HomeMy WebLinkAbout701201011 Geotech Assessment
$D(JOCo - 0005 i
l6 \ '1-'0 ( 0 li{
j1
STRATUM GROlJP
1451 Grant Street. Bellingham, 'Y\" A <.lS221
Phone (360) 7] 4-9409
; ~::;>>
o
2 2 2006
July 7, 2006
~tV+ E ~'
,.-1 I
'"'0"".. ,q'f \
_~^~~_~__~ Ut.~JtLI .vr'~ltll j
Gary Phillips
450 Penny Creek
Quilcene, WA 98376
S' ~,
fAj f..
Re: Geolog)' Hazard Assessment
Tax Parcel 701201011
Piper Road
Quilcene, W A
Dear Mr. Phillips:
Stratum Group is pleased to present the results of our geology hazard evaluation of the above
referenced property on a shoreline bluff The purpose of this geology evaluation was 10
qualitatively evaluate the risk of slope failures impacting the proposed building site on the
subject property and provide general site development and maintenance recommendations. This
evaluation included a visual inspection of the property and vicinity, a visual inspection of the
slopes on the property and vicinity, review of available geologic mapping in the area, and
inspection of the shoreline at the base of the bluff.
The property location is indicated on Figure 1. The property is located on the east side of the
Bolton Peninsula in the northeast quarter of Section 20, Township 27, Range 1 West W,i\iI, The
property extends from the shoreline of Dabob Bay on the east up the slope to an elevation of
approximately 150 feet on the west side of the property.
SCOPE OF SERVICES
The scope of our services included the following:
1) Conducted a site visit to visually inspect the subject property including the slope
conditions, shoreline conditions, and relevant conditions in the vicinity ol'the
property.
2) Observed surface soil conditions on the slope and on the uplands above the slope.
3) Prepared this report summarizing our findings, including a qualitative evaluation
of the slope stability in regards to the proposed building site on the property,
recommendations for site development, and recommendations f()l' further
investigation, if necessary.
GENERAL GEOLOGY
C\;2
July 7, 2006
Tax Parcel 70120101 I, Piper Road, Quilcene, \VA
Geology Assessment
Northwestern Washington has been occupied by continental glaciers at least four times during
the Pleistocene Epoch (1.6 million to 10,000 years ago). Durin.g these glacial and accompanying
interglacial periods, the underlying bedrock was eroded and a relatively thick layer of glacial
related and interglacial t1uvial sediments were deposited over the underlying bedrock in the
vicinity of the subject property.
The Geologic Map of Surficial Deposits in the Seattle 30' by 60' Quadrangle. Washingtoll
(Y ount, Minard, and Dembroff, 1993) indicates the bluff is underlain by undi fferentiated
Pleistocene deposits. These deposits are described as consisting of poorly exposed gravel. sand,
silt, and ciay of uncertain provenance. The Geologic Map of East~Central Jefferson CounJL
Washington (Birdseye, 1976) indicates the shoreline and bluff exposed 1.1n the subject property
and vicinity is underlain Pre- V ashon glaciation stratified deposits. Birdseye indicatcd that there
are three glacial till deposits on the bluff in the vicinity of the property. the PosscssionTi II. the
Double Bluff Till and the Vashon Till. The Vashon Till is the youngest of the ti lis and thl.:
Double BlutfTill is the oldest. The till deposits were deposited directly by glacial ice and consist
of a mix of rock fragments embedded in a matrix of fine grained material consisting of day and
silt sized rock particles. The Pre- V ashon stratified deposits are described as consisting of sand,
gravel, silt and clay deposited prior to the last ice age. These deposits were deposited by glacial
melt water and by interglacial streams and rivers.
Observations on the subject property, slopes in the vicinity of the property and upland arcas
above the subject property are consistent with the mapping described above. Most of thc property
is underlain by compact sand and graveL Glacial till was observed at two locations on the steep
slopes ofthe property and till deposits underlie the lower bluff slopes to the north of the subj~ct
property.
Compact and gravel deposits underlay most of the steep shoreline slope. A 'vcry compact sandy
silt with gravel glacial till unit is located along the mid section of the steep shorel i ne blu iT
The Relative Slope Stabilitv in East-Central Jefferson County. Washing!QD. (Birdseye, t 976) map
indicates slopes in the vicinity of the subject property are unstable and idemiHcs arcas of recent
landsliding in the immediate vicinity of the subject property.
Observations confirm that there are potential unstable slopes on the su~ject property <:lnd thl;.~
vicinity. The shoreline at the base of the bluffis erosional and thus the slopes above the shore are
potentially unstable and in some places are unstable. Unstable slopes are locatcd on the steep
slopes west (inland) of the subject property, particularly in areas of convergent topography near
the upper slopes of the Bolton Peninsula and along the steep sided ravines on the slopes of the
I'ilc ("i2,Oi>l)
Stratum (,roup
2
"._~"....~_._.._"'~--'
--I
July 7, 2006
Tax Parcel 70120 I 0 II, Piper Road, Quilcene, W A
Geology Assessment
Bolton Peninsula, However, a fairly large area of the upland area above the steep shorelinl: slope
has gentle slopes and divergent topography.
