HomeMy WebLinkAboutBLD2022-00363-09A-Geotech ReportGeological Hazard Assessment Geotechnical Report
August , 2022
Braverman Residence Renovation Project
Jefferson County Parcel # 801121001
552 Embody Road
Port Ludlow, Washington 98365
Prepared for:
Porchlight Woodworks, LLC
Prepared by:
Trent Adams, P.E.
Geotechnical Engineer
Geoview Engineering
110 W. 6
th Avenue, Unit 232
Ellensburg, Washington 98926
Phone: (360) 643-8533
Email:trent.adams@geovieweng.com
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TABLE OF CONTENTS
Page
OVERVIEW ................................................................................................................3
SITE BACKGROUND .................................................................................................3
Topography.....................................................................................................3
Septic System .................................................................................................3
Potable Water..................................................................................................4
Wetland...........................................................................................................4
Foundation and Structural Plans......................................................................4
Surface Hydrology...........................................................................................4
THE GEOLOGIC SETTING ........................................................................................4
Site Geology....................................................................................................4
Jefferson County Map of Geologically Hazardous Areas.................................4
Soil Types on the Property ..............................................................................5
Well Log Review..............................................................................................5
Field Observations and Shallow Subsurface Investigation...............................5
SEISMIC HAZARD ASSESSMENT ............................................................................5
Earthquake Types ...........................................................................................5
Earthquake Sources, Fault Locations, and Ground Shaking............................6
Ground Shaking Potential................................................................................6
Liquefaction Potential......................................................................................7
Lateral Spread Potential..................................................................................8
Seismic Site Class...........................................................................................8
GEOTECHNICAL CONCLUSIONS.............................................................................8
Seismic Hazard at the Property.......................................................................8
Development Feasibility from a Seismic Perspective.......................................9
GEOTECHNICAL RECOMMENDATIONS..................................................................9
Foundation Upgrade........................................................................................9
Subgrade Preparation for the Strip Footing ...................................................10
Wet Weather Precautions..............................................................................10
Stormwater Management ..............................................................................10
SUGGESTIONS FOR ADDITIONAL SEISMIC PROTECTION .................................10
THE LIMITATIONS OF THIS REPORT.....................................................................11
BIBLIOGRAPHY.......................................................................................................12
APPENDIXES...........................................................................................................27
Appendix A Septic System Map................................................................28
Appendix B Well Log.................................................................................30
Appendix C Wetland Map .........................................................................31
Appendix D Structural Plans .....................................................................32
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LIST OF TABLES
Table Page
1 Soil description of hand auger hole #1.....................................................13
2 Soil description of hand auger hole #2.....................................................13
3 Summary of earthquake fault scenarios at the subject property...............13
LIST OF FIGURES
Figure Page
1 Project location map................................................................................14
2 Slope map of the topographic situation....................................................14
3 Location of streams in the project vicinity.................................................15
4 Map of local geology in the project vicinity................................................15
5 Seismic hazard area map........................................................................16
6 USDA map of soil types in the project vicinity...........................................16
7 Hand auger location map.........................................................................17
8 Earthquake sources and fault map ..........................................................18
9 Ground shaking scenario for Southern Whidbey Island fault....................19
10 Ground shaking scenario for a full length rupture of the CSZ...................19
11 Ground shaking scenario for a northern rupture of the CSZ.....................20
12 Ground shaking scenario for the Seattle fault ..........................................20
13 Ground shaking scenario for the Canyon River-Saddle Mountain fault
zone ........................................................................................................21
14 Ground shaking scenario for the western section of the Darrington-
Devils Mountain fault zone.......................................................................21
15 Ground shaking scenario for the Nisqually fault.......................................22
16 Ground shaking scenario for the Seatac fault ..........................................22
17 Ground shaking scenario for a full length rupture of the Darrington-
Devils Mountain fault zone.......................................................................23
18 Ground shaking scenario for the Lake Creek-Boundary Creek fault zone 23
19 Ground shaking scenario for the Tacoma Fault .......................................24
20 Liquefaction susceptibility in the project vicinity .......................................24
21 Preliminary diagram of proposed continuous strip footing........................25
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OVERVIEW
SITE BACKGROUND
The existing partially constructed residence was constructed circa 2018 in the southwest corner
of the lot (Figure 1). Other existing structures on the property include barns, yurts, outbuildings,
a greenhouse, and fenced pastures.
