HomeMy WebLinkAboutBLD2008-00339 Geotechnical Report • • APPROVED
STORMWATER PLAN
STRATUM GROUP MAY 2 8 2009
P.O.Box 2546,Bellingham,WA 98227
Phone(360)714-9409 JEFFERSON CO
February 26, 2009 E' �• el �, i�i,//j-�
Allan Herold
26020 SE 154th Street MAR 2Q^
Issaquah, WA 98027
JEFFEUrn EGiJNI TED
Re: Stormwater Plan and Geotechnical Review
Parcel 701311021, East Quilcene Road
Jefferson County, WA
Dear Mr. Herold:
I have reviewed the proposed development plans for your property, reviewed the geotcechnical
reports for the site and reviewed and added information for the stormwater submittal to Jefferson
County. The proposed development can fully disperse stormwater consistent with the Ecology
(2005)BMP T5.30 with sheet flow dispersion from the driveway area consistent with Ecology
(2005) BMP T5.12 and infiltration trenches for roof run off consistent with Ecology (2005) BMP
T5.10. Stormwater management in this manner is consistent with the Ecology (2005) stormwater
manual for Western Washington and is consistent with the geotechnical reports previously
prepared for the site and is consistent with my own assessment of geologic conditions at the site.
Setbacks and Slope Stability
I reviewed the NTI (1993) and NTI (2002)reports regarding geology conditions at the site and I
visited the site and inspected the geology conditions at the site. Conditions I observed at the site
are generally consistent with the observations by NTI and I concur with the recommendations
made by NTI.
The initial geotechnical report NTI (1993) for the site recommended permanent structures should
be located a minimum of 50 feet back from the top of the steep shoreline bluff slope. The
proposed home will be located 3U feet back from the top of the steep shoreline slope. Based on
my inspection of the site, it is my opinion that this distance will be protective of the home for the
expected life of the home (at least 100 years). It is acceptable to have a deck within the 50-foot
setback area as long as the deck is not structurally connected to the home. The proposed deck
will be located no closer than 44 feet to the top of the bluff slope. The location of the deck 44
feet from the top of the bluff slope will not increase the risk to the proposed home nor will it
increase the risk of slope failures on the steep shoreline bluff slope.
NTI (1993) described a shallow bowl-shaped depression along the south side of the slope down
towards the intermittent stream. NTI indicated that the area may represent past shallow slumping
of soils, but noted that the potential slump area was not active. Past grading has taken place at
this location such that soil was removed from the upper part of the bowl like area and a rockery
• I
February 26,2009
Parcel 701311021,Jefferson County,WA
Stormwater Plan
wall was constructed between the bowl area and the proposed building area. No recent movement
was evident. Prior to the NTI site visit, the level area had already been graded and it is possible
that past grading had placed organic rich soils with no compaction on the slope and some
settlement had taken place. I also observed what may be a very old skid road across the
intermittent stream valley. NTI recommended a setback from this bowl shaped area of 25 feet.
The proposed home site will be located greater than 25 feet from the bowl area. The proposed
building area above this are is underlain by weathered bedrock soil. Based on my observations
placement of the house on the level building area as proposed will be appropriate and the home
will not be subject to settlement from soils on the slope.
Stormwater Management
The proposed stormwater plan meets the recommendations of NTI, and is consistent with the
Ecology(2005) stormwater manual adopted by Jefferson County. The proposed stormwater
management for the site is for full dispersion consistent with Ecology (2005) BMPT5.30. The
site will be left in greater than 65% tree and native vegetation and the amount of impervious
surfaces is less than 10% of the site. The stormwater generated by the driveway area is less than
the threshold requiring pretreatment and will be managed by BMP T5.12 (sheet flow).
Stormwater from the roof area will be managed by BMP T5.10 (infiltration).
The proposed stormwater management from the driveway area will be to allow sheet flow off of
the driveway surface towards the north consistent with Ecology (2005) BMP T5.12. The
following figure indicates where sheet flow dispersal should take place. Soils to the north of the
driveway area consist of well drained soils with topsoil and native vegetation. The soils are
Alderwood gravelly sandy loam. These soils are well drained and test pit observations on the
gentle slope below the driveway to the north are consistent with these soils being well drained.
