HomeMy WebLinkAbout3188; COLLEGE BLVD EXTENSTION; EXPANSIVE SUBGRADE SOIL; 1988-01-21Testing Engineers—San Diego
I,
A Division of United States Testing Company, Inc.
3467 Kurtz SL, P.O. Box 80985, San Diego, Ca: 92138 (619) 225-9641
2956 Industry Street, Oceanside, California 92054 (619) 757-0248
January 21, 1988 . . . Job No. 5259
RECEIVED
City of Carlsbad JAN 26 1983
2075 Las Palmas Drive
Carlsbad, California 92009-14859
MUNICIPAL PROJt
Attention Pat Entezari DIVISION)
Project: improvements to College Boulevard
Carlsbad, California
Subject Expansive Subgrade Soil
Under Center Median
Gentlemen :
We were requested to review the recommendation contained in our
November 14, 1987 letter about mitigating the effects of expansive
soils as there has been some upheaval and median cracking where
this recommendation was employed This revised recommendation is
conceptually similar but includes a reduction of the required
percentage of compaction in..,
n the median area The potential for
swell is generally reduced by compacting the material on the wet
side of the optimum moisture content and at the lowest density
possible. We would recommend that the material be wetted to
where it is three to five percent above the optimum moisture and
then compacted to only, eighty-five percent of its maximum dry
density. The reduction in percent compaction is based on the
fact that the only load on the median will be the four inch
concrete median itself so that load bearing capacity and strength
are not a prime concern Also the optimum moisture and density
have been established using ASTM-D-1557 commonly referred to as a
modified Proctor. Eighty-five percent of a modified Proctor
density is equivalent to ninety, percent-'of a standard Proctor
density and would give adequate support to such a lightly, loaded
area
From the standpoint of swell potential, the procedure of wetting
the soil and then compacting produces a soil with significantly
less swell potential than one where the soil is compacted and
then wetted The reasons have to do with the way clay particles
are arranged when compacted which are different on the dry and
wet side of optimum
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January 21, 1988
Page 2
Job No. 5259
There are three other important factors to consider. The first
is that swell is a relatively slow phenomenon. It is not likely
that the soil will have truly stabilized with respect to swell
when it' is compacted. Itis also unlikely that the moisture
content at which it is compacted .S: the final moisture content
that it will be maintained, at. ':Inaddition, it is possible that
changes could occur to deeper layers which may also reach a new
and different equilibrium position The recommendations are intended to mitigate the effeáts 'of 'swell, however, there can be
no guarantee, that all the effects of the expansive soil are
eliminated.
Another concern is that sections of the median are to be planted
with trees Tree roots seek out and remove moisture from soil,
consequently the possibility that either water restrictions could
be imposed during a drought or the roots themselves cause
variations in moisture content present additional maintenance
concerns
I reviewed some of the test data for the median strip which were
tests numbered 373 through .379. Of the seven tests, five were
below optimum moisture, one was at optimum and one was over
optimum. The densities also' exceeded the ninety percent value of
104L9 in place dry density. ..Using 'the same maximum dry density
of 116.5 and an eighty-five percent requirement would result in
an inp].ace dry density of99.0pcf being acceptable.
We appreciate this opportunity' to be of service. If you have any
questions regarding this letter, please contact this office
Very Truly Yours,
TESTING ENGINEERS-SAN DIEGO '
Aan KrGeotechni
h, R.C.E OO236 ical S rvices