Topography
The topographic situation in the immediate project vicinity can be seen in a slope map derived
from high-resolution LIDAR acquired in Figure 2. The 9.9-acre parcel has a gentle slope to the
northeast with gradients between 1% to 6%. In the southwest corner of the parcel, the ground
elevation is about 180 feet above sea level. In the northeast corner of the parcel, the ground
elevation is about 165 feet above sea level. Contours show a fan-like shape pointing east in the
vicinity of the property, with the residence being located on the northern flank.
Septic System
A 2-bedroom, mound-type septic system designed by Nathan Cleaver Septic Design, Inc.
(SEP2018-00011), was installed in 2020 on the property about 20 feet northwest of the
residence site (See Appendix A). Six soil logs are included with the septic system plans that
indicate medium to coarse sand with lenses of gravel is generally present to depths of 4 feet
below the ground surface in the vicinity of the septic system. Signs of mottling representing a
seasonal water table was commonly observed at around 4 feet in depth.
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Potable Water
The well serving water to the property is located within the southerly addition to the existing
barn. Insight into deep subsurface conditions is provided by the well log for the well (Well ID
ALS006) (Appendix B). Mixtures of sand are present to 63 feet below the ground surface.
According to the well driller, glacial till underlies the sandy layers to an unknown depth and the
static water level was 4 feet below the ground surface at the time of drilling in late November /
early December 2005.
Wetland
A wetland exists in the northern third of the property that is documented in an April 2019
Wetland Delineation and Habitat Management Plan prepared by Olympic Wetland Resources,
LLC (See Appendix C). Olympic Wetland Resources observed hydric soil near the middle of the
east property line boundary but found sandy soil in the middle and in the southern parts of the
property.
Foundation and Structural Plans
The home is currently supported by a stem wall that wraps around the perimeter of the structure
and by isolated footings located beneath the center of the house. Structural plans dated June
20, 2022 for the project, prepared by Pacific Northwest Structural Group, Inc., show that three
new isolated footings are proposed to be constructed in-between the existing isolated footings
(See Appendix D).
Surface Hydrology
The nearest stream to the residence is located nearly 200 feet southwest of the property.
3, fish-bearing stream. The second closest stream to the residence is actually located roughly
300 feet to the east, flowing south and parallel to the eastern property line.
A curtain drain is located on the property in the southwest corner, upslope of the residence and
wastewater drainfield. The curtain drain daylights about 20 feet southeast of the eastern building
line to the house. The location of the curtain drain is best seen in SEP2018-00011 in Appendix
A.
THE GEOLOGIC SETTING
Site Geology
According to the WA Department of Natural Resources (DNR) geologic map of the Port Ludlow
and southern half of the Hansville 7.5-minute quadrangles, Kitsap and Jefferson Counties,
Washington by Polenz et al. (2015), the site is principally underlain by geologic alluvial fan
deposits (unit Qaf) (Figure 4). Alluvial fan deposits typically consist of pebbles, sand, silt,
cobbles, and boulders that are loose, moderately to poorly sorted, stratified, and lobe-shaped
where streams emerge from valleys. Soil and sediment descriptions in the well log indicate the
fan deposit may extend to about 28 feet below the ground surface. The northern ~100 feet of
the property
Jefferson County Map of Geologically Hazardous Areas
within a seismic hazard area (Figure 5). Seismic hazard areas in Jefferson County are defined
as areas subject to severe risk of damage as a result of earthquake induced ground shaking,
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seismic hazard map boundaries are drawn based on areas with poorly drained soils that have
greater than 50% silt and very little course material; loose sand or gravel, peat, artificial fill, and
landslide materials; and soil units with a high organic content.
Soil Types on the Property
The Soil Survey of Jefferson County published by the Soil Conservation Service of the United
States Department of Agriculture (McCreary, 1975) indicates three soil types are present on the
property (Figure 6). The southern half of the property is mapped as having Swantown gravelly
hile most of the northern half of the property
is mapped as having Semiahmoo muck (unit Se). A sliver of land in the northwest corner of the
property is mapped as Kitsap gravelly sandy loam, 15 to 30 percent slopes (KsD).
The Swantown gravelly sandy loam unit is described in the survey as being formed from glacial
till.
inches below the ground surface.
The Semiahmoo muck unit is described in the survey as being formed from herbaceous organic
material located in depressions.
table at the ground surface.
