HomeMy WebLinkAboutBLD2000-00468 Geotechnical Report U '
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SLOPE STABILITY
WAKEFIELD PROPERTY
BTWATI R BAT
by
Gerald W Thorsen
Consulting Geologist
Septemter 1990
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SUM?IART
The 50-foot ban
k that fronts thy. Wakefield property is made up
essentially of silty , very hard , glacial drift . The slope
averages 45 to 50 degrees and _s almost entirely covered with
vegetation , some as old as 50 years . Neither direct nor
vegetative evidence for significant ground water concentrations
were found . The bank lines at the toe and along the top are
more-or-less linear , suggesting rather uniform retreat . Tree
ages on slides and at the bank edge suggest retreat rates of
about 2 inches-per-year , Eros: on is predominately by slow
continuous soil creep and small , sporadic landslides . Deposits
of both are being removed by light-to-moderate wave action . The
nature and rate of erosion here suggest that prudent residential
development should not he hindered by stability problems .
INTRODUCTION
Location : The property fronts on the mouth of Bywater Ray ,
between Hood Head and the Hood Canal Bridge , in Eastern Jefferson
County . It lies within the SW 1/ 4 , NE 1 /4 , Section 35 , Township
28 N - Range 1E ,
Scope of work : This report is Lased on a general knowledge of
the geology and geologic processes in the area and an on-site
examination on August 23 . (Precious examinations of shoreline
banks to the north and south helped provide additional
perspective on materials and processes at work here . ) Aerial
photos were also reviewed althou3h were not much help due to the
dense tree cover . Available geologic maps , including the Coastal
Zone Atlas were also examined .
Physical setting : The property has a general easterly slope
aspect , with slopes near ParedLse .Bay Road averaging about 3
degrees . About mid-property , Elopes approach 15 degrees in
places , and flatten toward the shoreline to about 7 degrees . The
bank is obscured by dense vegetation and slopes are not uniform ,
but seem to average between 45 and 50 degrees . Along the south
boundary , nearly horizontal slo )es extend for almost 350 feet
from the road . There , they steejren abruptly , and are covered by
extremely dense vegetation . Thus , an estimate here of 25 degrees
is essentially a guess . Further to the east (shoreward ) , slopes
again moderate , probably averag .ng about 10 degrees . The north
property boundry is paralleled b3 a large draw , apparently carved
by post-glacial drainage .
Materials making up the site appear to be all of glacial origin .
Exposures are sparse , but what can be seen on the uplands is
generally a mottled clayey and/or sandy silt with randomly
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scattered pebbles . The bank is also largely obscured , but in
places near beach level a gray \ ery compact non-stratified sandy
silt can be seen . Isolated boulders indicate its glacial origin .
The hard nature of the material is no doubt due to compaction by
the last continental ice sheet; , bete at least 4000 feet thick .
The relationship between the materials exposed on the upland
surface and at the toe of the bank is unclear . Both are probably
deposits of an older glaciation .
Wave action at the site is light to moderate . Hood Head provides
wave protection from westerly and northerly storms . In recent
years the floating bridge , less than a mile to the south ,
provides protection from storm: coming up Hood Canal , (Thus ,
estimates of long-term wave eros: on rates in this area may now be
mIslending . )
BACUROUND
In 1974 a large ancient landslice southwest of Termination Point
reactivated , cutting roads and severely impacting the beach
fronting the area . In 1977 the County Commissioners signed
Resolution 36-77 placing a bul..ding moratorium on the area of
that slide activity . In 197E the Coastal Zone Atlas for
Jefferson County , was published ( see mep ) . The Atlas identified
both ancient and recent landsliles , as well as unstable areas ,
that extended well beyond the limits of the moratorium , In
addition , it identified "critical areas" of instability , one of
which encompasses the bank fronting the Wakefield property ,
Extensive landsliding has since occurred in the Thorndyke Bay
critical area . )
In 1983 the County Commissioners signed Resolution 69-83
restricting development in a •eas both west and north of
Termination Point that were designated in the Atlas as "Urs"
( unstable , recent slide) and "'Jos" (unstable, old slide ) , The
Commissioners , apparently in recognition of the limitations of
the Atlas , also resolved that ' specific sites within this area
may be excluded from the terms of this resolution" if a soils
engineer or geologist experienced in such matters could assure
"that slope stability can be aci.omplished" . Since passage of the
Resolution it has been possible to also exclude areas within the
moratorium area that are not unstable or , at least , just as
stable as most other shoreline binks in Jefferson County .
Such exclusions do not imply that the Atlas was "wrong" , they
merely demonstrate the limitations of reconnaissance-1eve1'
mapping . The Atlas itself recognizes such limitations in stating
( p . 3 ) that "areas on slope stability maps contain local
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exceptions due to limitations of map scales , generalization of
mapping units , or lack of info 'mation" . Thus , it is stressed
that "these maps are not a substitute for professional
site-by-site analysis in the field" . Such disclaimers appear to
have been considered by the Commissioners in the drafting of
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Resolution 69-83 .
