HomeMy WebLinkAbout601324007 Geotech Assessment
STRATUM GROUP
P.o. Box 2546, Bellingham, W A 98227
Phone (360)714-9409
May 15,2008
Michele Farfan
Jefferson County
Department of Community Developemnt
621 Sheridan Street
Port Townsend, W A 98368
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Re: Geology Hazard Assessment
Parcel # 601324007
Thousand Trails Road
Quilcene, Washington
HAY 1 6 2008
JEfFERSON COUNTY DCD
Dear Ms Farfan:
I completed a geology hazard assessment on the above property in January 17,2008, I wrote a
report for the Scotts and sent them a copy of that report' dated January 17, 2008, I subsequently
sent a more detailed report dated February 8, 2008. I am sending you a copy of February 8 report,
The conclusions of the two reports are the same, The only difference is that I provide a detailed
geologic interpretation of the slope below the yurt to emphasize that the slope near the yurt is
stable and is not landslide area. As such no set back is required on the slope and no vegetation
buffer is required,
This is a bit of confusing site. The slope next to the yurt is not the. top of the steep shoreline bluff
slope. It is an ancient slope feature likely the result of time near the end of the last glacial period
when local sea level was higher prior to rebound from the ice load on the crust. This slope is
stable and is not subject to landsliding.
The top of the steep shoreline bluff slope which is subject to landsliding is located a considerable
distance to the west. A buffer including a vegetation buffer of 50 feet should be maintained at the
top of the steep shoreline bluff slope.
Should you have any questions regarding oUr rec()nnaissance please contact our office at (360)
714-9409,
Sincerely yours,
kJ:~
Dan McShane, M.sc., L.E,G.
Licensed Engineering Geologist
STRATUM GROUP
P.o. Box 2546, Bellingham, W A 98227
Phone (360) 714-9409
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February 8, 2008
Re: Geology Hazard Assessment
Parcel # 601324007 .
Thousand Trails Road
Quilcene, Washington
Ft'B
c 1 3 2008
JlffERSON COUNTY oeo
Colleen Scott
10258 Battle Point Drive NE
Bainbridge Island, W A 98110-1490
Dear Ms Scott:
Stratum Group is pleased to present the results of our geology assessment of the above
referenced property adjacent on a shoreline bluff. A yurt is already located on the property and a
new septic system is proposed'for the property. The purpose of this geology assessment was to 1)
evaluate the potential risk of slope failures impacting the existing yurt site and the proposed
septic system, 2) evaluate the risk posed by the construction of the new septic system to the
stability of the slope, and 3) provide general site development and maintenance recommendations
for the property adjacent to a potentially unstable bluff,
Based on my investigation of the site and vicinity, it is my opinion that the yurt is not at risk from
slope failures over the expected life of the structure (25 years) and that construction of the
proposed septic system will not be at risk nor with construction of the system 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, available geologic mapping in the area, and investigation of the shoreline
at the base of the bluff.
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 shoreline slope 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 residence on the subject property and the proposed
.J..,iL
February 8, 2008
Parcel 601324007, Jefferson County, WA
Geology Assessment
septic system, a qualitative evaluation of the shoreline bluff 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 10,000 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 MalJof Surficial Deposits in the Seattle 30' x 60' Ouadrangle. Washington (Yount,
Minard and Dembrof, 1993) and the Geologic Map of West-Central Jefferson County.
Washington (Birdseye, 1976) indicate the subject property is underlain by Vashon till at on the
upland area of the property, Vashon advance outwash and the preglacial stratified deposits with
an older till at the base of the bluff in the vicinity of the subject property. The till units consist of
a poorly sorted mix of clay, silt sand and gravel with occasional boulders. The preglacial
stratified sediments consist of interbedded oxidized brown, red-brown, and gray gravel, sand, silt,
ahd clay. The layers aremoderately.to well bedded and the unit contains minor amounts of ice-
contact deposits and outwash gravel and sand. Generally the unit is nonglacial and has abundant
peat and woody debris. These sediments were deposited sometime prior to the last glacial event
and include deposits of the Whidbey and Olympia nonglacial periods. The sediments have been
overridden and consolidated by glacial ice. The Vashon advance outwash was deposited by melt
water streams from the advancing Vashon ice sheet. The unit generally consists of sand ,and
gravel with a coarsening upward within the unit.
