HomeMy WebLinkAboutBLD2000-00316 Geotechnical Report • - SHA
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August 10, 2000
Mr.Carlos-Sierra
Sierra Martin Architects
555 116t'Avenue NE, Suite 155
Bellevue, Washington 98004
RE: GEOLOGIC SLOPE STABILITY EVALUATION FOR HUNT RESIDENCE
ADDITION, TOANDOS PENINSULA,JEtT ERSON COUNTY, WASH[NGTON
Dear Mr. Sierra:
At your request, we visited the site on June 16 and 30,2000,to evaluate the suitability of an
approximately 12-foot building-addition set back from the crest of the ravine on the south side of
the existing residence. We understand that Jefferson County requires a minimum building set
back from the crest of the ravine at the site of 30 feet, but the county would consider a smaller
set back based on a favorable professional geologic engineering evaluation. Therefore, the
purpose of our work was to evaluate the suitability of a smaller building set back than the 30 feet
required by Jefferson County.
Based on our observations at the site,it is our opinion that a building set back less the county
designated minimum of 30 feet is not appropriate for this site. Consequently,this letter provides
a brief summary of our observations and conclusions, Preliminary observations and conclusions
were provided to Mr. Sierra orally upon completion of our site work.
SITE DESCRIPTION
The existing residence is a two-story wood-frame structure located on a relatively broad ridge
between two ravines_ The ridge and the ravines (located on the north and south sides of the
ridge) slope down to the east-northeast. The ridge ends and the ravines coalesce a few hundred
feet east of the existing residence at the edge of Hood Canal.
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os Sierra SHANNON FSWILSON.INC.
Martin Architects
ugust 10, 2000
Page 2
Initial site plans included construction of an approximately 700-square foot addition on the south
side of the residence,which would extend to within about 12 feet of the crest of the ravine on the
south. At this location,it is estimated that the ravine is approximately 50 feet deep. The slope of
the upper 40 feet of the ravine was measured with an Abney level between 34 and 35 degrees.
Vegetation on this portion of the slope includes maple, alder, and fir trees (up to about 2 feet in
diameter)with an undergrowth of sword fern. About 10 above the base of the ravine,the slope
decreases to about 15 to 20 degrees. At this Iocation, the ground was wet and boggy,indicative
of springs and seeps. In addition, undergrowth on this lower portion of the slope is more
hydrophilic and includes thick nettles and devils club.
GEOLOGIC CONDITIONS
Published geologic maps of the area indicate that the ridge and ravines are underlain by
undifferentiated, pre-Vashon sediment. While these sediments may be both glacial, and non-
glacial in origin, they all were overridden by the Vashon Stade ice sheet, which is estimated to
have been on the order of 3,000 to 4,000 feet thick in this area. Consequently the underlying
soils have been compacted to a very dense or hard state.
The presence of these very dense or hard soils was confirmed at the building location.
Excavations into the ridge for construction of the existing residence exposed very dense sands
and gravels. A backhoe test pit was excavated at the crest of the ravine near the location of the
proposed addition to a depth of about 10 feet during our site visit on June 30. Loose topsoil was
encountered within two feet of the ground surface,below which very dense sandy gravel with
scattered cobbles was observed.
The wet and boggy ground conditions and hydrophilic vegetation within the bottom 10 feet of
the ravine suggest that the sands and gravels may extend from the building site,down to this
elevation, and below this elevation, subsurface soils may consist of relatively impermeable soils,
which cause the wet conditions. A recent slide was observed on the marine bluff on the south
side of where the ravine ends at Hood Canal, which is approximately 90 feet high by 50 feet
wide. This slide has exposed very dense sands and gravels, overlying hard clay and silt with
seepage occurring at the contact between the sand/gravel and the clay/silt, with the contact near
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los Sierra SHANNON&WILSON,INC.
• Martin Architects
August 10, 2000
Page 3
the same elevation as the wet conditions observed on the side of the ravine near the residence.
