HomeMy WebLinkAbout001301031 Geotech AssessmentNORTHWESTERN TERRITORIES, INC. Engineers ^ Land Surveyors ^ Geologists
717 SOUTH PEABODY STREET Construction Inspection ^ Materials Testing
PORT ANGELES, WASHINGTON Phone (360)452-8491 ^ Fax (360)452-8498
98362 ~~I 1-800-654-5545 E-Mail: info @ nti4u.com
www.nti4u.com
`~ ~` ~ _ i~
TO: Susan McNab t t DATE: June 23, 2006
252 Lake Dell Av ~ ~ ~~~ , ~ _~~~ ATTN:
Seattle; WA 98122 ` ! ~ SUBJECT: Geotechnical Re ort
. _ _ _ _. p
__~ ~.., `T `LIEN
We are transmitting:
^ One bound report.
^ Three handbooks: Slope Stabilization & Erosion Control Using Vegetation
Surface Water & Groundwater on Coastal Bluffs
Vegetation Management
Remarks:
^ If you have any questions, please don't hesitate to give us a call.
copy to: Allegra Bothel
Windermere PT
~= - ;.
Sent via: U.S. Mail
Signed Bill Payton, L.E.G.; by F. Epp
GEOTECHNICAL REPORT
Prepared For Susan McNab
June 23, 2006
For the Property Described as
Tax Parcel # 001301031, Lane De Chantal
Section 30, Township 30 North, Range 1 West, W.M.
Jefferson County, Washington
Prepared by
NORTHWESTERN TERRITORIES, INC.
717 S. Peabody Street
Port Angeles, Washington 98362
Phone 360-452-8491 Fax 360-452-8498
Web Site www.nti4u.com E-mail info@nti4u.com
coo
NORTHWESTERN TERRITORIES, INC.
_~
A JLS GROUP COMPANY
~ 717 SOUTH PEABODY STREET, PORT ANGELES, WA 98382
N~,, Engineers Land Surveyors Geologists
Construction Inspection Materials Testing
(360) 452-8491 FAX 452-8498 www.nti4u.com E-Mail: in(o(~Dnti4u.com
JLS GROUP,
INC.
Geotechnical Report
Tax Parcel # 001301031, Lane De Chantal
June 23, 2006
Susan McNab
252 Lake Dell Ave.
Seattle, WA 98122
Subject: Geotechnical Report for Tax Parcel # 001301031, Lane De Chantal
located in Section 30, Township 30 North, Range 1 West, W.M., Jefferson
County, WA
Dear Ms. McNab:
Background
At your request, NTI Engineering & Surveying, Inc. (NTI) performed a geotechnical
inspection of the above-mentioned property, which included office research and a site
visit for visual observations conducted on June 13, 2006. No subsurface exploration
was performed. The purpose of this inspection was to observe the marine bluff at the
subject site by visual means and make recommendations regarding the development of
the property in accordance with the Jefferson County Unified Development Code
(UDC). It is our understanding that you plan to sell the parcel and the adjacent parcel to
the east, which you also own, for future residential development.
Site Description
The subject waterfront property is located on Lane De Chantal overlooking Discovery
Bay (Figure 1). The property is bounded on the north, east and south by wooded
residential property, and on the west by Discovery Bay (Figure 2). There is an existing
driveway on the property that leads to a small clearing at the top of the bluff (Photo 1).
The upland is hilly and bisected by two ravines that trend in a southwesterly direction
and drain over the bluff near the south side of the property (Figure 3). The ravines were
dry at the time of the site visit. There is a trail that leads through the ravine and down
the bluff to the beach (Photos 2 & 3). The upland is well vegetated in young to mature
trees, shrubs and brush.
A traverse down the bluff face was made in order to measure slope angles and
elevations. Access to the beach was gained via the above mentioned trail. The bluff
height at the property is variable due to the ravine, but ranges from a low of
approximately 200' at the gully, to a high of approximately 290' at the clearing at the
north side of the property. The bluff height at the south side of the property is about
250'. The average slope angle of the bluff, as measured from Figure 3 is approximately
40°, while measurements made on site were generally in the 44° to 48° range.
