HomeMy WebLinkAboutGeo-technical Report BLD2024-01361 SFR - BLD Engineering / Geo-tech Reports - 8/6/2024 MASON COUNTY "ist
COMMUNITY SERVICES Geotechnical Report
_ &hiding.Planning,Environmental Health.Community Health
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 17 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 John Kilbourne Parcel# 32104-50-00045
Site Address 71 E Country Club Drive E, Union, WA 98592
(1) (a)A discussion of general geologic conditions in the vicinity of the proposed development,
Located on page(s) 1-2
(b) A discussion of specific soil types,
Located on page(s) 2 -6
(c) A discussion of ground water conditions,
Located on page(s) 5
(d) A discussion of the upslope geomorphology,
Located on page(s) 2
(e) A discussion of the location of upland waterbodies and wetlands,
Located on page(s) 2
(f) A discussion of history of landslide activity in the vicinity, as available in the referenced maps and records.
Located on page(s) 2 - 3
(2) A site plan which identifies the important development and geologic features.
Located on Map(s) Plan Sheets 1 -3
(3) Locations and logs of exploratory holes or probes.
Located on Map(s) 2
(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) 1 -4
(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) 4
(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) 5
(7) (a) Appropriate restrictions on placement of drainage features,
Rev. February 2018
Located on page(s 5-6
(b) Appropriate restrictions on placement of septic drain fields,
Located on page(s) 4
(c) Appropriate restrictions on placement of compacted fills and footings,
Located on page(s) 6
(d) Recommended buffers from the landslide hazard areas shoreline bluffs and the tops of other slopes.
Located on page(s) 5
(e) Recommended setbacks from the landslide hazard areas shoreline bluffs and the tops of other slopes.
Located on page(s) 5
(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) 6
(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) 6
(10) An analysis of both on-site and off-site impacts of the proposed development.
Located on page(s) 6
(11) Specifications of final development conditions such as, vegetative management, drainage, erosion control, and
buffer widths.
Located on page(s) 6
(12) Recommendations for the preparation of structural mitigation or details of other proposed mitigation.
Located on page(s) 6
(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) 1 - 3
i, Alan A. Tahja PE 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 Report, dated July 6`", 2024 and entitled
2, Kilbourne Geotechnical Report—Residential
F WA
meets all the requirements of the Mason County Resource Ordinance,
y� Geologically Hazardous Areas Section, is complete and true, that the
b assessment demonstrates conclusively that the risks posed by the
37784 � landslide hazard can be mitigated through the included geotechnical
OA�,s Is ER �`{' design recommendations, and that all hazards are mitigated in such a
SIONAL E�
manner as to prevent harm to property and public health and safety.
Page 2 of 2
Disclaimer: Mason County does not certify the quality of the work done in this Geotechnical Report.
TAHJA ENGINEERING,Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
Mason County DCD
615 W Alder St.
Shelton, WA 98584
John Kilbourne
611 S Yakima Ave.
Tacoma, WA 98405
(253)284-3170 Re; Alderbrook Golf& Country Club
Geotechnical Report
71 E Country Club Drive E
Block 2, Lot 4
Union, WA 98592
Mason Case No.s:
Parcel No.: 32104-50-00045
INTRODUCTION
I was contacted in the Spring of 2024 by Mr. Kilbourne who requested my investigation and
documentation of slope stability and landslide hazard issues relating to the residential
development of the Kilbourne property located in the plat of Alderbrook identified as Mason
County tax parcel number 32104-50-00045.
The site was visited on two occasions in July and August of this year to collect topographic
information and to verify measurements and feature locations. Measurements were made
using a Sokkia model No. 8047-55 inclinometer, 100 foot and 200 foot long fiberglass
surveyor's tapes and other field equipment common to surveying. Software employed in
creating a site plan and writing this report included AutoCADTM 2023, GALENA 7.2 TM
Slope Stability Analysis software and MicrosoftTM Office Word 2003.
