HomeMy WebLinkAboutGeoReport Review for BLD2005-00860 - BLD Engineering / Geo-tech Reports - 6/21/2005 y0N_STq, MASON COUNTY
- MCp
o PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER
7 S T Shelton,Washington 98584
~Z�OJ N Y DATE: June 21 st, 2005
1864
INTER-DEPARTMENTAL COMMUNICATIONS
TO: Tammi Clark, DCD—Planner TO BE KEPT IN T H E
PARCEL FILE
FROM: Alan A. Tahja,P/W-Co. Hydr. Engr. WO#PLG-05
SUBJ: Geo-Report Review NAME: Lambson SFR A Jensen Road
BLD2005-00860
Tammi,
The"Geotechnical Report"prepared for the proposed Art Lambson Single Family Residence
(SFR)to be constructed at"Lot C of Short Plat Subdivision#800, Shelton",has been received
and reviewed by Public Works.
The report appears to satisfactorily address County requirements for geotechnical reporting. From
the contents of the report, I recommend accepting the report as satisfying the County's
requirement(s) for stability investigation and geotechnical reporting. The 80-foot building
setback from the crest of the southern slope promoted in the report should be incorporated into
the site's development plans and made conditions for permit issuance. Other recommendations
and conclusions contained in the report should also be made conditions for project approval.
Erosion and sediment control measures are required elements of any land disturbing activity. The
applicant/contractor should be required to implement adequate Erosion and Sediment(E&S)
Control features to ensure neighboring properties and State waters are protected from potential
adverse stormwater runoff impacts. 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.
In summary,the geotechnical report appears acceptable, and development of the lot for
residential development should be allowed to proceed, subject to the engineer's
recommendations.
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. Tahja
File: H: \WP\GEO\Reviews\Lambson-2.doc
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RECEIVED
JUN 2 0 2005
_____________ __ 5wElei MASON COUNT
- - ---- 80'setback from 70 to�90%slop�e I Z O o
N Test pits/Native soils consist of a dense
light brown cemented glacial till
in a sandy silt matrix. Allowable
soil bearing capacity is 2500 psf
for"UNDISTURBED"condition. 80
No clays were found below.
Geotechnical Engineering Report
Parcel#321 307 590 143
24 General Geologic Conditions. The entire Puget Sound Region experienced Vegetation Removal. The steep embankment had already been clearcut before
for major glacial advances and recessions leaving glacial till deposits and the owner purchased the property but has since grown back with a 5 thick layer of �T
/ ga alluvial gravels and silts. These major glacial events help to create the Puget Soctchbroom and other vegetation. This vegetation protects the steep embankment
\ Sound basin due to their extreme weight since they were nearly a mile high. from soil erosion during heavy periods of rainfall and helps to hold the soils in place
Wood stake
� locations,O
locati ,OK ;`rj a Wood stake Specific Soil T through their extensive root system
\ 3iedrnOm feeidenCe ld P Type. The predominant soil type is a dense dark brown glacial
till with 1 to 3"smooth gravels and an allowable soil bearing capacity of 2500 psf Temporary Erosion Control Plan. Since construction will be occurring during the
for the"UNDISTURBED"soil condition. Once this material gets disturbed it takes summer months,the likelihood of heavy rainfall during construction will be minimal and,
several passes with a vibratory plate compactor to get to within 95%of its therefore,a temporary erosion control plan will not be required.
5 Datum Elev. natural dense state.
37 58' 00' Onsite and Offsite Impacts. It is important that surface water runoff from impermeable
Groundwater Condition. There was no evidence of surficial groundwater areas such as roofs,dnveways,and sidewvaliks are directed in the opposite direction
conditions such as seeps,ponding,or other wise wet areas on the embankment of the embankment to prevent soil erosion and slides on the steep slope. It is
O 0 O or at the bottom of the slope. No streams were observe at or near the base of recommended that the remaining trees on the property including those remaining
-------- ---------------
_-___-- - 490'Elev. the 70 to 90%slope. on both sides as well as at the south end of the property be preserved since their
root systems help to hold soils in place. Also,through the process of evapotranspiration T'ft 44
OR ISetback drairffield 31Y Landslide Activity. There was no indications of slope movement such as old or their roots absorb approximately 40 to 60 gallons of water per day helping to dewater JF N P. 4f +► �y��
from steep slope new slides,escarpments,leaning or bowing trees,ground fissures,or hummocky saturated soils in the periods of heavy continuous rainfall during the winter and spring
WASH�
-- --------- -- ------- ----___-_ _480'Elev. areas at the toe of the embankment. This area does not appear on the Coastal months, Gj� pF
Zone Atlas for Mason County which identifies known landslide areas especial)
[25 to 35%slope arm along shorelines. Final Development Conditions. It is recommended that remaining trees and h
vegetation be preserved especially on the 70 to 900A embankment. The owner
---- -----------______ Topedgeo *W-!loge_-__ - 470'Elev. Geologic Features. The main geologic feature that identifies this site had expressed interest in replacing the existing Scotchbroomon the embankment
-
'Landslide Hazard Area"per Mason County Ordinance 17.01.100 is the 70 to with another type of ground cover. This geotechnical engineer will need to be
90%slope on the south half of the property. This area has been identified in consulted first before any removal of vegetation is allowed on the embankment and "
`- ----------------------------------------- PI drawing replaced with another type of ground cover. It is important that any replacement O ,p0758�
_ __460�Elev. both the an view to the left of this drawn and the cross-section above. S7'GR F
vegetation be just as effective and provide erosion control protection as the
_-- ------------------------------------------ -- 459 Elev. Soil Logs. 4to 6 feet deep perk holes were inspected and served as soil logs existing vegetation. Downspouts from the roof will need to be directed into 5'x 5'x 5' SSIONAL�G
70 to 90%slope and their locations are identified both in the plan view and the cross-section drywells at least 2U from the foundation and away from the hillside to allow the water
-- ------------------------------------------ 440'Elev. drawing above. There was no variation in the soil type for the 3 perk holes to percolate back into the ground rather than flawing over the embankment which may
-------------- examined. cause soil erosion or a slide. ExFIR` L� 87
Cross-section. A detailed cross-section of the building site and the 70 to 909/o Location of the 3 Bedroom Residence. The current location of the residence
steep embankment has been prepared. The test holes indicated that the native as determined by the wood stakes in the cleared area at the top of the embankment
-- ------_ ___________________ 420'Elev. soils consist of a darkbrovndenseglacialtillinasandysifmatrixwithan o______________ provides 80 feet of buffer from the start of the 70 to 90/o slope and,therefore,is
allowable soil bearing capacity of 2500 psf for the"UNDISTURBED"condition. considered sufficient.
