HomeMy WebLinkAboutGeo-Tech Submittal Review for BLD2004-01845 - BLD Engineering / Geo-tech Reports - 5/10/2005 STgTF MASON COUNTY
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o A°o N PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER
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y N Y 2 Shelton Washington 98564
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,�4 DATE: Mav 10''. 2005
INTER-DEPARTMENTAL COMMUNICATIONS
TO: Chuck McCoy, DCD - Planner
FROM: Alan A. Tahja, P/W - Co. Hydr. Engr. WON PLG-05
SUBJ: Geo-Tech Submittal Review NAME: Spires-Gray SFR& Steelhead Drive
BLD2004-01845
Chuck,
The supplement to the geotechnical assessment prepared for the proposed Maryann Spires&
John Gray Single Family Residence (SFR)to be constructed at 61 Steelhead Drive North,
Belfair,has been received and reviewed by Public Works.
The geotechnical assessment's supplement provides the additional information required for the
original geotechnical assessment to be considered a report level document which satisfies the
County's requirements for geotechnical reporting.
The supplement includes a minimum building setback distance of 25-feet from the toe of the
westerly ascending slope. This and other recommendations made in the original report and the
supplement should be made conditions for project approval and permitting.
With the additional information provided in the supplement to the original assessment,
geotechnical investigation of the property may be considered complete and acceptable.
The applicant should be alerted to their responsibility to protect neighboring properties and State
waters from adverse stormwater runoff impacts related to construction activities.
Please feel free to contact me at County extension 461 if you have any questions regarding these
comments, or if you feel any features need further discussion or attention.
Sincerely,
A an A. Tah�
File: H: \WP\GEO\Reports\Spire-Gray.doc
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SUPPLEMENTAL GEOTECHNICAL REPORT
PROPOSED SINGLE-FAMILY RESIDENCE
61 STEELHEAD DRIVE NORTH
BELFAIR, WASHINGTON
MAY 3, 2005
FOR
MARYANN SPIRES, C/O JOHN GRAY
Supplemental Geotechnical Report
File No. 1 1 951 -001-00
May 3, 2005
Prepared for:
Maryann Spires c/o John Gray
6418 Pullman Place SW
Port Orchard, Washington 98367
Attention: John Gray
Prepared by:
GeoEngineers, Inc.
1101 South Fawcett Avenue, Suite 200
Tacoma, Washington 98402
(253) 383-4940 of Wash;
GeoEngineers, Inc.
2S22
Eric L. Smith, LG, LEG
ns
Senior Engineering Geologist ERIC L. MI H
Jon W. Koloski, LG, LEG
Principal
ELS:JWK:tt:jm
TACO:\11\1195 100 1\00\Finals\l 195100100SupplementalR.doc
Disclaimer: Any electronic form, facsimile or hard copy of the original document (email, text, table, and/or figure), if provided, and any
attachments are only a copy of the original document. The original document is stored by GeoEngineers, Inc. and will serve as the official
document of record.
File No.11951-001-00
TABLE OF CONTENTS
Page No.
INTRODUCTION AND SCOPE OF SERVICES ...........................................................................................1
SITECONDITIONS.......................................................................................................................................2
GENERAL............................................................................................................................................2
SLOPECONDITIONS.........................................................................................................................2
SLOPE STABILITY EVALUATION......................................................................................................3
CONCLUSIONS AND RECOMMENDATIONS.............................................................................................3
GENERAL............................................................................................................................................3
BUFFERS............................................................................................................................................4
REFERENCES..............................................................................................................................................4
LIMITATIONS................................................................................................................................................5
List of Figures
Figure 1. Vicinity Map
Figure 2. Soil Survey Map
Figure 3. Geologic Map
Figure 4. Schematic Cross Section A-A'
File No.11951-001-00 Page i GWENGINEERS�
May 3,2005
SUPPLEMENTAL GEOTECHNICAL REPORT
PROPOSED SINGLE-FAMILY RESIDENCE
BELFAIR, WASHINGTON
FOR
MARYANN SPIRES C/O JOHN GRAY
INTRODUCTION AND SCOPE OF SERVICES
We are pleased to present this supplemental geotechnical report in support of your permit application for
construction of a new single-family residence at 61 Steelhead Drive North in Belfair, Washington. This
report supplements our previous report, a"Geologic Assessment"of the property,dated March 11, 2005.
