Correspondence RECEIVED
G R
9725 SW Beaverton- Hillsdale Hwy. Suite 140
Beaverton. OR 97005 -3364 f
p1503 -641 -3478 f1503- 644 -8034 OCT 0 ti 2010
CITY OF TIGAtRD
RLANNINNG /ENGINEERING
October 4, 2010 5159 LOT 13 C.O. SERVICES LTR
,M 5T , /O -ck )/
Westlake Homes, Inc.
PO Box 1588
Lake Oswego, OR 97035
Attention: Ken Nelson
SUBJECT: Geotechnical Consultation during Construction
Lot 13, Mountain View Estates
City of Tigard Building Permit Number MST 2010 -00146
Tigard, Oregon
At your request, GRI provided observation services during excavation of the foundation for Lot 13 in the
Mountain View Estates residential development. As you know, GRI completed a geotechnical
investigation for the development, and conclusions and recommendations for design and construction of
the project are summarized in our November 30, 2004, report to LanPacific, Inc. entitled, " Geotechnical
Investigation, Mountain View Estates, Tigard, Oregon." Additional design recommendations regarding
foundations and retaining wall design were provided to Accent Homes in the following memoranda by
GRI:
"Retaining Wall Design, Mountain View Estates, Tigard, Oregon;" dated December 30, 2005
"Global Stability, Proposed Retaining Walls and Fill Slopes, Mountain View Estates,
Tigard, Oregon;" dated February 17, 2006
"House Foundation Design, Mountain View Estates, Tigard, Oregon;" dated March 20, 2006
"Retaining Wall #4, Mountain View Estates, Tigard, Oregon;" dated June 29, 2006
On September 20, 2010, Matt Shanahan with GRI met you, Steve Suckow with Prime -X Contracting and
Excavation, and Richard White with White Homes Design, for a preconstruction meeting at the site. We
reviewed the following setback recommendations for footings located adjacent to vertical excavation
benches:
a) Unsupported cuts below the interior and uphill perimeter footings should not extend
below a 2H:1V slope that extends downward from the edge of the footing. For
example, a footing on a bench above a 4-ft -high vertical cut should be located at least
8 ft back from the cut. The unsupported height of the cut may be reduced with
structural fill of compacted crushed rock or concrete.
b) In addition to the 2H:1V setback criteria, for interior footings we recommend a
minimum 2 -ft horizontal footing setback from unsupported vertical cuts. Uphill
perimeter footings should be set back a minimum 4 ft from unsupported cuts.
Providing geotechnical and environmental consulting services since 1984
GRI subsequently visited the site on September 21, 23, 28, and 30, 2010, to evaluate the foundation
subgrade prior to installation of the perimeter drains and backfilling the walls. At the time of our site visit
on September 30, 2010, the foundation excavation was complete.
This letter summarizes our observations and provides our conclusions and recommendations regarding the
suitability of the work performed.
Based on our observations at the site, the exposed subgrade consists of stiff to hard, clayey silt fill with
scattered gravel -size pieces of concrete and angular rock. The subgrade was lightly desiccated to a depth
of about 1 in., and scattered concentrations of loose soil and gravel -size materials were present at the
ground surface. These loose materials should be removed from the subgrade prior to completion of the
forms and placement of concrete. The contractor overexcavated soft fill soils encountered in the
southwestern corner of the excavation that appeared to be associated with grading completed during
construction of the neighboring home. The area was overexcavated to very stiff to hard silt. On
September 30, 2010, we observed that some loose slough had fallen into the westernmost deck footing
excavation and recommended removing the loose material to firm subgrade. The contractor indicated the
footings will meet the setback criteria summarized above or granular structural fill will be used to reduce
the height of unsupported cuts as necessary. The project plans indicate that drains will be installed around
the perimeter footing, and the retaining walls will be backfilled with free - draining granular material. The
backfill should be compacted as indicated in our 2004 geotechnical report. We also observed that the
contractor placed a fill up to 3 ft thick at the back of the house. The area of the fill was relatively small and
was not within 15 to 20 ft of the existing retaining wall. A layer of loose topsoil was placed across the
remainder of the yard to about the top of the existing retaining wall.
Based on our observations at the site and assuming the loose material was removed from the footing
subgrades, it is our opinion the footing excavations were accomplished in substantial conformance with
the plans and specifications and our recommendations for this project. It is also our opinion that the
prepared subgrades are suitable for support of the proposed building Toads. Please contact the
undersigned if you have any questions regarding this correspondence.
Submitted for GRI,
PR
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Matthew S. Shanahan, PE, GE David D. Driscoll, PE, GE
Senior Engineer Principal
This document has been submitted electronically.
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