HomeMy WebLinkAboutGeoTech Report for BLD2005-00023 - GEO General - 3/14/2006 MASON COUNTY
n o PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER
o N =; Shelton, Washington 98584
N y
DATE: March 14, 2006
INTER-DEPARTMENTAL COMMUNICATIONS
TO: Chuck McCoy PARCEL# 32226-34-00090
FROM: Alan Berbisco, Proiect Engineer-PW BUILDING PERMIT NUMBER: Bld2005-00023
SUBJECT: Geotech. Report Review NAME: Todd Wagner
Chuck,
The Geotechnical Report prepared for the proposed residence located at 1621 NE Blacksmith Dr. Belfair,
has been received and reviewed by Public Works.
The content of the report discussed more on stability of the site. There is no recommendation for final
location of the proposed building or residence. From the authors opinion,the proposed improvement is
not expected to result in any adverse on-site, or off-site impacts, and the proposed residential development
of property will not result in adverse impacts to either the subject property or to neighboring properties.
I recommend accepting this Geotechnical Report, but drainage, building setback, location of drain field,
storm water management features,vegetative management, erosion control recommendation, foundation
and others, should be addressed when the site development plan will be finalized or might submit
additional report addressing all development for the proposed residence.
Adequate Erosion and Sediment(E&S) control features need to be implemented during land disturbing
activities to protect neighboring properties and State waters from adverse stormwater runoff impacts. It
shall be the responsibility of the property owner and the permittee to ensure that accelerated erosion does
not occur during and after the project construction. The migration or release of silty water or mud from the
applicant's property will be considered a violation of County and State water quality protection
regulations.
Please feel free to contact me at 461 if you have any questions regarding these comments, or if you feel
any features need further discussion or attention.
Sincerely,
Afff r is
Pro 3et Engineer
00 0 c:
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
Plans include _-.------pages of-
Mason County DCD engineering dOr-Ur tents that must
Planning Division Yem,ain attacheiJ io approved pl"Ar's
PO BOX 279
Shelton, WA 98584 APPROVED ���
Todd Wagner MAS N ILDING INSPECTOR �� ``�
7402 Granite Lane SU J CT TO SRO"J C t, � �
Olympia, WA 98516b
RE: Geotechnical Report
FILE Parcel No. 32226-34-00090
Approx. Milepost 12,North Shore Rd
COPY Tahuya, WA 98588
INTRODUCTION
Mr. Todd Wagner wishes to develop the above identified five acre parcel of property for
a residence. His development plans include a partial timber harvest, and development of
a building site on the property along with connection to a drinking water source already
on the property, and other utility installations. The property contains a ravine or narrow
valley that accommodates a small non-fish bearing stream which traverses the parcel
from the property's northwesterly corner to the property's southeasterly corner. The
County road culvert and other obstructions downstream from the Wagner property have
placed the stream in a non-fish bearing classification. The timber harvest proposed in the
five acres' development will not result in any new stream crossing, and though timber
will be harvested from both sides of the stream,no logs are planned to be felled across
the creek or drug through it.
Mr. Joe Staley,with Washington Timber Management(WTM)has proposed a 50/70
timber harvest wherein roughly half the trees will be harvested with a target 70%of the
merchantable timber taken. The timber harvest plan is being prepared by WTM.
Mr. Wagner considers the southwesterly portion of the property the most easily, and the
most economical area of the property to develop for a home site. Though there will be
some difficulty in accessing the hillside in the southwesterly quadrant of the property, it
will provide a panoramic view of Hood Canal, and is very likely the best use of the
property. Two avenues of access to the proposed development area of the property were
pioneered in during past timber harvests.
Mr. Wagner has enlisted my services in providing a geotechnical report and related
documents needed to complete his Forest Practice Act Application.
File:C:IProjectsXagnerU Acre FPkGeo-Rpt.doc 1 of 5 2/26/2006
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
A site plan, cross section, and a stability analysis sheet are provided as an appendix to
this report.
The following ten 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 has medium aged second growth fir,
hemlock, and cedar tress growing in clustered areas of the property. Besides the conifers,
the property has a moderate covering of Salal, Oregon grape, ferns, and other vegetation.
The Series 1950,No. 9 SOIL SURVEY of Mason County indicates the site soils are an
Everett gravelly sandy loam (Ek)typically found on 15 - 30 percent slopes. The soil is
described as excessively drained and very variable. The soils observed on the site
conform with the gravelly descriptions of the soil and depths of 12 to 36-inches to the
glacial till/hardpan substratum. The Coastal Zone Atlas describes the slops as stable, but
susceptible to instability if disturbed.
The high gravel content of the soil places the soil in a coarse—grained soil classification
and for analysis purposes appears to be a GM or GP soil designation in the Unified Soil
Classification System.
