HomeMy WebLinkAboutGEO2024-00092 - BLD Engineering / Geo-tech Reports - 8/23/2024 G 80q--000Pa
RECEIVED
PLANNING AUG 232024
615 W. Alder Street
Geotechnical Report
Call Retaining Wall
200 NE Raintree Lane, Tahuya
Parcel No. 22331-52-00035
Mason County,Washington
August 23, 2024
Project#24283
Prepared For:
Jeffrey Call
2407 80`' Street E
Tacoma, Washington 98404
Prepared By:
Envirotech Engineering, PLLC
PO Box 984
Belfair, Washington 98528
Phone: 360-275-9374
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MASON COUNTY Sl_.ibm tt l Checklist
COMMUNITY SERVICES Geotechnical Report
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Instructions:
This checklist must be submitted with a Geotechnical Report and completed,signed, and stamped by the licensed
professional(s)who prepared the Geotechnical Report for review by Mason County pursuant to the Mason County
Resource Ordinance. If an item is found not applicable,the report should explain the basis for the conclusion.
Note: Unless specifically documented, this report does not provide compliance to the International Residential
Code Sections R403.1.7 for foundations on or adjacent to slopes, Section R403.1.8 for expansive soils or section
1808.7.1 of the International Building Code Section for Foundations on or adjacent to slopes.
Applicant/Owner Jeffrey Call Parcel#22331-52-00035
Site Address 200 NE Raintree Lane, Tahuya
(1) (a) A discussion of general geologic conditions in the vicinity of the proposed development,
Located on page(s) 5
(b) A discussion of specific soil types,
Located on page(s) 6
(c) A discussion of ground water conditions,
Located on page(s) 7
(d) A discussion of the upslope geomorphology,
Located on page(s) 3
(e) A discussion of the location of upland waterbodies and wetlands,
Located on page(s) 3
(f) A discussion of history of landslide activity in the vicinity,as available in the referenced maps and
records.
Located on page(s) 8
(2) A site plan which identifies the important development and geologic features.
Located on Map(s) Geotechnical Site Plan—Appendix A
(3) Locations and logs of exploratory holes or probes.
Located on Map(s) Geotechnical Site Plan and Soil Logs(Appendix B)
(4) The area of the proposed development,the boundaries of the hazard,and associated buffers and
setbacks shall be delineated(top,both sides,and toe)on a geologic map of the site.
Located on Map(s) Geotechnical Site Plan
(5) A minimum of one cross section at a scale which adequately depicts the subsurface profile,and
which incorporates the details of proposed grade changes.
Located on Map(s) Soil Profile(Appendix B)
(6) A description and results of slope stability analyses performed for both static and seismic loading
conditions.Analysis should examine worst case failures. The analysis should include the Simplified
Bishop's Method of Circles.The minimum static safety factor is 1.5,the minimum seismic safety
factor is 1.1,and the quasi-static analysis coefficients should be a value of 0.15.
Located on page(s) 10 Page 1 of 35
(7) (a) Appropriate restrictions on placement of drainage features,
Located on page(s) 19
(b) Appropriate restrictions on placement of septic drain fields,
Located on page(s) 20
(c) Appropriate restrictions on placement of compacted fills and footings,
Located on page(s) 14 and 16
(d) Recommended buffers from the landslide hazard areas shoreline bluffs and the tops of other slopes.
Located on page(s) 19
(e) Recommended setbacks from the landslide hazard areas shoreline bluffs and the tops of other slopes.
Located on page(s) 18
(8) Recommendations for the preparation of a detailed clearing and grading plan which specifically identifies
vegetation to be removed,a schedule for vegetation removal and replanting,and the method of vegetation
removal.
Located on page(s) 19
(9) Recommendations for the preparation of a detailed temporary erosion control plan which identifies the specific
mitigating measures to be implemented during construction to protect the slope from erosion, landslides and
harmful construction methods.
Located on page(s) 10
(10) An analysis of both on-site and off-site impacts of the proposed development.
Located on page(s) 13
(11) Specifications of final development conditions such as,vegetative management,drainage,erosion control,and
buffer widths.
Located on page(s) 14-20
(12) Recommendations for the preparation of structural mitigation or details of other proposed mitigation.
Located on page(s) 20
(13) A site map drawn to scale showing the property boundaries,scale, north arrow,and the location and nature
of existing and proposed development on the site.
