HomeMy WebLinkAboutGEO2020-00045 - GEO Geological Review - 9/16/2020 G EO 2°1,D- !e�
Mason County Review Checklist
For a Geological Assessment
Instructions:
This checklist is intended to assist Staff in the review of a Geological Assessment.1-fie",Assessment is reviewed for
completeness with respect to the Resource Ordinance. If an item is found to be not applicable,the report should
explain the basis for the conclusion.The Assessment is also reviewed for clarity and consistency. If the drawings,
discussion, or recommendations are not understandable,they should be clarified. If they do not appear internally
consistent or consistent with the application or observations on site, this needs to be corrected or explained. If
resolution is not achieved with the author, staff should refer the case to the Planning Manager or Director.
Applicant's Name: C���✓��hZ W
Permit#: r7l_�?2O Zd— 007 io l Parcel#'s: Z
Date(s)of the Document(s) reviewed: 9 l D
1. A discussion of geologic conditions in the general vicinity of the proposed development,with geologic unit
designation based on referenced maps.
OK? ✓ Comment:
2. (a) A discussion of the groundwater conditions at the site,
OK? l,/ Comment:
(b) A discussion of the estimated depth to water,
OK? Comment:
(c) A discussion�f the quantity of surface seepage;
OK? 7 Comment:
(d) A discussion of the upslope geomorphology,
OK?_ iz_Comment:
(e) A discussion of location of upland waterbodies and wetlands.
OK? ✓ Comment:
3. The approximate depth to hard or dense competent soil, e.g. glacial till or outwash sand.
OK? ✓ Comment:
4. A discussion of any geomorphic expression of past slope instability(presence of hummocky ground or ground
cracks, terraced topography indicative of landslide block movement, bowed or arched trees indicating
downslope movement, etc.).
OK? ✓ Comment:
5. A discussion of the history of landslide activity in the vicinity, as available in the referenced maps and records.
OK? ✓ Comment:
6. An opinion on whether the proposed development is within the landslide hazard area or its associated buffer or
setback and the potential for landslide activity at the site in light of the proposed development.
OK?_ /Comment:
7. A recommendation by the preparer whether a Geotechnical-Report should be required to further evaluate site
conditions and the proposed development of the subject property.
OK?X/Comment:
If the presence of a hazard is determined within 300 feet of the proposed development, then the following are
delineated on a geologic map/site map:
Pagel of 2 Form Effective June 2008
(a) the area of the proposed development,
OK? Comment:
(b) the boundaries of the landslide hazard area(top, both sides, and toe),
OK? Comment:
-(c) the associated buffers(top, both sides, and toe)
OK? Comment: .
(d) building or othersetbacks(top, both sides, and toe).
OK? Comment:
9. 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.
OK? Comment:
Are the Documents signed and stamped? By whom? 1'c ''► o�
License#: 3 License type:
FIRST REVIEW OApproved ❑ Need more info.
If not approved, what is the next action/recommendation for further action?
Reviewed by , on _fO Time spent in review: A44 :.;,.>
SECOND REVIEW/UPDATE ❑ Approved ❑ Need more info.
If not approved, what is the next action/recommendation for further action?
Reviewed by , on Time spent in second review:
THIRD REVIEW/UPDATE ❑ Approved ❑ Need more info.
If not approved, what is the next action/recommendation for further action?
Reviewed by , on . Time spent in third review:
Disclaimer: Mason County does not certify the quality of the work done in this Geological Assessment
Page 2 of 2 Form Effective June 2008
1,
,tK)000 5 RECEIVED
JUL 2 0 2020
Mason County Department of Community Developm W. Alders.,�� 1
Submittal Checklist For a Geological Assessment
Instructions:
This checklist must be submitted with a Geological Assessment and completed, signed, and
stamped by the licensed professional(s) who prepared the Geological Assessment for review by
Mason County pursuant to the Mason County Resource Ordinance. If an item found to be not
applicable, the report should explain the basis for the conclusion.
