HomeMy WebLinkAboutBLD1994-00484 GeoTech AtAUIASSOCIATED
EARTH
SCIENCES, INC
September 6, 1994
Project No. G94199A
Icy and Beth Brown
3620 227th S.W.
Brier, Washington 98036
5ubje,t: P.1•4:wn ikiai Ptiyetty
Lot 4, Discovery Bay Village
Case No. C.A.R. 94-0016
Jeffers,. a County, Washington
Dear Mr. and Mrs Brown.
h} Jefferson ( okirty, A ss-c:ciaied Earth Sciences T. c' (t F.-SV
;..;0104;ic hai.ai-di assessment of yohr resich-nt:d view property tt. kd et
- - applt.,,...etattely 3'4 mile south ef IU Pot iithe Bay V1128e
. ;Tose of our assessment was to evaluate whether po lions of the proposed
,vithln the minimum 30 foot laudslide aAra buffer may be '.7n,mplecd w.thout
tft iisk of a L.,rdidde and whether there is a current rik of a Ian(Nlidt.
of hazards a,sessruent included a geo!ogic. ti-_ • isJice or the site, a
\Linty's Cleologirnily T-Tazatdous Att:45 1ei1ioi ), a r.;:view of
., p -ing of this area and conversatHris with }c111 ,'oroernIng )ow proposed
rroject and Site lloceription
and surrounding area;
e r
the benches 1 ,f
asci FLu Road) arlaummiummainito
'le top of the piotnouttny •
,f ,,:di:ncrIS were exposed it the
..q. HJett along the west
in the eastern por-hm c th,: blvt,114-g We
. 'ai cA.cayation wilt tyu nacvs5a.r,) to teach the 1,-NI r, aaelePt
.44V threler heti4-t-,:tartr1 that the front ,elsto-r.v.1 va ,1 k proposed to be
the- sl.-pe and o n ycird drain near he1,1,or)sed horne Tt,e neo
ie ½ We ,,.oe orthc pooiontory slope rat,' A*tfc hc.rne ste. Thi.c
r fsw erode a r fet ffn'n the
•
q • • •_i 1 t.1 I 1 r_. -4 l 1 f-1 LI I_ • t_ f-i k`-' 1 1 1 1_ 1 :i Y . __� s_ _F I • 4
We unu::I r:itiltl ;l, t t'..ts septic drainfield for this home is located to the south of the southern
!acing ;1tti.li:16 a;ea. Also, the storm vv ilcr altni, nrt. i propose i to be
ughttincd rvr:i a1 ir:.i.pcx.:u.uble sui faces to the toe of the southern slope. We understand that
this is a County apps.ved storm water discharge area.
TC•pvbraphi<aJl j tt+•. tv.v tr.J;iding areas warm.re! tiv -ly level except fir th crevionSty raescribert
cut. bench t,,, tt-+li.,l ty c,:r1,,11ntling three sides of the home site Q,ere natural ;steep slopes that
rl...wn:.ti:•r,I f:,,4,1 apprC1xirnately 21I:1V (Horizontal:Vertical) tcwvard the east to near
?c Ii•1 V t:,wrtrd the nc=rth and cc,ilth. These mopes were covered with r7=oral .'egetatiot Near
tiC. ,VC tot sic c :!,erre was ant:dS .ort: til �:,..�. ('u.. Tl. ,ul to to eect
hiel: and e posed a eery dense mlxl.+re of Sand ail grovel with sume 'silt. The total elevation
change from the .ower garage arta to the upper home area was ar pro::llnatcly 35 to 40 feet.
To ;he t=a t of the pioposed lower g-ArAge shout 2R feet, the s'e drops off at approximately : 5N
to IV.