The Kitsap gravelly sandy loam unit is described in the survey as being formed from lacustrine
deposits and/or marine deposits located on terraces. The soil is considered to be moderately
Field Observations and Shallow Subsurface Investigation
We also examined the foundation and soil within the crawlspace beneath the residence
(Photograph 1). The foundation to the partially finished residence appeared to have been
founded on native gravelly sand similar to what was observed in our two auger holes.
SEISMIC HAZARD ASSESSMENT
As part of our effort to assess the overall seismic hazard at the site, we reviewed potential
earthquake sources and faults, ground rupture potential, potential ground shaking scenarios,
liquefaction potential, and lateral spread potential. Below is an overview of the three main types
of earthquakes that could occur.
Earthquake Types
Shallow Earthquakes
Shallow crustal earthquakes are the most common type of earthquake that can occur. They are
usually relatively low in magnitude but shaking near earthquake sources can be strong enough
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to damage buildings. In northwest Washington, there are about 10 different crustal fault sources
in the Puget Sound and Olympic Peninsula regions where shallow earthquakes could occur.
Geologists that have studied the earthquake histories of these faults currently believe there is a
15% chance that a magnitude 6.5 or greater shallow crustal earthquake will happen sometime
in the next 50 years.
Deep Earthquakes
In the last 100 years, six deep earthquakes with magnitudes greater than 6 have occurred about
every 10-30 years in the Pacific Northwest region. The most recent major deep earthquake to
occur was the 2001 magnitude 6.8 Nisqually earthquake. Geologists currently believe there is
an 84% chance that a magnitude 6.5 or greater deep earthquake will happen sometime in the
next 50 years.
Subduction Zone Earthquakes
Located about 70 miles offshore of the west coast of North America, the Cascadia Subduction
Zone is a major tectonic boundary that spans a distance of 620 miles (1,000 km) from northern
California to British Columbia. This plate boundary is currently in a locked state and is building
up stress until the next big subduction zone earthquake is triggered and releases the stress.
Unlike shallow crustal earthquakes and most deep earthquakes, subduction zone earthquakes
commonly have magnitudes greater than 8 and can cause continuous shaking that lasts several
minutes.
The last time a Cascadia Subduction Zone earthquake occurred was 1700 A.D. and historical
records of large tsunami waves in Japan indicate that it was a magnitude 9 earthquake. The
recurrence interval for magnitude 9 earthquakes varies along the length of the subduction zone,
but in northern Washington big earthquakes occur about every 480-505 years. Geologists
currently believe there is a 10-14% chance that a magnitude 9 earthquake will happen on the
Cascadia Subduction Zone sometime in the next 50 years and a 30% chance that the next
earthquake would be magnitude 8.
With the subject property being located 175 miles away from the subduction zone, ground
shaking from a Cascadia Subduction Zone earthquake would be less intense compared to
places located directly on the west coast, but the event would not go unnoticed in Port Ludlow.
In this scenario, intense shaking is expected to last about 5 to 6 minutes.
Earthquake Sources, Fault Locations, and Ground Rupture Potential
Major earthquake sources and fault locations mapped by the WA Department of Natural
Resources (Czajkowski & Bowman, 2014) in Washington are shown in Figure 8A and local
faults located within the immediate vicinity of the site are shown in Figure 8B. No known faults
are shown as crossing the subject property, but there are a handful of faults located within 20
miles that are either believed to be inactive or that have unknown fault histories.
Since no known active faults cross the property, it is unlikely that the ground will rupture at the
site.
Ground Shaking Potential
We reviewed WA DNR Earthquake Hazards Scenario Catalog for major earthquake faults
located in Washington State to assess the relative potential for structural damage to the house.
The scenarios show the predicted ground shaking on the Modified Mercalli Intensity (MMI) scale
(Figures 9 through 19). The scenarios are considered realistic, but some scenarios are
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hypothetical or speculative. The predicted ground shaking intensity for each of the scenarios is
summarized in Table 3. The MMI value for each of the earthquake scenarios correlates to how
noticeable the earthquake will be, whether structural damage should be expected, and whether
liquefaction will occur in soils that have the capacity to do so.
In three out of 11 (27%) of the scenarios (Lake Creek Boundary Creek fault, Tacoma fault,
Darrington Devils
break; large bells will ring; vibrations will be felt like a large train is passing close to the house.
