HomeMy WebLinkAbout937800119 Geotech Assessment (2005)
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STRATUM GROUP
1451 Grant Street, Bellingham, WA 98225
Phone (360) 714-9409
June 20, 2005
Katherine Kent
411 Coleman Drive
Cape George Colony, WA 98368
Re: Geology Stability Inspection
311 Victoria Loop
Cape George Colony, Washington
Dear Ms Kent:
I visited 311 Victoria Loop in May 2005 to evaluate the slope stability of the steep shoreline bluff
on the northwest side of the subject property. The purpose of the site visit was to assess the slope
and shoreline and determine an appropriate minimum setback distance for the construction of a
home on the site as well as other development recommendations. Based on my assessment of the
geology and geologic processes at the subject property a home sited on the property should be set
back at least 50 feet from the top of the steep bluff slope.
SCOPE OF SERVICES
The scope of our services included the following:
1) Conducted a site visit to visually inspect the subject property including the bluff
face slope conditions, shoreline conditions, and relevant conditions in the vicinity
of the property.
2) Observed surface soil conditions on the bluff face and on the uplands above the
bluff by excavating shallow hand dug test pits.
3) Prepared this report summarizing our fmdings, including an evaluation of the
feasibility ofbwlding a residence on the subject property, an evaluation of the
shoreline bluff stability, recommendations for site development, and
recommendations for further investigation, ifneeessary.
,
June 24, 2005
311 Victoria Loop, Cape George
Geology Assessment
GENERAL GEOLOGY
Northwestern Was~gton has been Qccupied by continental glaciers at least four times during
the Pleistocene Epoch (1.6 million to 10,000 years ago). During these glacial and accompanying
interglacial periods, the underlying bedrock was deeply eroded. The Surficial Geologic Map of
the Port Townsend 30- by 60-Minute Quadrangle, Puget Sound Region, Washington (Pessl,
Dethier, Booth and Minard, 1989) and the Geologic Map of Northeastem Jefferson County
(Gayer, 1976) indicate that the subject property is underlain by advance outwash deposits.
However, Pessl and others (1989) indicate the subject property is located very near the contact
between the advance outwash and Vashon glacial till. Gayer (1976) indicates the property is
located near the contact between advance outwash and Vashon ice-contact stratified drift. Pessl
and others (1989) and Gayer (1976) indicate that the bluff face is underlain by glacial and
nonglacial sedimentary deposits. Gayer (1976) provides a profile of the stratigraphy of the bluff
to the northeast of the subject property. The stratigraphic profile from the top of the bluff to the
shoreline is Vashon till, Kitsap Formation, Possession Till, Whidbey Formation and Double
Bluff Formation.
Site observations on the subject property, bluff face, and in the vicinity are consistent with the
above-described mapping. The scale of the Pessl and others (1989) map is larger than Gayer
(1976) and therefore the Gayer (1976) may provides more detail.
The Vashon ice-contact stratified deposits described by Gayer (1976) consist of a poorly sorted
mix of clay, silt, sand, gravel, cobbles and boulders that shows weak stratification and is very
compact. The Vashon deposits were deposited directly by glacial ice and melt water under the
glacial ice during the last glacial period, which ended approximately 13,000 years ago. The
Vashon deposits underlie the upland area of the property and vicinity and the uppermost few feet
of the bluff.
Layered silts and clays and some fine sand layers all in a very compact condition underlay the
Vashon deposits on the steep shoreline slope. The silts and clays are consistent with the
descriptions of the Kitsap Formation. The unit appears to be part of a river alluvial deposit with
rme silt and clay over-bank deposits with lenses of cut and fill sand and gravel deposits
representing stream and river channels. The unit appears to consist of a sequence of sediment
deposited between the most recent glacial period and the previous glacial period. The unit is
approximately 45 feet thick at the subject property.
Glacial till underlies the very compact silts and clays. The till consists of poorly sorted clay, silt,
sand and gravel with occasional boulders. Based on the stratrigraphy of the bluff, the till is likely
the Possession Till. The till unit is approximately 15 feet thick on the bluff face.