My observations on the bluff face and uplands of the subject property and vicinity are consistent
with the mapping described above. The Vashon till does not appear to be present along the very
top of the bluff where the yurt is located. This area is underlain by well rounded gravel with sand
and is consistent with the upper part of the Vashon advance outwash.
SPECIFIC SITE OBSERVATIONS
A site plan sketch showing pertinent features is provided in Figure 2. The subject property
consists of a nearly level upland area on the east that slopes back away from the top of the steep
slope on the west side of the property such that the upland area actually slopes towards the east
away from the steep shoreline bluff. Except for the cleared area immediately adjacent to the yurt
site and Sierra Trails Road that angles across the upland portion of the subject property, the
upland area is tree-covered primarily with Douglas fir and with an understory of salal and
Stratum Group
File:11.l5,07A
2
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February 8, 2008
Parcel 601324007, Jefferson. County, WA
Geology Assessment
FEB 1 3 2008
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evergreen huckleberry.
The Yurt is supported on concrete blocks, The closest block to the top of the steep slope to the
southwest is slightly more than 15 feet from the top of the steep slope. The proposed drainfield is
located to the east of the yurt. '
The upland is bounded on the west by southwest facing slope that slopes downward towards the
very steep shoreline bluff slope, This slope slopes downward at an angle of between 35 and 40
degrees. The top of the slope is at an elevation of approximately 420 feet. The slope is plainer
with a gradual curve to the south off of the subject property such that the slope faces directly ,
west south of the subject property. The slope is tree-covered primarily with Douglas fir with an
understory of salal and evergreen huckleberry. The trees on the slope are straight and there is no
indication of recent slope movement or slides. The slope is underlain primarily with sand and
gravel in a very compact condition and appears to be well drained.
Numerous old skid/logging roads cut across the slope in the'vicinity of the property. Except for
minor raveling along road cut slopes, the roads and past harvest activities do not appear to have
caused any slope stability problems and no landslide areas were observed,
The slope described above is bounded on the west by the steep shoreline bluff slope. The bluff
slope is much steeper and is vertical in many locations. The shoreline bluff slope is
approximately 80 feet high. The steep shoreline bluff is underlain by very compact sand, gravel
and silt and is very resistant to erosion due to the very compact nature of the soils.
No springs or seeps or seasonally wet areas are present on the bluff slope. 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 south of the subject property. Despite this
recent very high rain fall event I observed no indications of surface water flow on the property. A
narrow drainage swale that ends along the top of the steep shoreline bluff appeared to have had a
small flow of water from the recent heavy rainfall event as the soil at very top of the steep
shoreline bluff contains silt layers. No significant erosion was caused by the water flow and the
drainage was dry at the time of my site visit indicating that water flow in this drainage is very
rare. 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,
The shoreline bluff is eroded by a combination of wave action that undermines the base of the
bluff, ravelling of material off the exposed soils on the unvegetated portions of the bluff, and
periodic shallow soil failures, The material eroded from the bluff face is removed from the base
of the bluff and transported along the shore generally towards the north by prevailing wave
Stratum Group
File:l1.15.07A
3
RE<':EIVED
February 8, 2008
Parcel 601324007, Jefferson County, WA
Geology Assessment
FEB 1 3 2008
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action. The beach was in overall good condition and consists of sand and gravel. Erosion at the
base of the bluff appears to happen only during very high tides/storm events as the top of the
beach appears to be inundated infrequently,
CONCLUSIONS AND RECOMMENDATIONS
Based on my visual inspection of the subject property and vicinity, I conclude that the existing
yurt and the proposed septic system are not at risk from potential landslides or erosion induced
slope failures within the expected life of the yurt structure (25 years) as long as my
recommendations are followed.
The very steep shoreline bluff slope on the western most portion of the subject property is a
geologically hazardous landslide slope. The yurt is located approximately 500 feet east of this
steep shoreline bluff slope.
