by seismic events include submerged and confined, poorly graded
granular soils. Although gravel- and silt-sized soil particles could be problematic, fine and
medium grained sands are typically subjected to these types of seismic hazards.
4.4 Slope Impacts
Considering the planned construction as summarized in the Introduction Section of this report;
the aforementioned surface and subsurface conditions for the project; the slope stability
assessment provided herein; and our engineering conclusions, it is our opinion that the project is
not within a landslide hazard area, and that the proposed site alterations will not encourage a
landslide hazard.
Additional methods and practices that avoid and/or reduce slope impacts include maintaining as
much vegetation on the property as practicable on slopes exceeding 30%, and following best
management practices for site development drainage.
4.5 Upland/ Slope Drainage Control and Groundwater Control
Positive drainage should be provided in the final design for all planned residential buildings.
Drainage shall include sloping the ground surface, driveways and sidewalks away from the
project structures. All constructed surface and subsurface drains should be adequately maintained
during the life of the structure. If drainage problems occur during or after construction, additional
engineered water mitigation will be required. This may include a combination of swales, berms,
drainpipes, infiltration facilities, or outlet protection in order to divert water away from the
structures to an appropriate protected discharge area.
This project does not appear to experience significant subsurface drainage or surface runoff. It is
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Envirotech Engineering Geotechnical Report
PO Box 984 page 11 Parcel 995400031
Belfair, Washington 98528 Jefferson County, Washington
Ph. 360-275-9374 October 12, 2021
our opinion that groundwater or surface drainage is not a limiting factor for this project. Drainage
control per County development standards should be sufficient for this project. Drainage facilities
located on or influencing slopes greater than 15% is acceptable for this project. If large
stormwater plans are triggered for this development, Envirotech should review prior to approval
in order to determine if proposed drainage facilities will affect slope stability.
4.6 Vegetation Management
As previously mentioned, maintaining as much vegetation on the property as practicable on
slopes exceeding 30% should be employed. However, it is our opinion that clearing will not be
problematic with relation to landslides for these areas because the vertical relief on 30% slopes on
the property is less than 10 feet. Any tree should removed if a threat to life and/ or property.
Denuded areas should be re-vegetated per standard practice.
Conclusions in this report are based on the type and location of the anticipated development, and
existing on-site and off-site conditions. Site development that significantly deviates from the
anticipated improvements presented in this report, or nearby development that influences this
project may require geotechnical design recommendations.
4.7 Septic System Impacts
Septic drainfields were considered in our geotechnical evaluation. This includes septic drainfields
with relation to the observed soil conditions, expected vegetation removal, and existing and
proposed topography. Based on the aforesaid parameters, the septic drainfields are not expected
to adversely influence critical slopes. This is also based on compliance with all recommendations
in this report.
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Envirotech Engineering Geotechnical Report
PO Box 984 page 12 Parcel 995400031
Belfair, Washington 98528 Jefferson County, Washington
Ph. 360-275-9374 October 12, 2021
5.0 CLOSURE
Based on the project information and site conditions as presented in this report, it is Envirotech’s
opinion that additional geotechnical studies are not required to further evaluate this project.
Due to the inherent natural variations of the soil stratification and the nature of the geotechnical
subsurface exploration, there is always a possibility that soil conditions encountered during
construction are different than those described in this report. Therefore, it is recommended that
either a qualified engineer observes and documents the construction, or Envirotech is promptly
notified if project and subsurface conditions found on-site are not as presented in this report so
that we can re-evaluate our recommendations.
This report presents a geological/ geotechnical assessment, and is intended only for the owner, or
owners’ representative. Furthermore, this report is only valid for the project information and
location described herein.
The services described in this report were prepared under the responsible charge of Michael
Staten, a professional engineer with Envirotech. Michael Staten has appropriate education and
experience in the field of geotechnical engineering in order to assess landslide hazards,
earthquake hazards, and general soil mechanics.
Please contact Michael Staten at 360-275-9374 if you have any questions, comments, or require
additional information.
Sincerely,
Envirotech Engineering
Jessica Smith, M.S. Michael Staten, P.E.
Staff Geologist Geotechnical Engineer
APPENDIX A
SITE PLAN
APPENDIX B
SOIL INFORMATION