Located on Map(s) Geotechnical Site Plan
I,Michael Staten,hereby certify under penalty of perjury that I am a civil engineer licensed in the State of Washington
with specialized knowledge of geotechnical/geological engineering or a geologist or engineering geologist licensed in
the State of Washington with special knowledge of the local conditions. I also certify that the Geotechnical
b�'V CLYDZ ST Report,dated August 23, 2024 ,and entitled Call
Retaining Wall meets all the requirements of the
Mason County Resource Ordinance,Geologically Hazardous
43045 �, Areas Section, is complete and true,that the assessment
8/23/24 demonstrates conclusively that the risks posed by the landslide
hazard can be mitigated through the included geotechnical
Disclaimer:Mason County does not design recommendations,and that all hazards are mitigated in
certify the quality of the work done in such a manner as to prevent harm to property and public
this Geotechnical Report. health and safety.Page 2 of 2
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 TOPOGRAPHY.....................................................................................................................................3
2.1.1 Upslope Geomorphology............................................................................................................3
2.2 SURFACE DRAINAGE..........................................................................................................................3
2.2.1 Upland Water Bodies.................................................................................................................3
2.3 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.................................................................................................5
3.3.1 Groundwater............................................................................................................................... 7
4.0 ENGINEERING ANALYSES AND CONCLUSIONS......................................................................0
4.1 SLOPE STABILITY...............................................................................................................................0
4.1.1 Slope Stability Analysis..............................................................................................................2
4.2 EROSION.............................................................................................................................................2
4.3 SEISMIC CONSIDERATIONS AND LIQUEFACTION...............................................................................3
4.3.1 Liquefaction............................................................................................................................... 3
4.4 LANDSLIDE,EROSION AND SEISMIC HAZARDS CONCLUSIONS.........................................................3
4.5 RETAINING WALLS AND LATERAL EARTH PRESSURES....................................................................4
4.6 ON-SITE AND OFF-SITE IMPACTS......................................................................................................4
5.0 ENGINEERING RECOMMENDATIONS.........................................................................................6
5.1 BUILDING FOUNDATION RECOMMENDATIONS..................................................................................6
S.LI Bearing Capacity........................................................................................................................6
5.1.2 Settlement................................................................................................................................... 7
5.2 EARTHWORK CONSTRUCTION RECOMMENDATIONS........................................................................7
5.2.1 Excavation.................................................................................................................................. 7
5.2.2 Placement and Compaction of Native Soils and Engineered Fill............................................ 7
5.2.3 Retaining Wall Backfdl..............................................................................................................8
5.2.4 Wet Weather Considerations.....................................................................................................9
5.3 BUILDING AND FOOTING SETBACKS..................................................................................................9
5.4 SURFACE AND SUBSURFACE DRAINAGE............................................................................................9
5.5 VEGETATION BUFFER AND CONSIDERATIONS.................................................................................10
5.6 TEMPORARY AND PERMANENT EROSION CONTROL.......................................................................10
5.7 SEPTIC DRAINFIELDS........................................................................................................................10
5.8 STRUCTURAL MITIGATION...............................................................................................................10
6.0 CLOSURE.............................................................................................................................................11
Appendix A—Geotechnical Site Plan
Appendix B-Soil Information
Appendix C-Slope Stability
1.0 INTRODUCTION
Envirotech Engineering, PLLC. (Envirotech) has completed this geotechnical report for a
property located at 200 NE Raintree Lane, Tahuya, identified as parcel number 22331-
52-00035 in Mason County,Washington.
As presented herein, this report 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 soil descriptions in the Subsurface Investigation
Section; supporting documentation with relation to slope stability, erosion, seismic
considerations, and lateral earth pressures in the Engineering Analyses and Conclusions
Section; and, recommendations for foundation, settlement, earthwork construction,
retaining walls, erosion control, drainage, and vegetation in the Engineering
Recommendations Section.
An initial geotechnical evaluation of the project was conducted by Envirotech on August
23, 2024. It was determined that slopes in excess of 40% with a vertical relief of at least
10 feet were present within 300 feet of the planned development. Based on this site
characteristic, the proposed development will require a geotechnical report pursuant to
Landslide Hazard Areas of Mason County Resource Ordinance (MCRO) 8.52.140.
During the site visit by Envirotech, surface and subsurface conditions were assessed.
After completion of the field work and applicable project research, Envirotech prepared
this geotechnical report which,at a minimum,conforms to the applicable MCRO.
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 block retaining wall, earthwork, and other ancillary features typical of this
type of development . Approximate site development footprint with relation to existing
geological site features are illustrated in the Geological Site Plan in Appendix A of this
report.
1.2 Purpose of Investigation and Scope of Work
The purpose of this geotechnical investigation is to assess geological hazards, and
evaluate the project in order to provide geotechnical recommendations that should be
implemented during development. The investigation included characterizing the general
project surface and subsurface conditions, and evaluating the suitability of the soils to
support the planned site activities.