At MMI = 5, liquefaction is not expected to occur.
In six out of 11 (55%) of the scenarios (northern rupture of the Cascadia Subduction Zone,
Seattle fault, Seatac fault, Canyon River-Saddle Mountain fault zone, Nisqually fault, the
western section of the Darrington Devils Mountain fault zone), ground shaking intensity will
; MMI = 6 . The earthquakes will be felt by all; it will be
difficult to walk; many will be frightened and run outdoors; windows, dishes, and glassware may
be broken; books may fall off shelves; heavy furniture may be moved or become overturned;
plaster may fall; overall damage will be slight. At MMI = 6, isolated liquefaction could occur.
In two out of 11 (18%) of the scenarios (Southern Whidbey Island fault and a full length rupture
of the Cascadia Subduction Zone
the property. Under these earthquake scenarios, furniture could be broken; damage will be
negligible in buildings of good design and construction; slight-moderate damage in other well-
built structures; considerable damage in poorly built/badly designed structures; some chimneys
may be broken. At MMI = 7, liquefaction will be more intense and widespread.
Liquefaction Potential
Soil liquefaction occurs when shaking, usually from an earthquake, causes saturated or partially
saturated soil to behave like a liquid instead of a solid. When liquefaction occurs, the strength of
soils is substantially reduced and can lead to significant vertical settlement beneath structures.
Loose, well sorted, fine-grained, saturated sand has the greatest liquefaction potential, but loose
to medium dense gravel, silty sand, silt with low plasticity, and even some types of clays are
also susceptible to liquefaction. Liquefaction also commonly occurs at sites with water tables
near the ground surface.
Major factors that affect liquefaction susceptibility include grain size, degree of grain sorting, the
degree of compaction during sedimentation, age of deposit, and water table depth. Poorly-
sorted materials that are well-compacted have lower liquefaction susceptibility, whereas well-
sorted materials with relatively low compaction have higher liquefaction susceptibility. Holocene
age deposits are considered to be more prone to liquefaction compared to Pleistocene deposits.
To assess liquefaction potential at the property, we reviewed the WA DNR Liquefaction
Susceptibility Map of Jefferson County (Palmer and others, 2004), which rates liquefaction
susceptibility at the project site as low to very low (Figure 20). This designation was assigned by
WA DNR based on 1:100,000 scale geologic mapping that suggests the subject property is
underlain by glacial drift, undivided (geologic unit Qgu). According to this map, the southern
property line is located next to a geologic boundary in which the property located immediately to
glacial outwash (geologic unit Qg). About 1,000 feet east of the subject property, the area along
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presence of alluvium (geologic unit Qa). There is also a localized area about 500 feet southwest
WA DNR liquefaction susceptibility ratings are based on the type and age of geologic formations
(Palmer and others, 2004). The most recent and detailed WA DNR geologic map (Polenz and
others, 2015) shows Pleistocene to Holocene age alluvial fan deposits (geologic unit Qaf) at the
subject property instead of undivided glacial draft. The presence of an alluvial fan is also
supported by topographic maps and contours that clearly show a fan-like shape to the land.
Using the liquefaction susceptibility rating criteria of Palmer and others (2004), the liquefaction
presuming deposition of the alluvial fan was
predominantly during the Pleistocene and Holocene.
Lateral Spread Potential
Lateral spreading is a type of ground failure induced by liquefaction in which horizontal
displacement occurs in conjunction with the formation of fissures in the ground. Lateral
spreading typically involves lateral displacement of earth near vertical banks, marine bluffs, but
it can also occur on relatively flat sites with gently-dipping slopes. Sites with loose sand, a
shallow water table, and gentle slope gradients are capable of experiencing lateral spreading.
With the site having moderately loose sand with gravel near the surface, a shallow water table,
very low slope gradients, and low to moderate potential for liquefaction, the potential for lateral
spreading to occur appears to also be low to moderate.
NEHRP Seismic Site Class and Seismic Design Category
According to the WA DNR Geologic Information Portal, the assumed NEHRP Seismic Site
Class for the property is C-D . The data indicates the average shear wave velocity in the upper
100 feet corresponds to site class C and the mean shear wave velocity minus one standard
deviation within the upper 100 feet corresponds to site class D. Based on our site observations
and review of the well log of the property, the property should be assigned seismic site class D
and the seismic design category should be D1 .
