HomeMy WebLinkAboutBLD2006-00386 Geotechnical Report • •
Gary A. Flowers, PLLC
Geological & Geotechnical Consulting
19532 12th Avenue NE
Shoreline,WA 98155-1106
August 13, 2006
Project No. 06-053
Mr. Raymond Stelk
813 Thorndyke Road
Port Ludlow, WA 98365
Subject: Geological/Geotechnical Report Addendum
Proposed New Single Family Residence
Lot 11, Bridgehaven Division 5
821 Thorndyke Road, Port Ludlow, Jefferson County, Washington
Dear Raymond:
This letter is an addendum to our previous report for this project dated June 12, 2006. The
purpose of this addendum is to address concerns from Jefferson County Department of
Community Development in their letter to you dated July 31, 2006. Specifically, items to be
addressed include the following:
1) Extension of the ecology block retaining wall between the subject property and your
residential property at 831 Thorndyke Road
2) Location and depth of footings along the north end
3) Existing soils conditions at the northeast corner of the site
4) Staking the top of bank
5) Verification that the submitted site plan and top of bank staking are coincidental
In order to address the above issues a pre-construction meeting was scheduled at the site on
Thursday, August 10, 2006. Present at the meeting was your excavation and concrete contractor
(Harlan),Frank Hall and Fred Slota from Jefferson County,myself and you. All of the above
issues were discussed with the following outcomes and recommendations.
1) The retaining wall extension between the two properties simply involved placing several
additional ecology blocks onto the east end of the existing wall to extend it and even out the east
end. Total added length on the east end, closest to the slope, was on the order of 9 feet. The wall
is still located outside of the 25 foot setback established for original construction of your house
and wall. As such, the added wall length remains in compliance with the original soils report for
your property by Robert M. Pride and the wall should have no adverse impact on the subject
slope. Several additional blocks were also added at the west end of the wall, near the driveway,
to shore up a steep slope cut in that area. Placement of these blocks is also in compliance with
the original soils report for the site.
Gary A. Flowers,PLLC �►`jl� ,�
19532 12th Avenue NE Shoreline,Washington 98155-1106 206-417-7640
SON C11uiI°
• 40
Proposed SFR
821 Thorn' ke Road, Port Ludlow,Jefferson County, Washington Geotechnical Services Report
2) A total of 4 additional test pits were excavated and observed by all parties. Two of the pits
were excavated along the east edge of the building pad and the other two were located along the
north edge. The foundation along the north side of the planned building will be located within 3
feet of the existing ecology block retaining wall. It is critical that foundation loads from the
residence not impact the existing wall. Therefore, the new building foundations must be placed
at a depth no higher than the top of the bottom block of the retaining wall in order to mitigate any
loading onto the wall from the house foundations. All backfill between the new foundation wall
and existing retaining wall should be clean, crushed rock.
Based on the 2 test pits excavated near the northeast corner and near the center of the north wall
of the planned house, bearing soils in this area are on the order of 6 to 7 feet deep. Soils
encountered above the bearing soils consisted of old fill materials that are not considered suitable
for foundation support. Foundations placed at this depth will have no impact on the existing
retaining wall. The depth to bearing soils decreases towards the west. As such, the footings can
also step up to the west. However, the minimum depth of footing (top of bottom blocks in
retaining wall)must be carried throughout this side of the house.
At the northwest corner of the house the footings will be located within several feet of the
existing septic tank. At this location the foundation trench must remain at the same minimum
depth as on the north side of the house in order to avoid any foundation loads impacting the
septic tank. Once the footings are past (south of) the septic tank, the footings may be stepped up
as the depth to bearing soil shallows.
Along the east side of the house bearing soils were encountered about 41/2 feet below existing
ground surface. Along this side of the house we recommend that the footings be placed a
minimum of 5 feet below existing grade. This will not only assure that bearing soils are exposed
but it will also provide the required "structural" setback of 25 feet from the top of the slope for
the footings near the southeast corner.