Sheet flow off of the driveway to the areas indicted will not lead to any surface water flow across
the site and the soils will infiltrate all runoff before any water reaches the intermittent stream
located approximately 75 feet to the northeast.
RECEIVED_
ORMWATER PLAN JEFFERSON COUNTY DCD
k,kAY .. 3 2009
JEFFERSON la TY
Stratum Group DEPT.OF CO .�� ^IT )J'OP �-
SIGNATURE: File: 1.10.09
Ase
2
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1141,
February 26 2009 ,aA _�' ('_
Parcel 701311021,� Jefferson County,WA F `�. till,
�` ` ' 2J��
Stormwater Plan
OEnserales4 o JEFF f.
Z C{LDC
.1 •r a termittent
�, 9 '0 �e_ '�`, sµi ream
<+�` TOP OF SLOPE '
_ (TYP.) t r
1 Y
_�__ �� it 1 M
ii
V.
- I F NVt'-1} �1 20'BUILDING ! )Nk Infiltration
1 �� ► SETBACK LINE ^ f Trench
4,2
_0o'r, '. SLUMP EXIST STEPS y 1
�I 1 tti AREA . •oCKERY '
1 i SE B K .). _ �-A2 WATER STUB c
J _ I 56.-6 _ 1.1 ._ _
i
I I ``-.t
rl1
t't}; . sss Stormwater
:_ 9 `-) Y ' .-DRIVE'
Ail. ) . ,• f sheet flow off
T r' '. _ 1.3 7 ; driveway into
—I —
o-0- . 1 • --F '` native soils
r, ; v i and vegetation
1 1 1 i 1 2 34' i i ROE AT r
Stormwater from the roof of the proposed home should be routed to a dispersion/infiltration
trench located to the northeast of the home site. Stormwater from the roof should be tight lined to
the infiltration trench. The soils at the infiltration trench are well drained and the site is located
upslope of native vegetation with thick(2 feet)topsoil. This site is the most appropriate for
infiltration on the site as soils at this location are thick and well drained and increased water at
this location will not cause any slope stability issues for the shoreline bluff slope. Directing
stormwater to this area is consistent with the NTI recommendations to direct stormwater away
from the steep shoreline slope and is consistent with my own opinion that this location will not
cause any slope stability problems on the steep shoreline bluff slope.
The stormwater generating surfaces of the roof and driveway are below the threshold level of
10,000 sf and less than the 3/4 acre threshold of land has been cleared of native vegetation for
development requiring flow control (see Ecology, 200 eti the
A PROV D
Stratum Group STORMWATER PLAN File: .I°.°9
3
MAY 2 3 2009
JEFFERSON CO
DEPT.OF CO tUNITY D COO
• •
February 26,2009
Parcel 701311021,Jefferson County, WA
Stormwater Plan
infiltration trench should be at least 10 feet long for every 700 sf of roof area and should be
installed such that the dispersion pipe is level. A typical design per Ecology recommendations is
presented below.
•
Ripe O.D.
t'-0
mitt i rCh end cap o r p lug
1E4.1'7°06 oiran out w e from pipe
a
°a 7 o 4"or6"pert rated pipe taidflat�leoel
notched a °
grade board �` tia
2"..;2'notches c; F 4 type IC B wfsolid ccn,er Clcoking)
1FJ"O.G. 3 a a
rd rI ""*-
3' 4
'i a pF i is yn
C fbinfl�.0 Q.�tip
C F$n perdes trencht
4 4 94.
lila
v -clean out wya from pipe
4dd
PLAN RECEW ,D
pipe OD.
'.AR 1 (3 2 1 f gahcanized bolts % 1'-0 / 1'•Q
r1
wl D�'30' �jr(r' r��' I1(�rlllIfnJ , S WIi
,,,0'3 �0 ULi rD
1.tie`ri4 ut),")� u.J
fir'i:i n ei++ r;, i �qY.l a'a' O • Q_',
E6 '•'.`f .'F.. .''�.
