HomeMy WebLinkAboutGEO2020-00052 Slope Stabilization - GEO Geological Review - 9/11/2019 C-��o2020 - pp��2
� MASON COUNTY
F' Imo COMMUNITY SERVICES Shoreline Stabilization
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Note:This checklist must be submitted with a Shoreline Geotechnical Assessment and completed, signed, and
stamped by the licensed professional(s)who prepared it for review pursuant to the Mason County Shoreline
Master Program. If an item is found not applicable, the Assessment should explain the basis for the
conclusion.
MCC 17.50.340 B.2. New structural stabilization measures shall not be allowed except as follows:
a. To protect existing priknary structures:
i. New or enlarged;structural shoreline stabilization measures for an existing primary structure, including
residences, shall not be allowed unless there is conclusive evidence documented by a Shoreline Geotechnical
Assessment* that the structure is in danger from shoreline erosion caused by tidal action, currents, waves, or
sea level rise. Normal sloughing, erosion of steep bluffs, or shoreline erosion itself, without a scientific or
geotechnical analysis, is not demonstration of need. The geotechnical assessment shall evaluate on-site
drainage issues and address drainage problems away from the shoreline edge before considering structural
shoreline stabilization.
ii. The erosion control structure will not result in a net loss of shoreline ecological functions. This shall be
demonstrated in a Habitat Management Plan as required in MCC 8.S2.170(J).
I
iii. Primary structure means the structure or the only access associated with the principal use of the
property that cannot feasibly',be relocated. It may also include single family residential appurtenant structures
that cannot feasibly be relocated.
Applicant/Owner ;V L TOU /o I- Parcel
Site Address / 20 /`✓ Cushmoil R;o�a� tom, f, frx�a�sport WA
1'e6c
Iter>i`>,s to be included in assessment Located on
page(s):
Scope of review, including landowner concerns and site assessment objectives.
Discussion of site development, including the location and setback of important primary and
appurtenant structures, roads and utilities. f
Assessment of relevant wetlands and surface water drainage and control systems.
Assessment of site vegetation,,including species and communities present.
Assessment of regional/site geology, including relevant rock/soil unit descriptions,geologic
history and stratigraphic correlation.
Assessment of regional/5 to geomorphology, including the influence of wave or tidal processes in
controlling sediment tra sport and the evolution of the site's landform. Include assessment of
the drift cell sediment budget and local beach conditions.
May 2018
Evaluation of erosion and/or flooding issues, including assessment of causes, and rates or
timeframes associated with each issue. Include evaluation of how historic shoreline
modifications near the site may have impacted patterns of erosion.
Evaluation of the amount of time until primary structures or major utilities will be damaged by
wave or tidal action, based on evidence of erosion rates, a slope stability analysis or other !�
geotechnical considerations if no action is taken on the site.
Discussion of alternative approaches to reduce risk from erosion on the site, including the use of
nonstructural measures such as drainage and vegetation, and soft shoreline techniques such as
gravel berms, large woody debris, or other measures. Explain why these techniques would be
infeasible or insufficient to reduce the threat from erosion.
Discussion describing the proposed shoreline stabilization is the minimum size necessary, placed
as far landward as possible and will not result in a net loss of shoreline ecological functions.
A site map drawn to scale which identifies property boundaries,scale, north arrow and the
location of proposed development, geologic features and ordinary high water mark (OHWM).All
development setbacks should be delineated and marked.
Cross-section(s), identified on site map,showing both horizontal and vertical scales, and
identifying important features such as: MHHW and OHWM,top and bottom of bluff, location of
upland structures and utilities, and geologic units where relevant.
Supplementary photographs.
I, 7`P�C W ( , IW,t^fG 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 Assessment,dated
Se ,and entitled
r/
w`�sl1iy V7a li l�<<Ar" meets all the
requirements of the Mason County Shoreline Master Program,Shoreline
r 1 _
Stabilization Section (MCC 17.50.340), and is complete and true.
p- 30758IST
��SSfONAL
(Signature and Stamp)
*MCC17.50.020 Definitions:Shoreline Geotechnical Assessment. A scientific study or evaluation conducted by a qualified expert
that includes a description of the ground and surface hydrology and geology,the affected land form and its susceptibility to mass
wasting,erosion, and other geologic hazards or processes,conclusions and recommendations regarding the effect of the proposed
development on geologic conditions,the adequacy of the site to be developed,the impacts of the proposed development,
alternative approaches to the proposed development,and measures to mitigate potential site-specific and cumulative geological
and hydrological impacts of the proposed development,including the potential adverse impacts to adjacent and down-current
properties. Geotechnical assessments shall conform to accepted technical standards and must be prepared by qualified professional
engineers or geologists who have professional expertise about the regional and local shoreline geology and processes.
