HomeMy WebLinkAboutGeotechnical Report and Addendum for FPA2006-00015 - FPA Reports - 11/20/2006 TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
Mason County DCD
Planning Division
PO Box 279
Shelton, WA 98584
Art Conrad—Craftsman Properties, LLC
PO Box 1201
Silverdale, WA 98383
(360) 620-3300
Re: Geotechnical Report Addendum
Parcel Nos: 3222-76-00100 & 00110
Case No: FPA2006-00015
Mr. Art Conrad, representing Craftsman Properties, LLC, contacted me to request my re-
inspection of the subject property of my original geotechnical report to determine what
measures might be necessary to bring the completed property logging activities into
compliance with County requirements,and the recommendation contained in my original
report.
I visited the site on Friday, March 14, 2008 to inspect the completed logging operation
and to inspect the completed site for compliance with my original report. In my original
report I indicated that little or no clearing was anticipated as necessary to accomplish the
tree harvest. I found that the site had been clear cut,and most of the logging debris
(limbs, crush,tree tops)had been stacked in multiple piles distributed around the
property, and that fairly substantial skid roads had been constructed over many areas of
the property. Mr. Conrad had expressed a concern to me regarding the impact of the skid
roads,particularly in regards to stormwater runoff and hillside stability.
In my conversation with Mr. Conrad, he informed me that the skidder roads had been
constructed last year, and the site I was inspecting had weathered the December 2007
fairly well. My inspection of the"upper"area of the property noted two logging trails that
traversed down the hillside to Korpi Drive which generally bisects the Conrad property in
two. From the top end of the property, it was fairly evident where stormwater runoff had
accumulated and discharged over the crest of the steep hillside. For the most part,
stormwater tended to follow the skidder trails,resulting in a concentrated discharge to
Korpi Drive,which allowed the stormwater to continue down the hill and into the skidder
trails constructed down the hill from Korpi Drive. For the amount of rain and damage
done by December's storm,the Conrad site faired fairly well. Though the logging skidder
trails had become conduits for stormwater runoff,the amount of erosion on the entire site
was minimal,no mass wasting areas were evident, and the site appeared to be in a fairly
good condition.
File:C:IProjectslConrad FPAIGeo-Addndm 1 of 3 3/18/2008
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
I asked Mr. Conrad about observance of the stream buffers originally placed around what
was termed a Type V stream in my original investigations. Mr. Conrad informed me that
the flow in the streambed was very small (6-inch wide trickle), and that it was now being
considered an un-typed water course, or simply surface runoff.
The skidder roads,though more substantial than I would have expected, do not create any
stability problem issues for future development of the property as residential home sites,
unless building locations are chosen on the sloped areas of the properties. From the
evidence left from recent storm events and stormwater runoff trails, it appears evident
that the logging roads have become unintentional conduits for the site's stormwater
runoff,concentrating and directing stormwater runoff to the lower part of the property.
Stormwater runoff from the entire site naturally discharges to the lower part of the
property,but with the skidder trails intercepting and concentrating runoff flows,the
opportunity for stormwater runoff to be treated with natural vegetation is reduced.
I've discussed this issue with Mr. Conrad who agrees that this situation could be easily
remedied with the installation of water bars across the skidder trails placed on 50-foot
intervals. Spilling any stormwater runoff collected by the skidder trails on 50-foot
intervals will give the runoff a good opportunity to disperse into the low lying vegetation
left from the timber harvest, and provide treatment to the runoff prior to it being delivered
to the flow at the lower end of the property.
Mr. Conrad also indicated they were considering whether the logging trails should be
eliminated, or the hillsides restored to their pre-timber harvest condition. This could be
accomplished, but would likely result in a greater adverse environment impact than
simply letting the road prisms return to a natural condition on their own. The traversing
of the hillside slopes with cat trails may provide an advantage in providing groundwater
migrating down the hillside's shallow soils an opportunity to surface at the road cuts,thus
diminishing interflow through the shallow soil horizon. With the installation of water
bars as suggested, concentrated runoff flows could be avoided or diminished to near pre-
harvest conditions.
As a last step in closing out the site from the harvest operation,bare soils should be
protected from rainfall with a 2-inch layer of hay or straw after grass seed has been
broadcast over the bare areas. Areas on the slopes where concentrated stormwater
discharges are evident should be embellished and armored with a riprap or round stone
layer of armoring to protect the channels from erosion. Alternatives to armoring the
natural channels would be the collection and delivery of concentrated flows down the
slope in a pipe drain, or the construction of shallow level trenches at the top of the slope
which would be constructed to convert concentrated stormwater discharges back into a
shallow sheet flow regime.
With the installation of water bars on the skidder trails and dispersion of collected
stormwater runoff into vegetated areas of the property,the site should be considered
stable in terms of geographic stability and stormwater impacts.
File:C:IProjectslConradFPAIGeo-Addndm 2 of 3 3/18/2008
TAHJA ENGINEERING, Inc.