Slopes surrounding the property were measured as steep as 63%as identified on the
accompanying site plan. With slopes on or within 300 feet of the property exceeding 40%
with a height of 10 feet or more, Mason County requires a geotechnical report to be
developed to address stability issues relating to the property's development. The following
thirteen elements are required by Mason County to be addressed in geotechnical reports
prepared to comply with Mason County's Critical Resource Ordinance Sections 17.01.100
through 17.01.104.
1. SURFACE AND GEOLOGIC CONDITIONS:
The property has been logged in the past, and second growth timber has re-vegetated most of
the property. Inspection of the Geologic Map of Shelton 1:100,000 Quadrangle,
Washington', finds the property located in an area roughly 0.8 miles south of Hood Canal.
'DNR OFR2003-15 Geologic Map of Shelton 1:100,000 Quadrangle,Washington
File:C:IProjectsXilbournelGeo-Rpt 1 of 7 8/6/2024
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
A portion of the Geologic Map of Shelton l:100,000 Quadrangle, Washington Quadrangle
map on the following page reveals the site soils are classified as Continental Glacial
Deposits, Fraser Glaciation,Vashon Stade and is further described as follows:
Qgt Till, late Wisconsinan (Pleistocene)—Unsorted, unstratified, highly
compacted mixture of clay, silt, sand, gravel, and boulders deposited-
by glacial ice of the Puget lobe; gray; may contain interbedded strati-
fied sand, silt, and gravel; sand-size fraction is very angular and con-
tains abundant polycrystalline quartz, which distinguishes this unit
from alpine till; cobbles and boulders are commonly striated and (or)
faceted; although unweathered almost everywhere, may contain cob-
bles or small boulders of deeply weathered granitic rock.
As a portion of the Geologic Map of Shelton 1:100,000 Quadrangle on the following page
reveals,the Kilbourne property is located in the vicinity of a fault line identified on the map
as a fault line which follows Hood Canal is located approximately 5 miles west of the
Kilbourne site. The upslope geomorphology surrounding the site may be described as rolling
to mountainous. The presence of these features does not represent a hazard, and standard
seismic analysis of the site is intended to anticipate potential tectonic impacts. Washington
State Ecology Coastal Atlas records and the Washington State Interactive Geologic Map
were inspected, finding no stability issues apparent near the Kilbourne' property.
The only water body found near the site is a seasonal stormwater creek which runs sorthwest
to northeast and comes within 8 feet of the Kilbourne's northwest corner of the property as
shown on the attached site plans. The proposed residence is approximately 84 feet from this
runoff route as shown on plan sheet#3.
Further inspection and soil analysis finds the shallow surface soils on the level areas of the
property to generally conform to a weathered glacial till material classified under the Unified
Soil Classification System(USCS)as a GP-GC soil division because of its combination of
course and fine soil components. The weathered surface soil has a dry density of roughly 123
lbs/ft, a saturated soil cohesion strength of 200 lbs/ft, and an internal friction angle of 32°.
Beneath the surface soil layer which was found to extend down in depth to between 3 and 4
feet below the ground's surface was found a moderately cemented sandy clayey silt material
of fairly homogenous texture. This soil is classified under the USCS classification system as
an ML-CL type with a maximum dry density of 125 lbs/ft3, a saturated soil cohesion strength
of 300 lbs/ft2,and an internal friction angle of 32°.
File: C.WrojectsWilbournelGeo-Rpt 2 of 7 8/6/2024
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
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2. SITE PLAN WITH IMPORTANT DEVELOPMENT AND GEOLOGIC
FEATURES:
The attached site plan shows a conjectural residence location on the property, as well as the
location of the steep slopes to the northwest of the property.
No records were found on the County website for either an onsite septic system or for any
building permits.
The attached site plan shows the location of the existing 63% slope west of the conjectural
residence. Because of the glacial till substrate,the 63% slope is very stable and does not
represent a landslide hazard or threat to the Kilbourne property or planned improvements.
Nearly a third of the property lies within 50' within the required vegetated buffer,but only
roughly only 17 feet(428 sqr ft)of the proposed improvements encroach into the buffer.
A request to reduce the vegetated buffer required under Mason County's Code accompanies
this report.