These conditions observed in the slide scarp on the marine slope are consistent with postulated
subsurface conditions beneath the side of the ravine on which the existing residence and
proposed addition are located.
CONCLUSIONS
It has been our experience that the juxtaposition of the permeable sand and/or gravel soils over
impervious silt and/or clay deposit is a significant risk factor with respect to the potential
occurrence of relatively deep-seated slides and slope movement. Because the sand/gravel
deposits are relatively pervious, while the clay/silt deposits are not,perched groundwater
develops within the sand/gravel at the contact with the underlying clay/silt_ Where this contact
daylights on the face of a slope,springs and seeps develop from the perched groundwater. In
addition,pressure gradients or build-up of hydrostatic pressures associated with this perched
water often result in slope instability. The contact between the sand/gravel and underlying
clay/silt has historically been the location of numerous slides on slopes in the region. The
occurrence of a landslide on the marine slope farther to the east along Hood Canal on this site is
not untypical given the sand/gravel and clay/silt underlying the slope.
In our opinion,the risk of deep-seated slope instability is smaller for the slope along the ravine
than the marine slope where the recent slope movement was observed. This is because the
marine slope is steeper and is maintained in a steeper condition due to wave erosion at the toe of
slope, which is not,present at the toe of the slope on the side of the ravine. Nevertheless,because
of the apparent occurrence of perched groundwater at a sand/gravel over silt/clay contact on the
ravine slope, it is our opinion that a building sex back less than the County minimum of 30 feet is
not advisable at this site without more extensive subsurface explorations, engineering slope
stability analyses,and possibly construction or risk reduction measures(e.g.,drilled horizontal
drains, toe buttress, etc.).
Please note that regardless of the building setback,there is some risk of future instability
(shallow or deep-seated)present on all hillsides, which the owner must be prepared to accept.
Such instability could occur because of future water line breaks/leaks, uncontrolled drainage,
2t-1-09084-001-Li WP/M01 21-1-09084-001
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•s Sierra SHANNON&WILSON.INC
. Martin Architects
gust 10, 2000
Page 4
unwise development in adjacent areas, or other actions or events on a slope that may cause
sliding.
Limitations
The conclusions in this letter are based on site conditions visually observed during our site
reconnaissance, test pit exploration, and inferred from published geologic, topographic, and
hazard maps and assume that observed conditions are representative of the subsurface conditions
throughout the site; i.e.,the subsurface conditions are not significantly different from those
observed in the test pit,inferred from the site reconnaissance or indicated on geologic maps. If,
during subsequent site activities (e.g., construction), subsurface conditions different from those
inferred in this letter are observed or appear to be present, we should be advised at once so that
we can review those conditions and reconsider our conclusions where necessary.
Within the limitations of scope, schedule, and budget, the conclusions presented in this letter
were prepared in accordance with generally accepted geologic engineering principles and
practices in this area at the time this letter was prepared. We make no other warranty, either
expressed or implied.
This letter was prepared for the use of the Owner,Architect, and Engineering in the evaluation of
the stability of this site. With respect to possible future construction, it should be made available
for information on factual data only and not as a wan-anty of subsurface conditions, such as those
interpreted from the site visit and discussion of geologic conditions included in this letter.
Please note that the scope of our services did not include any environmental assessment or
evaluation regarding the presence or absence of wetlands or hazardous or toxic material in the
soil, surface water,groundwater, or air on,or below, or around this site. We are able to provide
these services and would be pleased to discuss these with you if the need arises.
Shannon &Wilson has prepared the attached, "Important Information About Your Geotechnical
Report," to assist you in understanding the use and limitations of our report.
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Sierra SHANNON ,WILSON,INC.
Martin Architects
{` 10,2000
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We appreciate the opportunity to provide geologic services to you,and are available to answer
any questions regarding our observations and conclusions contained in this letter.
