HomeMy WebLinkAboutBLD2008-00444 Geotechnical Report • •
EVE LOPME INEER(NG, PLLG
�cJ ' P.O.Box 446
Tacoma,WA 9840
Ph(253)228-0513
2 7 2°16 www.de-civil.com
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Olympic Ridge,Inc. Geotechnical Report
2871 NE Holman Avenue Proposed Residence El-Y :SON COUNTY
REV/SE-6-1 DCp
Poulsbo,WA 98370 201 Montgomery Lane
Ph: (360)302-0156 Pt.Ludlow,Washington
PN:990600152
Attn:Mr.E.J.Klinker Job: 16-067 Olympic Coatsworth.GR
INTRODUCTION
This report presents the results of our geotechnical investigation and engineering recommendations
for the proposed residence to be located at 201 Montgomery Lane in the City of Pt. Ludlow, in
Jefferson County,Washington.The general location of the site is shown on the attached Site Vicinity
Map, Figure 1.
Our understanding of the project is based on our ciscussions with you,and our March 18,2016 site
visit. We understand that you are in the process of applying for a building permit for the site and that
you plan on building a single family residence on the property this spring and summer. We further
understand that the future residence will likely be constructed utilizing conventional wood framing
and spread concrete footings, it also anticipated that the residence will include a daylight basement.
Clearing and grading is expected to be limited to that required to maintain access to the site and a
suitable yard area around the proposed residence.
SCOPE
The purpose of our services is to evaluate the surface and subsurface conditions at the site as a basis for
assessing potential adverse impacts to and from groundwater and loose soils located within the site
area. Specifically, our scope of services for the project included the following:
1. A visit the site to conduct a geologic reconnaissance to assess the site's slope,soil and ground
water conditions.
2. Reviewing the available geologic, hydrogeologic and geotechnical data for the site area.
Including the WADNR, Division of Geology and Earth Resources,Geologic Maps of Eastern
Jefferson County,the USDA,Soil Conservation Service,Soil Survey for Jefferson County,and
the DOE Coastal Zone Atlas for Jefferson County.
3. Assessing the landslide hazards in accordance with Jefferson County, Chapter 18.22,Critical
Areas Ordinance.
4. Providing stormwater recommendations for on-site stormwater controls of the roof water.
Subsurface explorations and/or laboratory testing were not completed as part of this assessment.
The services described in this report were prepared under the responsible charge of Mr.Glen Coad,
PE. Mr.Coad meets the qualifications contained in Title 18,Section 18.22.420 to prepare a
Geotechnical report. Mr.Coad understands the requirements of the current Critical Areas Chapter
18.22 and the definitions of the applicable terms contained within Chapter 18.22. Individuals under
the responsible charge of Mr.Coad have performed a landslide hazard geological assessment,
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April 14, 2016
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conducted a field investigation, and researched historic records on or in the vicinity of the above
referenced site.
SITE CONDITIONS
Surface Conditions
The subject parcel is located at 201 Montgomery Lane in the City of Pt. Ludlow in Jefferson County,
Washington. The parcel is rectangular in shape, measures approximately 168 feet in length (west to
east) by approximately 75 feet in width (north to south) and encompasses approximately 0.3 acres.
The building site has been cleared and is well vegetated with grass while the eastern portion of the
site is occupied by a steep slope which is well vegetated with trees and brush. Access to the site will
be from Montgomery Lane along the west end of the site. The building site is located in the mid
central portion of the lot and will likely include a daylight basement. The site is bounded by
established residential development on the north and south,by Montgomery Lane to the west and by
Puget Sound to the east.
As mentioned the building site is located in the mid central portion of the site and is in a gently sloped
area with grades of 10 to 20 percent. At the east shoreline side of the site is a top of a steep slope which
faces to the east dropping down to Puget Sound shoreline. The slope is at 100 percent to near vertical
grade and has an overall height of approximately 60 feet. Overall elevation change across the site is
approximately 70 feet including the shoreline bluff. A site plan was available for the project and is
included showing areas of 20 to 39 percent and greater than 40 percent slopes, Site Plan-Figure 2.
No seepage was observed at the time of our site visit. No evidence of deep-seated slope instability
was observed at the site or the adjacent areas at the time of our site visit. No erosion,or other soil
movement was observed at the time of our site visit.
Site Soils
The Soil Survey of Jefferson County(USDA Soil Conservation Service) indicates that the property is
underlain by the Swanton gravelly loam soils(SuB)that forms on 0 to 8 percent slopes. These soils
were derived from glacial till under a forest cover and are listed as having a "slight"erosion hazard. A
copy of the SCS soils map for the site area is included as Figure 3. We observed no evidence of
surficial erosion on the site or within 200 feet of the site at the time of our site visit.
