HomeMy WebLinkAboutKittinger's Block 6 - Geologic Reconnaissance - 2002.04.27py llu tr\ lll0
G. W. Thorsen, Consulting gist
927 56th Street, Port Townsend, WA 98368
360) 385-6002 (also fax)
thorcogw@olypen.com
April2T,2002
To: Appropriate Building Deparhnent Staff
Subject: Geologic recon of lots 1-8, Kittinger's Addition (NW quadrant, Block 6 at the intersection of 36th
and Logan) for Sasha Landis, 535 Redwood, Port Townsend (ph379-3244)
Purpose and Scope
My assignment was to provide a "letter report" outlining the findings of an on-site examination as well as a
review of relevant background mapping and reports. The need for a letter report was triggered by the
recognition by City staff that the site included slopes that met at least one of the criteria (slope) for a
geologically hazardous area" as defined in Port Townsend Municipal Code Section 19.05.100.
My examination of the site included study of the low cut-bank along Logan Street , making approximately 20
2- to 3-foot deep soil probes, and observing exposed sediments on the uplands. I also reviewed the USDA
soil maps, available slope stability mapping, and aerial photography in stereo. As the four homes proposed
have not yet been designed, their "footprints" are not shown on the attached map and the building sites are
approximate.
Summary of F'indings
Subsoils underlying Block 6 are glacia|till. Thus, they have good bearing strength but low permeability.
Whereas the northern portion of lots 1-4 meets one of the six criteria (slope-see map) for a "geologically
hazardous area",I found no evidence that it is actually ahazardous arca.
The geologic processes that formed these slopes have not been active for millennia (i.e., the topographic
features we see today arc "fossil" rather than continuing to develop).
The slopes are stable in their present condition, in spite of at least one earlier episode of logging.
The steeper (e.g.,25Yo-55%) portions of lots 1-4 could be destabilized or at least locally eroded by storm
runoff andlor depositing clearing debris onto the bank'
Carefirl development, including preservation of native ground cover and dispersal of any storm runoff from
developed surfaces away from the bank, can preserve pre-development conditions.
Response to ESA Guidelines and PTMC 19.05.100
A. Purpose
The proposed development of four home sites served by a common access from the 36th Street right-of-way
see map), given general adherence to Code design and performance standards, should pose no threat to
adlacent lands' or to "public health and safety".
B. Classification
The slopes along the norlhern portion ofthe property reach angles as steep as 55 percent in places, thus
meeting one of the criteria (5.) of a "geologically hazardous area" (Section 19.05.100). The other criteria do
not seem to apply. For example:
1. The Clallam series soils here are considered "well-drained", with a "slight to moderate" erosion hazard
McCreary, 1975). No evidence of surface runoff was found.
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Classifi catio n (c ont inue d)
2. No direct (zones of seepage) or indirect (horizons of water-loving plants) evidence of potentially
destabilizing ground water was found.
3. No evidence of mass movement, either topographic or vegetative, was found on-site or reported in the
previous mapping (Gayer, 1977).
4. The area is not subject to stream or wave erosion.
5. Slope (see above under "Summary..").
6. This is not an area "subject to severe risk of damage as a result of earthquakes", faulting, or ground failure.
No evidence for such exists, nor do local soil conditions meet those criteria listed in Section 8.6 (i.e., in
the event of a quake of historically unprecedented magnitude, strucfures here would be subject to severe
shaking but not ground failure).
Recommended measures
It is beyond the scope of my assignment for this letter report to address in detail Sections C (Regulated
Development) and D (Performance Standards for Development) of this section of the code (the homes,
access, and utilities have not been designed). However, a few general observations may be appropriate in
regard to Section E (Buffers and Setbacks). These are basic guidelines that would apply to any development
in such a geologic and topographic setting.
The agricultural soils (upper few feet) here are naturally well drained but are subject to compaction by
heavy equipment. Such traffic during clearing and construction should be minimized.