Stratum Group
File: 6.6,OSA
2
June 24, 2005
311 Victoria Loop, Cape George
Geology Assessment
The till is underlain by layered sand, silt and gravel deposits in a very compact condition. Based
on the stratigraphy of the bluff, the unit is likely the Whidbey Formation. The unit is very
compact, but portions of the sandy units lack silt and clay and therefore have low cohesion.
SPECIFIC SITE OBSERVATIONS
The subject property consists of a gently sloping upland area bounded on the northwest by a very
steep shoreline bluff that is approximately 140 feet high. The upland area of the property from
Victoria Loop road to the top edge of the steep shoreline bluff slope is 162 feet wide. A home is
located on the property. The home is currently vacant as the northwest side of the home is located
very close to the top edge of the bluff and the home is at risk.
The uppermost portion of the bluff is vertical and is approximately 15 feet high. A steeply
sloping bench is located below and then the slope angle approaches vertical again. The lower
slope of the steep shoreline bluff is underlain by landslide deposits. Wave action on the lower
part of the bluff has been causing the bluff to become over steep triggering periodic landslides.
ASSESSMENT OF GEOLOGIC RISK
Observations along the shore and the top of the bluff indicate that the primary cause of slope
instability is the undermining of the base of the bluff by wave action. As wave action erodes the
base of the bluff the lower slopes fail and the failed material with the exception of boulders is
readily washed away by the large waves that periodically hit the shoreline. As the base of the
bluff fails and the bluff becomes over steep the slope failures progressively move up the slope
until the uppermost part of the slope fails as well and the process repeats itself. This stretch of
shoreline bluff is in various stages of this process. The shoreline bluff southwest and northeast of
the subject property has failed in a manner that the bench area that is present on the slope of the
subject property is not present. Other sections of the shoreline in this area have very little or no
landslide debris along the base of the bluff. Whereas, an area to the northeast has a landslide
prism at the base of the bluff covered by a mature stand of alders that appear to be at least 30
years old.
The steepest sections of the bluff are most likely to fail during periods of heavy rainfall. The
most likely failure areas are not only the steep undermined sections but also areas with fine sandy
zones and areas where vegetation debris and topsoil has accumulated. I did not observe any
significant sandy layers anywhere along the bluff, so that the risk of a large scale sand blowout
on the bluff face appears to be unlikely. The presence of the bench appears to be the result
primarily of the presence of a very compact till with boulders. This bench area is not a prominent
feature on the bluff and is not present over a large distance because the primary geomorphic
Stratum Group
File: 6.6.05A
3
June 24, 2005
311 Victoria Loop, Cape George
Geology Assessment
action on the bluff is wave erosion at the base of the bluff.
A large rain-on-snow event struck the Puget Sound area on January I, 1997. This weather event
consisted of a period of cold snowy weather followed by a heavy snowfall that then turned to a
warm heavy rain. Slope failures took place throughout the Puget Sound area during that event
and over several weeks to months afterwards. This event impacted the subject property and led to
the home on the site being at great risk of failure. Approximately 5 to 10 feet of the upper
portions of the bluff collapsed onto the slopes below.
Weather events like that of January I, 1997 are not a common occurrence at the location of the
subject property, but should be expected, as well as periods of extended very wet winter weather.
Not all the steep slopes in Puget Sound failed during that event, and since the subject property
has recently experienced a slope failure that reached the top of the bluff it may be safer for a
time.
Overall though the bluff is very steep and erosion at the toe of the bluff should be expected to
continue. As the very steep slope weathers shallow slab type failures are likely to occur. Based on
my observations along the top of the bluff at the subject property and all along the top of the
bluff northeast and southwest of the bluff, it is my opinion that these slab type failures will be on
the order of a 5 to 10 feet.
Erosion rates along the eastern end of the Strait of Juan de Fuca are estimated to be on the order
of 6 inches per year. Because of the protection afforded by Protection Island, I estimate that
erosion rates along this section of coast to be somewhat less - on the order of 4 inches per year.