File:C:WrojectsWilbournelGeo-Rpt 3 of 7 8/6/2024
TAHJA ENGINEERING,Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
3. LOCATIONS OF LOGS OF EXPLORATORY HOLES OR PROBES:
The soil probe locations are identified on plan sheet 2 along with soil strata information. A 4'
soil probe was used to explore the soil horizons in various undisturbed areas of the lot which
found the property to consist of a relatively shallow layer(—36")of medium dense
reconsolidated glacial till overlying very dense cemented clay till. The cross-section plan
sheet indicates the depth of the medium dense surface soil overlying the deeper denser glacial
till layer composed predominantly of silty clayey gravel.
4. AREA OF PROPOSED DEVELOPMENT& BOUNDARIES OF HAZARDS,
BUFFERS,AND SETBACKS:
As reported later in this report,the 63% slope located along the northwest boundary of the
property is stable in its present configuration and condition,though under Mason County's
terminology this slope is termed a hazard area due to its gradient exceeding a 40%
downslope. The entire area west of the Kilbourne property is termed a hazard area due to its
steepness exceeding 40%. Mason County requires a vegetated buffer to be maintained
between any area defined as a hazard area and proposed development/improvements.
Without allowing a reduction in the 50-foot vegetated buffer,the property has a very small
area where a residence could be located on the property.
The septic system drain-field is planned for the area adjacent to E Country Club Drive.
My inspection of the slopes,trees and soil log holes found the proposed area suitable for an
onsite septic system drainfield and other septic components.
Modeling of the site,using conservative soil properties shows that the most critical potential
slide geometries provide factors of safety greater than required under Mason County's
Critical Resource Ordinance. Various initial failure circle locations were explored, all
indicating that factors of safety are realized exceeding the County's minimum requirements.
5. CROSS SECTION WITH PROPOSED GRADE CHANGES:
A cross section view is provided in the attached plan sheet#4.No grade changes are
currently being proposed on the slope below or above the planned residence's location.
6. SLOPE STABILITY ANALYSIS:
The steep slopes on or near the Kilbourne' property (composed of the undisturbed silty clay
layer) in their present condition contain no obvious stability problem indicators. Trees on the
hillside below the Kilbourne property were observed to be growing straight with no
indications of movement or slippage in the area. The proposed building area in the central
area of the property is generally level and based on the dense cemented till material and has
no discernable irregularities, settlements, fissures, or evidence of consolidation problems.
File: C:ProjectsXilbmrnelGeo-Rpt 4 of 7 8/6/2024
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
Unified Soil Classification System typing places the shallow hillside soils in a GP-GC
category due to its fines content constituting the majority of the soil's bulk. The top 3%feet
of the soil's surface has weathered to a density estimated at 123 #/ft and a cohesion
estimated at 200#1 ,and a phi angle of 32°. The buried glacial till soil is modeled as a
USCS ML-CL soil characterized as having a soil unit weight of 125 lbs/f 3, a phi angle of
32°, and a cohesion character of 300 lbs/ft2. A phreatic surface/ground water is assumed far
beneath the ground's surface, generally at sea level.
The slope geometry and soil characteristics provide a minimum factor of safety for the site in
a static condition of 1.88 and a minimum dynamic factor of safety of 1.38. Both values
exceed the County's minimum safety requirements.
7. RECOMMENDED SETBACKS & DRAINAGE:
The slope stability analysis indicates that the slopes present on the Kilbourne' site are stable
in their present geometries and conditions and do not represent a landslide or geological
hazard to the proposed Kilbourne' property improvements. Though the bank below the
proposed building location is steep and appear stable, a setback is being proposed at 10'.
With a recommended setback of 10-feet, I also recommend all structure foundations bear on
undisturbed ground or be placed on structural fill. The attached site plan shows the current
standard County building setbacks from property lines and easements.
Drainage from the present site sheets or flows to its natural drainage direction to the
northeasterly area of the property down the slope to the seasonal drainage immediately north
of the Kilbourne property. This drainage appears natural and should be preserved.