The scope of services included in this report is based on review of the Mason County Inter-Departmental
Communication, dated March 23, 2005, the Mason County Notification of Incomplete Application, dated
March 29, 2005, and discussions with you and with County representatives. Mason County has indicated
that a geotechnical report is required per Mason County's Resource Ordinance No. 77-93, Section
17.01.100"Landslide Hazard Areas." The proposed building site is a Category "A" designation based on
the presence of a slope steeper than 40 percent located very near the proposed building. We received
authorization for this supplemental geotechnical report consultation on April 22, 2005.
In addition to the research and the site reconnaissance performed for our March 11, 2005 report, we have
performed a slope stability analysis using the SLOPE/W slope stability program. The attachments to this
geotechnical report include the following figures:
• Figure 1 — a vicinity map showing the location of the property relative to local streets and
landmarks.
• Figure 2 — a topographic map of the geographic site area, with contacts showing the soil units
mapped by the USDA Soil Conservation Service.
• Figure 3 — a site-specific topographic map of the property, based on a sketch provided by the
owner and indicating our observed and interpreted geologic information together with the
geometric cross section on which our stability analysis is based.
• Figure 4 — a schematic cross section of the slope on the alignment shown on Figure 3 and with
additional information from our site reconnaissance and measurements.
Specifically, the services provided by GeoEngineers, Inc. for this geotechnical supplement includes the
following:
1. A site plan based on the plan provided by the applicant.
2. A schematic cross section,based on the plan provided by the applicant, supplemented by our field
measurements and our knowledge of the site geology.
3. An analysis-based slope stability evaluation.
4. A supplemental geotechnical report that includes the above information as an addendum to our
report dated March 11, 2005.
File No.11951-001-00 Page 1 GMENGINEERS�
May 3,2005
SITE CONDITIONS
GENERAL
For a detailed description of the project site, please refer to our previous report dated March 11, 2005.
The information in this section of this supplemental report is abbreviated and included herein to facilitate
description of the slope stability analysis.
The property is an existing residential site and is located within an area of other residential development.
The property consists of two residential lots that we understand will be consolidated into one. We
understand that the existing mobile home(see Figure 3)will be removed.
The proposed building site is located generally on the floor of Mission Creek valley at the base of the
valley wall slope. The specific building site is nearly level and has been cleared in conjunction with past
development. The valley wall slope rises about 120 feet in elevation to a level upland. The slope and the
adjacent upland are undeveloped and covered with trees and low brush.
SLOPE CONDITIONS
As indicated on Figure 4, the western portion of the property consists of a slope that rises to the west for
approximately 120 feet at an inclination of approximately 1.311 to 1.5H:1V (horizontal to vertical)
equivalent to a slope angle range of about 78 percent to 65 percent. The gradient and height of this slope
warrant its inclusion in the Critical Areas Ordinance for the County. We traversed this slope on February
2,2005 and found it to be of relatively uniform gradient and orientation, with no indications of large-scale
instability. The toe of this slope has been modified locally by past grading(cutting). It has been cut back
(truncated)to a gradient of approximately 1H:1V for a height averaging approximately 6 to 8 feet, locally
reaching a maximum of 12 feet. Minor surface sloughing was evident along the top and toe of this cut.
The vegetation on the slope includes a cover story of mature second-growth evergreens, up to 2 feet in
diameter and 50 feet or more in height, and mature second growth of deciduous trees ranging from
saplings to greater than 2 feet in diameter. The evergreens are mainly Douglas fir and western hemlock.
Cedar trees are notably absent. The deciduous trees are generally represented by big leaf maple and red
alder. The trunks of the evergreen trees are generally straight and vertical (not butt-bowed or distorted).
We also observed numerous large-diameter old-growth conifer stumps. The deciduous trees generally do
not appear to be growth-distorted except for light-seeking orientations. The ground cover generally
consists of a mixture of salal, huckleberry and sword fern. Ground cover and brush that prefers wet soil
conditions is also notably absent.
We did not observe any geomorphic expressions of past slope instability such as hummocky ground,
ground cracks, terraced topography or bowed or arched trees on the subject property. We did not observe
evidence of surface flow or groundwater seepage on the slope, and we did not observe standing water in
the project area at the time of our site visit.
We expanded our observations in adjacent areas to the north and south of the subject property along the
same generally easterly facing slope. While the northward and southward extensions of the subject slope
continue to be relatively uniform for several properties distant, we did observe evidence of seepage
locally through surface soils along contacts with denser underlying native materials,revealed primarily by
vegetation trends. These conditions appeared to be generally absent from the subject property.