Applicant/Owner Parcel# 22319-50-00116
Site Address xxx NE Tahuya Blacksmith Road
(1) A discussion of geologic conditions in the general vicinity of the
proposed development, with geologic unit designation based
on referenced maps.Located on page(s) 2
(2) (a)A discussion of the ground water conditions at the site,
Located on page(s) 2
(b) A discussion of the estimated depth to water
Located on page(s) 2
(c) A discussion of the quantity of surface seepage
Located on page(s) 2
(d) A discussion of the upslope geomorphology
Located on page(s) 2
(e) A discussion of location of upland waterbodies and wetlands.
Located on page(s) N/A
(3) The approximate depth to hard or dense competent soil, e.g. glacial till or outwash sand.
Located on page(s)
(4) A discussion of any geomorphic expression of past slope instability (presence of
hummocky ground or ground cracks, terraced topography indicative of landslide block
movement, bowed or arched trees indicating downslope movement, etc.).
Located on page(s) 2
(5) A discussion of the history of landslide activity in the vicinity, as available in the
referenced maps and records.
Located on page(s) 2
(6) An opinion on whether the proposed development is within the landslide hazard area or
its associated buffer or setback and the potential for landslide activity at the site in light of
the proposed development.
Located on page(s) 3
(7) A recommendation by the preparer whether a Geotechnical Report should be required to
further evaluate site conditions and the proposed development of the subject property.
Page 1 of 2 Form Effective June 2008
Located on page(s) 3
(8) If the presence of a hazard is determined within 300 feet of the proposed development,
then the following are delineated on a geologic map/ site map:
(a) the area of the proposed development,
Located on Map(s) n/a
(b) the boundaries of the landslide hazard area (top, both sides, and toe),
Located on Map(s) n/a
(c) the associated buffers(top, both sides, and toe)
Located on Map(s) n/a
(d) building or other setbacks(top, both sides, and toe).
Located on Map(s) n/a
(9) 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) Figure 4
Glen Coad, 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 Geological Assessment, dated Jan 19, 2020 and entitled Geological Assessment
Landslide Hazard Geotechnical Letter meets all the requirements of the
Mason County Resource Ordinance, Landslide Hazard Section, is complete and true, that the
assessment demonstrates conclusively that the risks posed by the landslide hazard can be
mitigated through the included geotechnical design recommendations, and that all hazards are
mitigated in such a manner as to prevent harm to property and public health and safety.
(Signature and Stamp)
GEC ,•,�
t, WA.Sffj, �
3
Disclaimer: Mason County does not certify the quality of the work done in this Geological
Assessment.
Page 2 of 2 Form Effective June 2008
DE-CIVIL, PLLC
P.O. Box 446
Tacoma,WA 98401
Ph (253) 228-0513
www.de-civii.com
RECEIVED
January 19, 2020
JUL 2 0 2020
Family Classic Homes Geologic Assessment: Landslide 615 W. Alder Street
60 NE Lake Drive Hazard Geotechnical Letter
Tahuya, WA XXX NE Tahuya Blacksmith Road
Mason County, Washington
Attn: Mr.Steve Ates PN: 22319-50-00116
Job: 20-005 Guernsey.LHGL
INTRODUCTION
This report presents the results of our geotechnical investigation for the proposed residential building to be
located at XXX NE Tahuya Blacksmith Road in Mason 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 discussions with you, and our January 17, 2020 site visit. We
understand that you are in the process of applying for a building permit for a new residence to be located on
the above mentioned lot. The building will be situated in the center of the parcel and will most likely be
comprised of a mix of concrete and wood frame construction. Clearing and grading is expected to be limited to
that required to develop sufficient access to the new building site and the proposed septic system.
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 the slopes located within the site area. We understand that because of
the height and inclination of the slopes on the site,a geotechnical assessment is being required by Mason County
to address geologic hazards at the site. Specifically, our scope of services for the project included the following:
1. Visiting the site and conducting a geologic reconnaissance to assess the site's soil,groundwater,
and slope conditions.
2. Assessing the landslide hazards in accordance with the current Mason County ordinances.
No subsurface explorations or laboratory testing were completed as part of this assessment. However,we were
able to observe the near surface soil conditions in exposed soils and open septic test pits on the site.
The services described in this report were prepared under the responsible charge Glen Coad, PE. Mr. Coad
meets the qualifications contained in the Mason County codes to prepare a landslide hazard geological
assessment. Mr. Coad understands the requirements of the current Landslide Hazard Area Chapter and the
definitions of the applicable terms contained within it. Individuals under the responsible charge of Mr. Coad
has performed a landslide hazard geotechnical assessment, conducted a field investigation, and researched
available historic records on the above referenced site.In our opinion,the scope of services completed for this
project is adequate to meet the requirements of the Department and it does not appear that an active
landslide hazard area exists within 300 feet of the site.
