HomeMy WebLinkAbout502311020 Geotech AssessmentSTRATUM GROUP.
P.O. Box 2546, Bellingham, WA 98227
Phone (360) 714-9409
February 11, 2008
Larry Howe
1705 West Gunstock Loop
Chandler, AZ 85286
Re: Geology Hazard Assessment
Parcel # 50231.1020
Case # MLA07-00595
313782 Highway 101
Brinnon, Washington
Dear Mr. Howe:
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Stratum Group is pleased to present the results of our geology assessment of the. above
referenced property located within a potential geologic hazard area. The purpose of this geology
assessment was to 1) evaluate the potential risk of slope failures impacting the proposed building
area, 2) .evaluate the risk posed by the construction of the new septic system to the stability of the
slopes on the property, and 3) provide general site development and maintenance
recommendations for the property adjacent to a potentially unstable slopes.
Based on my investigation of the site and vicinity, it is my opinion that gentle sloping area on the
property where the building and septic drain- field are proposed is not at risk from slope failures
over the expected life of the structures (100 years) and that construction of a home on the site or
the drain field will no increase the risk of slope failures at the site or off site provided the
recommendations presented in this report are followed. This assessment included a visual
inspection of the property, slopes on and in the vicinity of the property, and review of available
geologic mapping in the area.
SCOPE OF SERVICES
The scope of our services included the following:
1) Conducted a site visit to visually inspect the subject property and vicinity
including the bluff slope conditions, slope conditions on and in the vicinity of the
property and shoreline conditions.
2} Observed surface soil conditions on the slopes and upland areas by excavating
shallow hand dug test pits.
3) Prepared this report summarizing our findings, including an evaluation of the risk
of slope failures impacting the proposed development on the subject property and
February 11,.2008.
Parcel 5023 1 1 020, Jefferson County, WA
Geology Assessment
the proposed septic system, an evaluation of the slope- stability, recommendations
for site maintenance, and recommendations for further investigation, if necessary.
GENERAL GEOLOGY
Northwestern Washington has been occupied by continental glaciers at least four times during
the Pleistocene Epoch (1.6 million to 1.0,000 years ago). During these glacial and accompanying
interglacial periods, the underlying bedrock was eroded and a relatively. thick layer of glacial
related. and interglacial fluvial sediments were deposited over the underlying bedrock in the
vicinity of the subject property. .
The Geologic Map of Surficial Deposits in the Seattle 30' x 60' Quadrangle, Washington (Yount,
Minard and Dembrof, 1993) indicates the subject property and vicinity is underlain by Vashon
till and Vashon Recessional Outwash. The till units consist of a poorly sorted mix of clay, silt
sand and gravel with occasional boulders. The Vashon Recessional Outwash was deposited by
melt water streams from the retreating Vashon ice sheet. The unit generally consists of sand and
gravel with a fining upward within the unit.
My observations on the subject property and in the vicinity are consistent with the mapping
described above. However, much of the lower portion of the property is underlain by pre Vashon
silts, sand and gravels. The unit is very compact indicating that it has been over ridden by glacial
ice. Most the subject property, including the proposed building area is underlain by glacial till
with perhaps a veneer of recessional deposits.
SPECIFIC SITE OBSERVATIONS
A site plan sketch showing pertinent features is provided in Figure 2. The property slopes up
from the shore of Hood Canal and Highway 101 at an angle of approximately 25 degrees. This
slope is tree covered and is generally plainer. A cleared, grass-covered area with an existing
building is located on a very gentle sloping area above this slope. The cleared area is underlain by
glacial till and includes a couple of large erratic boulders with Canadian derivation. The slope of
the cleared area slopes very gently with no slopes exceeding 14 degrees except where a minor cut
slope is present.
The proposed building area on the gentle slope area is accessed via a gravel road spur off of a
gravel road to the east of the property. This same gravel road cuts across the northern portion of
the property. A seasonal stream flows across the northern portion of the property. A second
seasonal stream is located west of the subject property. This second stream appears to have been
incising down through the slope in an area west of the property and two small shallow soil
slumps were located above the incised portion of the stream. ~~~~~~
Stratum Group
2 ~'E~222~
File:12.15.07
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February 11, 2008
Parcel 5023 1 1 020, Jefferson County, WA
Geology Assessment
No unstable slopes or landslide hazards are present on the subject property. The only
unstable/landslide hazard area observed was on the steep slope of the incised stream drainage to
the west of the subject property.
