HomeMy WebLinkAbout935100007 Geotech Assessment AddendumGeoResources, LLC
Ph. 253-896-1011 5007 Pacific Hwy. E., Saite 20
Fa. 253-896-2633 ~ ~° ~~ I~~ Fife, Washignton 98424
JEfEEDSDlV C~~1~1Y DCD June 22, 2005
Mr. Terry ENingson
Ms. Elaine Morgan
5145 NW EI Camino Boulevard
Bremerton, Washington 98312
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Geotechnical Review
Proposed Infiltration System
Deck Foundation Considerations
Lot 8, Division 5, Bridgehaven
Thomdyke Road
Jefferson County, Washington
Job No. Ellingson.Thomdyke.01 RG
We are wrifing this letter to respond to review comments generated by Jefferson County in
their June f 4, 2005 letter regarding the above referenced project. Jefferson County has asked that
GeoResources, LLC evaluate the proposed Stormwater Pian, prepared by Callagan Engineering.
We have reviewed the plan and find the plan conforms to our geotechnical report prepared for the
project.
Jefferson Couni)r's staff have also asked that we comment on the proposed deck foundations
which will be located within the 30-foot setback from the crest of the steep sbpe as requin~ by the
County. Our geotechnical report recommended that a 30-foot setback be maintained between the
slope face and the outer edge of any foundation. However, in accordance with the provisions of the
IBC, we also recommended that the setback be measured at the bearing elevation of the foundation.
Effectively, this would alkrw the foundations to encroach within the 30-foot horizontal distance at the
top of sbpe, as long as the foundation el~nents are deepened to maintain the minimum 30-foot
horizontal separation, when measured at the foundation bearing elevation. This topic was discussed in
greater detail in our design report in the "Buffer Modification" and "Building Setback" sections of our
design report. We have included excerpts from those sections in this letter for the reviewer's
convenience.
We trust this letter is sufficient for your present needs. Should you have any questions, or require
additional information, please contact us at your convenience.
GeoResources, LLC
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Kurt W. Groesch PE
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Elli~soNMorgan-Thomdyke
January 17, 2005 /~-
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Excerpted Sections (GeoResources, LLC January 17, 2005 Report)
Building Setback
Jefferson County requires a building setback and buffer area from all landslide and erosion
hazard areas. The buffer distance is tabulated based on the vertical height of the slope. Aftemativey, a
structural setback may be recommended by a licensed geotechnical engineer. Vegetation in the buffer
may be enhanced, if approved or n~uired. Clearing, grading and filling within the setback area is allowed
if it can be demonstrated that the buffer vegetation will not be adversey impacted, or if the impacts can
be mitigated.
Based on our geotechnical evaluation of the site, we n~ommend a Structural Setback of 30
feet where footings will be founded in the medium dense native soils. The Structural Setbadk may be
measured horizontally from the bottom of the footing th the face of the sbpe where sbpes are 30
percent or steeper, as described in the "Buffer Modification' section of this report.
Buffer Modification
Where necessary, the Structural Setback may be measured from the bottom of the footing
to the face of the steep slope, in accordance with UBC/IBC. Where the Setback Modification is
utilized, the foundation elemenrts should be extended verticaly th meet the necommended setback
criteria. This modification is based on the foundation elements extending to and being fourxled in
medium dense to dense soils. Maintaining the prescribed setback in this manner provides the
conventional 2 to 1 bearing prism. A schematic section is provided as Figure 6.
Roadways and driveways are exempt from the Building Setback criteria.
Conventional earthwork guidelines should be utilized with the recommended geobec~nical design
criteria for cuts and fills in the roadway and driveway areas, as described in the "Earthwork
Section" of this report.
As previously discussed, weathering, erosion and the resulting surficial sloughing and
shallow tendsliding are natural processes that affect steep slaps areas. As noted, bcali~ed evidence of
surficial ravel'utg and riling. were not observed in the steeper portions of the site owing to the dense
ground cover. To mifigafie and reduce the potential for these natural processes th become
exacerbated, we recommend the following: No drainage of concentrated surface water or signifir,~rt
sheet flow onto or near the sbpe area. Drainage from the roof and driveway area should be collected
and tightlined to the site's on-else infiltration sysfiem, or sheet fkrwed to densei~r vegetated portions of
the site. -
• No filling within the setback zone unless retained by engineered retaining waNs or
constructed as a monitored and engineered fill.
• No peroolation of surface water within 30 feet of the top of the steep slope unless the
seepage rafie is controlled tD a sbw seep (similar to septic systems).
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