GEOTECHNICAL CONCLUSIONS
Seismic Hazard at the Property
In eight of out eleven (73%) of the potential seismic scenarios discussed herein, ground shaking
will be sufficiently strong at the site that localized liquefaction may occur. In these scenarios, a
small, but not insignificant amount of differential settlement could occur beneath the existing
isolated footings supporting the structure.
In a Cascadia Subduction Zone full rupture scenario or Southern Whidbey Island fault zone
scenario, ground shaking will be more intense at the site compared to the other scenarios.
Liquefaction will be more widespread and a significant amount of differential settlement could
occur beneath the existing isolated footings supporting the structure.
The wetland area located in the northern half of the property more than 280 feet from the
residence location is not susceptible to liquefaction, but it does represent a high seismic hazard
due to the presence of compressible organic soil and peat. A significant amount of settlement
could occur if a structure were to be placed on organic soil and peat.
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At the residence location, although the liquefaction maps reviewed herein indicate a relatively
low potential for liquefaction based on the landform type mapped at the property, the moderately
loose and predominantly sandy texture of the material encountered in our auger holes has the
capacity to liquefy during an earthquake due to the texture of the material and the presence of a
shallow water table. Materials described in the septic logs and well log are also somewhat
liquefiable, especially the well-sorted sand layer observed 28- to 63-feet below the ground
surface. The capacity for liquefaction is relatively lower in the dry summer months of the year
when the water table is at its lowest, but during the wet season, when the water table is close to
the ground surface the potential for liquefaction is much higher. Thus, in our opinion, the overall
potential for liquefaction to occur beneath the house is actually moderate.
Based on our site observations of geotechnical conditions and review of earthquake sources
and faults, it is our opinion that under normal conditions the overall seismic hazard at the
location of the residence is low to moderate. In the majority of potential earthquake
scenarios, ground shaking is not anticipated to cause major structural damage, but our
qualitative assessment does indicate localized liquefaction could cause differential settlement to
occur below the existing and proposed isolated footings.
Under extraordinary circumstances (e.g., a Cascadia Subduction Zone earthquake), the long
duration of ground shaking that is expected may result in unpredictable and unavoidable
damage to the residence. This risk of unpredictable and unavoidable damage is shared by most
residents living on the Olympic Peninsula and in the Puget Sound.
Placement of fill or grading operations do not appear to have been involved in construction of
the residence to date.
Our conclusion of the overall seismic hazard at the site is based on a qualitative and
conservative assessment of site conditions. Seismic hazard at the site could be investigated
quantitatively by conducting standard penetrometer testing or cone penetrometer testing at the
property and with more detailed engineering analyses, but in our experience, the results would
likely confirm that the overall seismic hazard at the site is low to moderate.
Development Feasibility from a Seismic Perspective
From our reconnaissance-level qualitative assessment of the project site and review of existing
geologic maps, septic logs, well logs, earthquake fault locations, and potential seismic
scenarios, it is our opinion that the proposed development is feasible from the geotechnical
perspective, provided that the geotechnical recommendations of this report are incorporated into
the structural design and followed during construction.
GEOTECHNICAL RECOMMENDATIONS
Foundation Upgrade
To reduce the potential for liquefaction-induced differential settlement and lateral spreading to
occur beneath the isolated footings, instead of adding three additional isolated footings, a
continuous strip footing should be constructed that spans the full length of the house (Figure
21). The strip footing will substantially reduce the potential for differential settlement to occur
and mitigate seismic hazard. Existing isolated footings may remain, but steel reinforcement
must be used to splice existing footings together into the continuous strip footing. If it is more
economical and practical from the builders perspective, the existing footings may be replaced
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entirely. The strip footing should be designed in accordance with applicable provisions of the
2018 International Residential Code.
Subgrade Preparation for the Strip Footing
For the new strip footing, the native subgrade should have a firm and unyielding surface. If the
native subgrade surface unexpectedly shows soft or weak material, the weak material should be
over-excavated and replaced with compacted fill.
Wet Weather Precautions
Every effort should be made to construct the strip footing during dry summer weather when the
groundwater table is relatively low. Wet weather work will add substantially to the cost of the
project with no long-term benefit.
If weak material or a high groundwater table is encountered during excavation for the footing,
please contact Geoview Engineering so that we may provide additional geotechnical
recommendations to remedy the situation.