Stratigraphy
is Jridcruan by twu alstlut;t c 10..�111 .tenIt• Th.r P3 `-rr,c.et
unit exi)+i:t.•'� ,t :vim of ills 1rv+:,e foundettion cit .ati Vn•sl,=>n ago till. 'T'ill ie at e ,s"rted
111Xi,ULC wa.s CILi!U91LGt3 at. th.; a 1ac:er durinF, thz mot r::`cent
glacial advance :roto the Puget Sound region un the t=i.1cr of 12,0(Y) yearn ago. The low
thio unit &ra kl•,a roe,'it of tt-r. whip range of gernrnent grain ci7eS
n ,d C 'r oti,�ati_rt the v night of approximately 40(0 fec-t of glncinl ice I...!n'e1 vying the tilt
and exposed aie'tg the ,:ut face near the proposed ga:dde cicvaticr., was a mixture of sand and
,ravel wi:h some si?;. We understand that this cut was made approximately 17 years ago. This
t71t Is interpreted to be Vasl,oit advance outwash that was deposited from meltwater streams in
runt of the ad%a .:_:ng Vashon j'racier This geologic unit has also .lydergone consolidation from
^i:a1 as was Lie till., thus accounting for Its dense shite"
v� givulit: wales seeps or evidence of past ground water seeps were noted at the time of our
field work.
Cita
flhc= watinrtc
• f r f 1 !or:11
ion
hint+
l d it
On tfCSC siopcs.
Out re-'onn,+,ixsar,cc of the site indicates that the proposed areas for the lower garage
,tl'.L Ll►G 11CW e10a11C r'=i17 L4 Dltlai
T1,4, 01r.rvk 1,40,Arghei,n HIP
IC;tYCa garage area and he new home site consisted of dense to very dense glacial sediments that
2
•
•
4 T ..1 E 1 r . 4 f4 ,_ !F.-• LSC • E H F: T H =. C- I 8 2 i — =. 4 4 F' . '1 4
are judged to be in place and covered by some small alder trees, meaning that there was no
indication of past deep seated earth movement in this area of the site. Furthermore, even though
thr cut slope has not been protected from erosion for the past 17 years, apparently ve, ,:ttic
material has eroded off from the cut face as exemplified by the small amount of . -ent
accumulation at the toe of the cut slope.
The recent and past site grading has produced two areas of till. The most recent filled area was
rear the top of slope surrounding the new home site. We understand that this fill will be
removed down to the rratura' glacial sediments prior to construction of the new home The older
T that was placed 17 years ago was located east of the proposed lower garage. This fill area
epic ars to have not changed over the past 17 years based on information provided by you arv:
is not addressed furtl-.er by this study.
Conclusions and Recommendations
•
1 Ind recommendations presented in this report are o f'tired as a mea. ,�=
and redact r~ '.5 associated with development near a steep slope. Elimination ., 1 .
(,idcntificd or not identified) is no', puss'st,te. De'.elotnne, site
ilcl orgy pri..cerd if tt,, v vnt;:i dt.,Lepts full tvsputtsiUility fvl any damage iii the event of slope
�uwernent.
i3 tsed on the conclus on t :at glacially consolidated sediments underlie your proposed building
area and that the slope was tnincated approximately 17 years no, we would assign a low to
moderate risk of slope failure. The low risk would apply under st:.ttic (current) conditions
raise to moderate under seismic conditions.
Due to the limited building foutptint available for the new home and the lack of evidence of
landslide activity in the vicinity, we would recommend that a minimum 10 foot building setback
line be establishes: from all steep slopes for the new home.
Approximately 4 to 3 feet of soil has been or will be removed from the home area prior to
construct;on. The weight of the new home should be much less than the weight of the soil
rernoti ed from tyle lop of the slope. Therefore, the global stability of the slope should not be
OCi l'Pd ce.L1 by the proposed construction.
We would recommend that all exposed soils be re-vegetated as soon as pos •ible or otherwise
protected from surface erosion during and after construction. A local landscaper should be
consulted to det?rrnile. the most appropriate type of permanent vegetation for erosion control in
3
•
.. r. 4 l- f_I E: 1 E. : 4 4 ►-a •_• E I R T' H =:. C. I - _ 4 2 4 F . _+
this area. R:ir.off should be controlled to prevent water from discharging on or above the slope
at all times.
Should any changes in .he slope occur that cause you concern, o:;r office should be consulted
immediately. These ;tianges may require that erosion protection or slope protection measures
would need to be implemented.