The stream and existing gravel road that traverse the property have a grade of
approximately 10%. The road and creek bed create a small valley floor which is
generally a level grade across the valley at any location. The hillside slopes emanating
from the valley floor climb to the east and west with slopes ranging from 15 to 20%on
the southerly end of the property to 100% in localized areas.
It appears that very little site grading will be required to situate a house on the back half of
the property, or on the front quarter of the property. Installation of the septic system will
probably include the installation of the gravel trenches along the naturally existing contours
of the site. Stormwater runoff from the house construction and the septic system installation
can be expected to direct itself to the lower portion of the property along Kokanee Ridge
Drive. Silt fences may need to be installed to intercept stormwater runoff if construction is
undertaken during wet weather. Hay bales installed to act as water dams are an alternative to
silt fence installation.
2. DEVELOPMENT AND GEOLOGIC FEATURES:
A site plan has been developed for this property and is attached in the appendixes. The
final location of the residence will be decided once the timber harvest has been
completed, and will be addressed in a subsequent geotechnical report.
File:C:IProjectsiWagnerU Acre FPkGeo-Rpt.doc 2 of 5 2/26/2006
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
Development of the property is proposed to use as much of the existing site features as
possible. Past logging and grading on the property has carved out avenues of access to
various portions of the property. These old skid trails will be used again, and the one
that curls up the hill from behind the well house will be improved by widening it, and
surfacing it with gravel driving surface. The area where logs will be yarded to and
loaded will probably be in the location of the hairpin behind the well house, at the well
house elevation.
3. LOCATIONS OF LOGS OF EXPLORATORY HOLES OR PROBES:
The property contains several areas with cut banks made to accommodate past road
construction. These cuts reveal the variation of the substrate soil horizons, and indicate that
the soil is generally similar over the entire site. The top 2 to 5-feet of soil typically contains
the topsoil,vegetation root mat, and usually 2 to 3-feet of medium density soil, usually
derived from weathering of the glacial till materials underlying the entire site.
The"valley"floor is generally a cleaner gravel than the hillsides,mainly because it's been
delivered there by the stream during high flow conditions.
4. CROSS SECTION WITH PROPOSED GRADE CHANGES:
Cross sections are included in the appendix. The building area for a residence is still
conceptual, but a south-facing daylight basement structure would appear to be a likely
choice for the building location and style.
5. SLOPE STABILITY ANALYSIS:
The steepest slopes on the property in its present condition contain no obvious recent
mass landslides or stability problem indicators.
Classification by the Unified Soil Classification System indicates the soils are
considered a coarse grained soil (GP&/or GM)with a dry density between 120-
1351bm/ft3 and a typical effective stress friction angle(0)>34 to 37°. An effective
stress angle of 36'will be used in stability calculations.
Slope stability calculations are addressed on an Excel spreadsheet following the plans.
Foundations bearing on the medium dense Everett gravelly sandy loams can expect to
realize bearing capacities of the natural soil in the 1000 to 2000 pounds per square foot
range,with settlement for residential foundations within half an inch overall.
6. RECOMMENDED SETBACKS & DRAINAGE:
The proposed timber harvest and house site development grading will occur on steeply
sloping areas of the property. The timber harvest will be performed so as to produce a
minimal disturbance to the site and existing vegetation.No soil should be allowed to "spill"
over the old road beds in widening them, but should be placed on properly cleared and
leveled receiving benches if material is placed on the lower side of any grading,whether for
File:C:Projects)Wagnerl5 Acre FPkGeo-Rpt.doc 3 of 5 2/26/2006
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
the road widening,or for roughing out a temporary logging landing and potential building
location. Grading will be expected to comply with environmental setbacks from the stream
so far as they apply,but a major portion of the proposed residential driveway already
parallels the stream for approximately 300-feet. Widening of the road paralleling the stream
will be on the side of the road away from the stream.
The site soils are not particularly erodible because of their high gravel content and moderate
percentage of fine soils. The existing road generally drains to the vegetated areas alongside
the road, and eventually to the creek. Drainage for the residence will be addressed when the
site plans are finalized, but will be developed to minimize or eliminate stormwater runoff
from the residence's development area. The log road intended to be improved as a driveway
will include a ditch on the uphill side of the road. Road runoff from the lower side of the road
will be allowed to sheet over the road's fill embankment. Water collected in the driveway's
ditch will be conveyed beneath the access road,and delivered to a temporary settling pond
for treatment.(settling of silt)prior to release to the stream. The pond will be designed as a
short swale(ditch)with an elevated outlet, and may not be needed if construction is
performed during the dry season.