Stormwater Management
Stormwater runoff from the roof of the residence may be dispersed on-site using simple splash
blocks.
SUGGESTIONS FOR ADDITIONAL SEISMIC PROTECTION
Ensure that the house is adequately bolted to the existing stem wall foundation to prevent the
structure from shifting off its foundation during shaking from an earthquake.
Free-standing and unsecured water heaters can topple from ground shaking, which can cause
broken gas and water lines, flooding and/or fire. To ensure the water heater remains standing
during an earthquake, brace the water heater with heavy-gauge metal straps that are wrapped
around the tank 1.5 times.
Should a free-standing wood stove ever be installed in the home, steel straps should be used in
conjunction with lag bolts to secure the flue to the wall and the stove should be anchored to the
floor as well.
During an earthquake, unsecured propane tanks with rigid supply lines may tip over and break
their connections, causing fire. Should a propane tank ever be installed next to the home, a
flexible gas line should be installed. If a rigid gas line is the only option, an automatic gas shut-
off valve should be installed.
Given the number of potential earthquake sources in the Pacific Northwest, it is advisable to
prepare emergency kits for your home, automobile, and work before the next earthquake
happens.
If an earthquake occurs while you are inside:
1. Drop to the floor and move away from objects that may topple,
2. Take cover beneath large, sturdy furniture, and
3. Hold on until the shaking subsides.
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If an earthquake occurs while you are outside, move out into the open and avoid anything that
might fall on you, including buildings, chimneys, trees, light posts, and power lines.
THE LIMITATIONS OF THIS REPORT
The observations and conclusions of this report apply only to the subject property and they
are not transferable to nearby or adjoining property. This report is the property of Porchlight
Woodworks and may be used by others only with their permission. Unknown subsurface
conditions that were unseen could affect the outcomes of the study. Users who need a high
level of reliance on the observations and conclusions of the study may wish to obtain further
investigations.
Geoview Engineering, PC warrants that this study and the related report were conscientiously
prepared in accordance with the practice of geotechnical engineering and according to the
principles of geoscience. No other warranty, neither express, nor implied, is provided herewith.
The opinions expressed in this report are those of Geoview Engineeri
represent the opinion of any other organization.
Please call on the undersigned Geotechnical Engineer if you have questions about the contents
or the meaning of this report or if additional recommendations are needed.
Sincerely,
Geoview Engineering
Trent Adams, P.E.
Geotechnical Engineer
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BIBLIOGRAPHY
Czajkowski, J.L.; Bowman, J.D., 2014, Faults and earthquakes in Washington State:
Washington Division of Geology and Earth Resources Open File Report 2014-05, scale
1:750,000, 1 sheet.
Jefferson County Central Services geographic information system (GIS):
http://maps.co.jefferson.wa.us/Website/mspub/viewer.htm?mapset=esa (accessed August,
2022).
McCreary, F.R., 1975, Soil survey of Jefferson County Area, Washington: United States
Department of Agriculture, Soil Conservation Service. 105 p.
Palmer, S.P., Magsino, S.L., Bilderback, E.L., Poelstra, J.L., Folger, D.S., and Niggemann, R.A.,
2004, Liquefaction susceptibility and site class maps of Washington State, by county:
Washington Division of Geology and Earth Resources Open-File Report 2004-20, 78
sheets, 45 p. (available at ftp://ww4.dnr.wa.gov/geology/pubs/ofr04-20/).
Polenz, M., Favia, J.G., Hubert, I.J., Paulin, G.L., Cakir, R., 2015, Geologic map of the Port
Ludlow and southern half of the Hansville 7.5-minute quadrangles, Kitsap and Jefferson
Counties, Washington: Washington Division of Geology and Earth Resources Map Series
2015-02, 1 sheet, scale 1:24,000, 40 p. text. (available at
http://www.dnr.wa.gov/publications/ger_ms2015-
02_geol_map_port_ludlow_hansville_24k.zip).
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TABLES
Table 1.Soil description from hand auger hole #1.
Depth below ground surface (inches) Description of
Gray-brown, moderately sorted, sand with
subrounded, pebble-sized gravel.
Gray-brown, damp, moderately sorted, sand with
); gravel content
Table 2.Soil description from hand auger hole #2.
Depth below ground surface (inches) Description of
Gray-brown, moderately sorted, sand with
subrounded gravel and some silt.
