Report (708) 06/23/00 FRI 15:50 FAX 503 244 0417 MILDREN DESIGN GROUP PC 1002
06/29/00 FRI 15:31 FAX 002
GEODESLGN, INC, -
GEOTECNNICAL. ENVIRONMENTAL. AND OLOLOOICAL CONSULTANTS � �
, 35 - 2 P\`
June 23, 2000
Bw
Pacific Northwest Properties 0 :7
9665 SW Allen Boulevard, Suite 115 •
Beaverton, Oregon 97005
Attention: Mr. Paul Gram
Geotechnical Engineering Services
Supplemental Information
72 Avenue' Office Building
Tigard, Oregon
Fite: PNWP -18
INTRODUCTION
GeoDesign prepared an October 26, 1999 geotechnical engineering report for the
proposed 72nd Avenue Office Building site. The approximately 2.6 -acre site is located at
the northwest comer of SW Cherry Drive and SW 72n Avenue in Tigard, Oregon. Based
on our June 22 telephone conversations with Mr. Jack Kriz of Mildren Design Group, P.C.
and Mr. Bob Poskln of the City of Tigard, additional information is required prior to Issuing
the building permit.
SITE LAYOUT
As stated in our October 26, 1999 geotechnical engineering report, relatively consistent
subsurface conditions were encountered in the explorations. The new site plan indicates
that the building will be located within the middle third of the site, extending east -west from
SW 72n Avenue to SW 74 Avenue. Based on review of the explorations associated with
the prior site layout, it is our opinion that additional explorations are not necessary to
evaluate the new plans.
SHALLOW FOUNDATIONS
Our report was prepared based on the preliminary plans for two single -story structures at
the site. We assumed that column toads would be less than 150 kips and wall loads would
be less than about 4.5 kips per linear foot, which is typical for concrete tilt -up construction.
The current plans are to construct one two-story structure at the site. Based on a
June 23, 2000 fax from Mr. Gerard Navarra of Afghan Associates, Inc., the maximum
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column and exterior wall loads for the two -story structure will be 220 kips and 8.5 kips per
linear foot, respectively. Based on the results of our explorations,. laboratory testing and
analyses, it is our opinion that the design allowable bearing pressure of 2,500 pounds per
square foot is applicable to the new foundation Toads_
LIQUEFACTION
Our report included recommendations for the applicable Uniform Building Code seismic
design criteria. Mr. Poskin has requested that we also include an evaluation of the risk of
liquefaction- induced damage to the structure. A relatively high risk of liquefaction is
indicated in the publication "Portland Metropolitan Relative Earthquake Hazard Maps" by
M.A. Mabey (DOGAMI IMS -1, 1997). However, recommendations are based on widely
interpolated results rather than site specific information.
Liquefaction can be defined as the sudden Toss of shear strength in a soil due to an
excessive buildup of pore water pressure. Liquefied soil layers generally follow a path of
least resistance to dissipate pore pressures, often resulting in sudden surface settlement,
sand bolls or ejections, and/or lateral spreading in extreme cases. Clean, loose, uniform or
silty, fine- grained, saturated sands are particularly susceptible to liquefaction. Lateral
spreading is a liquefaction - related seismic hazard. Areas subject to lateral spreading are
typically gently sloping or flat sites underlain by liquefiable sediments adjacent to an open
face, such as riverbanks. Liquefied soils adjacent to open faces may "flow" in that
direction, resulting in lateral displacement and surface cracking.
Based on our subsurface exploration and laboratory testing, the near - surface soils are
comprised of 8 to 12 feet of stiff to very stiff silt underlain by medium dense to dense sand
with varying amounts of silt. We met practical refusal in TP -10 when we encountered a
large boulder at approximately 11 feet. Our experiences from sites in the immediate
vicinity indicate that basalt bedrock is present at depths less than 30 feet.
• Research concerning liquefaction potential conducted over the past several years (Stark
and Olson, 1995, R.B. Seed, 1996, and Finn, 1996) has determined that soils with high
fines content (35 percent or greater) have a much reduced potential for liquefaction.
Accordingly, the surface silts have a low risk of liquefaction. We performed an analysis of
liquefaction based on the results of our explorations, relative soil fines content,
groundwater levels, and anticipated earthquake ground acceleration_ Based on our
analyses, the underlying sand layer has a low risk of liquefaction. Based on the low risk of
liquefaction of surface soils, the risk of lateral spreading at the site is low.
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We appreciate the opportunity to work with you on this project. If you have questions
concerning the information provided, please call.
Sincerely, �IP ►
GeoDesign, Inc.
•
George P. Saunders, P.E. 4t �
Principal _ p. IW
to IZcoo2.
cc: Mr. Jack Kriz, Mildren Design Group; P.C.
Mr. Gerard Navarra, Afghan Associates, Inc.
GPS:kt
Document ID: PNWP- 18- GeorAdd
Two copies submitted
PNWP.18: 062300