Page 2 of 2
Disclaimer:Mason County does not certify the quality of the work done in this Geotechnical Assessment.
Prepared for: Taylor Cushman LLC
c/o: Kyle Taylor
22311 161" Ave E
Graham, WA 98338-8567
Prepa, ed by: MET Engineering, PLLC
1018 E. Wishkah St.
Aberdeen, WA 98520
(360) 310-0270 bus
(360) 289-0958 cell
(360) 861-8493 fax
(800) 590-0958
Contact: Steven P. Morta, P.E.
P.
WASI"4
Sep. 11, 2019
Rev: Oct. 18, 2019
Rev: Apr. 13, 2020
30758
NAL
Sep. 24. 2019
Apr. 13, 2020
Page 1 of 6
To: Tylor Cushman,LLC
c/o: Kyle Taylor
22311 161" Ave E.
Graham, WA 98338-8567
Subject: SLOPE STABILIZATION ASSESSMENT for Property Located at 1520 N
Cushman Ridge Dr.,Hoodsport,WA 98548 (Parcel#: 42331-50-01918)
Ref: Cushman Hwlroelectric Proi'ect FERC Project No. 460: "Shoreline Uve Specificatioln and
Permitting Guidelines", dated March 2014
Scope of Review
The landowner is concerned with continual shoreline soil erosion that is gradually eroding the
beach and working its way toward the residence further on up the embankment. A wood
bulkhead has been designed to minimize the affect of shoreline soil erosion due to fluctuating
water levels that tend to soften up sometimes even the hardest glacial till soils and then when the
water level recedes, fines are then gradually removed. This results in the loosening of the caravels
and cobbles along the shoreline. A minimal length bulkhead has been designed to minimize this
gradually shoreline erosion,
On Monday September 24, 2018, this geotechnical engineer registered as a professional civil
engineer in the State of Washington visited the above site to evaluate the condition of the existing
slopes especially along the shoreline of Lake Cushman. This geotechnical engineering report
documents the results of this site visit by this engineer. Digital photographs were taken at the site
and have been incorporated into this geotechnical engineering report.
All over-the-water structures as shown in the attached drawings, i.e., A-1/6 through A-6/6, will
be removed. All new over-the-water construction will consist of a 5' x 22' pier, a 3" x 10' ramp,
and a 1,8- ' float with the total over-the-water coverage of 306 square feet.
General Site Description
The road and utilities are located on the west side of the property. The residence itself is located
upslope from proposed bulkhead.. pier, and floating dock.
This geotechnical engineer examined the condition of the slope especially near the toe along the
west shoreline of Lake Cushman where unraveling of the gravelly till soils was observed. An
existing bulkhead was observed constructed of post and plank but was in disrepair and part of the
bulkhead appeared to be missing.
Existing structures were observed on the water including an existing pier and floats that will
replaced so that they will be safe to walk on. Also,there was existing debris on the shoreline that
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Sep. 24, 2019
Apr. 13, 2020
Page 2 of 6
will be removed per',Tacoma Power's Shoreline Use Specifications Guidelines (SUSPG) per the
above reference.
Also, there was solve evidence of slide activity especially near the toe of the slope where parts
of the existing bulkhead are missing due to wood decay over time. This geotechnical engineering
report will describe in more detail the results of the slope stability analysis and the need for a
replacement bulkhead to ensure long term stability of the vegetated slope.
Geology
Most of the Pacific(Northwest area had been covered with extensive glaciers that advanced and
receded at least four(times in the last 10,000 to 40,000 years ago leaving sands, silts, and gravels
during the last recession. Also. prior to this period there had existed an inland sea that existed
several million yearn ago and left compacted silt and sand deposits. The last glaciation advance
and recession resulted in the formation of Puget Sound due to the scouring action of the thick ice.
Evidence of these glaciers can be seen in the smooth pebbles, cobbles, and boulders that were
ground smooth by the sandpaper-like action of the glacier ice and the broken and fractured rock
in the glacier's path'! The last glacial advance and recession took place over 11,000 years ago
with thicknesses of over half a mile high.
Site Geology
The smooth cobble$ present within a sandy silt matrix indicates glacial deposition and recession
within the last 40,00,:0 years. The Lake Cushman depression may have been caused by the scour
of the last retreating glaciers which simultaneously ground up the rocks within the glaciers
themselves resulting! in smooth cobbles and pebbles. Cut banks along the local roads indicate
deep depositional layers of very dense glacial till soils within a sandy silt matrix.