The 35 to 40 degree slope between the level to gently sloping upland area and the very steep
shoreline bluff slope is stable and is not subject to landslides or other slope movement beyond
very minor soil creep. This interpretation is not consistent with the Washington Coastal Atlas
which is based on the Relative Slope Stability in East-Central Jefferson County. Washington by
Birdseye (1976). Birdseye (1976) identified this feature as a former slide area with unknown
stability. LIDAR imagery as well as topographic mapping indicates a broad convergentatea on
this slope; however, there is no evidence of slope failures or landslide deposits anywhere on the
slope or below the slope. The entire slope as well as the shoreline bluff slope to west is underlain
by undisturbed preglacial deposits. This feature may be associated with a very ancient past
failure, but the feature is not definitive and there is no clear field evidence of this feature being a
, slide area.
If the feature is a slide area, it is clearly a very old feature and may have been associated with
slope erosion that would have taken place shortly after the glacial ice had retreated out of the area
and relative sea-level was higher than at present prior to the region rebounding from the removal
ofthe glacial ice through isostatic rebound. Relative sea levelis estimated to have been up to
600 feet above the current sea level shortly after the glacial retreat (Geologic Map of the Freeland
and Northern Part of the Hansville7.5~minute Quadrangle. Island County. Washington. Polenz,
Schasse and Peterson, 2006). Based on the location of the site, relative sea level may have been
on the order 100 to 200 feet in the area. The former higher relative sea level has formed a variety
of elevated shoreline features around Puget Sound and may be the cause of the broad convergent
landforms in this area. Regardless of the feature observed, there is no evidence in the area of
recent slope movement with the exception of the steep shoreline bluff itself and there are no
landslide deposits anywhere on the slope.
Stratum Group
File: I L15.07A
4
,,1._. ,.
February 8, 2008
Parcel 601324007, Jefferson County, WA
Geology Assessment
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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 of35 to 40 degrees is stable due to the fact that the slope is underlain by
very well ,drained and very compact sand and gravel. Given the compact nature of the
preglacial sand and gravel underlying the slope, this slope angle is stable. '
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.
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.
Building Setbacks
It is my opinion that the steep slope to the southwest of the yurt 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, if a home were to built on the site, I recommend
that base of the home foundation be set back from the slope 7.5 feet as measured horizontally
from the base of the foundation to the intersection of the slope in order to minimize the risk of
differential settlement along the outer edge of the slope. No fill soils are present along the outer
edge of the slope.
The very steep shoreline bluff slope is a geologically hazardous landslide slope due to erosion at
the toe of the bluff and the very steep slope conditions, The yurt is located approximately 500
feet back from this steep slope. This distance is well back from the slope and will provide'
adequate protection to the yurt.
Shoreline
Because the shoreline bluff is an eroding bluff, slope failures should be expected to occur on,a
periodic basis along the bluff shoreline. The only way to prevent continued erosion of the base of
the bluff is to construct hard armoring at the shoreline at the base of the bluff. However, the
eroding bluff acts as a feeder bluff (erosion of the bluff provides sediment) for the beaches east
and west of the subject property, and any shoreline armoring will have a negative impact on
properties down drift from the bluff. The construction of shoreline armoring on other properties
Stratum Group
File:Il.15,07A
5
February 8,2008
Parce1601324007, Jefferson County, WA
Geology Assessment
may cause an increase of erosion of the subject property. The owner of the subject property
should contact Jefferson County officials regarding any proposed shoreline protection projects
along the bluff or any shoreline protection construction. There are currently no shoreline-
armored areas along this stretch of coastline.
Storm and Septic Drainage
Soils on the upland portion of the property are well dniined..The soils appear to consist primarily
of coarse grained deposits. The present manner of dispersed drainage should be maintained. Any
additional buildings if constructed should have roof drains and any other drainage should be
discharged into a level dispersion/infiltration trench located to the east of the structure.
The proposed septic drain field should not cause any problems for slope stability as long as the
drain field is located at least 30 feet back from the top of the steep shoreline slope.
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.'
A 50-foot buffer of trees and native vegetation should be maintained along the top of the steep
shoreline bluff slope, If trees are harvested or thinned or limbed for view purposes on the slope
between the buffer and the upland area, all debris should be removed from the slope.
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 as required,
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
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lYan McShane, M.Sc., L.E.G.