Stormwater runoff discharged from residential downspouts should be dispersed at the corners
of the structure and allowed to dissipate into the vegetated areas of the property. The glacial
till layer underlying the site prevents the effective infiltration of stormwater, so infiltrating
stormwater runoff above the easterly facing slope of the property would adversely saturate
the shallow soil horizon above the steep slope. Concentrated discharges of stormwater runoff
should not be directed onto the sloping areas of the property, but should be dispersed into
sheet flow before it is allowed to proceed to its natural relief locations.
8. CLEARING, GRADING,AND VEGETATION MANAGEMENT:
General site grading and preparation for further development of the property has already
been accomplished, and very little, if any, additional clearing will be necessary to proceed
with the residential development of the property.
Once the residence is completed,the property can be expected to be maintained with a lawn
and landscape shrubbery of the owner's choosing. Deep rooting vegetation should be
promoted on the slopes surrounding the development area.
File:C.•IProjects1KilbournelGeo-Rpt 5 of 7 8/6/2024
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
9. EROSION AND SEDIMENT CONTROL:
Inspection of the Soil Survey of Mason County found the site to be located in an area with
Hoodsport(Hd& He) soil type(s), which are not identified in the County's Critical Resource
Ordinance as susceptible to severe erosion when disturbed.
Nearly all stormwater runoff will exit the Kilbourne property down the hummocky slope
north and northwest into the seasonal drainage sown on my drawings.
A silt fence should be installed during construction along the clearing areas' boundaries in
any areas where stormwater runoff may try to collect and exit the property in a concentrated
flow fashion over the steep slope.
10. ON-SITE AND OFF-SITE IMPACTS:
Construction and completion of the proposed Kilbourne' residence is not expected to result
in any adverse on-site or off-site impacts. The proposed residential development of the
Kilbourne' property will not result in adverse impacts to either the subject property or
neighboring properties.
11. SPECIFICATIONS OF FINAL DEVELOPMENT:
Upon completion of the property improvements,the site can be expected to be maintained as
a permanent residence,possibly with a lawn and other landscaping shrubbery. Vegetation
should be encouraged on the westerly & easterly slopes.
12. RECOMMENDATIONS FOR THE PREPARATION OF STRUCTURAL
MITIGATION OR DETAILS OF OTHER PROPOSED MITIGATION:
Modeling of the westerly and easterly steep slopes indicates that the location of the proposed
residence will be safe, and no structural fills,measures or mitigation appear necessary, other
than placing the bottoms of the footings on the underlying dense till layer.
13. SITE PLAN WITH THE LOCATION OF EXISTING AND PROPOSED
DEVELOPMENT ON THE SITE:
A site plan is attached as an appendix to this report and is hereby made a part of this report
along with the cross section view of the property and the stability analysis printout.
File:C.1ProjectsXilbournelGeo-Rpt 6 of 7 8/6/2024
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
Summary & Conclusions:
The proposed Kilbourne' property development,though located in the vicinity of hazard area
slopes, may be considered safe and the completion of the development features that Mr.
Kilbourne plans will not represent a hazard and no adverse impacts are anticipated from
development activities. Please feel free to contact me if there are any questions or concerns
regarding this report or my findings.
Sincerely,
T
WA
�p 30784 9
IONAL S
Alan A. Tahja, P.E.
Attached: Geotechnical Report Checklist
3 Plan Views& 1 Cross Section Sheet
GALENATM Stability Analysis Printout
Hazard Area 50-foot Vegetated Buffer Reduction Request
File:C:IProjectsWi1bourne1Geo-Rpt 7 of 7 8/6/2024
Revision# Description Date TAHJA ENGINEERING,INC Applicant:
Alan Tahja,P.E. John Kilbourne Kilbourne
PO Box 235 611 S Yakima Ave.