File No.11951-001-00 Page 2 GWENGINEER�
May 3,2005
We have interpreted soil conditions on the subject property based on our observations of soil exposures in
nearby road excavations and on a relatively recent near-vertical cut slope on the adjacent property
immediately south of the Spires property. The adjacent recent cut slope is standing about 25 to 30 feet in
height at an angle of about 0.5H:IV. Soils underlying the subject slope consist of a surface layer about
2-to 3-feet thick composed of loose gravelly sand, underlain by very dense mixed sand and gravel with
occasional included thin layers or lenses of hard sandy silt. We interpret the dense material to be
Advance Outwash deposited in front of an advancing glacier. The loose sur cial material is colluvium
(top soil) weathered from the underlying dense soil. The upland is capped by a layer of glacial till a very
dense, unsorted mixture of silt, sand and gravel deposited directly by a glacier. Our observations are
consistent with published geologic and soils maps of the area.
SLOPE STABILITY EVALUATION
Figure 3 is a site plan provided by the applicant. We have shown the approximate distribution of the
surface geologic units mapped in this area on this base map. Also shown is the location of the prepared
Cross Section A-A'. Figure 4 shows the approximate topographic profile along the cross section
alignment shown on Figure 3, supplemented by reconnaissance-level field measurements taken during our
initial site visit. The estimated location of the contact between Advance Outwash and till is also depicted
on this schematic cross section.
We evaluated the stability of the slope shown as Cross Section A-A' on Figures 3 and 4 using the slope
stability program SLOPE/W, developed by GEO-SLOPE International, Ltd. of Calgary, Alberta, Canada.
SLOPE/W is a software product that computes factors of safety against sliding, based on limit
equilibrium theory, to evaluate the stability of earth slopes, identifying the critical slope surface with the
lowest factor of safety for a large number of potential slip surfaces. A range of circular trial slip surfaces
is typically specified. Each circular slip surface is defined in the analysis by its center and radius. A grid
pattern is typically used to search for the circle origin and the radius that results in the lowest safety factor
against sliding(critical slip surface)for each condition evaluated.
We performed our analysis using estimated soil conditions based on our literature review, our geologic
reconnaissance and our experience. The soil parameters we used are based on our experience with similar
materials and on published values (Koloski, et al., 1961). We modeled the subsurface conditions in two
dimensions using geometry inferred from the topographic data in the supplied site plan and estimated
subsurface topography as illustrated on the cross sections. In our analysis, the structure loading
parameters were input into the program using area loads typical for a residential structure. Stability
analysis for existing and proposed static conditions was performed for the site. We also performed a
pseudo static analysis of the developed conditions using a horizontal seismic coefficient applied to the
critical slip surface developed from our static analysis. We used an average horizontal seismic coefficient
of 0.15g in our analysis(g is the vertical acceleration due to gravity).
Our stability analyses for the site indicates the computed factor of safety for the critical slope failure
surface is greater than 2.0 for static conditions and is approximately 1.6 for theoretical seismic loading.
CONCLUSIONS AND RECOMMENDATIONS
GENERAL
Based on our supplemental geotechnical evaluation, site reconnaissance and experience in the area, it is
our opinion that the site is generally suitable for the proposed development from an engineering geologic
File No.11951-001-00 Page 3 GMENGINEERS�
May 3,2005
and geotechnical engineering standpoint,provided the recommendations presented in this report(and our
previous report dated March 11, 2005)are followed.
Based on our site observations, our stability analyses, and experience with the subject geologic units and
the site area, it is our opinion that the potential for gross slope instability is low. The slope to the west of
the proposed residence is relatively uniform plane and gradient, with no indications of gross instability.
However, the base of the slope is partially truncated by an approximately 6- to 8-foot high 1H:1V cut.
While this cut does not render the slope grossly unstable, it does undermine the more porous upper soil
layer which, when saturated, may be prone to surficial raveling, erosion, or local slumping. A building
setback from the base of the slope is recommended as noted below. Alternatively, an engineered
retaining wall (with adequate back drainage, foundation embedment and reinforcing) can be installed
along the base of the westerly ascending slope.
We recommend that representatives of GeoEngineers, Inc. be retained to review the plans and observe
grading activities,foundation excavations and installation of designed drainage devices.
Conventional erosion control features such as silt fences and loose straw ground cover, sand bags,
trenching may be appropriate for this site, depending upon the design and grading options chosen.
Review of the need for such structures was beyond the scope of this report.