In order to fulfill the purpose of investigation, the geotechnical program completed for
the proposed improvements of the project include:
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PO Box 984 page 1 Parcel 22331-52-00035
Belfair,Washington 98528 Mason County,Washington
Ph. 360-275-9374 August 23, 2024
• Review project information provided by the project owner and/ or owner's
representative;
• Conduct a site visit to document the site conditions that may influence the
construction and performance of the proposed improvements of the project;
• Define general subsurface conditions of the site by observing subsoils within test
pits and/ or cut banks, review geological maps for the general area, research
published references concerning slope stability, and review water well reports
from existing wells near the project;
• Collect bulk samples, as applicable, at various depths and locations;
• Perform soils testing to determine selected index and/or engineering properties of
the site soils;
• Complete an engineering analysis supported by the planned site alterations, and
the surface and subsurface conditions that were identified by the field
investigation, soil testing, and applicable project research; and,
• Establish conclusions based on findings, and make recommendations for
foundations, drainage, slope stability, erosion control, earthwork construction
requirements, and other considerations.
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Vicinity Map from Mason County Website
Envirotech Engineering Geotechnical Report
PO Box 984 page 2 Parcel 22331-52-00035
Belfair,Washington 98528 Mason County,Washington
Ph. 360-275-9374 August 23,2024
2.0 SURFACE CONDITIONS
Information pertaining to the existing surface conditions for the project was gathered on
August 23, 2024 by a representative with Envirotech. During the site visit, site features
were documented that may reveal potential geological hazards.
This Surface Conditions Section provides information on topography, drainage and slope/
erosion conditions for the project and surrounding areas. An aerial photo of the project
and immediate vicinity is provided on the following page.
2.1 Topography
The topographic information provided in this section was extrapolated from a public lidar
source, and incorporated observations and field measurements. Where necessary, slope
verification included measuring slope lengths and inclinations with a cloth tape and
inclinometer. See the Geological Site Plan in Appendix A for an illustration of the
general slope indicators with respect to the planned development.
Maximum sloping grades within 300 feet of the proposed development area with a
vertical relief in excess of at least 10 feet were observed to be 40%.
2.1.1 Upslope Geomorphology
The landform that is upland from property is considered a hillside of glacial origin
with centuries of weathering overburden. Additional geomorphology that is
pertinent to both upslope and downslope areas are provided in the Subsurface
Investigation Section of this report.
2.2 Surface Drainage
The majority of the stormwater runoff originating upslope from the anticipated
development is expected to be minimal to moderate . Significant scour, erosion and
sediment transport was not apparent near the project.
2.2.1 Upland Water Bodies
There is not an upstream water body that would influence the project.
2.3 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,
Envirotech Engineering Geotechnical Report
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Belfair,Washington 98528 Mason County,Washington
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• 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.
These slope instability indicators or other significant mass wasting on the property or
within the general vicinity of the project were not observed. Indications of past
landslides, current unstable slopes, deep-seated slope problems, or surficial slope failures
were not observed during the site visit.
Aerial Photo from Mason County Website
Envirotech Engineering Geotechnical Report
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Belfair,Washington 98528 Mason County,Washington
Ph. 360-275-9374 August 23,2024
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 August 23, 2024
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. See Appendix B for a soil profile cross-sections, soil logs, and other
appropriate information as necessary.
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. More specifically for the project location, a caption
from the Washington State Department of Natural Resources is as follows:
Map Unit: Qgt
Age: Quaternary
GeoMaterial: Glacial Till
Description: Sand and gravel, silt, deposited during the advance of glaciers; sandy units
commonly thick, well sorted,and fine grained, with interlayered coarser sand, gravel, and
cobbles; locally contains nonglacial sediments and deposits mapped as transitional
between glacial and nonglacial.
3.3 Specific Subsurface Conditions
The following subsurface conditions are estimated descriptions of the project subgrade
Envirotech Engineering Geotechnical Report
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Belfair,Washington 98528 Mason County,Washington
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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.
The project is primarily composed of undisturbed, native soils, without indications of
substantial fill. Within native, undisturbed areas, competent bearing soils were
encountered at 12 inches below the existing ground surface,in locations where the ground
was probed. For engineering purposes, these native soils consist of distinguishable layers,
as presented below.
Soils within the upper 4 feet of natural ground were observed to be primarily moist,
brown silty sand with gravel (SM) . Soils below this upper soil strata were observed to
be the same glacial outwash material. The outwash is believed to extend to depths of at
least 50 feet. This is based on nearby well reports, site geology, and/or knowledge of the
general area.
Not withstanding the potential for any old fill, the relative densities are suitable for
foundations as provided above.Expanded and specific subsurface descriptions, other than
what is provided in this section, are provided within additional soil information located in
Appendix B of this report.
According to the "Soil Survey of Mason County," by the United States Department of
Agriculture, Soil Conservation Service (SCS), the site soils are described as Alderwood
gravelly sandy loam with 8%- 15% slopes. See the soil map below.