FINDINGS AND DISCUSSION
The location of the "break" between stability categories here , as
shown in the Coastal Zone Atlas , was apparently based largely on
the published topographic map of the area ( see map) . note that
in this general area the width ( in horizontal distance) of the
bank is shown to be nearly the same as that of the more gently
sloping uplands . Clearly , this . s an artifact of the r,nntouring
process . (The contours of the bunk here would "run together" as
a single broad line if drawn t( scale . ) Later contour maps for
other areas solved this problem by only showing top and bottom
contours in areas of high steep banks . Thus , recon mappers for
the Atlas had to contend with a base map that was misleading in
such instances , as well as tree cover so dense that aerial
photography was of limited value
The bank in this area is an area of old slides ("Uos" ) , as
mapped . (Shoreline banks and b .uffs bordering Puget Sound and
Hood Canal are all a result of tie combination of landsliding and
wave action , ) Just as some mappers categorized wooded bluffs as
areas of slides , "Uos" , they considered bare bluffs as evidence
of recent slides "Urs" . There , also , were commonly shown
exaggerated due to map limitatiols .
Current bank erosion at the site is predominately in the form of
small landslides ( photos C an(l G) and soil creep ( photo H) .
The soil creep is slow, on the order of inches per year , and
involves only the weathered m;Lterial and vegetation mat that
overlies the ice-compacted g: acial materials . (Heavy rains
and/or concentrated surface runoff can saturate such materials
and trigger shallow debris avalanches , but none were seen on the
site . ) In the long-term , soil creep can be an important mode of
bank erosion , especially in su :h settings where the underlying
materials are difficult for tree roots to penetrate . Even
shallow cuts , such as for a trail , can accelerate creep rates of
the soil/vegetation mat .
The two recent landslides obse •ved illustrate the mode of bank
failure more important to humaft occupancy . The shape and depth'
of these landslides suggest 'Oat they are in the category of
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slumps rather than debris avalanches . They are deeper than a
typical debris avalanche in su :h a setting , the one in photo
G , near the south property boundary , cutting almost 10 feet into
the bank (measured perpendicula ' to the surface ) . The one near
the north property boundary apparently moved rapidly , as
indicated by the large silt fragments deposited well out onto
the beach surface . This combination of deeper-seated and sudden
movement are the primary condern for the safety of future
structures on the adjacent uplands .
Fortunately , these slumps are snail (no more than 25 feet wide )
and discontinuous . Neither hail a significant impact on the
adjacent uplands . . The age of the alder on the southern one
(photo G) suggests that there ray be a fairly long reccurrance
interval between such events et given site . For example , the
alder transported to the beat} by that slump apparently was
growing for at least 55 years on the mid-bank slope above its
present location . The relatirely straight bank edge also
suggests a long recurrence interval between such events . ( e . g . ,
two slumps of the upper bank at the same site in a short period ,
say 10 years-or-so , would yield a pronounced indentation or
"scallop " in the upper bank line . )
A problem with arriving at a safe bank setback for a home is the
question "Is what we see going on today typical?" (Landslides
are episodic events , so short-term averages can be misleading . )
Based on what can be seen today , I feel that current processes
are representative of at leas : the past 50 �+/
bank-edge scallops near the bi fir p years • Both the
ge of
vegetation from the southern slice suggest oerosion to B) �rates nd eofaabout
2 inches per year for that period . Based on this figure and the
for a substantial safety factor , a bank setback of at least 1need
50 feet would seem prudent . This would yield a shoreline setback j
Of more than 90 feet (horizontal ) and a shoreline-to-structure
angle of about 30 degrees .
- The general topography and abrup ; change in slope angle along the
upland south property boundary suggests that this particular area
could be the left flank of a la -ge deep-seated landslide . If so ,
little appears to remain of the slide mass . This absence of
"slide mass topography " and the presence of old-growth stumps
on what appears to be a scarp , indicate that it is an ancient
feature , possibly thousands of years old . The extremely dense
vegetation makes it difficult to learn more without detailed
investigation , possibly incluling borings . Lacking further
information , it should be assumtd that this is a slide scarp . As
there is little glide mass left to reactivate , that aspect is of
less concern than the oversteepened scarp area . Because of the
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unknowns involved , this bank should be treated with the same
• respect as the shoreline bluff , in regard to setbacks . Any
drainage from the swampy uplands here should be routed around
this topographic feature rather than drained into it . There
should be no excavation or construction on or near the toe of
this possible E1IIcI.C11L .7l„dl N wJ t.h ue the guidance of a coilo
engineer .
RECOMMENDATIONS
A . Minimize soil and vegetation disturbance and "dead weight"
loading on the hank hy :
- preserving all vegetation practical
- removing any trees cut for view enhancement
- avoiding deposition of clearing or other debris on bank
- a community beach access (rather than lot-by-lot)
- using stairs and ramps , rather than cuts , for trail
B . Maintain and enhance natural bank toe protection :
- do not cut downed trees on upper beech until they die
- stabilize future bare slide masses by planting
C . To minimize rilling of uplands or gullying of bank :
- coordinate and control storm drainage
-plant bare surfaces as soon as practical
- conduct any runoff from uplands to beach in pipe
- carefully plan any drainage of swampy area , SW corner
b : To maintain apparent stability of probable ancient scarp ,
south edge of property :
- avoid cutting and/or any construction on or below scarp
- divert upland runoff fron apparent slide area
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