Upslope geomorphology indicates relatively steep heavily treed slopes with Douglas Firs and
pines growing near '(vertical indicating mostly stable slopes over the life of these trees. Maple
trees were also observed near the toe of the slope. Any downward slope movement due to
potentially heavy rainfall and high winds would have caused the tree trunks to curve into a pistol
butted handle shape.! There was little to no evidence of curved tree trunks upslope of the project
site.
Groundwater Conditions
Little to no surface or subsurface groundwater or surface water drainage was observed on the
site. Also, no upland waterbodies from the project site were present when this geotechnical
engineer visited the site. Little to no significant signs of seeps or wetland areas along the face of
the slope was observed. The slope was heavily vegetated which helps to protect the soils from
heavy rainfall which can soften unprotected soils and potentially can cause a slide event in the
future. It is recommlended that all trees and vegetation be preserved on the slope.
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Sep. 24, 2019
Apr. 13. 2020
Page 3 of 6
Upland Geomorphology
The area above the stair landing and pier consist of treed and vegetated 40 to 50% slopes. Tile
existing trees were estimated to be between 60 to 80 years old and were growing nearly vertical
indicating slope stability for at least this length of time. As a result, there's been little to no
change in the upland geomorphology over time. The source of the insitu soils is the deposition
from the last glacial advance and recession that occurred approximately 10,000 to 20,000 years
ago.
Geomorphology of the Lake Cushman Shoreline
On an annual basis the level of Lake Cushman gets filled due to the Cushman dam that has
artificially produced the man-made lake. Then typically, in the fall and winter, when electrical
power is required for heating, then the elevated water conditions is used to run the electrical
generators within the dam and, as a result,gradually lowers the lake level.
This annual rising and lowering of the lake level has a tendency to saturate the otherwise dense
granular material that comprises the local soils. The smooth, rounded cobbles and some boulders
indicate that this material is glacial deposits as a result of the last major glacial advance and
recession. Once this very dense till soils get saturated,there is then a tendency for the binding silt
matrix that holds the cobbles and boulders together to loosen and slough away into the lake.
This, in turn,can result in the potential erosion of the shoreline and the toe of the hillsides.
Other alternative methods such as planting additional vegetation is not effective since this still
leaves the toe of the slope vulnerable to the fluctuating lake level with results as described above.
Soil Exploratory Borings
According to the USDA Web Soil Survey, the soils in this slide prone area have been identified
as rough moamkrir7oats land, Hoodsport soil material (Re) per the online Web Soil Survey prepare
by the USDA Natural Resources Conservation Service. The first soil exploratory site was at the
toe of the slope which indicated smooth gravels embedded in sandy loam soils for a depth of at
least 6 feet or more. There was some evidence of slope movement near the toe due to the
steepness of the slope and also the scouring wave action of Lake Cushman along the shoreline.
The second soil exploration site was located approximately a third of the way up the slope where
some slide activity exposed more sandy silty soils with gravels. Relatively heavy ground cover
that included salal and indigenous brush is providing same soil protection from heavy rainfall that
can occur in the Pacific NW. Reference is made to the attached drawing Sheet 1.0 which shows
the location and cross-sections for the two borings.
Slope Stability Analysis
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Sep. 24, 2019
Apr. 13. 2020
Page 4 of 6
A detailed slope stability analysis was completed for the site as indicated on Engineering Sheet
2.0 which took into account the height of the slope,the density of the insitu till soils, the cohesion
of the native soils, and the internal friction angle of the slope material. For the static or non-
seismic case,the resulting factor of safety was determined to be 0.934 and since this was less than
the normally accepted value of 1.500, the slope was considered unstable.
It is to be noted that::a new wooden bulkhead would increase the overall longterm stability of the
65% slope and would also help to mitigate a slide event by reinforcing the toe of the slope, thus,
minimizing the potential of another slide event since the reinforced toe helps to hold the slope in
place especially in the case of a seismic event. The following analysis determines the stability of
the existing slope in the case of an earthquake event.
Using similar inputi parameters as was input for the static case, the resulting factor of safety
resulted in 0.785 and since this is less than the normally accepted value of 1.100, the slope was
considered unstable. Again,a new bulkhead would help to stabilize the toe of the slope which in-
turn would increase the longterm stability of the hillside.