Envirotech Engineering Geotechnical Report
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Belfair,Washington 98528 Mason County,Washington
Ph. 360-275-9374 August 23, 2024
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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 50 feet below the current ground surface. Surface seepage
or perched groundwater at shallow depths was not observed on-site, nor indicated
on the well reports.
Envirotech Engineering Geotechnical Report
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Belfair,Washington 98528 Mason County,Washington
Ph. 360-275-9374 August 23,2024
4.0 ENGINEERING ANALYSES AND CONCLUSIONS
The following section includes slope stability, erosion, seismic considerations, and
impacts to both on-site and off-site properties.
4.1 Slope Stability
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 with relation to
current engineering protocol. 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, repeated surficial slope movements, if not repaired, may present a threat to the
structural integrity of the slope. If any slope movement arises, the slope should be
inspected by an engineer. Subsequently, maintenance may be required in order to prevent
the possibility of further surficial or deep seated slope movements that may be damaging
to life and property.
According to the Coastal Zone Atlas of Mason County, Washington, the project is
located outside of its mapping limits.
within and near terrain labeled `Stable' and `Unstable' regarding potential landslide
activity. Descriptions of these mapping units may be found in the aforesaid Atlas. A
Stability Map from the Coastal Zone Atlas for the general area of this project is provided
below:
According to the Resource Map from the Washington State Department of Natural
Resources (DNR), the project is not within terrain labeled `highly unstable' relating to
soils. DNR labeled portions of this project as medium and high slope instability with
relation to slopes. A Resource Map from the DNR Forest Practices Application Review
System is provided below:
Project
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Resource Map from Washington State Department of Natural Resources Website
WADNR PUBLIC FP Soils
Soils-Highly Erodible
......... ...............
WADNR PUBLIC FP Soils
Soils-Highly Unstable
WADNR PUBLIC FP Soils
Sc,i!s-Hydrlc
Envirotech Engineering Geotechnical Report
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Belfair,Washington 98528 Mason County,Washington
Ph. 360-275-9374 August 23, 2024
4.1.1 Slope Stability Analysis
The Simplified Bishop Method, utilizing `STABLE' software, was used to
analyze the static stability of the site slopes. Seismic conditions were estimated
utilizing worst case scenario values from the static analysis, a quasi-static analysis
coefficient of at least 0.15, and applying the applicable values to STABLE
software. Various radii's and center points of the circle were automatically
selected, and produced factor of safeties in a graphical and tabular format. Worst
case scenario values were used in the slope stability analysis in regards to
topography, surcharges, water content, internal friction and cohesion of the site
soils. STABLE software has been repeatedly checked with manual calculations,
and consistently proved to be a very conservative program. The following soil
properties were used in the analysis, and are based on observed conditions,known
geology, and/or published parameters:
Soil unit weight: 140 pcf
Angle of internal friction: 40 degrees
Cohesion: 400 psf
Based on the slope stability analysis, unacceptable factors of safety could be
present on and near the critical slope, but do not reflect conditions where
development is expected to occur. For this project, at the location of the proposed
development, minimum factor of safeties for static and dynamic conditions were
estimated to be at least 1.5 and 1.1, respectively. See the slope stability
information in Appendix C for a depiction of minimum factors of safety away
from the project.
4.2 Erosion
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.' This project is
not within an erosion hazard area as defined by the MCRO. Erosion hazard areas are
those with USDA SCS designations of River Wash (Ra), Coastal Beaches (Cg),
Alderwood Gravelly Sandy Loam on slopes 15% or greater(Ac and Ad), Cloquallum Silt
Loam on slopes 15% or greater (Cd), Harstine Gravelly Sandy Loam on slopes 15% or
greater(Hb), and Kitsap Silt Loam on slopes 15%or greater(Kc).
It is our opinion that minor erosion control recommendations provided in this report is
sufficient for the development of this project, and additional engineered erosion control
plans are not required. Temporary and permanent erosion control measures are required
for site development. Extents of temporary 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 site conditions and planned disturbance
Envirotech Engineering Geotechnical Report
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Belfair,Washington 98528 Mason County,Washington
Ph. 360-275-9374 August 23, 2024
of the project include wind-borne silts during dry weather, and sediment transport during
prolonged wet weather. Sediment transport could be from stormwater runoff or tracking
off-site with construction equipment.
The Temporary and Permanent Erosion Control Section (Section 5.6) of this report
consist of specific erosion controls to be implemented. Additional erosion control
information and specifications may be found in the latest addition of the "Stormwater
Management Manual for Western Washington," prepared by the Washington State
Department of Ecology Water Quality Program.
4.3 Seismic Considerations and Liquefaction
There are no known faults beneath this project. The nearest Class `A' or 'B' fault to this
property is the Tacoma Fault Zone , which is not located on the property or adjacent
properties. This information is supported by the USGS Quaternary Fault and Fold
Database for the United States.