Hoodspoft, WA 98548 Tacoma, WA 98405 1 of
Mason Co.Prel No.: 32104-50-00045 (360)877-9512 (253)284-3170
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TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
UO
520
500
480 -3-112'Weathered Till 3oi
480
Consolidated Glacial Till �'� 5� i Consolidated Glacial Till
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440
Phreatic 3urtacv
420
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300
0 50 100 1. 200 250
1 GALENA vw 12
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GALENA 7.2 Analysis Results Version: 7.2.3.05 Licensee:
Tahja Engineering, Inc.
File: C:\Program Files (x86)\GALENA\Kilbourne Static.gmf
Processed: 06 Aug 2024 16:20:58
DATA: Analysis 1
Material Properties (2 materials)
-------------------
Material: 1 (Mohr-Coulomb Isotropic) - Weatheres Glacial Till
Cohesion Phi UnitWeight Ru
150.00 32.0 123.00 Auto
Material: 2 (Mohr-Coulomb Isotropic) - Consolidated Glacial Till
Cohesion Phi UnitWeight Ru
300.00 32.0 125.00 Auto
Water Properties
----------------
Unit weight of water: 62.400 Unit weight of water/medium above
ground: 62.430
Material Profiles (2 profiles)
-----------------
Profile: 1 (10 points) Material beneath: 2 - Consolidated Glacial Till
0.00 473.00 40.00 470.00 71.00 460.00
85.00 450.00 117.00 434.00
130.00 440.00 160.00 460.00 173.00 465.00
220.00 473.00 261.00 473.00
Profile: 2 (10 points) Material beneath: 1 - Weatheres Glacial Till
0.00 469.50 40.00 466.50 71.00 456.58
85.00 446.50 117.00 430.50
File:C.-TrojectsWilbournelGALENA Printout 1 of 2 8/6/2024
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
130.00 436.50 160.00 456.50 173.00 461.50
220.00 469.50 261.00 469.50
Slope Surface (10 points)
-------------
0.00 473.00 40.00 470.00 71.00 460.00
85.00 450.00 117.00 434.00
130.00 440.00 160.00 460.00 173.00 465.00
220.00 473.00 261.00 473.00
Phreatic Surface (2 points) ----------------
0.00 430.50 300.00 430.50
Failure Surface ---------------
Circular surface defined by: XL,XR,R
Intersects: XL: 130.00 YL: 440.00 XR: 173.00 YR:
465.00
Centre: XC: 141.38 YC: 469.91 Radius: R:
32.00
Probability Analysis (1 material) -------------------
Material: 1 - Weatheres Glacial Till
Cohesion (Defined) : 150.00 Standard Deviation: 0.00 Phi
(Defined) : 32.0 Standard Deviation: 0.00
Simulations: 100 (using the critical failure surface from a critical search)
- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
RESULTS: Analysis 1
Probability Analysis - Spencer-Wright Method of Analysis
--------------------------------------------------------
Simulations using the Critical Failure Surface from a Multiple Circle Search
Initial failure surface approximation - Factor of Safety: 1.996 Final
Angle of Interslice Forces (Theta) : 20.6 degrees
Simulation Summary
There were: 100 successful simulations from a total of 100 simulations (Theta
filter applied)
Resultant Factors of Safety - Minimum: 1.88 Maximum: 1.88 Mean:
1.88 Standard Deviation: 0.000
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File:C.WrojectsWilbournelGALENA Printout 2 of 2 8/6/2024
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
540
520 4=�
500
480 -3-1/2•Weathered Till Soi
400
Consolidated Glacial Till '� S�o� consolidated Glacial Till
1
440
Phceatic Sucfacv
-----------------------�'=----------------------------
420
400
380
380
0 50 100 150 200 250
RQW 1 GALENA w.rwn
F/a:c:Wopa.n Faw mp a"Mvasn"2 Rol.WM
GALENA 7.2 Analysis Results Version: 7.2.3.05 Licensee:
Tahja Engineering, Inc.
