HomeMy WebLinkAboutBLD2003-00536 Geotech Review - GEO General - 6/12/2003 MASON COUNTY
PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER
Shelton,Washington 98584
DATE: June 12'', 2003
INTER-DEPARTMENTAL COMMUNICATIONS
TO: Rick Mraz, DCD - Planner
FROM: Alan A. Tahja, P/W - Co. Hydr. Engr. WO#PLG-03
SUBJ: Geo-Tech Review NAME: Seamans SFR @ 1191 Larson Boulevard
BLD2003-00536
Rick,
The geotechnical report prepared for the proposed Seamans' residential development of 1191
Larson Boulevard, Befair, has been received and reviewed by Public Works.
The report appears to satisfactorily address County requirements for geotechnical reporting. The
report's author indicates that, in his opinion,the proposed development of the property will not
cause stability problems for either the subject property or neighboring properties, and he feels
additional reporting is unwarranted. From the contents of the report, I am inclined to agree with
the author, and recommend accepting the report as satisfying the County's requirement(s) for
stability investigation. Recommendations contained in the report should be incorporated into the
site's development and made conditions for permit issuance.
With these recommendations incorporated into the site's development, stability issues appear to
have been adequately addressed.
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.
Si cerely,
Al n A. Tahj
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Allen L. Hart Engineering Geologist
1720 North Oakes, Tacoma, Washington 98406
(253) 752-8963
RECEIVED
June 5,2003 Project No.0305034 jUN 0 9 2003
426 W: CEDAR ST+
Victor Seamans
2700 NE Old Belfair Highway tiS dt143J ;M 9Z17
Belfair,Washington 98528
Report of Soil Study and Geological Assessment togZ 6 0 �n �
Proposed Single-Family Residence Building Site Q 3 n 1 3 0 3 8
1191 Larson Boulevard
Belfair,Mason County,Washington
This report presents the results of a soil and slope review and geologic assessment directed at evaluating
potential landslide hazards at the referenced site.The scope of this study was developed based on our
telephone conversations and my site visit on May 16,2003.Written authorization for this study was
received from you on May 28,2003.Fieldwork for the project was completed on May 30,2003.
The subject site is located on the east side of Larson Boulevard between northeast Saber Drive and
Cutlass Way and approximately four miles west of the center of Belfair.The site location is shown on the
Vicinity Map,Figure 1 of this report.The subject lot is roughly rectangular in shape with the proposed
building site sitting approximately 18 to 20 feet above the level of Larson Boulevard. Slopes to Larson
Boulevard range from 33 degrees to 37 degrees(65%to 75%)with slope heights ranging from about 10
to 18 feet.A slope of lesser height and grade(30 degree to 32 degree slope,three to four feet high)
parallels a portion of the south property line,as shown on the attached Site Sketch,Figure 2.The
proposed building site and yard area have been roughly graded and driveway access to the building site
developed as shown on the Site Sketch.Based on our conversations and the sketch provided, it is my
understanding that you propose to place a 14 foot by 70 foot mobile home on the rear of the lot and
construct a carport in the southeast lot corner adjacent to the home.As shown on Figure 2 the home will
sit about 60 feet back of the highest portion of the west facing slope and roughly 20 feet from where the
slope terminates at the upper end of the driveway. Within 10 feet of the termination point the slope is less
than three feet in height.
GEOLOGIC AND GEOTECFMCAL CONSULTING
Project No.0305034
1191 Larson Boulevard,Belfair,WA
June 5,2003
Page No.2
It is understood that the primary and reserve septic drainfields well be located between the home and the
top of the west facing slope.It is anticipated that little if any additional grading will be required for site
development.With the exception of a proposed low retaining wall along the up hill side of the driveway,
site slopes will generally remain undisturbed.It is anticipated that storm runoff from the residence will
be dispersed on the site using splash blocks or collected and disposed of in the ditch along Larson
Boulevard.