Geologic materials (in-place subsoils) here should have ample bearing capacity for the proposed residences
These materials (glacial till), if excavated for basements, will require careful engineering for use in any
significant fill. (Till will not re-compact if too wet or too dry.)
Other than isolated cutting or selective limbing of trees for view enhancement, bank vegetation should not
be disturbed.
Storm runoff should be dispersed as far as practical from the bank. No concenfrated runoff should be
allowed to reach the bank.
There are no geologic reasons why the minimum buffer along the steep north slopes (25 feet) could not be
reduced where appropriate for design requirements (e.g., daylight basements on the two northem sites).
A minimum foundation setback of at least 10 feet is recommended for those sites.
Whereas I am prepared to discuss any aspect of this letter report with City personnel, the client (Mr. Landis)
or engineers and contractors, I see not further role for a geologist here and feel that conditions do notjustiff
further subsurface exploration or a formal geotechnical report.
Gerald W. Thorsen
References cited:
Gayer,M. J.,1977, Quaternary and environmental geology of northeastem Jefferson County, Washington;
North Carolina State University M.S. thesis, 140 p.
Grimstad, Peder; Catson, R. J., 198 1, Geology and ground-water resources of eastern Jefferson County,
Washington: Washington Deparhnent of Ecology Water Supply Bulletin 54,125 p., 3 plates.
McCreary, F. R., 1975, Soil suwey of Jefferson County area, Washington: U.S. Department of Agriculture,
Soil Conservation Service, 100 p., 70 plates [sheets].
t
City of Port Townsen
Department of Building and Community Dev
Waterman-Katz Building
l8l Quincy Street, Suite 301, Port Townsend, WA 98368
360) 38s-0644 FAX (360) 38s-767s
email: jwalat(@ci.port-townsend.wa.us db r1
May 29,2002
Sasha Landes
Scott Landes
535 Redwood Street
Port Townsend, WA 98368
RE: Block at the northwest comer of 36th and Logan Streets, legally described as Kittinger's
Addition, Block 6, Lots I through 8.
Dear Sasha and Scott,
This is to document a site visit on May 24th,2002 to the above property with Gerald Thorsen, a
consulting geologist who wrote the attached letter dated April 17, 2002 concerning the steep
slopes on your site. My understanding is that you own Lots 2-4 and 5-7. Davis Pratt owns Lots
I and 8. You have individually proposed a total of four single-family residences for the 8 lots.
Lots 5 through 8 are relatively level; there is no evidence of any environmentally sensitive areas
ESA) on those lots. As Mr. Thorsen's letter indicates and as the site visit confirmed, there are
portion of slopes in excess of 40o/o grade on Lots I - 4. Slopes of 40% or gleater by definition
constitute an ESA. Because of the overall stability of the site, Mr. Thorsen noted in his letter that
a l0-foot buffer from the top of slope would be appropriate. The purpose of the site visit was to
document the location of slopes in excess of 40o/o in the vicinity of the proposed building
locations and to identifu the location of the l0-foot buffer.
During the site visit, we identified areas in the vicinity of the proposed building sites that were in
excess of 40% slope and staked the ten-foot buffer from the top of slope. In those areas with
greater than 40%o slopes, the l0-foot buffer should not be disturbed nor trees or other vegetation
rernoved. No clearing or other construction activities may take place in the buffer. Stakes
should be left in place at least until final building permit approval is obtained.
For those proposed building sites where the slope is less than 40%io, no buffer is required, because
other conditions indicating unstable conditions are not present, according to the Thorsen letter.
However, the northernmost extent of the proposed building sites on slopes less than 40%owas
also staked. If any building is proposed northward of the staked sites, additional evaluation may
be required because the lots get steeper towards the northem property lines. During the site visit
we looked only at those portions of the site identified as potential building locations.
I have also enclosed information about the City's interim tree clearing ordinance which you
should read before undertaking any clearing. If you have any questions, please call me at 38t
0644. Thank you for your cooperation.
Sincerely,,
rJ,/j
M. Walat
Planner I
CC: Davis Pratt
Gerald Thorsen