Potential landslides due to seismic hazards have been considered, and are addressed in
the Slope Stability Analysis Section provided earlier in this report.
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.
4.3.1 Liquefaction
The potential for liquefaction is believed to be low for this project. This is based,
in part, on the subsurface conditions such as soil characteristics and the lack of a
permanent shallow water table. Subgrade characteristics that particularly
contribute to problems caused from liquefaction include submerged, confined,
poorly-graded granular soils (i.e. gravel, sand, silt). Although gravel- and silt-
sized soil particles could be problematic, fine and medium grained sands are
typically subjected to these types of seismic hazards. No significant saturated sand
stratifications are anticipated to be within the upper 50 feet of the subsoil for this
project.
4.4 Landslide,Erosion and Seismic Hazards Conclusions
DNR did not indicate historic landslide activity near the project. Mapped slope
conditions, as delineated by the Departments of Ecology and/or Natural Resources, were
considered in our slope stability assessment. Based on the proximity and severity of
Envirotech Engineering Geotechnical Report
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Belfair,Washington 98528 Mason County,Washington
Ph. 360-275-9374 August 23,2024
mapped delineations with respect to the proposed development, results of the aforesaid
slope stability analysis, observed surface conditions, and other pertinent information, it is
our opinion that the proposed development may occur in accordance with the
recommendations in this geotechnical report.
4.5 Retaining Walls and Lateral Earth Pressures
Envirotech observed the project site after the excavation was underway and the retaining
wall system partially constructed.
The lateral earth pressures exerted through the backfill of a retaining wall are dependent
upon several factors including height of retained soil behind the wall, type of soil that is
retained, degree of backfill compaction, slope of backfill, surcharges, hydrostatic
pressures, earthquake pressures, and the direction and distance that the top of the wall
moves.
An equivalent fluid unit weight used for structural design may be estimated as the
product of the backfill soil unit weight and the earth pressure coefficient for at-rest
pressures. Retaining walls should be designed to resist a lateral earth pressure based on
an equivalent fluid unit weight of the following:
At-Rest Active
Native Soils 46 pcf 26 pcf
Engineered Fill Soils 42 pcf 24 pcf
The values provided above shall be increased by 1 pcf for every 1 degree of backfill/
natural slope angle. These equivalent fluid unit weight values do not include lateral earth
pressures induced by earthquakes, groundwater, or surcharges from live loads.
Earthquake pressures should be added to the wall analysis as determined by the structural
engineer per standard practice utilizing the design earthquake. See the Earthwork
Construction Recommendations Section for details concerning the use of native soils,
engineered fill and placement of backfill.
4.6 On-Site and Off-Site Impacts
From a geotechnical position, it is Envirotech's opinion that the subject property and
adjacent properties to the proposed development should not be significantly impacted if
all recommendations in this report are followed. This opinion is based on the expected
site development, existing topography, existing nearby development, land cover, and
adhering to the recommendations presented in this report. Future development or land
disturbing activities on neighboring properties or properties beyond adjacent parcels that
Envirotech Engineering Geotechnical Report
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Ph. 360-275-9374 August 23, 2024
are upslope and/or downslope from the subject property could cause problems to the
subject property. For this reason, future development or land disturbance near the subject
property should be evaluated by a geotechnical engineer.
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5.0 ENGINEERING RECOMMENDATIONS
The following sections present engineering recommendations for the proposed
improvements of the project. These recommendations have been made available based on
the planned improvements as outlined in the Introduction Section of this report; general
observations including drainage and topography as recapitulated in the Surface
Conditions Section; soil/ geologic conditions that were identified from the geotechnical
investigation that is summarized in the Subsurface Investigation Section; and, project
research, analyses and conclusions as determined in the Engineering Analysis and
Conclusions Section. Recommendations for the project that is provided herein, includes
pertinent information for building foundations, earthwork construction, building and/ or
footing setbacks, drainage,vegetation considerations, and erosion control.
5.1 Building Foundation Recommendations
The recommended allowable bearing capacities and settlements as presented below,
consider the probable type of construction as well as the field investigation results by
implementing practical engineering judgment within published engineering standards.
Evaluations include classifying site soils based on observed field conditions and soil
testing for this project. After deriving conservative relative densities, unit weights and
angles of internal friction of the in-situ soils, the Terzhagi ultimate bearing capacity
equation was utilized for determining foundation width and depth. Foundation parameters
provided herein account for typical structural pressures due to the planned type of
development. A structural analysis is beyond the scope of a geotechnical report, and a
structural engineer may be required to design specific foundations and other structural
elements based on the soil investigation. Stepped foundations are acceptable, if warranted
for this project.
5.1.1 Bearing Capacity
Existing in-situ soils for this project indicates that the structure can be established
on any foundation type deemed necessary by the structural engineer.