With the exception of the recent grading and driveway construction the property does not appear to have
been previously developed.Grading of the development area appears to have been done as a rough cut-
fill balance with a two to three foot high cut being made along the east side of the area and spoil
materials being spread westward to the top of the slope. Although some material has been stockpiled
adjacent to the top of the west facing slopes, it does not appear that a significant amount of material was
pushed over the top of the west facing slope.
No vegetation remains in the graded area.Outside of the graded area vegetation is generally comprised of
sparse brush,vines,weeds,grasses and shrubs with a few small evergreen trees in the southwest portion
of the lot.
No existing slopes will be steepened or cleared and all areas of bare soil will be re-vegetated and
landscaped such that the potential for the post-construction development of erosion and/or landslide
hazards is minimized.
Methodology
Soil and slope conditions of the subject property were visually examined and evaluated using available
slope exposures and road cuts in the site vicinity. Additionally four soil logs for water wells in the
general site vicinity were reviewed.On site information combined with various soil exposures around the
site provided a basis for a general understanding of the local soil and geologic conditions as discussed
below. In addition to the observation of available soil exposures a reconnaissance of the area was
completed and readily available geologic and soil maps reviewed.
Resources used:
1. "Washington State Coastal Zone Atlas,Mason County volume"
2. "Soil Survey of Mason County Washington",United States Department of Agriculture, 1960
3. U.S.G.S. 7.5 minute series topographic map`Belfair, Washington Quadrangle"
Allen L.Hart Engineering Geologist 1720 North Oakes,Tacoma,Washington 98406
Project No.0305034
1191 Larson Boulevard,Belfair,WA
June 5,2003
Page No.3
4. "Relative Slope Stability of The Southern Hood Canal Area, Washington,"map I-853-F,
Washington State Department of Natural Resources,Division of Geology and Earth
Resources, 1977.
5. Lidar and aerial photographs of the site area.
Geologic Setting
The site is located in the south-central portion of Puget lowlands,an elongate,north-south trending
structural depression and topographic trough. The Puget lowlands have been filled several times by
glacial ice,resulting in many topographic features that are a result of both glacial and post-glacial erosion
and deposition. Site soils,topography and geology result primarily from the advance and retreat of the
Vashon Stade(most recent glacial advance)of the Frazier Continental Glaciation. In the project area the
typical soils appear to be a dense to very dense lodgement glacial till capping the upland areas,which is
underlain by Advance Outwash deposits and locally overlain by Recessional Outwash deposits(the
Advance and Recessional Outwash equate to the Soil Conservation Service soils of the Everett and or
Indianola Series). The glacial till in the area is generally comprised of a mixture of sand, silt and gravel
picked up by the ice sheet as it moved, then deposited at the base of the glacier and overridden and
densified by some 1,500 to 3,000 or more feet of ice.The general appearance of the till is similar to a
very dense, lean concrete. Outwash exposed in the site area is comprised primarily of gravels and sands
with some scattered siltier layers.Typically near surface materials are found to be loose near the surface,
becoming denser with depth. The unweathered glacial till and outwash materials are generally dense to
very dense.
Surface Conditions
As previously indicated the proposed building site is located on a graded bench flanked by level ground
to the north and east and overlooking a steep slope to the west. To the south of the bench the ground
drops down a low, gentle slope to the adjacent lot and flat ground. The area of steep slope varies from
about 10 to 18 feet in height with slope grades generally in excess of 40%. There is no evidence of any
slope instability on the lot or adjacent properties and in discussions with the neighbor to the east it was
learned that he knows of no slope stability, erosion,or ground water problems in the area.
Referencing the Coastal Zone Atlas of Washington,Mason County volume, the site is located on the
edge of the mapped area and is considered to be stable. Similarly the map"Relative Slope Stability of the
Southern Hood Canal Area, Washington' indicates that the site is in an area where the slopes are
considered to be class 1 (stable)slopes and overlooking steeper Class 2 slopes to the west. Class 2 slopes
Allen I—Hart Engineering Geologist 1720 North Oakes,Tacoma,Washington 98406
Project No.0305034
1191 Larson Boulevard,Betfair,WA
June 5,2003
Page No.4
are"Areas believed to be stable under normal conditions,but may become unstable if disturbed...over
steepened—or subjected to strong seismic shaking."Neighbors report no slope soil movement during the
6.1 magnitude earthquake of 2001.