Land Slide Activity in the Area
The shoreline appears to be unraveling with loose cobbles and gravels especially where the
existing stairs and Ianding are located. The seasonal fluctuations of Lake Cushman water level
which appears to loosen the glacial till type soils which initially are very dense at nearly the
density of concrete. However, when the water level is at its highest point, the water has a
tendency to saturat, these very dense soils and the fine silts which acts as a binder will be
removed. As a result, the larger gravels and cobbles become loose since there is nothing holding
them together.
As the toe of the slope gradually gets undermined due to loose gravels and cobbles.then the rest
of the slope will now, have a tendency of becoming unstable due to unraveling at the toe of the
slope. Thus, it is important that an adequate bulkhead gets properly designed and constructed that
will help to protect the toe of the slope from further soil erosion.
Storms also can generate significant wave action which in turn can cause soil erosion especially
along unprotected areas of the shoreline. Again, a well-designed bulkhead can help to protect the
shoreline. A follow-�on engineering analysis and design will be prepared per the SUSGS.
It is to be noted that this geotechnical engineer has observed a number of slide areas on both the
east and west sides of the Lake Cushman due to items already listed in this section as well as due
to the steepness as Well as the height of the slopes around Lake Cushman. It is to be noted that
the lake was created)by the construction of the dam which in turn has resulted in the formation of
a large lake surrounded by relatively steep slopes and resulting residences. Thus. any land-
disturbing activities(during the construction of access roads and the building themselves can have
the potential of initiating a landslide if the necessary geotechnical precautions are not adhered to.
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Sep. 24, 2019
Apr. 13, 2020
Page 5 of 6
If no action is taken there can be continual beach erosion which could adversely affect utility
lines and potentially the residence itself if nothing were done to mitigate the continual shoreline
soil erosion. The proposed bulkhead is located as far landward as possible to preserve shoreline
ecological functions.
Land Slide Mitigation
Mason County's Geotechnical Checklist has been filled out and attached to this geotechnical
engineering report and provides guidelines to mitigate future landslides. Also, groundwater can
be a cause of slides if not properly mitigated, i.e., the groundwater or seep would need to be
channeled away from the proposed construction area. It is to be noted that no groundwater or
seep or wet areas were observed on the uphill side of the slope.
Bio-retention Bulkhead Replacement
This "soft" solution is not applicable for this slope since the base of the slope consists of gravel
and cobblers and that any plantings in this area is not feasible. For this steep toe condition, the
most effective solution is a mechanically stabilized wood bulkhead. it is to be noted that a
proposed fixed pier and dock is planned at the toe of the slope,
Vegetation
The site in General is vegetated with firs and ground vegetation which helps to protect the
relatively steep slope located between proposed new bulkhead, pier, and dock and the single-
family residence. It is important that the trees and vegetation on the slope be preserved. If a
view is required, it is much better to trim the limbs than to top the trees which preserves the
health of the trees.
On-Site and Off-Site Impacts(if any)
It is anticipated that there will be little to no on-site impacts since the only construction activity
is mostly the recontiguration of the existing pier and floats in the area. Also, there is no off-site
impacts since all the construction activity remains onsite.
Final Development Conditions and Structural Mitigation
All existing shoreline structures will be removed and replaced with news structures.
The planned bulkhead replacement location is shown on Sheet 1.0 and will be constructed on
both sides of the existing stairs. Currently, there is severe soil erosion occuITing under the stairs
and, therefore, these existing stairs will be removed. It is to be noted that since the bulkhead and
the pier will be at the same elevation, access to them will be straight and level without the need
for stairs.
Sep. 24. 2019
Apr. 13, 2020
Page 6 of 6
Structural Mitigation Plan
The primary s,�rLlCtural mitigation plan will be to upgrade the existing wooden bulkhead to a
length of at least 10 eet on both sides of the existing stairs and landing for a total of 20 feet of
upgraded wooden bu khead. This bulkhead will be constructed of Alaskan Yellow Cedar posts
and AYC planks atta(lied to the uphill side of the posts.
This geotechnical engineering report also satisfies Mason County's requirements checklist
which has been attached to this report as well. Any questions or comments regarding this
geoteclinical ei)ginecring report can be addressed by calling (360) 310-070 or by e-mail at
Respectfully Subrni�ted.
Steven P. Morta, 4.
OV WAS
30758
(GIS-TV
S'IONAI,
References:
I. Online Soil Web Survey prepared by the USDA Natural Resource Conservation Service
that provides a detailed description of the soils and soil strata as well as its origin.
2. Mason 6unty Washington GIS Web Map
3 Stability Irarts for Uniform Slopes by Radowslaw L Michalowski, F.ASCE from the
Journal of Geo echnical and Geoenvironniental engineering, April 2002.