20-005 Guernsey.LHGL
January 19, 2020
Page 2
SITE CONDITIONS
Surface Conditions
The subject parcel is located at XXX NE Tahuya Blacksmith Road in Mason County, Washington. The parcel is
rectangular in shape and measures approximately 200 feet in length(north to south) by 140 feet in width(west
to east)and encompasses approximately 0.73 acres.The lot is currently undeveloped and is well vegetated with
second growth forest. Access to the site will be from NE Tahuya Blacksmith Road, a public road. The site is
moderately sloped with grades going down from west to east. From the road,grades rise up at 20 to 30 percent
with a height of about 6 feet and then leveling off to 10 to 20 percent grade through the building and septic
areas. The back of the lot(west side)gets steeper at 30 to 40 percent grade with an overall height of about 14
feet and continuing off site. The site is bounded by similar residential properties on the west, north and south,
and NE Tahuya Blacksmith Road to the east.
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.At the time of our site visit the property was well vegetated with trees,typical forest
understory. From the septic test pits we noted a till layer at a depth of 3 to 4 feet below surface.
Site Soils
The Natural Resource Conservation Service Web Soil Survey for Pierce County mapped the soils in the area of
the site as Alderwood gravelly sandy loam (AB, Ac). The Alderwood soils are derived from glacial till under
conifers slopes of 8 to 15 percent and 15 to 30 percent and are listed as having a "slight" and "moderate
potential for erosion respectively. Typically, these soils have very limited infiltration capabilities. A copy of
NRCS Web Soil Survey map for the site vicinity is attached as Figure 2.
Site Geology
The Geologic Map of the Lake Wooten 7.5 minute Quadrangle, Mason County,Washington by Robert E. Derkey,
Nicholas J. Hehemann and Katelin Alldritt,July 2009 indicates that the site is underlain by pre-Vashon glacial till
(Qpt). This glacial soil was deposited prior to 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 over-consolidated,which typically exhibit high strength
and low compressibility characteristics. An excerpt of the above referenced map in included as Figure 3.
No groundwater seepage was observed on the site or adjacent areas.
Landslide Hazard Indicators—per Mason County,Chapter 8
The Code of Ordinances for Mason County, Chapter 8, Section 52,Article 140 states that a landslide hazard area
is any of the following:
A. Areas with an indications of earth movement such as debris slides,earthflows, slumps,and rock falls.
B. Areas with artificial over steepened or un-engineered slopes. i.e., cuts or fills.
C. Areas with slopes containing soft or potentially liquefiable soils.
D. Areas over steepened or otherwise unstable as a result of stream incision, stream bank erosion, and
undercutting by wave action.
E. Slopes greater than fifteen percent(8.5 degrees)and having the following:
a. Hillsides intersecting geologic contacts with a relatively permeable sediment overlying a
relatively impermeable sediment or bedrock(e.g., sand overlying clay); and
b. Springs of groundwater seepage.
20-005 Guernsey.LHGL
January 19, 2020
Page 3
F. Any area with a slope of forty percent or steeper and with a vertical relief of ten or more feet except
areas composed of consolidated rock. A slope is delineated by establishing its toe and top and
measured by averaging the inclination over at least ten feet of vertical relief.
The following information may be used as a guide by the county to indicate areas that have a higher likelihood of
meeting the classification criteria previously listed:
A. The areas identified on the Mason County Soil Survey Map as having slopes greater than fifteen
percent.
B. The areas identified on the Coastal Zone Atlas,Volume 9, of Mason County,Washington as:
a. Unstable—"U"
b. Unstable Old Slides—"UOS"
c. Unstable Recent Slides—"URS"
d. Intermediate Slopes—"I"
e. Modified Slopes—"M"
C. The areas identified as Class 2, 3, 4, or 5 of the maps: "Relative Slope Stability of the Southern Hood
Canal Area, Washington" by M. Smith and R.J. Carson, Washington State Department of Natural
Resources, Division of Earth Resources, 1977; "The Geological Map of North Central Mason County,
Washington," by R.J.Carson 1976, U.S. Geologic Survey OFR 76-2.