Alternatively, foundations may be constructed on selective re-compacted native
soil or compacted engineered fill as described in the Earthwork Construction
Recommendations Section of this report.
For a bearing capacity requirement of no more than 2000 psf,foundations shall be
placed at a minimum of 12 inches below the natural ground surface atop
unyielding soils and away from any potential old fill or disturbed soils.
Foundation recommendations are made available based on adherence to the
remaining recommendations that are provided in this report.
Envirotech Engineering Geotechnical Report
PO Box 984 page 6 Parcel 22331-52-00035
Belfair,Washington 98528 Mason County,Washington
Ph. 360-275-9374 August 23, 2024
Alterations to the aforementioned foundation recommendations may be completed
upon a site inspection by a geotechnical engineer after the foundation excavation
is completed.
5.1.2 Settlement
Total and differential settlement that a structure will undergo depends primarily
on the subsurface conditions, type of structure, amount and duration of pressure
exerted by the structure, reduction of pore water pressure, and in some instances,
the infiltration of free moisture.
Based on the expected native soil conditions, anticipated development, and
construction abides by the recommendations in this report, the assumed
foundation system may undergo a maximum of 1.0 inch total settlement, and a
maximum differential settlement of 0.75 inch per 50 linear feet.
5.2 Earthwork Construction Recommendations
Founding material for building foundations shall consist of undisturbed native soils to the
specified foundation depths. Compacted engineered fill, or selective re-compacted native
soils may be used to the extents provided in this Earthwork Construction
Recommendations Section. The following recommendations include excavations,
subgrade preparation,type of fill, and placement of fill for building foundations.
5.2.1 Excavation
Excavation is recommended to remove any excessive organic content, old fills/
disturbed soils or other deleterious material, if present,beneath foundations and to
achieve appropriate foundation depth. Additional sub-excavation will be required
for this project if the soils below the required foundation depth are loose,
saturated, not as described in this report, or otherwise incompetent due to
inappropriate land disturbing, or excessive water trapped within foundation
excavations prior to foundation construction.
All soils below the bottom of the excavation shall be competent, and relatively
undisturbed or properly compacted fill. If these soils are disturbed or deemed
incompetent, re-compaction of these soils below the anticipated footing depth is
necessary. Excavations shall be completely dewatered, compacted, and suitable
before placement of additional native soil,engineered fill or structural concrete.
5.2.2 Placement and Compaction of Native Soils and Engineered Fill
For engineered fill or disturbed native soils that will be utilized as fill material
directly beneath foundations, observation and/ or geotechnical testing is required
Envirotech Engineering Geotechnical Report
PO Box 984 page 7 Parcel 22331-52-00035
Belfair,Washington 98528 Mason County,Washington
Ph. 360-275-9374 August 23, 2024
prior to foundation construction. The following placement and compaction
requirements are necessary.
For disturbed native soils or engineered fill beneath foundations, limits of
compacted or re-compacted fill shall extend laterally from the bottom edge of the
foundation at a rate of one horizontal foot for each foot of compacted or re-
compacted fill depth beneath the foundation.
Both engineered fill and native soils used as compacted fill should be free of roots
and other organics, rocks over 6 inches in size, or any other deleterious matter.
Because of moisture sensitivity, importing and compacting engineered fill may be
more economical than compacting disturbed native soils. Engineered fill shall
include having the soils retained on the No. 4 sieve crushed (angular), and should
consist of the following gradation:
U.S. Standard Sieve % Finer(by weight)
6" 100
3" 60— 100
No.4 20—60
No. 200 0- 8
Table 1
Particle Size Distribution of Engineered Fill
Compaction shall be achieved in compacted lifts not to exceed 6 inches for both
native soils and engineered fill, respectively. Each lift should be uniformly
compacted to at least 95% of the modified Proctor maximum dry density (ASTM
D 1557) and within 3% of optimum moisture content. Each lift surface should be
adequately maintained during construction in order to achieve acceptable
compaction and inter-lift bonding.
Temporary earth cuts and fill slopes exceeding 4 feet in height should be limited
to a slope of 1:1 (horizontal:vertical). Utility trenches or other confined
excavations exceeding 4 feet should conform to OSHA safety regulations.
Permanent cut and fill slopes shall be limited to a slope of 2:1, unless otherwise
approved by an engineer.
5.2.3 Retaining Wall Backfill
Native soils may be used as retaining wall backfill for this project if the total wall
height is 4 feet or less and the recommendations below are followed. Native soils
for retaining walls exceeding 4 feet in height must be approved by the local
authority or evaluated by an engineer.