Pressure `% av
treated grade 'g :^ i � �..� p a % . f :• 4"or 6"perforated
', f+
boardto
`' 11, op. 9 t pipe laid flak
j NOTES:
tY
" C 4' sup Ott pera-t I r;`� a 1.Thy trenoh shall te constructzd so
clean %tir s
•`� b_ '" �`'`5e a�h prevent pointdisc•harge and,hr
td a q r erasrn
4, 2.Trenctr_s rnanr te plated no closer
filter .Gr i -' ' '''` " than 50 feet t one anott a r.(100 feet
a.br,,fbwline)
"15% nr �cturf•fla controt,Vater� aslih' 3.Trenchandgrate beard must be
level.A Iig n to follow contours of
treatrrrent in wi J a.rvm,
SECTION x x 4.Support post spacing as nag uired by
J'1'!A boar)
A F nV 'D
STORMWATER PLAN
Stratum Group
File: 1.10.09
4 v1AY 2 2009
JEFFERSON CO Y
DEPT.OF COMM NITY D' EL
SIGNATURE:
i , S
February 26,2009
Parcel 701311021,Jefferson County,WA
Stormwater Plan
Conclusion
Based on my site assessment and the investigation by NTI, it is my opinion that development of
the site and the following of the stormwater plan will not cause any adverse impacts to the
stability of the site or adjoining sites. Furthermore, the stormwater plan utilizing full dispersion
of stormwater will not cause any significant degradation to streams or the shoreline.
Stratum Group appreciates the opportunity to be of service to you. Should you have any
questions please contact our office at (360) 714-9409.
Sincerely yours,
Stratum GroupCZC) Vg
i_I. 4hN".P. Attu .:ib
Dan McShane, M.S., L.E.G. �t ' ' ��
Licensed Engineering Geologist +,,, .�\,t'„L °:.,�r ' .'
DenicM
RECEIVED
MAR 1 1 r
JEFFERSON COUNTY HD
APPRO 'ED
STORMWATER PLAN
Stratum Group
Ile I I o 09
MAY 2 2009
J
JEFFERSON Ce Y
DEPT.OF CO 0 NIT DE Lid EN
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• •
Herold Residence
Infiltration Trench Details 3962 E Quilcene Rd
Parcel#701311021
M LA08-00363
pipe F.D.
1%0 1`-0
rain min ,..,end cap or plug
IFl„c o,,._-clear!outwr from pipe
a a
°a 99,, 0 -'4"orb"perfaratd pipe la.idfiatilevel
ched _
not p 6-
grade board Ea `a
215c 2'notches v, 9 .,.type I C B w//olid cover(Ice king)
iS'O.C. A 1 a
a r[
r+l-
a influent pip= (maxdesign
�qp fiow .0.8CF$pertr'ench)
7. clean outw'e from pipe
oa a
PLAN ""'":: �® ,
10 feet of length required per 700 kl; 0. .
square feet of roof area ' �r'�`"�'' "' � '`f xp`P,r
0 1373 /✓ ci
i pipe C.G.
galvanized bolts 1 0 j 1'
0
00 ,I--7T SFr'' . NOTES:
b 3' '• "
2'x12" `1' c,r, r finis.o r n•;9 h4h :i 1.The tprah stall tp is c harge an sr)
` ,,.; r.•b• a '£ ` er s prevent point discharge andk�r
pressure �' ���/�:��� a a'
''VI"' 9 •'• erosion.
treated grade . p n n , i,.4"ore'pert ri d 2 Trenctes lie be pI?e ed no clo r
bJal'd D c?.,...,4pips Idid fiat than50 itr_t ID one.Krbiher. 10lj fit
n $' a a a?
4"Y.4 support pert t r 3 Trench nd gr.--,de board roust be.
�a C1�3J1{-�,,•�, tires) Hel Align n_:folimoonvur$ofsiV.
tp 4 a S ,. 4.Support post spacings rxauir d by
d a A. ' 9Dilc.orditinstoensufegrdeboard
fitter brie �'' ` remains Mosel.
%15% rrrixfa-r fl cw ccritrol eater ganity
treatment in rural aree . rr��
SECTION A-A
mike i p,� r-1Pri�,
JEFr�nt,nil! 0�p,IT n
E! ',,.l;a t?,,!?IY