Would be significantly less if the glacial till material was disturbed due to
excavation for the footing and,therefore,would need to be recompacted with
410'Bev. a vibratory plate with at least 6 passes with the compactor. No grade changes
---------------------------------------- will be made to the site or the slope.
Slope Stability Analysis. Refer to engineering sheet 2 of 2 for the slope Kim
stability analysis for both the static and dynamic cases. v ffi
-- -- 400'Elev. Drainage Features. Runoff from the roof shall be directed into 5'x 5'x 5' U) $�
---'-"--'---- dry wells filled with 1 to 3"diameter smooth drainroclk to allow roof runoff to ' IF 85
percolate back into the ground. Each of the four corners of the residence a C
Scale: 1" = 50 ft should have its own downspout that is tightlined into a drywell located at least a �E
_ 20 feet away from the foundation of the structure. v ¢ O
--�
0 NY 100' Septic Drainfields. As shown it the drawing to the left the septic drainfields W w N
shall be setback from the steep embankment by 30 feet to protect the 70 to 90% ^
slope from slope saturation and soil erosion. O5�
Setbacks. As shown on the engineering drawing to the left,an 80 foot setback g
_-_- -------------- -----
___- ____---- 390'Elev. is recommended from the top of the steep embankment to the south side of the a $
-
residence. The owner had wood corner stakes driven into the ground to
identify the four corners of the residence. These wood stakes satisfies the 80 ft
setback as recommended by this geotechnical engineer a
r
Slope Stability Calculations (at Cross-section AA
15ft
Weight(\M
Native Soil
ZZ VVedge
p,0�
- Pha - ot�a\F6
Compute factor of safety(FS)against slipping-Static Case(Ref: "Geotechnical and Foundation Engineering-
Design and Construction", by Rebert W. Day, 1999, page 10,28
1. FS= Resisting force/Driving force
=[c'L+ N'x Tan(phi')]/[W x sin(alpha)]
=[c'L+ (W x cos(alpha)-u x L)tan(phi')]/[W x sin(alpha)]
where: FS= Factor of Safety against sliding(dimensionless parameter)
c'=drained shear strength of the slide plane(c' -300 psf for dense glacial tills)
phi'=angle of internal friction of glacial tills-40 degrees
L=length of the slip plane surface(feet) =40 ft
N=normal force, i.e., the force acting perpendicular to the slip surface[(N=W x cos(alpha)]
W=total weight of the failure wedge(i.e., W=gamma times the area of the wedge), Ibs
for a unit length of slope=(140 pcf)(40 ft x 10 ft)/2=28,000 Ibs=28.0 kips
u=average pore water pressure along the slip surface(psf)=0 for gravelly loam sands
alpha=slip surface inclination(degrees)=39 degrees
FS=[(.30 x 40)+[28.0 x cos(39)-(0 x 40)]tan(40)]/[28.0 x sin(40)]_[(12.0+ 18.3)]/18.0= 1.68>1.50 OKAY
2. Compute factor of safety against slipping for the Dynamic(seismic)Case, (Ref: Robert W Day, page 10.64)
seismic factor= .3
FS=[c'L+ (W x cos(alpha)-b x W x sin(alpha)-u x L)x tan(phi')]/Mx sin(alpha)+b x W x cos(alpha)]
=[.3 x 40+(28.0 x cos(39)-.3 x 28.0 x sin(39) -0 x 40)tan(40)]/[28.0 x sin(39)+ (0.3 x 28.0 x cos(39)]
=[ 12.0+(21.8-5.3-0)0.84]/(17.6+6.5) = 25.9/24.1 = 1,08 > 1.00 OKAY
fir"rs�"*•w
4 Afo v�►
Slope Stability Analysis for Building Site located
at 1580 E. Jensen Road, Shelton, WA 98584
T.P#321 307 590 143
r
30758 .r �"� Prepared for Art and Mary Lambson
O,�RF:;ISTLRF� fit _. 2041 E.Brockdale Road
.
' Shelton,WA98584,(360)427-1187,5201�-3652 c.
`SS'01NAI.C'C
* Prepared by: Morta Engineering&Testing,POB 250,Ocean Shores,WA98569
ExPipEs �6=J 800 590-0958 Smorta OL.00m
June 19,2005 Analyzed by SPM Page 2 of 2