No springs or seeps or seasonally wet areas are present on the slope of the cleared area on the
property or on the slope below the cleared area. The time of my site- visit was a few days after an
extreme heavy rain on snow event where as much as 16 inches of rain was recorded over a two
day period a few miles from the subject property. Despite this recent very high rain fall event I
observed no indications of surface water .flow on the property except for some minor surface
flow along an old logging road now overgrown with trees. I would note that other seasonal
drainages in the vicinity including one a-half mile from the subject property had very high
damaging water flows during the storm event of early December 2007.
CONCLUSIONS AND RECOMMENDATIONS
Based on my visual inspection of the subject property and vicinity, I conclude that the proposed
development on the cleared area with gentle slopes of 14 degrees or less are not at risk from
potential landslides or erosion induced slope failures within the expected life of the development
(100 years) as long as my recommendations are followed.
The 25 degree slope on the south portion of the property is stable and is not subject to landslides
or other slope movement beyond very minor soil creep. The Washington Coastal Atlas indicates
the area is unstable. Jefferson County's Critical Areas Landslide Hazard Map indicates that the
landslide hazard on the southern two-thirds of the property is slight and that the northern third of
the property has a moderate landslide hazard. An area of high landslide hazard is located to the
west of the property.
The high hazard area located to the. west of the property is located in an area of steeper slopes
with recessional outwash perched above impermeable till along with cut slopes from past road
construction. These conditions are not present on the subject property as the slopes on the subject
.property are much less steep and recessional units are not present at least with the proposed
development area.
My investigation and observations of the slope and soils on the slope indicate that the slope is
stable and is not a landslide hazard. Evidence observed on the site to support, this conclusion:
The slope angle of 25 degrees is stable due to the fact that the slope is underlain by very
compact silts, sand and gravels. Given the compa e,,p,~,~i~ re lacial sand and
gravel underlying the slope, this slope angle is sta (~~~i' 11~
Stratum Group ~~®~ 9 ~(~na File:12.15.07
JEEfEOSON COUNTY DCD
February 11, 2008
Parce1 5023 1 1020, Jefferson County, WA
Geology Assessment
All. trees on the slope and stumps on the slope are very straight indicating minimal soil
creep.
Past logging of the slope did not cause any slope failures as there are no slide deposits
anywhere on the slope or at the base of the slope.
Road cuts on the slope from past logging activity have been stable with the exception of
minor raveling.
No springs or seeps are present anywhere on the slope.
The only potentially unstable slope on near the property is the slope to the west of the property
down towards the incised seasonal stream located west of the subject property. Shallow slides are
present on portions of this slope and future shallow soil failures should be expected on this slope.
Building Setbacks
It is my opinion that the 25-degree slope on the south side of property is stable and there is no
risk of landslides on this slope under present conditions. The steep slope is stable and no specific
setback is required from this slope. However, because. logging slash is present along the lower
part of the 14 degree slope, I recommend that base of the home foundation. be founded on native
soil and no construction take place on the slash underlain area unless the slash is entirely
removed. '
The steep slopes down to the incised seasonal stream located just west of the property are
potentially unstable due to the incision of the stream and the steepness of the slope down to the
stream. Any development should be set back from the top of this slope at least 30 feet. This
distance would encompass only the. westernmost portion of the property as most of the slope and
setback area is on the properly to the west.
Storm and Septic Drainage
Soils on the upland portion of the property are well drained. The soils appear to consist primarily
of coarse grained deposits. Any additional buildings if constructed should have roof drains and
any other drainage discharged into a level dispersion/infiltrationuenches located to the east of
the structure.
The proposed septic drain field should not cause any problems for slope stability or erosion as
long as it is not located on the 25 degree slope. ~~~~~~ T~~
Stratum Group
4 FEB 2 2 200
File:12.15.07
JE~~E~SDN CGUNtYDCD
February 11, 2008
Parce1502311020, Jefferson County, WA
Geology Assessment
Vegetation
Site grading soils or debris, landscape debris, or any other material should not be disposed of
over any slopes greater than 30 degrees. Over time debris will break down and form an unstable
mass that will slide damaging the slopes below.
Please note that there are inherent risks associated with building on lots near or adjacent to steep
slopes. These are risks that the building owner 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 asrequired.
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
Dan McShane, M.Sc., L.E.G.
Licensed Engineering Geologist
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Filed 2.15.07
Stratum Group
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Figure 1. Site Location 47°37.113' N, 122°58.790' W WGS84
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