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• a rl s on Test Inc • Construction In & Related 'ref
Geotechriical Consulti:-
P.O. Box 23814
February 18, 1997 Tigard. Oregon 97281
Phone (503) 684-3460
FAX (503) 684 -0954
CTI JOB NO. 96 -6022
Cascade Communities, Inc. _
13535 SE 145th Avenue
Clackamas, OR 97015
FINAL REPORT OF EARTHWORK OBSERVATION AND TESTING
MOUNTAIN HIGHLANDS HI
TIGARD, OREGON
Reference: MORNINGSTAR PROJECT, SOILS AND GRADING REPORT,
Oakley Engineering,lnc., April 28, 1994.
Carlson Testing Inc. (CTI), has conducted on -call inspection services for the earthwork at the
above residential development. This final letter briefly summarizes our observations and
testing during construction, and the as -built soil conditions to the best of our knowledge. This
letter also provides recommendations for foundation design and soil
construction of the single - family homes. g guidelines during.
SITE PREPARATION AND FILL PLACEMENT
Based on our visual observations and our density test results, after stripping vegetation and
topsoil, engineered fill using on -site soil was moisture - conditioned and placed on the following
areas to a maximum depth of approximately 15 feet:
Lots 42 to 49
Based on the in -place soil density results and our visual observations, it is our opinion that the
engineered fill on the above lots was generally placed in conformance with the standards of
the City of Tigard.
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Lots 26 to 41 and 50 to 52 are cut lots or lots left near natural grade.
The backfill placed in the sanitary sewer easement between Lots 46 and 47 and to the laterals
was not compacted and has settled more than one foot.
STRIPPINGS AND /OR EXCESS FILL
Based on our observations, it appears that most of the topsoil strippings
fill material was removed from the site. The strippings were stockpiled very h gh Lots 4
through 51 before they were apparently removed. During our site visit on 2 -14 -97 it was
observed that approximately 12 inches of trench spoils and strippings were spread across the
fronts of Lots 42 to 52. There is a 2 to 3 foot high debris pile on the front of Lot 33 in the
northwest corner.
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CTI 96 -6022
Page 2 of 4
SLOPE STABILITY
The original geotechnical investigation for this project was performed by others. CTI engineers
observed the base of the fill placed along Lot boundaries 43/44 and 44/45 which included
subdrains and were started with quarry run fill material before any soil fill was placed. The
subdrains are flowing presently and these fills have performed well this winter compared to
nearby natural slopes. We also observed the recent slope repairs on Lots 43, 46 and between
Lots 28 and 29. More recently a new failure, initiated by excessive uphill storm runoff has
occurred just east of Lot 29.
Generally, the slopes near the creek become less stable up- canyon or in the southerly
direction. We do not anticipate any stability concerns on Lots 47 through 52. Lots 31
through 34 have fill placed during previous phases and some thin dumped fill by builders;
shallow slope creep was noted on Lots 31 and 34. Lots 30 and 35 through 41 will likely
require cuts to construct future homes. We recommend that these cuts be made in the
summertime to minimize the potential for adversely affecting existing residences above; any
cut exceeding 8 feet in depth should be observed by a geotechnical engineer during
excavation.
The following Lots should definitely require a lot -by -lot geotechnical review:
Lots 26 -29 and 42 through 46
We anticipate that Lots 26, and 27, may be founded on spread foundations, with some special
considerations. These lots would be more stable with daylight basement homes. Lot 29 may
likely be founded on spread foundations with a proper setback from the east slope where a
recent failure occurred. Lots 28, and 42 through 46, are more steeply sloping or are very near
steep slopes that have a moderate to high probability of failing in the future; these lots may
require deep foundations or horizontal anchors to improve stability or they may be founded on
spread foundations, subject to a geotechnical engineer's review.
HOUSE EXCAVATION GUIDELINES
We anticipate deeper than typical foundation excavation depths on most of the lots due to the
sloping nature of the site. If excavated material is spread around the lot and is expected to
support appurtenant structures such as deck footings and sidewalks, it should be placed,
compacted, and tested as engineered fill. Under no circumstances should nonengineered fill
be placed without observation over ground sloping steeper than 20 percent, or on flat ground
more than 3 feet deep.
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CTI 96 -6022
Page 3 of 4
STANDARD FOUNDATION RECOMMENDATIONS
The proposed residential buildings on simple; relatively flat lots, will likely be founded on
shallow spread footings bearing on competent native soils or engineered fill. Spread footing
design and construction should generally conform to Chapter 4 of the CABO One and Two
Family Dwelling Code (CABO), except where we specifically recommend otherwise.
For protection against frost heave, s pread footings on nonexpansive soils should have a
minimum embedment depth of 12 inches for exterior grades on level ground. The
recommended minimum widths for continuous wall footings are tabulated below:
Minimum Width for
No. of Stories Continuous Footing
(floors supported) (in)
1 -story 12
2 -story 15
3 -story 18
We anticipate that the allowable bearing pressure can be taken as 1,500 lb/ft for footings
bearing on competent native subsoils or engineered fill to a maximum of 15 kips for column
footings. ,For heavier column loads and masonry chimneys, a Soil Engineer should be
consulted. The coefficient of friction between on -site soil and poured -in -place concrete may
r be taken as 0.40. The maximum anticipated total and differential footing movements
(generally from soil expansion and /or settlement) are 1 inch and % inch over a span of 20 feet,
respectively:
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CTI 96 -6022
Page 4 of. 4
CLOSING AND LIMITATIONS
Our reports pertain to the materials tested /inspected only. This letter should be made available
to each builder in the development: however, information contained herein is not to be
reproduced, except in full, without prior authorization from this office. This letter should not
be construed to relieve or lessen the responsibility of the contractor or owner's site
representative for this site work, but is provided for the minimum required governmental
assurance. Our support was given on an as- needed basis as requested. If conditions are
encountered during foundation excavation which differ from this report, then the
developer(Cascade Communities,lnc.), the contractor (Clearwater Construction), and CTI
should be allowed to review the condition before corrective actions are taken. Corrective
work performed by the builder without notifying the above parties will be interpreted as an
acceptance of the conditions encountered.
Respectfully submitted,
CARLSON TESTING, INC.
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James D. Imbrie, P.E.
Geotechnical Engineer
JLK
Attachments: Summary report of in -place soil density tests
cc: City of Tigard
Clearwater Construction Company