Licensed Engineering Geologist
Stratum Group
File:II,15.07A
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STRATUM GROUP
P.o. Box 2546, Bellingham, WA 98227
Phone 060) 714-9409
January 17,2008
L>~i'''
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Colleen Scott
10258 Battle Point Drive NE
Bainbridge Island, W A 98110-1490
Re: Geology Hazard Assessment
Parcel # 601324007
Thousand Trails Road
Quilcene, Washington
Dear Ms Scott:
Stratum Group is pleased to present the results of our geology assessment of the above
referenced property adjacent on a shoreline bluff. A yurt is already located on the property and a
new septic system is proposed for the property, The purpose of this geology assessment was to 1)
evaluate the potential risk of slope failures impacting the existing yurt site and the proposed
septic system, 2) evaluate the risk posed by the construction of the new septic systemto the
stability of the slope, and 3) provide general site development and maintenance recommendations
for the property adjacent to a potentially unstable bluff.
Based on my investigation of the site and vicinity, it ,is my opinion that the yurt is not at risk from
slope failures over the expected life of the structure (25 years) and that construction ofthe
proposed septic system will not be at risk nor with construction of the system 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, available geologic mapping in the area, and investigation of the shoreline
at the base of the bluff.
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 shoreline slope 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 residence on the subject property and the proposed
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FEB - 6 2008 Il~~::J I
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;l"','~'[OPi'J1ENT I
January 17,2008
Parcel 601324007, Jefferson County, WA
Geology Assessment
septic system, a qualitative evaluation of the shoreline bluff 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 10,000 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) and the Geologic Map of West-Central Jefferson County.
Washington (Birdseye, 1976) indicate the subject property is underlain by Vashon till at on the
upland area of the property, Vashon advance outwash and the preglacial stratified deposits with
an older till at the base of the bluff in the vicinity of the subject property. The till units consist of
a poorly sorted mix of clay, silt sand and gravel with occasional boulders. The preglacial
stratified sediments consist of interbedded oxidized brown, red-brown, and gray gravel, sand, silt,
and clay. The layers are moderately to well bedded and the unit contains minor amounts of ice-
contact deposits and outwash gravel and sand. Generally the unit is nonglacial and has abundant
peat and woody debris, These sediments were deposited sometime prior to the last glacial event
and include deposits of the Whi~bey and Olympia nonglacial periods. The sediments ' have been
overridden .and consolidated by glacial ice, The Vashon advance outwash was deposited by melt
water streams from the advancing Vashon ice sheet. The unit generally consists of sand and
gravel with a coarsening upward within the unit.
My observations on the bluff face and uplands of the subject property and vicinity are consistent
with the mapping described above. The Vashon till does not appear to be present along the very
top of the bluff where the yurt is located. This area is underlain by well rounded gravel with sand
and is consistent with the upper part of the Vashon advance outwash.
SPECIFIC SITE OBSERVATIONS
A site plan sketch showing pertinent features is provided in Figure 2, The subject property
consists of a nearly level upland area on the east that slopes back away from the top of the steep
slope on the west side of the property such that the upland area actually slopes towards the east
away from the steep shoreline bluff. Except for the cleared area immediately adjacent to the yurt
site and Sierra Trails Road that angles across the upland portion ofthe subject property, the
upland area is tree-covered primarily with Douglas fir and with an understory of salal and
Stratum Group
File: I J.l5,07A
2
January 17,2008
Parcel 601324007, Jefferson County, WA
Geology Assessment
evergreen huckleberry.
The Yurt is supported on concrete blocks. The closest block to the top of the steep slope to the
southwest is slightly more than 15 feet from the top of the steep slope. The proposed drainfield is
located to the east of the yurt,
The upland is bounded on the west by southwest facing slope that slopes downward towards the
very steep shoreline bluff slope. This slope slopes downward at an angle 'of between 35 and 40
degrees. The top of the slope is at an elevation of approximately 420 feet. The slope is plainer
with a gradual curve to the south off of the subject property such that the slope faces directly
west south of the subject property. The slope is tree-covered primarily with Douglas fir with an
understory of salal and evergreen huckleberry, The trees on the slope are straight and there is no '
indication of recent slope movement or slides, The slope is underlain primarily with sand and
gravel in a very compact condition and appears to be well drained,
Numerous old skid/logging roads cut across the slope in the vicinity ofthe property. Except for
minor raveling along road cut slopes, the roads and past harvest activities do not appear to have
caused any slope stability problems and no landslide areas were observed,
The slope described above is bounded on the west by the steep shoreline bluff slope. The bluff
slope is much steeper and is vertical in many locations, The shoreline bluff slope is
approximately 80 feet high. The steep shoreline bluff is underlain by very compact sand, gravel
and silt and is very resistant to erosion due to the very compact nature of the soils.