BUFFERS
Ordinance 17.01.100, "Landslide Hazard Areas," of the Mason County Resource Ordinance, Section D-6
"Buffers" states that: "A 50-foot (15.25-meter) buffer of vegetation is required around the Landslide
Hazard Area." The subject building pad area has already been cleared, apparently years ago in
conjunction with placement of the existing mobile home and septic waste treatment system. Due to the
excavated toe of the westerly ascending slope, the surficial soils immediately upgradient of the cut are
undermined and may ravel somewhat with time. We,therefore,recommend that all structures be set back
a minimum distance of 25 feet from the toe of the westerly ascending slope. We also recommend that the
slope vegetation be maintained to reduce the incidence of surficial erosion and sloughing.
It is a relatively common occurrence for large or tall trees on steep slopes to blow over or topple with
storm or wind events or due to normal aging, health (disease) or decay processes. This can present a
special hazard to people and structures. Our slope stability analysis and our recommendation for the
buffer/building setback from the base of the slope does not consider the potential risk,to people or
structures from the toppling of trees. We recommend that an arborist be consulted regarding any large
trees which could topple in the direction of proposed structures.
REFERENCES
1960, Bishop, A.W. and Morgenstern, N., "Stability Coefficients For Earth Slopes", Geotechnique, Vol.
10,No.4,pp 164-169.
1965, Garling, M.E., et al., "Water Resources and Geolgy of the Kitsap Peninsula and Certain Adjacent
Islands," State of Washington Department of Conservation, Division of Water Resources Water
Supply Bulletin No. 18(with contributions by the U.S. Geolgoic Survey).
1989,Koloski,J.W.,Tubbs,D.W. and Schwartz, S.D., "Engineering Properties of Geologic Materials",in
Engineering Geology of Washington, Vol. II, Bulletin 78, Washington Division of Geology and
Earth Resources,Richard W. Galster,Editor.
File No.11951-001-00 Page 4 GWENGINEER�
May 3,2005
1960, Ness, A.O., "Soil Survey of Mason County, Washington," U.S. Department of Agriculture, Soil
Conservation Service.
1977, Smith, M. and Carson, R.J., "Relative Slope Stability of the Southern Hood Canal Area,
Washington,"U.S. Geolgocial Survey Miscellaneous Investigations Map 1-853-F, Scale 1:62,500.
Slope/W 2004, Geo-Slope International, 2004,Calgary,Alberta.
LIMITATIONS
We have prepared this report for the exclusive use by Maryann Spires, John Gray and their design
consultants for the proposed single-family residence at the subject property.
Within the limitations of scope, schedule and budget, our services have been executed in accordance with
generally accepted practices in the field of engineering geology in this area at the time this report was
prepared. No warranty or other conditions, express or implied, should be understood. Please refer to the
"Report Limitations and Guidelines for Use"in our March 11, 2005 report for further guidance on use of
this supplemental report.
File No.11951-001-00 Page 5 GWENGINEERS�
May 3,2005
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a VICINITY MAP -61 STEELHEAD DR. NE, BELFAIR, WA
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SOIL SURVEY MAP -61 STEELHEAD DR. NE, BELFAIR, WA
GWENGINEERS /
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Ln 2. This figure is for informational purposes only. It is intended to assist in the identification of features APPROXIMATE SCALE IN FEET
discussed in a related document. Data were compiled from sources as listed in this figure. The data GEOLOGIC MAP
i sources do not guarantee these data are accurate or complete. There may have been updates to the
data since the publication of this figure. This figure is a copy of a master document. The master hard 61 STEELHEAD DR. NE, BELFAIR, WA
0 copy is stored by GeoEngineers, Inc. and will serve as the official document of record. GMENGINEER� FIGURE 3
a Reference: Drawing created from sketch plot plan provided by John Gray.
PROFILE ESTIMATED FROM
FIELD MEASUREMENTS AND
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m 2. Refer to Figure 2 for location of Section.
3. This figure is for informational purposes only. It is intended to assist in the identification of features SCHEMATIC CROSS SECTION A-A'
discussed in a related document. Data were compiled from sources as listed in this figure. The data 61 STEELHEAD DR. NE, BELFAIR, WA
i sources do not guarantee these data are accurate or complete. There may have been updates to the G M E NG I N E E R�
data since the publication of this figure. This figure is a copy of a master document. The master hard ji FIGURE 4
a copy is stored by GeoEn ineers, Inc. and will serve as the official document of record.