Envirotech Engineering Geotechnical Report
PO Box 984 page 8 Parcel 22331-52-00035
Belfair,Washington 98528 Mason County,Washington
Ph. 360-275-9374 August 23, 2024
Backfill may consist of engineered fill, as presented in this report, or borrow
material approved by a geotechnical engineer. Compaction of these materials
shall be achieved in compacted lifts of about 12 inches. Each lift should be
uniformly compacted to at least 85%, and no more than 90% of the modified
Proctor maximum dry density(ASTM D 1557).
If pavement or building loads are planned to be located within retaining wall
backfill, then 90% compaction is required. In addition, heavy construction
equipment should be at a distance of at least '/2 the wall height. Over-compaction
and limiting heavy construction equipment should be prevented to minimize the
risk of excess lateral earth pressure on the retaining structure.
Envirotech recommends that retaining wall backfill is compacted with light
equipment such as a hand-held power tamper. If clean, coarse gravel soils are
utilized as engineered fill, and surcharges will not influence the retaining wall,
compaction may be achieved by reasonably densifying granular soils with
construction equipment.
5.2.4 Wet Weather Considerations
Due to the types of subsurface soils, additional provisions may be required during
prolonged wet weather. Every precaution should be made in order to prevent free
moisture from saturating the soils within excavations. If the bottom of excavations
used for footing placement changes from a moist and dense/hard characteristic as
presented in this report to muck or soft, saturated conditions, then these soils
become unsuitable for foundation bearing material. If this situation occurs, a
geotechnical engineer should be notified, and these soils should be completely
removed and replaced with compacted engineered fill or suitable native material
as presented in this section.
5.3 Building and Footing Setbacks
Provided that assumptions relating to construction occur and recommendations are
followed as presented in this report, the factor of safety for slope stability is sufficient for
this project, and setbacks are not needed.
5.4 Surface and Subsurface Drainage
Positive drainage should be provided in the final design for all planned structures.
Retaining wall backfill drainage shall be determined by the structural engineer as
necessary.
Envirotech Engineering Geotechnical Report
PO Box 984 page 9 Parcel 22331-52-00035
Belfair,Washington 98528 Mason County,Washington
Ph. 360-275-9374 August 23,2024
If impervious thresholds are exceeded per the prevailing agency code, then engineered
stormwater management plans are required for this project. The drainage engineer must
coordinate with a geotechnical engineer for input with relation to slope stability prior to
submitting drainage plans. If stormwater management plans are not required for this
project,then the following recommendations should be followed.
5.5 Vegetation Buffer and Considerations
For this project, we believe that a detailed clearing and grading plan is not warranted
unless the prevailing agency thresholds are exceeded, and basic vegetation management
practices should be adhered to.
Vegetation Buffer — Vegetation is not restricted on this property with regards to
geological hazards.
5.6 Temporary and Permanent Erosion Control
Erosion control during construction should include minimizing the removal of vegetation
to the least extent possible. Erosion control measures during construction may include
stockpiling cleared vegetation, silt fencing, intercepting swales, berms, straw bales,
plastic cover or other standard controls. Although other controls may be used, if
adequate, silt fencing is presented in this report as the first choice for temporary erosion
control. Any erosion control should be located down-slope and beyond the limits of
construction and clearing of vegetation where surface water is expected to flow. If the
loss of sediments appears to be greater than expected, or erosion control measures are not
functioning as needed, additional measures must be implemented immediately.
Permanent erosion control is necessary if substantial vegetation has not been established
within disturbed areas upon completion of the project. Temporary erosion control should
remain in place until permanent erosion control has been established. Permanent erosion
control may include promoting the growth of vegetation within the exposed areas by
mulching, seeding or an equivalent measure. Additional erosion control measures that
should be performed include routine maintenance and replacement, when necessary, of
permanent erosion control, vegetation, drainage structures and/or features.
5.7 Septic Drainfields
Septic drainfields are not an element of this project.
5.8 Structural Mitigation
With respect to landslide alleviation or slope improvements, structural mitigation is
necessary as a structurally engineered retaining wall, which is the subject of this report.
See the Retaining Wall Section of this report for geotechnical parameters.
Envirotech Engineering Geotechnical Report
PO Box 984 page 10 Parcel 22331-52-00035
Belfair,Washington 98528 Mason County,Washington
Ph. 360-275-9374 August 23, 2024
6.0 CLOSURE
Based on the project information provided by the owner, the proposed development, 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. It is not
recommended that a qualified engineer performs a site inspection during earthwork
construction unless fill soils will influence the impending foundation. However, if native,
undisturbed subsurface conditions found on-site are not as presented in this report, then a
geotechnical engineer should be consulted.
This report presents geotechnical design guidelines, and is intended only for the owner,
or owners' representative, and location of project described herein. This report should not
be used to dictate construction procedures or relieve the contractor of his responsibility.