No active springs or seeps were observed during the site reconnaissance.
Soil Profile and Subsurface Conditions
Based on the results of my fieldwork,reconnaissance,and review of readily available maps and water
well soil logs it appears that the site is underlain by outwash type materials.The soil logs for water wells
in the site vicinity indicate that interbedded sands and gravels similar to those exposed on the site may
reach depths ranging from roughly 50 feet to 160 feet or more.A generalized soil log for the lot
developed from the northeast property corner to the southwest corner at Larson Boulevard indicates that
below a 2.5 foot thick mantle of topsoil,tree roots and loose sand with minor gravel the lot is underlain
by more than 24 feet of interbedded gravelly sand,sand and gravel,and sand with lenses of gravel.As is
typical for similar deposits,near the surface the soils are loose but rapidly become denser with depth.
Mapping by the Soil Conservation Service(SCS)indicates that the lot is on or near the contact between
the Indianola loamy sand, 5%to 15%slopes and the Everett gravelly loamy sand, 5%to 15%slopes.The
SCS indicates that the parent material for both of these soil types is a glacial outwash.
Based on the soils exposed on and around the property and shown on the water well logs, it is my opinion
that the site soil conditions are generally consistent with those shown on the readily available maps of the
area.
Conclusions
It is my opinion that the site area meets the requirements of the Mason County Resource Ordinance
Chapter 17.01.100 for classification as a landslide hazard area. Specifically, section
• 17.01.100,A"Classification", If- "Any area with a slope of 40%or steeper and with a vertical
relief of 10 or more feet...."
Based on my site observations and review of pertinent materials, it is my opinion that although the west
facing slopes on the subject site meet the ordinance definition for a landslide hazard area the potential for
landsliding on the lot is slight;the steep slopes are of minimal height,the site soils are well drained,and
below the weathered/disturbed zone the insitu soils appear to be medium dense to dense. It is further my
Allen L.Hart Engineering Geologist 1720 North Oakes,Tacoma,Washington 98406
Project No.0305034
1191 Larson Boulevard,Belfair,WA
June 5,2003
Page No.5
opinion that with completion of the proposed development the present stability of the site slopes will not
be jeopardized nor will the stability of adjacent properties be jeopardized.
Ordinance section 17.01.100,D,6`Buffers"requires that a 50 foot wide buffer be maintained around
landslide hazard areas.Although as indicated above the west facing site slopes meet the ordinance
definition as a landslide hazard area it is my opinion that the maintenance of a 50 foot wide buffer is
excessive under the given site conditions and development plans.At present there is only sparse
vegetation on the slope and no vegetation at the top of the slope. With completion of the project,
landscaping of the yard and vegetating of the slope areas, it is my opinion that the intent of the ordinance
buffer requirement will be maintained.
As indicated above the yard area will be landscaped and the slopes vegetated with completion of the
development. With regard to the establishment and maintenance of slope vegetative the Washington State
Department of Ecology(DOE)has three publications,which may be helpful in developing long-term
slope vegetation maintenance/protection and landscape plans.
1. "Slope Stabilization and Erosion Control Using Vegetation:A Manual of Practice for
Coastal Property Owners",May 1993,Publication 93-30.
2. "Vegetation Management: A Guide for Puget Sound Bluff Property Owners",May 1993,
Publication 93-31.
3. "Surface Water and ground Water on Coastal Bluffs:A Guide for Puget Sound Property
Owners",June 1995,Publication 95-107.