File: C:\Program Files (x86)\GALENA\Kilbourne Dynamic Analysis.gmf
Processed: 06 Aug 2024 16:04:30
DATA: Analysis 1
Material Properties (2 materials)
-------------------
Material: 1 (Mohr-Coulomb Isotropic) - Weatheres Glacial Till
Cohesion Phi UnitWeight Ru
150.00 32.0 123.00 Auto
Material: 2 (Mohr-Coulomb Isotropic) - Consolidated Glacial Till
Cohesion Phi UnitWeight Ru
300.00 32.0 125.00 Auto
Water Properties
----------------
Unit weight of water: 62.400 Unit weight of water/medium above
ground: 62.430
Material Profiles (2 profiles)
-----------------
Profile: 1 (10 points) Material beneath: 2 - Consolidated Glacial Till
0.00 473.00 40.00 470.00 71.00 460.00
85.00 450.00 117.00 434.00
130.00 440.00 160.00 460.00 173.00 465.00
220.00 473.00 261.00 473.00
File:C:TrojectslKilbournelGALENA Printout I of 2 8/6/2024
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
Profile: 2 (10 points) Material beneath: 1 - Weatheres Glacial Till
0.00 469.50 40.00 466.50 71.00 456.58
85.00 446.50 117.00 430.50
130.00 436.50 160.00 456.50 173.00 461.50
220.00 469.50 261.00 469.50
Slope Surface (10 points)
-------------
0.00 473.00 40.00 470.00 71.00 460.00
85.00 450.00 117.00 434.00
130.00 440.00 160.00 460.00 173.00 465.00
220.00 473.00 261.00 473.00
Phreatic Surface (2 points) ----------------
0.00 430.50 300.00 430.50
Failure Surface ---------------
Circular surface defined by: XL,XR,R
Intersects: XL: 130.00 YL: 440.00 XR: 173.00 YR: 465.00
Centre: XC: 141.38 YC: 469.91 Radius: R: 32.00
Earthquake Force
----------------
Pseudo-static earthquake (seismic) coefficient: 0.150
Probability Analysis (1 material)
--------------------
Material: 1 - Weatheres Glacial Till
Cohesion (Defined) : 150.00 Standard Deviation: 0.00 Phi
(Defined) : 32.0 Standard Deviation: 0.00
Simulations: 100 (using the critical failure surface from a critical search)
- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
RESULTS: Analysis 1
Probability Analysis - Spencer-Wright Method of Analysis
--------------------------------------------------------
Simulations using the Critical Failure Surface from a Multiple Circle Search
Initial failure surface approximation - Factor of Safety: 1.443 Final
Angle of Interslice Forces (Theta) : 18.7 degrees
Simulation Summary
------------------
------------------
There were: 100 successful simulations from a total of 100 simulations (Theta
filter applied)
Resultant Factors of Safety - Minimum: 1.38 Maximum: 1.38 Mean:
1.38 Standard Deviation: 0.000
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File:C:IProjectslKilbournelGALENA Printout 2 of 2 8/6/2024
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
Mason County DCD
Planning Division
615 W Alder St.
Shelton, WA 98584
John Kilbourne
611 S Yakima Ave.
Tacoma, WA 98405
(253)284-3170
Re; Alderbrook Golf& Country Club Geotechnical
Report Hazard Area Reduction Request
71 E Country Club Drive E
Block 2,Lot 4
Union, WA 98592
Mason Case No.s:
Parcel No.: 32104-50-00045
Mason County's geotechnical document review process will determine that the proposed
Kilbourne property improvements will result in encroachment inside the County's 50-
foot Vegetated Hazard Area Buffer required under Resource Ordinance 17.01.100.
Stability analysis of the slope below the conjectural development area indicates that the
site stability will satisfy Mason County's minimum landslide hazard safety requirements,
and will not result in any adverse effects.
Therefore,on behalf of Mr. Kilbourne ,I request that the anticipated future property
improvements be allowed to be sighted inside the hazard area's buffer as promoted in my
site plans and geotechnical report dated 8/6/2024.
This request effectively reduces the hazard area's buffer from 50-feet down to
approximately 40-feet and represents a request for a 10-ft development intrusion into the
hazard area buffer.
Sincerely,
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Alan A. Tahja, P.E.
File:C:IProjectsl Kilbourne(Hazard Area Reduction Rqst 1 of 1 8/6/2024