SPECIFIC SITE OBSERVATIONS
A. sketch map ofthe property with pertinent features indicated is provided in Figure 2. The
property is accessed via a spur road off of Piper Road. The access road traverses down towards
the shoreline bluff along a gentle ridge area between two dry drainage swales.
The slopes on the upland portion of the lots are between 5 and 20 degrees. Except for the access
road the upland are is tree-covered with mostly mature Douglas fir with an understory of salal
and ferns. Soils in the upland area consist of sand and gravel to cobbles.
The top of the steep shoreline slope breaks very sharply. The shoreline slope slopes down to the
beach below at an average angle of approximately 60 degrees. This slope is underlain by rnostly
compact sand and gravel with a layer of sandy silt with gravel that] interpret to be glacial till.
Despite the steepness of the slope and compact soils trees are scattered on the slope.
The steep shoreline slope down to the beach has had shallow surface soil failures in the past. This
slope is primarily tree-covered but areas of bare soil are present on portions of the very steep
slope ,vhere slides have taken place in the past An approximately 4-foot thick layer of soil and
logs from a past slide is located on the upper beach at one location of the subject property. This
slide deposit has trees and brush growing on it No springs or seeps of water were present
anywhere on the steep shoreline bluff slope of the property and there are no indications or
seasonal wet areas on the bluff slope on the subject property.
There was no evidence of surface water flow in either of the two dry drainage swales located on
the upland portion of the property. The very porous soils underlying these low arcas appears to
preclude surface water flow. These dry drainages may be the result of initial drainage of the area
shortly after the last glacial period. Alternatively, the drainages may conduct water only during
extreme weather events such as rain on snow events. I was at this section of shoreline during a
very hard continuous rainfall that had been preceded by a period of very wet weather and did not
note any surface water flow on the bluff or within another similar drainage to the south. ,
A metal st.aircase leads from the top of the bluff do\\-n to a landing on the beach. The stairs ar).'
supported at locations that are presently stable and spans most of the bluff face such that the
stairs are not causing any problems to bluff stability.
Stratum Group
J'ik (, i 2.06D
3
_.."...,-~- ....~._.-.~,---"._-_...,--"~-'-".-..>
. .
July 7, 2006
Tax Parcel 7012010 I \ ,. Piper Road, Quiicene. W A
Geology Assessment
CONCLUSIONS AND RECOMMENDATIONS
Based on my inspection of the subject property and vicinity, J conclude that a residence can be
located on the upland portion of the property in the site plan sketch. Any residence should be
located at kast 40 feet back from the top of the shoreline bluff. A residence located at this site
should not be at risk from bluff erosion or landsliding that would threaten the structure within the
expected life of the structure (100 years) as long as our recommendations are follow'cd.
No debris should be placed on the top of the bluff or on the steep bluff slope. Thinning and
trimming of trees on the bluff slope or within the 40-foot bluff setback should be limited to
topping of alders and maples and limbing for view corridors of Douglas lir. Any debri s from
thinning on the bluff slope itself should be removed from the slope.
Storm water run off from the roof gutters should be routcd into dispersion trenches to the south
and north of the building area, Water should be able to readily infiltrate into the ground at these
sites.
The septic drain field may be located will not cause any stability problems on the steep slopes as
long as the drain field is located at least 40 feet back from the steep shoreline bluff slope.
CLOSURE
Please note that there are inherent risks associated with lots near or adjacent to steep slopes.
These are risks that the building o\-vner should recognize and be willing to accept. If conditions
appear different than those described in this report, or other concerns arise, we request that we be
notified so we can review those areas and modify our recommendations as required.
We appreciate the opportunity to be of service to you. Should you have any questions regarding
please contact our office at (360) 714-9409.
Sincerely yours,
TI:u-
Dan McShane, M.Sc., L.E.G.
Licensed Engineering Geologist
hIe: 6.12.06D
Slrutllll1 Gmup
4
. .
122.850000 W
Figure 1, Site Vicinity Map
122.833330 W 122.816670 VI! WGS84122,800000
ni"~
~':. :\' .';:~) :
, 7
,
"",". /' 1 \. \ ") I' J
. . .' . ,,\ "" .j "
(\'\'" '1 'J1 "" """~ '"" . I -'.~ ~.:I .
: t ~~}' " \."''-1, .'
7:' ~"~:;'-~".*~~,~ 'j ",,' :!. ,..,'/./
,~. <\1~(","'",; \ /'.>0'
,~.{1-~\\ ' " ( - , I."
drr ~,_"j? ,.,~<-:' ~
~ 'f\. .:, ~'~f?~' \,
At ~)
.(':r,',":j'~-:~
~~~:(~~;.~~:~.
ill.....
):c\..-',
":.'Ii;
,,:,,:r, (;"
,';
,,(, i.\;\';"
i I ,\
. \ (,;::'
\"
, ,
',',j
,
.'
1
-j!