~
No springs or seeps or seasonally wet areas are present on the bluff slope. 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 south of the subject property. Despite this
recent very high rain fall event I observed no indications of surface water flow on the property. A
narrow drainage swale that ends along the top of the steep shoreline bluff appeared to had a small
flow of water from the recent heavy rainfall event as the soil at very top of the steep shoreline
bluff contains silt layers, No significant erosion was caused by the water flow and the drainage
was dry at the time of my site visit indicating that water flow in this drainage is very rare, 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.
The shoreline bluff is eroded by a combination of wave action that undermines the base of the
bluff, ravelling of material off the exposed soils on the unvegetated portions of the bluff, and
periodic shallow soil failures, The material eroded from the bluff face is removed from the base
of the bluff and transported along the shore generally towards the north by prevailing wave
action. The beach was in overall good condition and consists of sand and gravel. Erosion at the
Stratum Group
File:l1.l5.07A
3
-,: --.-'--.
FEB - 6 2008
January 17, 2008
Parcel 601324007, Jefferson County, WA
Geology Assessment
base of the bluff appears to happen only during very high tides/storm events as the top of the
beach appears to be inundated infrequently.
CONCLUSIONS AND RECOMMENDATIONS
Based on my visual inspe~tion of the subject property and vicinity, I conclude that the existing
yurt and the proposed septic system are not at risk from potential landslides or erosion induced
slope failures within 'the expected life of the yurt structure (25 years) as long as my
recommendations are followed,
Building Setbacks
The present yurt located slightly more than 15 feet at its closest point to the top of the steep
slope. This distance is adequate for the protection of the yurt over the expected life of the yurt
(25 years). It is my opinion that the steep slope to the southwest of the yurt is generally stable and
the risk of slope failures on this slope is very low under present conditions,
Shoreline
Because the shoreline bluff is an eroding bluff, slope failures should be expected to occur on a
periodic basis along the bluff shoreline. The only way to prevent continued erosion of the base of
the bluff is to construct hard armoring at the shoreline at the base of the bluff. However, the
eroding bluff acts as a feeder bluff (erosion of the bluff provides sediment) for the beaches east
and west of the subject property, and any shoreline armoring will have a negative impact on
properties down drift from the bluff. The construetion of shoreline armoring on other properties
may cause an increase of erosion of the subject property, The owner of the subject property ,
should contact Jefferson County officials regarding any proposed shoreline protection projects
along the' bluff or any shoreline protection construction. There are currently no shoreline-
armored areas along this stretch of coastline.
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. The present manner of dispersed drainage should be maintained. Any
additional buildings if constructed should have roof drains and any other drainage should be
discharged into a level dispersion/infiltration trench located to the east of the structure. '
The proposed septic drain field should not cause any problems for slope stability as long as the
drain field is located at least 30 feet back from the top of the steep shoreline slope,
Stratum Group
File:11.I5.07A
4
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January 17,2008
Parcel 601324007, Jefferson County, W A
Geology Assessment
FEB - 6 2008
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Vegetation
Site grading soils or debris, landscape debris, or any other material should not be disposed of
over the bluff face or placed at the top of bluff. Over time debris will break down and form an
unstable mass that will slide damaging the slopes below.
Native vegetation, particularly tress and low native brush on the steep slope should be disturbed
as little as possible. If trees are thinned or limbed for view purposes on the slope, debris should
be removed from the slope. Some thinning of small Douglas fir trees growing on the slope
should not cause any problems as long as the native brush understory is left in place, At least one
tree per 30-foot center should be left on the slope.
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 as required,
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,
11: Group
Dan McShane, M.Sc., L.E.G.
Licensed Engineering Geologist
Stratum Group
File:lI.I5.07A
5
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