Sincerely,
SHANNON& WILSON,INC.
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William J.Perkins, R.P.G.
Principal Engineering Geologist
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Enclosures: Important Information About Your Geotechnical/Environmental Report
c: Mr.Kevin Russell, Jefferson County
OI-LI/MVP/MGI 2 1-1-09084-00 1
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County Department of Community Development August 14, 2000
-.� Street. Port Townsend, WA 98368
7 450 CElldi
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FINDINGS AND CONDITIONS OF APPROVAL AS REQUIRE t'ERe ? 3
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JEFFERSON COUNTY INTERIM CRITICAL AREAS ORD NA E i
JEFFERSON COUNTY
DEPT. OF COMMUNITY DEVELOPMENT
Applicant: EARL B HUNT
2001 KILLARNEY DR SE w i--"*"^BELLEVUE WA 98004-7019
Critical Area Review Case Number: CAR00-00212
Project Description: Single family dwelling (addition)
Parcel Number. 601271002 S-T-R: 27-26N-01W
Site Address: 12477 COYLE RD
QUILCENE WA, 98376
FINDINGS:
In accordance with the authority provided under subsection 4.101 and the requirements contained in Subsection
5.402 of the Jefferson County Interim Critical Areas Ordinance, the following are findings of fact relating to the
referenced application:
1.) The application was reviewed by Jefferson County Permit Center staff on 5/19/00 for the potential presence
of critical areas regulated under the provisions of the Ordinance, and the following critical areas were
confirmed as potentially present on the subject property: Wetlands, Seismic Hazard Area, Eagles, Shoreline
(Conservancy), Erosion, Streams (Type 4 and 5), Landslide Hazard Area (1,11,111), and Susceptible Aquifer
Recharge Area.
2.) Acting upon the above information, Jefferson County staff conducted a site inspection of the subject
property on June 8, 2000 and confirmed the existence of the following critical areas and/or their associated
buffers on the property: Susceptible Aquifer Recharge Area, Erosion Hazard Area, and Landslide Hazard
Area.
3.) A ravine measuring 50-feet in depth is located on the subject property. The applicant proposed to site a
residential addition approximately 8-feet from the top of the ravine. A site investigation conducted by staff
concluded that a Geotechnical Report was required. The submitted report recommended a 30-foot setback
from the top of the ravine to the proposed building envelope.
4.) The ravine contains a type 5 stream. However, the stream is located outside of any designated setback or
proposal.
5.) The potential for an earthquake of a high magnitude occurring in western Washington in the near future is
considered a nigh probability by regional experts.
6.) Aquifer recharge areas in Jefferson County are characterized by porous geologic formations that allow
percolation of the surface water into the soils and the underlying zone of saturation. Aquifers are geologic
formations that contain sufficient saturated permeable material to yield significant quantities of water to
wells and springs. Aquifers serve as the source of drinking water within most of the rural portions of
Jefferson County.
7.) Susceptible aquifer recharge areas are those with geologic and hydrologic conditions that promote rapid
infiltration of recharge waters to groundwater aquifers. This includes any portion of Jefferson County with a
DRASTIC index of greater than or equal to 180 points as determined using the U.S. Environmental
Protection Agency DRASTIC methodology: Drastic: A Standardized System for Evaluating Ground Water
Pollution Using Hydrogeologic Settings (EPA 600/23-87-035).
8.) On June 8, 2000 a request was sent to Washington Department of Fish and Wildlife to determine whether
an eagle management plan was needed. Although wintering bald eagles use the subject property for
perching sites, no active nests are located within a quarter mile of the parcel, therefore an eagle
management plan is not required.
9.) The proposed development does not encroach upon the designated shoreline, therefore, the Jefferson
County Shoreline Master Program does not apply to this particular proposal.
, .ous Areas in Jefferson County are characterized by slope, soil type, geologic material,
that may combine to create problems with slope stability, erosion, and water quality during
struction or during natural events such as earthquakes or severe rainstorms.