-<
"')i,,; ',,"i,""
"-
li\ "
()i)~~'::~)~'i\
'f});<, "'i'
':,n,;: ..\< "'J.
',"'''''''';;';'..';'
.,'.",',.' '"..",
}"i': :\,;. .' ""'"
, "'. "," ',..",'..,j'.
;: ,>;' , .." ';'.';:
,,' "..., ,', ',..,.,.','.;);
SUBJECT PROPhRTY ",...",.;,', ,;<.. >)"
)i,",; ',,", ,',,',', .y,
,",', ".:,,}
;, """"',"'",,,;,. '",,; i ;.~
.. .;:;,...,..,,>: z
"";"'.,,';.'<'"": ~\," " ..
;. .".',..,..'...,,\.. ,..'
..;"t ,'" '. ,','. "'f-'
,\'J', ", ".,
: "', '\,{~.i ,'; . "}"i ~
<~\.....;'\:\l',}~:" .,; ,>r\;.~
';":"\");.':;:";:~
r.... . .';...,.',....','i.'. '\ .\~~I
"i. ','.' ,
',',...}, (/"/" <,; ,q: "" ',.~
{~'~" '" ~. '>i'/
'i:i' ,..
,,', i> z
· ".. "
Y'" ,.',','..' g
,." ',," ".'~ ',,' .0
i Iii rl'G'f:;>,-~ ~iriE<' ~\
'/,'IT:>,' /<,"\2, > i.?
J".,
~:,'~ ~i
",;' .','
.'1 :;',1'
~9 r?'\
"
/ ,.".. !',
\'
(~.,::?J
..J'./"
'.; ;{:
z
'[ E;'
~~~)~ \
"
(\')
(Y)
(Y)
(Yj
m
;--...
oo;t
I.
;
.t.
" ,
.,r !.\ ,',
i,
. '.
. i, .\t.:
"~"
:?~;j
" -'
,;'-;, ;:",'/ :";',.-.,.
'< .re"'" .' 'f.
_."\\. 2J
/1; ;;(,:: ;~':;>:I\J <~ <
",:":;,,.:,~"I-'
c' /':"- ~'''- .,.. \. '~Iti
, z
,\;~::-.~-
':,;:\~<::i~1. rl"
~~\\
o
1""'.
It>
<D
...-<
(JJ
z
o
r.....
>.0
.()
....
aJ
"
v
\-
.~~~
'j
",
z
"
o
o
o
o
co
"
v
\'{
~"':-!'~::':' '.' :::~':~\,:::~~~:: '
=$.ti'l \\~
~ \~\~;
~~:~~;?,
. ,
"
;
t,;:"
I,
-'<<:..'\', ~ ,.
122.85000" W
122,833330 W 122.816670P,Il,;,
~O-::~~__ __.._~.._..__ __~._~..,__1 Mill I
~(I)J fHT 9 __..,. .......~ __..~'ooo MUHlS
M_~ ~--....." .~", H.J."O"'" -== 1>ra.'JoD44' O'''''FPN;-r1l'1nY~J;;Jf~{ '.
VVGi>5'1l' 1 J;l228CW1r\qo Vy' ,
...... i, i z:. ....JJ..-/v , "'- -""
I ,-
..~
TNt jMN
11 180
, df.VELOPr.,,'ENT
-r
I ;:'
'-'
,-..
,....-t-
(f) -TJ
-0 LO'I
o c
::::::l ,
ill
\I
V"',
()
Q
,,-
"
.S:>
::.
~
-
o
o
~
~
o
f\
~-
"
~
f".
~
~
'"
~
~~ ,
1
~-
r'"
---::>2
r<"'~'~'-.',.~
v. +, ..t.,.
--ac
o ~ \
~..J'i">
"" ~g
""fl ,+-
~'^
~
I.r:........
it"~
!'l ('-
~ ;;;.,-.
~. -,
~"- ,,~ - ~-:::::. '-,. .'
~;;-~';:!~':j=~_.~ .~
~-...... : I
~ '.. .... ",.
\
-" ~,
\ ~ -"
)'
"<
\ . /'
.f~
it)
_/
"'-!
i
j
l
I
0/
I> I
~ ;'
-I
\ (
, \
I \
I i
! \
l l
\. .... -- \ "~,,~,
'~~ - -.. "' ~.",..."./
h-..., ~, "-
. ~tT-r---,"'r" ~"'<' .... ~
j "~, \ ...... --""'-
! I ~>.,)., i
I ~ S:~ .~l~,li ..",I-
t ""', g. . ~
., + ~
l~:t,- -'_.'-'~'-r--,.
-.-'------~-------., k\ i
--"'\----l
\A
\
\
~/
"'" ....' ;'
x
/
\,'\
"-""" ~
~ I
~.........\
"-...
os
S)
"<
~
5)
'\::.--
v
\"}---
VI
4-.
S.
,,'
\A
-l-
v' ___--
~-rr'.'r"" \
^) I i
~' i['
)"'" ~ }
II
\
I
----- :r-
~ - ~--;;- n