The bluff face vegetation typically consists of fir, cedar, and madrona trees in the ±12"
range with a brushy understory. Some of the trees are leaning and/or have curved
"pistol but" trunks, which indicates downslope movement. There has been a recent slide
on the bluff below the clearing at about the 50' elevation, which damaged a portion of
the trail (Photo 4). This slide is roughly 90' wide and 5' deep. The bluff face in the slide
area is void of vegetation, exposing predominantly sandy soil. There has also been
another recent slide north of the subject property that occurred in February of 2000
(Photo 5). This slide is roughly 180' wide near the head scarp, and extends from the top
of the bluff all the way to the beach. The top edge of the bluff may have receded as
much as 30' to 40' in the middle of the slide.
Site Geology
The Washington State Department of Ecology's Coastal Zone Atlas maps the upland
soil in the area of the subject property as Vashon advance outwash (Qva) over
Undifferentiated stratified sediments older than Vashon lodgement till (Qpf) (Figure 4).
These soils are generally composed of stratified sand and gravel, with some silt and
clay and are glacially derived. The Atlas also maps the stability of the bluff as Unstable
old slide (Uos) and Unstable recent slide (Urs). The upland is mapped as Unstable (U)
(Figure 5).
The Washington State Department of Ecology's "Geology and Ground-Water
Resources of Eastern Jefferson County, Washington", April, 1981 gives essentially the
same description of the soils in the area of the subject property.
The Soil Survey of Jefferson County Area, Washington (United States Department of
Agriculture, 1975), classifies the soil in the area of the property as being predominantly
Hoypus gravelly loamy sand (HuD), with Cassolary sandy loam (CfD) mapped in the
northeast corner of the property, and Coastal Beaches (Co) mapped along the beach.
The Hoypus soil formed in glacial outwash and consists predominantly of sand and
gravel, with areas of silt loam. Runoff is listed as medium, and the hazard of water
erosion as moderate. The Cassolary soil formed in reworked glacial and marine
sediments and consists predominantly of silty sand, with areas of clay. Runoff is listed
as medium, and the hazard of water erosion as moderate.
Visual observations made in the area were consistent with the above soil descriptions.
2
Mechanics of Bluff Recession
There are many forms of bluff recession that occur in the coastal regions of northwest
Washington. Two common processes are the erosion of the toe of the bluff by wave
action, and the sloughing of upper bluff soils due to saturation of the soil during the rainy
season. Commonly, this upper bluff sliding occurs in sandy soil that overlies an
impervious layer such as silt or clay as was the case with the large slide north of the
subject property.
When waves attack the toe of an unprotected bluff, the lower bluff soils. are eroded
away (Photo 3). Eventually, this erosion will oversteepen the bluff to a point where the
soil can no longer support itself at such a steep angle. Then the bluff soils will slough
off, depositing material at the toe of the bluff. This will have the effect of temporarily
reducing the angle of the bluff to a more stable angle, and then the whole process will
start over again.
Many of the landslides that occur in our region happen in the winter or spring when the
ground is saturated with water, and especially after heavy rainfall events. When the soil
becomes saturated, there is a decrease in the cohesion between the soil grains and an
increase in the pore-water pressure. This condition can trigger landslides and debris
flows on slopes. For this reason, it is important to control on-site drainage and runoff in
order to minimize the infiltration into and resulting erosion of the soil. It is also important
to maintain vegetation on the bluff face in order to reduce erosion of the bluff soils. The
bluff at the property is mostly vegetated, except for the bare areas mentioned above. It
would be difficult to establish vegetation in the bare areas that exist on the bluff at the
subject property due to continued erosion at the toe and due to the steepness of the
bluff in the bare areas.
Another factor to consider with regards to bluff stability is the "angle of repose" of the
soil. The "angle of repose" is defined as the maximum angle at which loose,
cohesionless material remains stable (assuming no destabilizing forces). This angle
commonly ranges between 33 and 37 degrees. The majority of the bluff at the property
is over the typical angle of repose. However this may be the angle of repose for this
particular soil given its current state of compaction from past glaciation. But when
loosened by weathering, frost heave or root action, the soil may become unstable and
typically assumes the lower angle of repose.
Conclusions and Recommendations
Due to the inexact nature of the geological sciences, it is difficult or impossible to predict
if or when a landslide will occur, or how large a slide might be. There have been recent
slides on the bluff at the subject property and in the vicinity, and the potential for future
slides exists.