7. CLEARING, GRADING, AND VEGETATION MANAGEMENT:
The only intentional clearing will be in the vicinity of the conjectural house location,and
clearing needed to improve the condition of the existing skidder trail. WTM has
recommended a 50/70 harvest,which will realize the commercial value of the site's timber
while maintaining portion of the timber for a sustained harvest plan.
A volume of less than 200 cubic yards is expected to be needed to be wasted off site or
delivered to the site to accomplish the planned road upgrading and other site work.
8. EROSION AND SEDIMENT CONTROL:
As indicated earlier in this report,the site's soils are not considered erodible, and minimal
Erosion and Sediment(E&S) Control measures should be adequate to protect neighboring
properties and State waters.Access to the site already exists off of the North Shore Road,and
was once used for a residence on the one acre parcel of ground immediately south of the
Wagner property.
If wet weather or a storm arrives during construction activities,a pond or swale may be
needed to treat the runoff from the newly graded road areas to ensure silty water doesn't
reach the stream or Hood Canal.
When the logging operations and road construction are completed,bare soils will need to be
protected from erosion and rain. Scattering hay bales/straw 2-inches thick, applying hydro
seed, applying commercial erosion protection products or covering bare areas with visqueen,
are a few of the Best Management Practices (BMPs)typically employed to help mitigate,and
help minimize adverse stormwater runoff impacts.
File:C:IProjectsXagnerU Acre FPAIGeo-Rpt.doc 4 of 5 2/26/2006
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
9. ON-SITE AND OFF-SITE IMPACTS:
Construction and completion of the proposed improvements are not expected to result in
any adverse on-site, or off-site impacts. The proposed residential development of the
property will not result in adverse impacts to either the subject property or to neighboring
properties.
IQ SPECIFICATIONS OF FINAL DEVELOPMENT CONDITIONS:
The completed site will be allowed to return, for the most part,to a naturally vegetated state.
The main ground cover will the native vegetation already inhabiting most of the site.
In the future development of the property, discharging concentrations of collected
Stormwater runoff within 30 feet of a property boundary should be avoided.
Conclusions and Summary;
The commercial timber harvest and residential development of the Wagner property
located near Tahuya, Mason County, Washington does not appear to present any stability
problems that cannot be overcome,and will not result in any undesirable impacts.
The property itself provides an adequate vegetated buffer area to protect neighboring
properties and State Waters from adverse stormwater runoff impacts. The stormwater
related recommendations made in this report should be adequate for both temporary and
permanent stormwater runoff management and concerns.
I hope this report meets the needs of the Wagners, and Mason County in planning the
property's development.
wA. �h�
Sincerely,
Alan A. Tahja, P.E. 078�
` �hr
Attached: Site Plan (Plan Sheet 1) ts: 1!7/Zr,22(p -
Cross Section(Plan Sheet 2)
Stability Analysis(Excel Spreadsheet)
File: C:'Projects'Wagner'5 Acre FPA1Geo-Rpt.doc 5 of 5 2/26/2006
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Revision# Description Date TAHJA ENGINEERING,Inc. N Shr 5 Acre Site Plan 9C"j„=100'
Alan Tabja,P.E. Todd Wagner
PO Boa 235 7402 Granite Lane "'A.
Wagner 5
HoodvoA WA 98548 Olympia, WA 98516
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Revision# Description Date TAHJA ENGINEERING,Inc. N Shr 5 Acre Site Plan
Alan Tabja,P.E. Todd Wagner Wagner 5
PO Box 235 7402 Granite Lane
Hoodsport, WA 98548 Olympia, WA 98516 sb- 2 of2
TAX PRCL NO.32226-34-00090 (360)877-9512 (360)456-5206
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Wagner Slope Stability Considerations and Analysis
North Shore Rd Approx. M.P. 12.0
With an internal angle of friction in the 360 range, the soil could theoretically be constructed
with a fill slope approaching 2:1 (V:H based on 0/2 +450 =630).
The steepest proposed constructed slopes will be constructed at a 2:1 (H:V).
In areas where the existing skidder roads need to be widened, the widening will be done
by excavating into the hillside. In some areas, sections of ecology block retaining walls,
or large quarry rock may be used to gain vertical height if a fill slope is impractical.
Either rockeries, or ecology block walls should be assembled with a minimum
1:6 cant(lean) into the slope they're intended to hold or protect.
In some areas the road widening may be accomplished by employing a cut bank with
up to a 1:1 backslope in compliance with Mason County road construction standards.
Beneath the surface soil layer, weakly cemented glacial till was found usually at
depths of less than 5-feet. So long as any constructed footing for any structures
are excavated to bear on the underlying hardpan, the structure could be model as
though it was founded on a soft or weak rock subgrade.
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