3) Existing soils in the northeast corner include approximately 2 cubic yards of materials that
were spread to level this area. Natural ground is exposed at the corner with the overlying recent
fill materials visible above. This small amount of soil will have no impact to the stability of the
slope. These soils are loose and dry and should be wetted, slightly regraded to a flatter
inclination and compacted once construction begins.
Current plans included additional filling near the northeast corner to provide a level structural
pad for a concrete patio. Approximately 3 feet of fill would be required to bring the site to
grade. This fill would extend several feet into the 25 foot setback zone and should not be placed.
Alternatively, a wood deck should be planned for this area and further grading eliminated.
4) As agreed with Mr. Slota and Mr. Hall, staking the top of the bank was not completed at this
time. The 25 foot setback line was previously established for the site and remains visible int e
Gary A. Flowers,PLLC - __
19532 12`"Avenue NE Shoreline,Washington 98155-1106 206-417-7640
2 OHM MUD
• •
Proposed SFR
821 Thorndyke Road, Port Ludlow,Jefferson County, Washington Geotechnical Services Report
form of a yellow line on the foundation of the adjacent Ciani house to the south. The setback
line extends from the yellow line to the southeast corner of the existing Stelk residence.
All building foundations will be located such that the distance from the outside edge of the
footing to the top of steep slope will be a minimum of 25 feet. Deck footings may extend into
the setback zone but should be founded on suitable bearing soil. No backfilling will occur within
the setback area.
5) Based on the current plans that were reviewed by all parties on the site, the submitted site
plan and the 25 foot setback from the top of steep slope coincide with each other.
Existing Retaining Wall
We were also asked to address the existing, approximate 4 foot tall, wood, retaining wall located
just east of the southeast corner of the house. This existing wall was constructed by the former
property owner, Mr. Peter Ciani, to level the property in this location for the former carport that
was located in this area. The wall is located within the 25 foot setback from the top of the steep
slope. Replacement of the somewhat unsightly wall may be desirable to coincide with
construction of the new house.
The existing wall appears to be stable with no visual indications of distress. As such it could
remain, as is. Alternatively, the wall can be removed and reconstructed in the same location or
closer to the new house location. No construction activity should occur closer to the top of the
slope. The wall should be kept to a maximum height of 4 feet and therefore should not require
further engineering. A further option would be to remove the wall and regrade the existing fill
materials to a gentle slope back to the new house.
Since this wall is a part of a larger wall that extends onto the Ciani property, the north end of the
remaining wall must be braced at the property line to prevent any movement of the wall when
the portion on your property is removed. Suitable bracing may require a wing wall be
constructed at the north end of the remaining wall. The wing wall should extend at least 5 feet
back to the west. Other methods of bracing may also be considered. Any disturbance to the
existing Ciani drain pipe at this location must be immediately corrected.
SUMMARY
We should be contacted to provide a follow-up inspection once all foundation trenches are
completed and prior to installation of reinforcing steel for the footings. The inspection will be to
verify that the recommendations presented herein, and in our original soils report for the site,
have been suitably followed. Other inspections may also be required by the County.
Gary A. Flowers,PLLC
19532 1211'Avenue NE Shoreline,Washington 98155-1106 ;,,r. 206404640
3 affiZON tD.0 NTY MP
. • •
Proposed SFR
821 Thorndyke Road, Port Ludlow,Jefferson County, Washington Geotechnical Services Report
Copies of this letter and any follow-up inspection reports will be provided to Fred Slota,
Building Official for Jefferson County Department of Community Development.
Our findings and recommendations provided in this report were prepared in accordance with
generally accepted principles of engineering geology and geotechnical engineering as practiced
in the Puget Sound area at the time this report was submitted. We make no other warranty, either
express or implied.
Thank you for the opportunity to be of service on this project. Should you have any questions
regarding this matter,please do not hesitate to contact our office.
Sincerely,
oWasti1
'e(1 ,4".,!%;„4.\.\
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'1 d Ge°‘o
I Gary A. Flowers
Gary A. Flowers, P.G., P.E.G.
Engineering Geologist
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AUG 15 2006
JFFFFRSON COUNTY OCP
Gary A.Flowers,PLLC
19532 12°'Avenue NE Shoreline,Washington 98155-1106 206-417-7640
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