Based upon our observations of current conditions at the site, we recommend a
minimum building setback of 100 feet from the top of the marine bluff. The approximate
3
top of bluff and 100' setback are sketched on Figure 6, as are two potential building
sites, one in the northeast corner of the property and another in the southeast corner of
the property.
Site #1, at the northeast corner, and site #2, at the southeast corner are similar in area
at roughly 5000 square feet each. These potential building site locations and
measurements were taken from Figure 6 and are just rough approximations. For more
accurate locations and dimensions, a topographic survey would be needed. There is an
existing driveway leading past site #1 and continuing on to the clearing at the bluff edge,
mentioned earlier in the report. There is also a driveway leading close to site #2. Both of
these driveways cross the adjacent property to the east of the subject property and may
need an easement. Also, the driveways may need to be improved in order to access the
sites with vehicular traffic.
Both sites will probably need some excavating work to provide a level building site. We
recommend that any excavated material be hauled off site rather than be pushed over
the slope, which can cause destabilization of the slope, and that the house be founded
in a full bench cut rather than cut/fill. The descending slopes around site #1 are
generally in the 20° to 26° range, while the slope north of site #2 is in the 32° to 36°
range. We recommend that no construction take place on slopes steeper than 30°, and
that a 15' building setback line be established from where the slope becomes steeper
than 30°. Further, the foundation requirements of the International Building Code (IBC)
will need to be followed (Figure 7). The full text of the code can be found in Section.
1805 of the code.
The following recommendations should also be considered with regards to the proposal:
1. It will be necessary to maintain ground cover to reduce erosion from surface
runoff. Any bare areas that develop should be revegetated. Native deep-rooted
vegetation that requires little or no irrigation would be the most beneficial.
2. Vegetation on the bluff face and ravine slopes provides stabilization to the soils.
Existing established vegetation should be left in as natural a state as possible.
If a better view is desired, selective tree removal, thinning and pruning should
be done in such a way that minimizes disturbance to the soil and root zone and
that insures the continued health of the vegetation. Damaged trees or trees that
are in danger of falling, due to undermining of the roots for example, should be
evaluated for removal because of the damage potential to the bluff caused by
the root mass of a large tree being pulled out of the ground, and also due to the
risk of a tree falling on the house. If trees are ,removed, the trunks should be left
in place so that the root mass can continue to provide stabilization to the soil. A
tree expert could provide valuable consultation in this matter should the need
arise.
4
3. Heavy irrigation or other activities that would contribute large quantities of water
to the soil should be avoided.
4. Surface water should not be allowed to flow over the face of the bluff as an
uncontrolled or concentrated flow and cause erosion of the bluff face. This can
be controlled with vegetation and using berms or swales to direct runoff to a
drainage system. Please see the attached DOE publications for more
information on this subject.
5. Surface runoff f~'om hard surfaces such as roofs, driveways, walkways and
patios should be controlled and routed to a drainage control system such that
surface water discharge to adjacent properties does not exceed
predevelopment conditions.
6. Silt fences or other sediment control devices may be needed during
construction such that sedimentation to adjacent properties does not
significantly exceed predevelopment conditions.
7. All drainage control devices should be maintained in good working order and
inspected at least once a year.
8. An engineered drainage, sediment and erosion control plan should be
developed for this property to address items 4 through 7 above. NTI can
accomplish this task if requested. A contractor knowledgeable in the
construction of these types of features should be retained for this work.
9. The septic drainfield should also be located behind the recommended building
setback lines.
10. We recommend that footing drains be incorporated in the design of the house.
Footing drains should be kept separate from roof drains.
Based on the findings, recommendations and limitations of this report:
1. Coastal bluffs are inherently unstable, however there appears to be minimal
landslide hazard to the proposal due to the recommended setback distance to
the proposed house.
2. Observations of slope stability indicate that the proposal should not be subject to
risk of landslide under the current conditions that exist at the site.
3. The proposal would not increase surface water discharge rates or sedimentation
to adjacent properties beyond predevelopment conditions.