Site Geology
According to the Geologic Map of Surficial Deposits in the Seattle 30'x60'Quadrangle,
Washington, by James C. Yount,James P. Menard and Glenn R. Dembroff, 1993, as well as
the earlier Geologic Map of the Uncas-Port Ludlow Area Jefferson County, Washington by
Kathryn L. Hanson, OFR 76-20,the site is underlain by glacial till. These glacial soils were
deposited prior to and during the most recent Vashon Stade of the Fraser Glaciation,
approximately 12,000 to 15,000 years ago. The glacial till consists of a mixture of clay,silt,
sand, and gravel that was deposited at the base of the prehistoric continental glacial ice mass
and were subsequently over-ridden. Glacial soils overridden by glacial ice can be considered
overconsolidated, which typically exhibit high strength and low compressibility
characteristics. An excerpt of the above reference geologic maps is included as Figure 4a &
4b.
A review of the Coastal Zone Atlas for Jefferson County indicates that the site is located in an area
mapped as"S"for stable with the shoreline bluff area mapped as Urs and Uos. No evidence of active
and ongoing shoreline erosion was observed along the shoreline or evidence of larger deep-seated
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April 14,2016
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rotational failures were observed. The areas of mapped recent(Urs)or older(Uos) landslides are
shown on the Coastal Zone Atlas for the site vicinity.A copy of the Coastal Zone Atlas is attached as
Figure 5.
Based on our site observations,the near surface soils are generally consistent with the mapped glacial
soils. Groundwater seepage was not observed on the site or adjacent areas.
GEOLOGICALY HAZARDOUS AREAS
The Jefferson County Municipal Code, Chapter 18 classifies a geologically hazardous area based upon
a combination of erosion, landslide,and seismic hazards. The following erosion, landslide,seismic,
and channel migration zone (CMZ)hazard areas shall be subject to the standards of Article V
(Geologically Hazardous Areas).These areas may be identified by the presence of any of the following
indicators:
Erosion Hazard Areas—per Jefferson County Section 18.22.160
Areas containing soils or soil complexes described and mapped with the United States Department of
Agriculture,Soil Conservation Service,Soil Survey for Jefferson County as having severe or very severe
erosion hazard potential.
Landslide Hazard Areas—per Jefferson County Section 18.22.160
Areas potentially subject to mass movement due to a combination of geologic,topographic,and
hydrologic factors including:
(i) Areas of historic failures or potentially unstable slopes,such as:
(A) Areas described and mapped as having severe or very severe building limitations for dwellings
without basements within the United States Department of Agriculture,Soil Conservation Service,Soil
Survey for Jefferson County;
(B) Areas described and mapped as recent or old landslides or slopes of unstable materials within
the Washington State Department of Ecology Coastal Zone Atlas of Jefferson County;and
(C) Areas described and mapped as areas of poor natural stability,former landslides and recent
landslides by the Washington State Department of Natural Resources, Division of Geology and Earth
Resources;
(ii) Areas potentially unstable as a result of raoid stream incision,stream bank erosion,or
undercutting by wave action;and
(iii) Areas with any indications of earth movement,such as:
(A) Rockslides;
(B) Earthflows;
(C) Mudflows;and
(D) Landslides.
Seismic Hazard Areas—per Jefferson County Section 18.22.160
Areas subject to severe risk of damage as a result of earthquake-induced ground shaking,slope
failure, settlement, soil liquefaction,or surface faulting. These areas are identified by the presence
of: poorly drained soils with more than 50 percent silt and very little coarse material; loose sand and
gravel, peat,artificial fill and landslide materials;or soil units with high organic content.
Channel Migration Zones(CMZs)—per Jefferson County Section 18.22.160
Areas subject to the natural movement of stream channel meanders. In the delineation high risk CMZ
area, channel migration is likely within the next 100 years. Areas protected from channel movement
due to the existence of permanent levees of infrastructure improvement such as roads and bridges
S
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constructed and maintained by public agencies are excluded from the high or moderate risk
designation.
CONCLUSIONS
Jefferson County,Chapter 18.22,Section 18.22.160 uses the above referenced indicators to define a
geologically hazardous area. Based on our observations of the site and review of published
information,we offer the following comments.
Erosion Hazard Classification
No soils or soil complexes onsite are described or mapped as having severe or very severe erosion
hazard potential.
Landslide Hazard Classification
The shoreline bluff areas were mapped on the Coastal Zones Atlas as having historic failures or
potentially unstable slopes. However,we did not directly observe areas such as:Areas described and
mapped as having severe or very severe building limitations for dwellings without basements within
the United States Department of Agriculture,Soil Conservation Service,Soil Survey for Jefferson
County; or Areas described and mapped as areas of poor natural stability,former landslides and
recent landslides by the Washington State Department of Natural Resources, Division of Geology and
Earth Resources. No areas potentially unstable as a result of rapid stream incision,stream bank
erosion,or undercutting by wave action were observed. No areas with any indications of earth
movement such as rockslides,earth flows,mudflows,and landslides were observed on site at the
time of our visit.