4/27/2019 Web Soil Survey
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Area of Interest(AOI) Soil Data Explorer Download Soils Data Shopping Cart(Free)
Printable Version'. Add to Shopping Cart
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Warning:Soil Map may not be valid at this scale. __--
Printable Version!
Report—Map Unit Description
Mason County,Washington
https://websoilsurvey.sG.egov.usda.govIApp/WebSoilSurvey.aspx 1/2
4/27/2019 Web Soil Survey 5
Rc—Rough mountainous land,Hoodsport soil material
Mason County,Washington(WA�45) Map Unit Setting
Mason County, Washington (WA64�) National map unit symbol:2hms
Elevation: 160 to 1,800 feet
Map Acres percent Mean annual precipitation: 1S to 60 inches
Unit Map Unit Name in of AOI Mean annual air temperature:48 to 50 degrees F
Symbol AOI Frost-free period: 150 to 180 days
RC 5G 1.9 65.0% Farmland classification:Not prime farmland
._ -.o:l Map Unit Composition
Rough mountainous land and similar soils: 50 percent
W ]..0 35.0% Hoodsport and similar soils:40 percent
Estimates are based on observations,descriptions,and transects of the
Totals for Area of 2.9 ,100.0% mapunit.
Interest iDescription of Rough Mountainous Land
Typical profile
HI -0 to 7 inches:gravelly sandy loam
H2- 7 to 60 inches:stratified extremely gravelly sandy loam
Properties and qualities
Slope:30 to 60 percent
Depth to restrictive feature: More than 80 inches
Natural drainage class:Excessively drained
Capacity of the most limiting layer to transmit water(Ksat): High
(1.98 to 5.95 in/hr)
Depth to water table:More than 80 inches
Frequency of flooding: None
Frequency of ponding:None
Available water storage in profile:Very low(about 2.7 inches)
Interpretive groups
Land capability classification(irrigated):None specified
Land capability classification(nonirrigated): 7e
Hydrologic Soil Group:A
Hydric soil rating: No
Description of Hoodsport
Setting
Landform:Till plains
Parent material: Basal till
Typical profile
HI -0 to 6 inches:stony medial sandy loam
H2-6 to 28 inches:extremely gravelly medial sandy loam,very
gravelly sandy loam
H2-6 to 28 inches:gravelly sandy loam
H3-28 to 60 inches:
Properties and qualities
Slope:30 to 60 percent
Depth to restrictive feature:20 to 40 inches to densic material
Natural drainage class:Moderately well drained
Capacity of the most limiting layer to transmit water(Ksat).Very low
to moderately low(0.00 to 0.06 in/hr)
Depth to water table:About 18 to 36 inches
Frequency of flooding: None
Frequency of ponding: None
Available water storage in profile:Low(about 3.2 inches)
Interpretive groups
Land capability classification(irrigated):None specified
Land capability classification(nonirrigated): 7e
Hydrologic Soil Group:B/D
Hydric soil rating: No
Description—Map Unit Description
https://websoilsurvey.sc,egov.usda.gov/App/\�ebSoilSurvey.aspx 2/2
NORTH
SITE PLAN AND SOIL EXPLORATORY BORINGS 25�
w E
,If,
#l' 8'x10' and 10'x12'
FLOATS TO BE
-�L REMOVED
EXISTING STAIRS
AND LANDING
(TO BE REMOVED) PIER&FLOAT ��'
TO BE REPLACED y�
NOTE: ALL OTHER REMAINING OVER-THE-WATER
STRUCTURES TO BE REMOVED.
To increase long term slope stability it is recommended
that all vegetation and trees,that are not within the stair/landing 780'
footprint shall be preserved. Setbacks and buffers are already
in place.
760,
y rr g �
= ff lE .r,�.h�i ✓. ..
1 -' 736 FT (APPROX.)
tiN R MO 100' 80, 60' 40' 20' 0'
��ti OF WAS/I �PJ9
�0A
6 TO 10"MINUS SMOOTH COBBLES AND GRAVELS IN A LOOSE SANDY LOAM MATRIX
30758
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VERY DENSE 4 TO 8"GRAVELLY SANDY LOAM SOILS
Site Plan and Soil Exploratory Borings Prepared by: MET Engineering,PLLC,1018 E.Wishkah St,Aberdeen,WA 98520-2937
(360)289-0958, (800)590-0958,(360)310-0270 text.
Prepared for:TAYLOR CUSHMAN,LLC (Kyle Taylor,Owner) Nov.4,2018 Site Visit on Thu.0-24-2018 SHEET 1.0
Rev: Dec.9,2018 at 10:00 to 11:30 am