4. The proposal would not decrease slope stability on adjacent properties.
5. The proposal would be stable under normal geologic conditions.
5
For further information please review the three attached copies of booklets published by
the Washington State Department of Ecology (DOE) entitled: "Slope Stabilization and
Erosion Control Using Vegetation", "Vegetation Management: A Guide for Puget Sound
Bluff Property Owners" and "Surface Water and Groundwater on Coastal Bluffs". These
publications are now out of print but can be reviewed at the DOE website at:
http://www.ecy.wa.gov/biblio/sea html under the 1993 and 1994 year heading. The DOE
website also contains more useful information regarding slope stability and site
development and is highly recommended.
Limitations
This report has been prepared exclusively for you and your agents in conjunction with
the above referenced project. The report has not been prepared for use by others or for
other locations. Others may use it only with the expressed written permission of the
Engineer.
Within the limits of scope, schedule and budget, this report was prepared in general
accordance with accepted professional engineering and geological principles and
practices in this or similar localities at the time the report was prepared. No other
warranty, expressed or implied, is made as to the conclusions and professional advice
included in this report.
The observations, conclusions and recommendations presented in this report were
based on our visual observations of the subject property at the time of our site visit;
laboratory tests were performed. Soil and geologic conditions can vary significantly
between test holes and/or surface outcrops. If there is a substantial lapse of time,
conditions at the site have changed or appear different than those described in this
report, we should be contacted and retained to evaluate the changed conditions anc
make modifications to our report if necessary. _ _ _
Sincerely,
NORTHWESTERN TERRITORIES,. INC.
Robert A. Leach, P.E., MBA
Principal Engineer
,~~.
Bill Payton, L.E.G.
Engineering Geologist
G:\Gen\Bill\Reports\MCNSD601.b1uff stability.30(30-1).Discovery Bay.doc
~~
191
6 ~Willlam C. Payton Jr,
Expires 11/06/06
no
~~z3~CG
/-.~~c
EXPIRES 12/30/2006
if ii ~r ~ 4 r F ~.
f' F 0.
•v-~~~.. r -. ~ ~ -°f --"i"'-~+`°"° /5 0 _ a-a~ _ 'tea- f _ ~ ~.._.
t 4 • -..-
e m.
i ~ T i~
V
;.
,,~
_ _
tfr' - `~ j~y~ _ - ~ I
_ iE
~'~ ~{+ i
f ~ "ice _~~
_ _ _ J _ _ I
'y' '-~®_
$UbJ~ = ~
__
~E ~ I' _ ~ ~
~~ ~~~ ,
'~.,'~._.~ ~ Figure 1
~.
u~ ~~~ c~~~~ Deny c ~ ~~ ~ ~ ~ -
1 ~ ~~r`~~ ~ tia _~ti s
i~ ~ / r„~ o
`s> I
y
20' Qvtt ~ ,~' ~ /
'
Qvt1
35 ~„
,
30" `~"a '~ ,sti Geology
! ~ ~ x ;
' ' -
.6
i 1~
' i.
/~
~
40
45' .
'~ .~
~
,,~
-Qo
S l
' ~ ~ ~'
A
~
~ »\
6°~ti' .etas %: - i ~~ ._ _. -- - - y- -
~ - ~,
a5'
L1~ ~ a ___ ~ ,
~•'` ~+,
'
: ~
~
~
;
F ..~
~. ~ ~ .'~ =y.
40 ~~xY bb ~
~,
,..:
„, ,~
.
~f
~ llos
;
Y
.
,
,
.
4 °a i.~~ a.
3 /~ '~
` -
lam
J~. o b
~
~~¢ ~
George ` Cape George ,
_
0
.. _ -
~~~"~1~~ ~~
~
m
m
~
3~
~~~~, r-~ ~ ~ =
`~
r
~
~~~ .rt- -:
~`~
~
~
QPf. , e
l .
~ '\
\
,~
o
4
'~ ~
Qpd , a i
`
~,
i ~
`
.
%
~; '
~~~ i$ s~
_
~_
3. U
';
_
~ , ~
'a ~
', ~ ~
€
,
~
'~' Uos
~
f ~ ~ /
c~~~' ~ - o
l ~~ + 0
~
'
1 =-
Washington State Department of Q- ~.,.~
Ecology's Coastal Zone Atlas
Glos.
~'~~,.
'4"- ;~
'~ ~ :.`
~~~ 1{D 4
g;'
~~~`.~~, Figure 4
\\ I .