Gray-brown, damp, well sorted, medium to coarse
sand with subrounded pebble-sized gravel.
Gray-brown, wet, well sorted, medium to coarse
sand.
Brown, wet, poorly sorted, clayey sand with
subrounded gravel.
Gray-brown, wet, moderately sorted, subrounded
gravelly sand; gravel
Table 3.Summary of earthquake fault scenarios at the subject property. Predicted ground
shaking intensity on the Modified Mercalli Intensity (MMI) scale is provided for each scenario.
Fault Scenario Earthquake
Type
Earthquake
Magnitude
Ground Shaking
Intensity
(MMI)
Southern Whidbey
Island fault zone
Shallow 7.4 7
Cascadia subduction
zone (full length rupture)
Subduction Zone 9.0 7
Cascadia subduction
zone (northern rupture)
Subduction Zone 8.3 6
Seattle fault Shallow 7.2 6
Canyon River Saddle
Mountain fault zone
Shallow 7.4 6
Darrington Devils
Mountain fault zone
(western section)
Shallow 7.4 6
Nisqually fault Deep 7.2 6
Seatac fault Deep 7.2 6
Darrington Devils
Mountain fault zone
Shallow 7.1 5
Lake Creek Boundary
Creek fault
Shallow 6.8 5
Tacoma fault Shallow 7.1 5
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FIGURES
Figure 1.Project location map.
Figure 2.Slope map of the topographic situation at the project site.
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Figure 3.Location of streams in the project vicinity (Jefferson County, WA GIS). A type 3 stream
intersects the eastern side of the property and another type 3 stream flows southeast near the southern
property line. A type 5 stream also flows from the west towards the southwest corner the parcel, but the
stream turns south abruptly at the parcel corner.
Figure 4.Map of local geology in the vicinity of the project site (Polenz et al., 2015).
WA DNR Stream Type
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Figure 5.Seismic hazard area map (Jefferson County, WA GIS). Potential seismic hazard areas are
shown with a white transparent layer.
Figure 6.USDA map of soil types in the vicinity of the project site (McCreary, 1975).
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Figure 7.Hand auger location map for field work conducted on July 22, 2022.
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Figure 8.Earthquake and fault location map. Major earthquake sources and faults in Washington State
(A) and faults located in the immediate vicinity of the site (B) (WA DNR).
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Figure 9.Ground shaking scenario for Southern Whidbey Island fault (Magnitude 7.4).
Figure 10.Ground shaking scenario for a full length rupture of the Cascadia Subduction Zone (Magnitude
9.0).
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Figure 11.Ground shaking scenario for a northern rupture of the Cascadia Subduction Zone (Magnitude
8.3).
Figure 12.Ground shaking scenario for the Seattle fault (Magnitude 7.2).
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Figure 13. Ground shaking scenario for the Canyon River-Saddle Mountain fault zone (Magnitude 7.4).
Figure 14. Ground shaking scenario for the western section of the Darrington-Devils Mountain fault zone
(Magnitude 7.4).
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Figure 15.Ground shaking scenario for the Nisqually fault (Magnitude 7.2).
Figure 16.Ground shaking scenario for the Seatac fault (Magnitude 7.2).
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Figure 17.Ground shaking scenario for a full length rupture of the Darrington-Devils Mountain fault zone
(Magnitude 7.1).
Figure 18.Ground shaking scenario for the Lake Creek-Boundary Creek fault zone (Magnitude 6.8).
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Figure 19.Ground shaking scenario for the Tacoma Fault (Magnitude 7.1).
Figure 20.Liquefaction susceptibility in the vicinity of the project site (Palmer and others, 2004).
Approximate location of the project site is represented by the red star.
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Figure 21. Preliminary diagram of proposed continuous strip footing (shown in red). Structural
layout adapted from Sheet 3 of 7 of the Braverman Residence Renovation plans prepared by
Pacific Northwest Structural Group, Inc.
Strip footing to replace
isolated footings for
seismic protection.
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PHOTOGRAPHS
Photograph 1.View of space beneath the house of the existing isolated footing and stem walls
supporting the house.
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APPENDIXES
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APPENDIX A
SEPTIC SYSTEM MAP (SEP2018-00011)
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APPENDIX B
WELL LOG (ALS006)
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APPENDIX C
WETLAND MAP
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APPENDIX D
STRUCTURAL PLANS (SHEET 3 OF 7)
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