Geotechnical Letter Kiehn 10-2-2019.pdfSHANNON 6WILSON
GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS
October 3, 2019
Mr. Shane Kiehn
23615 99th PI W
Edmonds, WA 98020
RE: GEOTECHNICAL EVALUATION OF STEEP SLOPE, 23615 99TH PLACE WEST,
EDMONDS, WASHINGTON
Dear Mr. Kiehn:
This letter presents our observations and recommendations regarding the steep slope at
23615 99th PI W, Edmonds, Washington. The toe of the slope is about 30 feet east from the
back of the residence. The slope is about 40 feet tall, and slopes down to the west at an
average slope of about 40 percent. Residences on 971h PI W are at the top of the slope.
Because the slope is 40 percent or more, the City of Edmonds ("City") has classified the
property as a landslide hazard area. Based on a phone call with you, we understand that
the City requires a geotechnical evaluation of the slope so that you can proceed with cutting
alder trees at the base of the slope. A certified arborist has reviewed the cutting plan and
has recommended replanting the slope with two new trees for each tree removed.
Our scope of services included:
■ Reviewing publicly -available geologic data, including geologic maps, and previous
subsurface explorations;
■ Visiting your property to evaluate the condition of the slope;
■ Qualitatively evaluating the impact of tree trimming on the stability of the slope; and
■ Preparing this letter.
The following sections summarize our findings.
EXISTING SUBSURFACE DATA
We reviewed our own files and the Washington State Department of Natural Resources
(DNR) online portal to evaluate the site subsurface conditions. We did not locate existing
subsurface explorations within 500 feet of the property. Based on DNR geologic maps, the
site sits near the contact of Vashon advance outwash and Vashon till. Advance outwash
generally consists of very dense, relatively clean sand. Till is a very dense/hard unsorted
Project No. - Letter.docx
400 North 34th Street ■ Suite 100 ■ PO Box 300303 ■ Seattle, Washington 98103-8636 ■ 206 632-8020 ■ Fax 206 695-6777
■ www.shannonwilson.com ■
Mr. Shane Kiehn
October 3, 2019
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IIISHANNON 6WILSON
mixture of clay, silt, sand, and gravel. Both the advance outwash and till were overridden
by the most recent glaciation.
SITE VISIT
We visited your property on September 23, 2019. The soil on the slope was typically gray
silt/clay with sand and scattered cobbles (Exhibit 1). The soil appeared to be consistent with
weathered Vashon till. The upper 6 to 12 inches of soil was soft and moist to wet; the soil
density increased with depth. We did not observe seeps or springs on the slope.
The slope was vegetated with shrubs, low brush, and trees (Exhibit 1). Trees on the slope
ranged from about 6 to 24 inches in diameter. Most of the tree trunks were vertical and did
not show signs of downslope soil creep; however, about four trees in the mid -slope were
bowed downslope.
The bottom 15 feet of the slope was about 45 degrees. The 45 degree section was confined to
the property; to the north and south, the toe of the slope was retained by rockeries up to
about 4 feet tall (Exhibit 2). Above the 45 degree section, the slope angle varied from about
23 to 35 degrees.
Exhibit 1: Exposed soil (a) and typical vegetation (b).
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Mr. Shane Kiehn
October 3, 2019
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IIISHANNON 6WILSON
Exhibit 2: Rockery at toe of slope to north of subject property (a) and toe of slope on subject
property (b).
We observed two pipe lines on the slope. At the south side of the slope, we observed a
sealed garden hose attached to an approximately V2 inch diameter black hose that was
buried in the slope. At the north side of the slope, we observed an approximately 2-inch
diameter black polyvinyl chloride pipe that extended downslope. The pipe was exposed for
about 6 feet and otherwise buried.
SLOPE STABILITY
The very dense advance outwash and till typically form stable slopes at slopes of 50 percent
and flatter. Consistent with this observation, the DNR maps do not show active landslides
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Mr. Shane Kiehn
October 3, 2019
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on or near the site. Based on our site observations, we did not see evidence of recent
landsliding. We did observe indications of soil creep, which is the downslope movement of
loose surficial soils under the effects of gravity that is typical for top soil on steep slopes.
IMPACTS OF TREE CUTTING ON SLOPE STABILITY
The Edmonds Community Development Code (ECDC) 23.40.005 states that cutting or
removing healthy trees within a landslide hazard area is considered an alteration of a
critical area. ECDC 23.80.060 and 23.80.070 present development standards for alterations
of critical areas. The development standards require that a registered professional engineer
evaluate the impacts of the proposed tree cutting on the critical area and adjacent properties.
Based on our site visit and review of geologic data, we have qualitatively evaluated the
stability of the slope and the potential impacts of the proposed tree cutting and replanting.
In our opinion, the proposed activities would likely not increase the landslide hazard at the
site and would likely not affect adjacent properties.
In our opinion, the proposed tree cutting and replanting conforms to the requirements in
ECDC 23.80.060 and 23.80.070.
CLOSURE
We based our evaluation on our literature review and site visit. We did not perform
subsurface explorations as part of this study.
As with any site located on or near a slope, there are always risks of instability that present
and future owners must be prepared to accept. In addition to natural factors (soil and
groundwater conditions, steep topography, heavy rainfall events, etc.), the risks are linked
to events such as:
■ Upslope water leaks,
■ Pipe breaks,
■ Improper drainage,
■ Lack of maintenance of drainage facilities,
■ Unwise excavations into the hillside,
■ Removal of trees and vegetation and/or poor maintenance of trees, and
■ Similar or unknown circumstances that may cause sliding.
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Mr. Shane Kiehn
October 3, 2019
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IIISHANNON 6WILSON
Because of these inherent risks, our evaluation described herein is not a guarantee or
warranty of future stability.
The conclusions and recommendations presented in this letter report are based on observed
site conditions as they presently exist. Shannon & Wilson has prepared the enclosure,
'Important Information About Your Geotechnical Report' to assist you and others in
understanding the use and limitations of our reports.
Sincerely,
SHANNON & WILSON
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Jeremy N. Butkovich, PE
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Letter.docx 104070-001