>, ~ ~-
~ ` ~ r ¢ ~ - ~ Slope Stability - .~ ' r Y ~'
pl Dios
c
~t 4l~ Uri. ~~z, ~.l '
.. c ' i Iw.. ~ f' .. ~.Ji~ ,
~~.~$ f ~
,UOS, f ~ , ~ ~~ ~'~t~r eb i:~ g / r
7~~, :z ~ ~~~~ -stn •axY I' "~-:~ _t
V - I. ~ `.
~.~b i r- '
a R ;j.~ ~ ~ ~ara ~2
4 i~ irs •,-`.~`n
Cape George _,z=`' 1 • ,- Capp Georg ~ - ~~
l-,r - ~ I _ z, _
m \ _~ - ,~
.~ .
~' p}jx~ ~'_ t -'o ~ ,
,a ~ ,
~ ~~ ~~
`. ~.~
~~1 }
3 ,~{..
'~
i , ~3:.
i 1 ~~ G'./
I ~.. 1
Urs ~ ~ , -
- ~~G -;~~~:~o. l~ ~/
Gr.-
! ~ 1~ w
~'I~a. ~ ~:, ~' d ~~~ ,~~
Uoa j' ~ ~~'1 .
i;~ i..
~ ~/ I ~~ 1
Uos <C,~ ~, / ~ ~ ~~~ ,> ~~
~ {31
Ur$` ,= ip~ ~`
! ~ ,CS
t~f~,$ r,Fq~ ' 1I c ~Ja
Beck UrS ~ `~.... q~` 6eckett Pt ;. "^~
3~J ~ . ~
~'C ~ • Urs -_-_,' .I
/ 1 ~~ -
•~ I i
.i i ~
'' Ovt11° Property I
Vicinity Urs ~ ;
Washington State Department of
Ecology's Coastal Zone Atlas ~ Figure 5
t,, .,, .,
,--. ,-
r ~
~;' i t S` 4
p;,_ ~ j ~ ~
- ',
'~ ;~° _~
Approximate y
` ~ top of bluff ~ ,, -,
''1~~I
3 t ~ y1, c, 5
~ !
~ ~', ~ r 331 i(.11 ~ '
i ' fl 1
~ ~i ~ ~~ ~ ~ I~ ~ ~ ~ ~ ~,
~y
~~~ I ~ li ~ ~1,~~~
~,~~ X1;3 ~{ ~(~ 1
I ~~ ~ 1i ;IIE ~ti~ ~
i (I~',€ 111( '1y
jj~ ~~ i i} 5 i ~~ i~ t
II ' (t itl~l jf~~l ~'2 S lie
~ti~ 1i i~' ~ '~
~~' it ~ I I I ~i ~ I~ } ~ I 1
I~f '~ ~~~•Itx5 ~ ' ~
locations and dimensions
~~~ ,,~
are approximate '' . ~'
'N~u crv.•dxt try,e'~sa-i ~• ~~s saes GI,S
i, l_?~.~~~.~... ~ ~~.
_ ~~
~~
~
,,i
~
~~
x-
~~~
1 1'R ~ 3i ~
Potential building "~ ', - -- --
site #1 ~~, ~ ',~
~
~ ~ _ _ ,may"„ I ~I ~
~
~ .~..,
~
~ ;
e.-
r yJ_ 1
.~ ~'
'E '.`~
~_-
~ S~
'~
'
... A }
I
ttom~
/ } Potential building ~,,~ ~
~, site #2 i~ ~ I
it =`~ !t
V
u; ~.
~ ~ ~~
' ~~
~
I ,
Approximate 100' `
,
setback line from ~ ~ ~~
top of bluff
:K 1 ~
.
Figure 6
.. ~,, ~ k _
2003 International Building Code
FACEOf
FOOTfNQ
TOP OF
SLOPE
-~"
_..__._
FRCEOF f-~t :............ J~
~S7AUCTi~1E ~\`//
TiOE OF N!9 BUT NEED NOT N
^.aLOY E EJCGEED 40 FL
MAX.
8UT NEED NOT EXGEED T5 FT. MAX.
For SI: I foot -304.8 mm.
FIGURE 18053.1 Fi ure 7
FOUNDATION CLEARANCES FROM SLOPES g