Based on the above, it does appear that the site has one of the indications of a geologically hazardous
area related to the Coastal Zones Atlas designations of Uros and Urs.
Seismic Hazard Classification
As stated above,the site is underlain by the Vashon glacial till. These soils are very dense and are not
susceptible to settlement due to earthquake events. Based on the above, it is our opinion that the
site does not meet the technical criteria of a Seismic Hazard Area.
Based on our observation and the subsurface units mapped at the site,we interpret the structural site
conditions to correspond to a seismic Site Class"C" in accordance with Table 1613.5.2 in the 2006 IBC
(International Building Code) documents. This is based on the likely range of equivalent SPT(Standard
Penetration Test) blow counts for the soil types observed in the site area. These conditions were
assumed to be representative for the conditions based on our experience in the vicinity of the site.
Channel Migration Zone Classification
Based on the location of the site,the site is not within an area susceptible to the natural movement of
stream channel meanders.
Recommended Setback
The Jefferson County building department may require a building setback in accordance with IBC
(International Building Code) standard requirements. IBC section 1805 requires a building setback
from slopes that are steeper than 3H:1V(Horizontal:Vertical)unless evaluated and reduced, and/or a
structural setback is provided, by a licensed geotechnical engineer. The setback distance is calculated
based on the vertical height of the slope. The typical IBC setback from the top of the slope equals one
third the height of the slope while a setback from the toe of the slope equals one half the height of
the slope. Vegetation in the setback area may be enhanced, if approved/required by the Jefferson
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April 14,2016
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County. Clearing,grading and filling within the setback area is allowed if it can be demonstrated that
the existing vegetation will not be adversely impacted or that it can be mitigated (enhanced).
Based on our site observations, in accordance with IBC guidelines,we recommend a setback distance
of H/3 for slopes greater than 10 feet in height to a maximum of 40 feet from slopes greater than 33
percent. It is our understanding that the proposed house location will be over 40 feet from the top of
bluff and will therefore be meeting any recommended or prescriptive setback criteria.
Weathering,erosion and the resulting surficial sloughing and shallow land sliding are natural
processes that affect steep slope areas. As noted, no evidence of surficial raveling or sloughing was
observed at the site. To manage and reduce the potential for these natural processes,we
recommend the following:
• No drainage of concentrated surface water or significant sheet flow onto or near the
steep slope area.
• No additional fill should be placed within the setback area.
• Grading should be limited to providing surface grades that promote surface flows away
from the top of slope to an appropriate discharge location beyond the toe of the slope,
such as into Puget Sound.
Site Drainage
All ground surfaces should be graded such they slope away from the proposed residence. We
recommend conventional roof and footing drains be installed for the residence,and that any
subgrade wall be provided with a drain to prevent the build-up of hydrostatic pressure.The roof drain
downspouts should not be connected to the footing drain unless adequate measures are taken to
prevent a surcharge of the footing drain. Roof water should be tightlined down the bluff to the
shoreline and discharged to an appropriate location. Collected runoff from the driveway area may be
directed to adjacent vegetated areas as sheet flow or tightlined to the roof drainage system.
LIMITATIONS
We have prepared this report for Olympic Ridge, Inc. and other members of the design team for use
in evaluating a portion of this project. Subsurface conditions described herein are based on our
observations of exposed soils on the parcel. This report may be made available to regulatory agencies
or others, but this report and conclusions should not be construed as a warranty of subsurface
conditions. Subsurface conditions can vary over short distances and can change with time.
Within the limitations of scope,schedule and budget,our services have been executed in accordance
with generally accepted practices in this area at the time this report was prepared. No warranty,
express or implied,should be understood.
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April 14, 2016
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We trust this is sufficient for your current needs. Should you have any questions,or require
additional information,please contact us at your earliest convenience.
Respectfully submitted,
Development Engineering, PLLC
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Glen Coad, PE
Owner
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Doc ID:16-067 Olympic Coatsworth.GR
Attachments: Figure 1—Vicinity Map
Figure 2—Site Plan
Figure 3—SCS Soli Survey
Figure 4—USGS Geology Map
Figure 5—Coastal Zones Atlas
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DEVELOPMENT
DEN%INEER.IN , PLC Site Vicinity Map
P.O.Box 446 Proposed Residence
Tacoma,WA 98401 201 Montgomery Lane
Ph(253)228-0513 Port Ludlow, Washington
www.de-civil.com
Job No:16-067 April 2016 Figure 1