PO Box 235, Hoodsport, WA 98548
(360) 877-9512
Please feel free to contact me if you have any questions regarding this addendum, my
original report, or there are any other issues that should be addressed at this juncture of
the property's development.
Sincerely,
v °F WAS!
Alan A. Tahja, P.E.
pow 30 TIS
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�sslONAL E�C'1
EXPII�S
File: C:IProjectslConrad FPAIGeo-Addndm 3 of 3 3/18/2008
TAHJA ENGINEERING, Inc.
PO Box 235,Hoodsport, WA 98548
(360) 877-9512
Mason County DCD
Planning Division
PO Box 279
Shelton, WA 98584
Art Conrad—Craftsman Projects, LLC
PO Box 1201
Silverdale, WA 98383
RE: Geotechnical Report
Parcel Nos. 32226-76-00100 & 00110
FPA2006-00015
NE Southridge Drive
Tahuya, WA 98588
INTRODUCTION
Mr. Art Conrad,representing Craftsman Projects, LLC, contacted me regarding the
timber harvest and residential development of two 5+acre parcels of undeveloped ground
in the Belfair View Estates plat area. Mr. Conrad wishes to develop the above identified
ten+acres by harvesting merchantable timber from the site, clearing two residential
building sites with individual Onsite Septic Systems (OSS), and preparing for other
residential amenities.
The proposed timber harvest plan is to harvest the merchantable timber easily available
on the upper/westerly half of tracts 10 & 11 in a Large lot subdivision in the NW 1/4 of
the NE 1/4 and the NE 1/4&NW 1/4 of section 26, T 22 N, R 3 WWM, Mason County.
Mr. Conrad considers the westerly portion of the property the most easily, and the most
economical area of the properties to develop for home sites. The easterly third of the two
properties contains slope approaching 80%and steeper, so little or no activities are
currently proposed in that portion of the properties.
Mr. Conrad requested my services in providing a geotechnical report and related
documents needed to complete his Forest Practice Application. 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
File: C:IProjectslConra&Geo-Rpt.doc 1 of 5 11/20/2006
TAHJA ENGINEERING, Inc.
PO Box 235,Hoodsport, WA 98548
(360) 877-9512
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 slopes 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.
Very little site grading will be required to locate residential structures on the westerly portion
of the properties. Installation of the septic system will probably include the installation of the
gravel trenches along the naturally existing contours of the site. The surface soil horizon was
observed to be less than 5-feet in depth,and pressure OSS are likely to be employed.
Drainfield areas would be located on the upper/westerly area of the property. Stormwater
runoff from the house construction and the septic system installation can be expected to
direct itself to the lower portion of the property. 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.
A Type V stream has been identified on the lower portion of the property, and is shown on
the attached site plan along with a 75-foot no-cut buffer. The stream courses through the
steeper portion of the properties, in an area not targeted for tree harvesting due to the
steepness of the vicinity slopes and inaccessibility of the area in general.
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 residences and onsite septic system components will be decided once
the timber harvest has been completed, but can be expected to be in the general areas
depicted on the accompanying site plan. 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 where convenient.
3. LOCATIONS OF LOGS OF EXPLORATORY HOLES OR PROBES:
The property contains several areas with cut banks made to accommodate past road
construction and local grading. 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
File:C:TrojectslConra&Geo-Rpt.doc 2 of 5 11/20/2006
TAHJA ENGINEERING, Inc.
PO Box 235,Hoodsport, WA 98548
(360) 877-9512
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 cut bank excavated when Korpi Drive was constructed confirms the soil stratifications
already described.
4. CROSS SECTION WITH PROPOSED GRADE CHANGES:
Cross sections are included in the appendix. The building area for a residence is still
conceptual, but an easterly facing structure would appear to be a likely choice for the
building location and orientation.No major grade changes are required or proposed to
effect the timber harvest, or to develop the properties with residences.
5. SLOPE STABILITY ANALYSIS:
The properties could be described as being fairly level on the upper/westerly third of the
property,but the easterly half of the properties falls to the east at slopes approaching 80%. A
private road, Korpi Drive,bisects both lots in a north-south direction,with the majority of the
developable area lying up gradient and west of this road.A cross section is attached which
shows the different slope areas of the property. The steepest slopes on the property in its
present condition contain no obvious recent mass landslides or stability problem indicators
other than the slow migration of the shallow surface soils typically termed surface creep
associated with freeze/thaw cycles.No development susceptible to the shallow surface creep
of the shallow soil horizons is proposed at this time.
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 house site grading will occur on the fairly level westerly third of the properties.
Residential structures should be located no nearer than 10-feet from the crest of the currently
existing vegetated slope.The westerly half of the property will be most aggressively
harvested, with little or no harvesting activities proposed on the easterly third of the property.
The timber harvest will be performed so as to produce a minimal disturbance to the site and
existing vegetation.
File:C:ProjectslConra&Geo-Rpt.doc 3 of 5 11/20/2006
TAHJA ENGINEERING, Inc.