Any and all content of this geotechnical report is only valid in conjunction with the
compliance of all recommendations provided in this report. Semantics throughout this
report such as `shall,' `should' and `recommended' imply that the correlating design
and/or specifications must be adhered to in order to potentially protect life and/ or
property. Semantics such as `suggested' or `optional' refer that the associated design or
specification may or may not be performed, but is provided for optimal performance. The
recommendations provided in this report are valid for the proposed development at the
issuance date of this report. Changes to the site other than the expected development,
changes to neighboring properties, changes to ordinances or regulatory codes, or
broadening of accepted geotechnical standards may affect the long-term conclusions and
recommendations of this report.
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
_Cz �,,
Michael Staten,P.E.
Geotechnical Engineer
Envirotech Engineering Geotechnical Report
PO Box 984 page 11 Parcel 22331-52-00035
Belfair,Washington 98528 Mason County,Washington
Ph. 360-275-9374 August 23, 2024
APPENDIX A
GEOTECHNICAL SITE PLAN
SCALL I IN:N W FEET
COLLINS LAKE
v
:a
'ROPERTY LINE
CRIT€CAL SI IPES (40%+) TPI
or PP2pPERTY. SETBACKS
OR BLFF£RS ARE NOT
NEEDED AS SLOPES
DEEMED STABLE.
ENTIRE PRaPERTY IS
BUILDABLE FROM A
�' GEOTECHNICAL
E PERSPECTIVE, WITH NO
! SETBACKS OR BUFFERS
`a
E �` � LN
I MN
L NTDM&S RF E NOT PREPARED RY A LICENSER LAB SPCE.AI
CONTTJ OW TEll r El17RMA%KNMATE3 FRR�A i`,.AI E'D IN-HE GEI AND
RE.M P6iA7ED F7E�.0.NE'.ASIIKXFxTE AS ESPLA7F G€N'iit Fa£QfEC+M:CAi..
P-DM;
Cr➢LTD E rE*S.WERE VIIT PREPARED 1T A.SLOKLICENSED TEIF OF S LO<llrT[ E
C E77E FEA";AES TYA1`ARE SiiDMN NCRE.SUCH K TDP OF SlII'CS..T� LCC�t'N
CF SL.00P`S,WATER FEATURE$.ETC..,WITH REI,ArM M THE PRO-ERTY .....�_,_..._,�..,..,__,........_ _
LINES mLst w WERO'DED IT THE DW+ER 1t[CnIB.XYUtS®/d IN THE ENuI Er"b
GEOTEENNIZAL REPORT PROWL SEINACKS,XFFERi.DE?THS.ETC-WITH ENVIR137TCX.ENGINEERING
RLLA-IM M GEOLOGIC FEAT'W".NOT PRO+LRTY LIW-'&THESE GEOLD7C PO DOS 9R+
FEATURES MY K LOCATED ON ttf SUIIFCT PROPFRTY OR NEI'J!HCR N'.r _-� $LOVE MtATOR
PRGPERIIE1 BELFAIR.WASH•.N57DN 9852N
tXKTING:rxNTOLit
T'PI TEST IT SITE PLAN
APPENDIX B
SOIL INFORMATION
VEQITf WN HDRt2IL UAI
G NCH +M1
RETAINING WALL WITH
ASSOCIATED CUT AND
FILL FOR LAND USE
MEDIUM DENSE SILTY SAND
WITH GRAVEL (SM)
COLLINS LAKE
DENSE GLACIAL TILL
SECTION A-A
NOTES-
0153MEM
D HEWN�jRADE EHANTi IE REWIRED IN EHIDEB 10 ACHIEVE EMVmOTECH ENGINEERING
POSITIVE ERAD\AuE PO BOX 964
eI THE SOL PROFILE n ACCLRATE FOR THE DEPTH EF SELTAIN,VASHEWWON 90WO
THE OBSERVED TEST PITS AT THE SPECIFIED LOCATIONS. 3ED-2)5-4571
LOVER DEPTHS ARE BASED 014 SITE 0mLMY.
YELL 1.06M,AND/OR DYERDOLE IN THE UEHAL AREA SOIL PROFILE
TEST PIT LOG
(TP-1)
Date of Log: August 23, 2024
Location: 200 NE Raintree Lane, Tahuya
Logged By: RJM
Depth USCS Description
0—4ft SM Silty Sand with Gravel.
Medium brown and moist
Medium dense to dense.
Gravel is primarily coarse and subrounded
Sand is mostly medium
No plasticity
Cemented hardpan at 4 ft
APPENDIX C
SLOPE STABILITY
. Z
2 +L
1 . 4 0
2 _ 5 0
-70
S SO
a 0 O
14`}q
t
Slope Stability: Dynamic
0
b
CD
� I
w
; c
1 1P
'
F+ M Ij N
0 0 tD J Gt qA GY N h' 0
0 0 0 0 00 O O O O