Additional Studies:
Item 7 under"Content of the Geological Assessment'asks for a recommendation whether a geological
report should be required to further evaluate the site and proposed property development. Referencing the
geotechnical report applicability portion of the Resource Ordinance,section 17.01.100,E, 1,the site meets
the requirements of category a: "Areas with slopes greater than 40%(21.8 degrees)will require a
geotechnical report..."However,adjacent properties have been previously developed without the aid of
geotechnical reports and the actual purpose or usefulness of completing the geotechnical report for this
specific lot,other than fulfilling Resource Ordinance requirements, is questionable. It has been shown
that the property is in an area where the slopes are considered to be stable,the lot is not in an area
evidencing a past history of landslide movement,the soils are well drained and there are no reports of
soil movement or erosion on the lot.Additionally,the residence will be set back from the top of the slope
on the order of three to four times the slope height,approximately 40 to 60 feet depending upon where
Allen L.Hart Engineering Geologist 1720 North Oakes,Tacoma,Washington 98406
Project No.0305034
1191 Larson Boulevard,Belfair,WA
June 5,2003
Page No.6
the measurement is made.It is my opinion that additional geotechnical studies are not required for the
proposed development of this lot.
Report Limitations
This report has been prepared for the exclusive use of Victor Seamans for use in development
considerations on the subject property.The conclusions in this report are based on my visual
observations, interpretation of site conditions as they presently exist,and the expectation that the
exploratory efforts adequately define the subsurface conditions throughout the site.The soil conditions
described in this report and the conclusions and recommendations contained in this report are provided
for this specific site only and should not be expanded for use on adjacent sites or properties without
additional exploration and review of those sites by this firm.
NOTE: Although I have reviewed subsurface conditions as part of this study,I have not conducted
analytical laboratory testing of any samples obtained,have not evaluated the site for the potential
presence of contaminated soil,and have not evaluated or addressed ground water conditions or concerns
except as noted in this report.
The owner and the contractor should make themselves aware of and become familiar with applicable
local,state,and federal safety regulations, including current O.S.H.A. safety standards.Construction site
safety generally is the sole responsibility of the contractor.The contractor shall also be solely responsible
for the means,method,techniques,sequences,and operations of construction operations.The firm,Allen
L. Hart Engineering Geologist(including consultants and subcontractors)is providing the preceding
information and recommendations solely as a service to Victor Seamans.Under no circumstances should
the provision of this information be construed to mean that the firm Allen L. Hart Engineering Geologist
(including consultants and subcontractors)is assuming responsibility for construction site safety or the
contractor's activities; such responsibility is not implied and should not be inferred.
Within the limitations of scope,schedule,and budget for this work,it is warranted that the work has been
done in accordance with generally accepted practices followed in this area at the time this is report was
made.No other warranty,expressed or implied is made.
Allen L.Hart Engineering Geologist 1720 North Oakes,Tacoma,Washington 98406
Project No.0305034
1191 Larson Boulevard,Belfair,WA
June 5,2003
Page No.7
Should you have any questions or if I may be of additional assistance, please call my office at(253)752-
8963.
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Sincerely,
Engineering Geologist
sa4 t
Allen L. Hart,CPG, RG/CEG
Licensed Engineering Geologist AL. HART
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FIGURE 1-Vicinity Map
ALLEN L. HART ENGINEERING Location: 1191 Larson Boulevard,Belfair,Mason County,
GEOLOGIST Washington
1720 North Oakes,Tacoma,Washington 98406 Job No. : 0305034
253-752-8963 Client: Victor Seamans
Date: 6/5/03
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Allen L. Hart Engineering Geologist FIGURE 2- SITE SKETCH
Location: 1191 Larson Boulevard,Belfair,Mason County,Washington
1720 North Oakes,Tacoma,Washington 98406 Job No. : 0305034
253-752-8963 Client: Victor Seamans
Date: 6/5/03
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Allen L. Hart Engineering Geologist FIGURE 3- CROSS SECTION LOCATIONS
1720 North Oakes,Tacoma,Washington 98406 Location: 1191 Larson Boulevard,Belfair,Mason County,Washington
Job No. : 0305034
253-752-8963 Client: Victor Seamans
Date: 6/5/03
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Allen L. Hart Engineering Geologist FIGURE 4— CROSS SECTIONS
Location: 1191 Larson Boulevard,Belfair,Mason County,Washington
1720 North Oakes,Tacoma,Washington 98406 Job No. : 0305034
253-752-8963 Client: Victor Seamans
Date: 6/5/03
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