PO Box 235,Hoodsport, WA 98548
(360) 877-9512
The site soils are not particularly erodible because of their high gravel content and moderate
percentage of fine soils.Drainage for the residence will be addressed when the site plans are
finalized, but will be developed to minimize or disperse stormwater runoff from the
residence's development area.
7. CLEARING, GRADING,AND VEGETATION MANAGEMENT:
The only intentional clearing will be in the vicinity of the conjectural house locations, and
clearing needed to improve the condition of the existing driveway into the properties.
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 driveways 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 Southridge Drive,a
private road, and is stable in its present condition.Additional gravel may be added to the
driveway access point at Southridge Drive to minimize the potential for mud or silt to escape
from the logging area and logging landing location.
When the logging operations and driveway construction are completed, bare soils will
need to be protected from erosion and rain. During the winter months from October
through March, stabilization is required within 2 days of completion of activities. During
the summer months,this time requirement is enlarged to a week. 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. At a minimum, grass seed should be broadcast over bare soils
and covered with straw as a temporary measure, which will also provide permanent
stability.
9. ON-SITE AND OFF-SITE IMPACTS:
Completion of the proposed timber harvest and residential development of the properties are
not expected to result in any adverse on-site, or off-site impacts.
I Q 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 be the expansion of the native vegetation already inhabiting most
of the site. In the future development of the property, discharging concentrations of collected
Stormwater runoff within 20 feet of a property boundary should be avoided.
File:C:IProjectslConra&Geo-Rpt.doc 4 of 5 11/20/2006
TAHJA ENGINEERING,Inc.
PO Box 235,Hoodsport, WA 98548
(360) 877-9512
Conclusions and Summary;
The commercial timber harvest and residential development of the Craftsman Project, LLC
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 Mr. Conrad, and Mason County in planning the
property's development.
Sincerely,
A.WAS
Alan A. Tahja, P.E.
7SA s�
Attached: Site Plan(Plan Sheet 1) j�jkAl '-
Cross Section(Plan Sheet 2)
Stability Analysis(Excel Spreadsheet) MMREs: l l t1f 1
File:C:IProjectslConra&Geo-Rpt.doc 5 of 5 11/20/2006
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Alan Tabja,P.E. An Comwd-Qaft man Project;LLC Conrad
PO Box 235 PO Box 1201
HoodspoA WA 98548 Si7veidale, WA 98383-1201
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Art Conrad-Craftsman Projects, LLC Stability Analysis
Mason County Parcel Number 42226-76-00100&00110
Slope Stability Analysis Section A-A
Area: 1670 CF/FT
Soil Density:130#/CF
Slip Angle: 22.6°
Slip Plane Length:180,
KORPI
DRIVE
Area: 720 CF/FT
Soil Demity:130#/CF
771 Slip Angle: 33.1-
Slip Plane Length:126'
Factor of Safety(FS) is to be calculated against sliding -Static Conditions
FS= Resisting force/Driving force
[c'L+ N'x Tan(A)]/[W x sin(a)]
[c'L+ (W x cos(a)- U x L)x tan(A)]/[W x sin(a)]
vhere;
FS= Factor of Safety against sliding N A.
c'= 0.30 cohesion-drained shear strength of the slide plane AS
A= 36.00 angle of internal friction (ML&CL soil classifications) Q'
L= 180.00 Length of slip plane surface
a= 1670.00 Failure wedge area
g= 130.00 Soil density Ibs/c.f.
W= 217.10 total weight of failure wedge 3 704�
N = 200.43 normal force : perpendicular to slip plane(N =W x cos(a)) 'ssJN ; £�
u= 0.00 average pore water pressure along the slip surface (psf)
a= 22.60 Slope/Slip Plane Angle
Slope#A Hillside below proposed residence-Static Analysis
FS= 2.392651 OK if> 1.5
Factor of Safety(FS)against sliding- Dynamic Conditions (Seismic Factor b= 0.3)
FS= [c'L+ (W x cos(a)-b x W x sin(a)-U x L)x tan(8)]/[W x sin(a)+ b x W x cos(a)]
= 1.263838 OK if> 1
TAHJA ENGINEERING, Inc. Pg 1 of 2 11/20/06
r �
Art Conrad-Craftsman Projects, LLC Stability Analysis
Mason County Parcel Number 42226-76-00100 &00110
Slope#2 Hillside below Korpi Drive-Static Analysis
L= 126.00 Length of slip plane surface
a= 720.00 Failure wedge area
W= 93.60 total weight of failure wedge(130 pcf x Area/1000')
a= 33.10 Slope/Slip Plane Angle
N = 93.60 normal force : perpendicular to slip plane(N =W x cos(a))
FS= 2.069922 OK if> 1.5
Factor of Safety(FS)against sliding - Dynamic Conditions(Seismic Factor b=0.3)
FS= [c'L+ (W x cos(a)-b x W x sin(a)- U x L)x tan(6)]/[W x sin(a)+ b x W x cos(a)]
= 1.120435 OK if> 1
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TAHJA ENGINEERING, Inc. Pg 2 of 2 11/20/06
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