We are confident that the above information will meet your currea project needs. Should you
ever have any additional questions, please contact our office at an., time.
Sincerely,
ASSOC1AI..ED EAR 1ft SCIENCES, INC.
B .
26890 A_
•
/2"21/
Ronald Parker, P. Bruce I.. Blyton, P.E.
Principal Senior Project Engineer
RAP/Id
094199k 1
WP 8/20/94 Id
4
. • 7. .
I --I
,‘ 9
i .
4
„
4ACT 2 / 1((---• . .c/ e,7d
.- ------------. 1.--- ta."
i 2/710.0 xiy.
/---
( ) 9
•
-Ns
EA..,
.../71'........ *Wil 1.7 r.?
(04 I 1
• . C 7- V11.1%, .
filli 1 i
,‘Zftikw 1 1 1 I I .5cidzy. ,..,_
k.0 74:..1.0 S in/ eC,I
(....) ..e - 7-a..z.... vibin , 1 ji " '"'"*----'
) .2 6 i ' I
41 I ‘ %
. .Iti
( .
N
;-,,, , ...t 1 ,
\
\\ 1'' \\'‘.
I.n\D s •r 4 z i77/- cicala. 69
(-_.•'z :.7. .!-.2./ "Ilt` . )-.
ilb
.
------ /lvi
__„___
* ,
1/---__Sta. .e 77' j.. \ .c>.-_'•.... 8x4' 7- ,
I' \ ,e,
Con c. 'a_St r voiri ---"-'5 .(-\N. YZ/Cada
_
\ . \ (...1 - • Der 44-...zi/ -Iiit / • 5.° Ell
N, 4 ''''''', \
\ ,
e: v
N ,,-; / */ / ),.
,
N „ / , .. ‘ .,
.. ? L,1§-z.t-, ..e,,,,, \ \ I
•-•,7 CS-5 5 ....i.a CI 7:_g , ,.
.,_ eE on . :t •
\ t, a
5c Cohstr c‘. 4c/ 4'1F' N., -'1 1
65, 0-haps N.. ‘. 0 ,e---- 16 / 7 C3`*'
\ ii) 5t .. ..,.7_5-7, 7‘ -,. .- i;
-t. '...6//-9'7,7 5 1 k I
c/(/: e P7C/TRACT ; \*----,,i,„),‘, _('1,'.5"/67;:r21/-;e7a0/: .0 , , . ...„.._ .,4\ , i.
.,.,
1
0 .. , ..„.. ..,„.,_.,,. . ,,•,„„g..,_
„, , ," G‘
\ ,,()
' I
.\ 1/4,
' r 1) , . ••••• V.
••••• ..
68 /
•
Corn bli7atio ri - • 67 /
-----• 1,.‘ \ / • •.•.:
- ..t., ,- ./,•-,s..•••
—''•
Ki..
-. - 6. -6' - -- N.,.--- •,, • 16. •-"...y,..:$.•:t.:...•::..:4
..;,::,-,.,.•--,,.:..
• ,-,i111. Y.. .::::*:: ::.::::.:•::...:7....
.-----.
sta. .1",c5-0 -) / A). ,,,,e'rSItili;:.:::•:•':.,\:•:.'•::: :::::::.:::.:;.;•::::::::.•:-.Y-•:
\ 1/1 taf;658°. / p ,) -. ..).:. "ifr::4, ....A-g:‘,::::,:.•.;:‘,.•,:.s.,••
/, \ ,,-.....4:. : ...e...:::.-4::: .::•::a... :-: .:..::...i.77:
V::::%.: :: .'...v..e:::::;:.:
.,--, -,-,, .c/) (.7"J
-..\- IlvdL 4381L387 .'
,4 / e' IS'' ...::.::.::::i..::.::1 : 1::::::i.r:4::.:'.:::::A:..:;‘•::..: :i.,.'..'.k.::::.:..:.::Z:::I
\43 . .:.'..:.::::::::.::.::::::::.?::'.:!•:::--:::-.:.::.::::-::::-:::1:55::.:.:.:.::-:.::•::::,:::.:: ::.:.:. .44