:-, from the provisions of the Jefferson County Interim Critical Areas Ordinance is granted subject to the
conditions. Any deviation from these conditions shall result in this waiver being revoked and the
',. becoming subject to further review.
„'. om0NS OF APPROVAL:
-� A vegetive buffer of 30 feet shall be permanently maintained from the top or edge of the landslide hazard
area. All buffers shall be measured perpendicularly from the top of the landslide hazard area.
,t„. 2.) Landslide hazard areas and their associated buffers shall remain naturally vegetated. Should buffer
disturbance occur during construction, replanting with native vegetation shall be required by the Critical
Area Administrator. No alteration shall be made to the landslide hazard area or its associated buffer without
prior authorization by the Critical Area Administrator.
3.) Among the native conifer species which may be used are: grand fir (Aibes grandis), sitka spruce (Picea
sitchensis), shore pine (Pinus contorta), Douglas-fir (Pseudotsuga menziesii), western redcedar(Thuja
plicata), western hemlock (Tsuga heterophylla).
4.) Among the native tree species which may be used are: vine maple (Acer circinatum) big-leaf maple (Acer
macrophyllum), red alder(Alnus rubra), Pacific madrone (Arbutus menziesii), quaking aspen (populus
tremula), black cottonwood (Populus trichocarpa), bitter cherry (Prunus emarginata), Oregon white oak
(Quercus garryana), cascara (Rhamnus purshiana), Pacific willow (Salix lasiandra), Scouler's willow (Salix
scouleriana).
5.) Among the native shrub species which may be used are: service-berry (Amalanchier alnifolia), red osier
dogwood (Cornus stolonifera) salal (Gaultheria shallon), ocean spray (Holodiscus discolor), indian plum
(Oemlaria cerasiformis), Pacific ninebark (Physocarpus capitus), red flowering currant (Ribes sanguineum),
wild rose (Rosa gymnocarpa), Nootka rose (Rosa nutkana), swamp rose (Rosa pisocarpa), willows (Salix
ssp.), red elderberry (Sambucus racemosa), snowberry (Symphoricarpos albus), evergreen huckleberry
(Vaccinium ovatum), red huckleberry (Vaccinium parvifolium).
6.) Prior to and during approved construction activities, the perimeter of the landslide hazard area buffer shall
be marked with a temporary sign. The sign shall be placed in such a manner that it can be directly
observed from the construction area. The sign shall contain the following statement: "Landslide Hazard
Area and Buffer- Do not Remove or Alter Existing Native Vegetation."
7.) All construction activities, including the storage and preparation of materials, shall not encroach upon the
designated geologically hazardous area, or its associated buffer.
8.) Damage to vegetation to be retained on site shall be minimized by directional felling of trees away from
these areas.
9.) The applicant shall permanently separate the boundary of the landslide hazard area buffer from the
remainder of the subject property through installing and maintaining one of the following; logs, trees, a
hedge row, or other prominent physical marking. The separation method identified by the applicant shall be
approved by the Critical Area Administrator prior to planting or installation.
10.) All trees and understory lying outside of approved construction limits shall be retained during clearing for
roadways and utilities. Understory damaged during approved clearing operations may, however, be pruned.
11.) The building envelope for the residential addition must maintain the setbacks stipulated through the
Geotechnical Report. The report concluded that a 30-foot setback must be maintained from the top of the
ravine to the proposed building site.
061./t/V4 I ir g/210
Warren Hart, A CP Date
Critical Areas Administrator
I understand that the granting of the above waiver from the provisions of the Jefferson County Interim Critical
Areas Ordinance is made subject to my complying with the above listed conditions, and that any violation of said
conditions shall result in revocation of the permit or approval and further review of the project under the Critical
Areas Ordinance.
Please `. n and return to the Department of Community Development.
Of-_____
Applicant's Signature Date
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