D. Areas mapped as landslide deposits (Map Unit Qls) on the" Geologic map of the Longbranch 7.5-
minute quadrangle, Thurston, Pierce, and Mason Counties, Washington by R. L. Logan, T. J. Walsh,
and Michael Polenz. One sheet,scale 2:24,000, 2003; Geologic map of the Squaxin Island 7.5-minute
quadrangle, Mason and Thurston Counties, Washington, by R. L. Logan, Michael Polenz, T. J. Walsh,
and H. W. Schasse. One sheet, scale 1:24,000, 2003; Geologic map of the Shelton 7.5-minute
quadrangle, Mason and Thurston Counties, Washington, by H. W. Schasse, R. L. Logan, Michael
Polenz, and T. J. Walsh. One sheet, scale 1:24,000, 2003; and Geologic map of the Summit Lake 7.5-
minute quadrangle,Thurston and Mason Counties,Washington, by R. L. Logan and T.J.Walsh. Forty-
two by thirty-six inch.Color sheet, scale 1:24,000, 2004.
CONCLUSIONS
Based on our site evaluation,it is our opinion that the site is suitable forthe future construction of the proposed
residence and related appurtenances. It is also our opinion that an active landslide hazard area does not exist
on or within 300 feet of the site.
Landslide Hazard Indicators—per Pierce Mason County Chapter 8
Mason County, Chapter 8, Section 52, Article 140 uses the above referenced checklist to define a landslide
hazard area. Based on our observations of the site and review of published information,we offer the following
comments.
No evidence of landslide activity, or significant erosion was observed at the site at the time of our site visit.
Some slopes steeper than 15 percent were observed on or near the site, but no intersecting contacts were
observed or are mapped in the site vicinity. There were no slopes steeper than 40 percent with more than 10
feet in height located on the site. The site is not mapped on the Coastal Zones Atlas. We did not observe slopes
greater than 30 percent with more than 10 feet of height on the site associated.Some clearing and grading may
occur on the steeper slopes occurring on the site. Provided appropriate soil erosion control and site stabilizing
measures are employed, we do not anticipate that it will be a limiting feature to site development. No
groundwater seepage was observed at the site. No planes of weakness or rockfall hazards were observed at the
site. No other landslide hazard criteria were observed at the site or the immediate adjacent areas. Given our
observations no further geotechnical work is warranted.
20-005 Guernsey.LHGL
January 19, 2020
Page 4
LIMITATIONS
We have prepared this report for Family Classic Homes for the Guernsey's 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.
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
GL
Y` �Y
Glen Coad, PE
Owner
WGC
Doc ID:20-005 Guernsey.LHGL
Attachments: Figure 1:Vicinity Map
Figure 2:USDA Soils Map
Figure 3:USGS Geological Map
Figure 4:Site Plan
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Approximate Site Location
Not to Scale
DE-CIVIL, PLLC
P.O. Box 446 Site Vicinity Map
Tacoma, WA98401 Guernsey Residence
Ph(253) 228-0513 XXX NE Tahuya Blacksmith Road
www.de-civil.com
Mason County, Washington
Job No:20-005 January 2020 Figure 1
b Soil Map—Mason County,Washington
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Approximate Site Location 0
Not to Scale
DE-CIVIL, PLLC USDA Soils Map
P.O. Box 446
Tacoma, WA 98401 Guernsey Residence
Ph(253) 228-0513 XXX NE Tahuya Blacksmith Road
www.de-civil.com
Mason County, Washington
Job No:20-005 January 2020 Figure 2
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DE-CIVIL, PLLC
P.O. Box 446 Geological Map
Tacoma, WA 98401 Guernsey Residence
Ph(253) 228-0513 XXX NE Tahuya Blacksmith Road
www.de-civii.com
Mason County, Washington
Job No:20-005 January 2020 Figure 3
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DE-CIVIL, PLLC Site Plan
D P.O. Box 446
Tacoma, WA 98401 Guernsey Residence
Ph(253) 228-0513 XXX NE Tahuya Blacksmith Road
www.de-civil.com
Mason County, Washington
Job No:20.005 January 2020 Figure 4