The top of the bluff will not systematically retreat 4 inches per year, but the bluff will
progressively get steeper and mid slope failures will take place and eventually another failure
will take place at the top of the bluff. As noted above the size of top of bluff failures should be
expected to be on the order of 5 to 10 feet.
The recurrence interval of top of bluff failures that send material to the base of the bluff appears
to be on the order of 25 to 30 years based on the age of tree stands along the steep shoreline
slopes. This estimate is complicated by a several factors: I) the bluff height to the southwest
decreases, 2) the bluff height increases to the northeast, 3) clearing of all mature large trees from
the top of the bluff may have altered the frequency of slope failures and increased erosion rates
because of the loss ofhu.ge woody material in the landslide debris, and 4) drainage and slope
management on the developed properties and undeveloped properties likely has varied. Hence,
towards the northeast where the bluff is higher and there has been less development, the
recurrence interval of slope failures effecting the top of the bluff appears to be longer, whereas to
the southwest the recurrence interval appears to be more frequent. In my opinion this difference
is due primarily to the height of the bluff. The lower bluff height to the southwest produces
Stratum Group
File: 6.6,05A
4
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June 24, 2005
311 Victoria Loop, Cape George
Geology Assessment
smaller slides that more readily erode and thus the frequency of bluff failures is higher to the
southwest. Whereas the higher bluff to the northeast produces larger slides that take longer to
erode after the slide and thus the upper slopes fail on a less frequent basis. Slight changes in the
geologic units can also playa role in erosion rates and slope angle. Portions of base of the bluff
to the northeast of the subject property are underlain by glacial till so that the rate of erosion may
be slightly lowered. '
CONCLUSION AND RECOMMENDATIONS
I recommend that all structures sited on the property should be set back at least 65 feet from the
top of the steep shoreline bluff. Structures that should be kept at least 65 feet back from the edge
of the steep shoreline bluff should include the septic tanks and drain field and the home. Decks
may be closer as long as they are not structurally attached to the home.
This setback distance is based on an assumed erosion rate of 4 inches per year. The erosion rate
estimate is based on conservatively high estimates of slope erosion rates. The 4 inches of erosion
per year would cause the top of the bluff to move back 33 feet. I added two maximum slab
failures of 15 feet to the 33 feet to yield 63 feet and rounded up to 65 feet. My site observations
along the top of the bluff indicated that the past slab failures on the bluffhave caused to bluff to
retreat a maximum of 10 feet with most observed failure scarps being less than 10 feet.
As describe above most of the factors impacting the bluff stability over time are beyond the
control of a property owner. However, some actions can be taken to reduce the frequency of
slope failures.
Based on site observations and my assessment of the geologic risk, I make the following
recommendations:
. Native vegetation, particularly tress and low native brush on the slope should be disturbed as
little as possible. No trails or grading should be performed on the bluff face.
. Waste and other deleterious material should not be placed on or over the slope. Such material
smothers vegetation, retains water and acts to destabilize the slope.
. Storm water from roof run-off and driveway run-off should be dispersed as much as possible
through allowing sheet run off and lateral dispersion spreaders such that storm water is not
concentrated anywhere on the property and onto the bluff face.
Stratum Group
File: 6.6.05A
5
,1
June 24, 2005
3]] Victoria Loop, Cape George
Geology Assessment
As noted above the stopping erosion and slope failures at the site is not feasible. The farther back
from the top of the slope the longer structures will last and the greater the factor of safety. The
total width of the upland portion of the lot is 162 feet.
Stratum Group appreciates the opportunity to be of service to you. Should you have any
questions regarding our reconnaissance please contact our office at (360) 714-9409.
Sincerely yours,
Stratum Group
&M-
Dan ~e, L.E.G., M.Sc.
Licensed Engineering Geologist
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Daniel McS"'~'. .:]
Stratum Group
File: 6.6.05A
6
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