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Smith, Robert (ed.) / The Wisconsin engineer
Vol. 70, No. 6 (March 1966)

Rinehardt, Greg
The use of soil-cement for airport runways,   pp. 18-27


Page 20


Fig. 1-Shows A, B and C horizons
of a soil profile (from  Soil-Cement
Handbook)
           Soil Types
  Soils can be roughly classified
into three broad divisions:
  Sandl Soils-Sandy    and  grav-
elly soils are best sllite(l for soil-
cement and    re(quir-e the  least
amoint of Cement for hardening.
The amount of fines ( pass a No. 4
sieve ) contained  in this type of
soil is a very important factor in
hoxv wevll the soil conforms to use
in soil-cement. If at least 55 per-
cent of the soil vill pass a No. 4
sieve,  it  is  wVll  silitcol  for  soil-
cement. Sand soils wvhich lack the
re1i(iired amount of fines reojuire
more cement, l)ut they still work
(ell.
  Silty and  Cla eti Soils-These
soils can be successfully used in
soil-cellment; howiever, they re(luire
more Cement tflan sandy soils. It
is sometimes found that using bor-
row soil woutld be more )ractical.
In general, when these soils re-
(Ilire cement contents of 16 per-
cent or more, one should consider
the constrnction costs of the na-
tive soils as compared to those of
horrow  soils and decide which is
more economical. Another uinfavor-
able characteristic of silty or clayey
soils is that they can be extremely
hard to pulverize. As a rule of
thull, if it is possible to pulverize
a soil of this type, thenr the soil is
suitable for use in   soil-cement.
Construction with silty and clayey
soils is also more dependent on
the weatlher, as large amounts of
moisture in these soils makes con-
striuction difficult.
  Organic Soils-As mentioned be-
fore, organic soil is usually unsuit-
able for soil-cement because of the
large amount of cement required
for hardening. Borrow    soil is
usually used in areas were this
type of soil is predominate.
   TESTS FOR SOILS AND
   SOIL-CEMENT MIXTURES
   For an effective and efficient
soil-cement, various tests should be
performed. The objective of these
tests is to determine:
  1. How much portland cement
    is needed to harden the soil
    adequately.
  2. How much water should be
     added.
  3. To what density the soil-
    cement should be compacted.
The number of tests required and
their thoroughness vary greatly
with the type of job. For a major
project, very thorough soil sam-
pling and identification tests would
be conducted, and detailed tests
on the soil-cement mixture itself
would be carried out. In this ma-
jor project, it would be desirable
to determine the minimum cement
content which could be used, for
the savings due to the lower ce-
ment requirements would far out-
weigh the costs of the tests. But
for a small project, the primary
concern would be to determine a
suitable, but not necessarily mini-
mum, cement content which could
be used. The time and cost of the
tests involved would not be com-
pensated by the small savings in
the amount of cement used.
  The primary objectives of the
various tests performed on both
soils and soil-cement mixtures will
be discussed; however, a complete
detailed coverage of the proce-
dures involved is beyond the scope
of this report. The Portland Ce-
ment Association's publication,
"Soil-Cement Laboratory Hand-
book", thoroughly covers the de-
tails and methods of soil-cement
testing, and should be consulted
for further information.
      Preliminary Soil Tests
  The primary soil work is identi-
fying the soil type and collecting
suitable samples for use in soil-
cement mixture tests.
20
     Soil Sampling-Getting accurate
   soil samples is of the utmost im-
   portance. If the sample obtained
   is not a true representation of the
   soil which will be used in the soil-
   cement, all the tests performed on
   the sample will be worthless and
   will give misleading results. These
   results will cause problems when
   contruction begins.
     A 75 pound sample is usually a
   sufficient quantity for laboratory
   tests. If the sample is to be taken
   from a runway subgrade which has
   been graded, it is usually taken by
   digging a trench from the center-
   line to the edge of the runway,
   and to the same depth as the pro-
   posed soil cement base. When tak-
   ing samples from a runway which
   has not yet been graded, they are
   usually taken at exposed cuts or
   from  the surface with an auger.
   A sample should be taken so that
   only the soil horizon which is to be
   processed is represented.
     On small projects, wvhiere the
   principal concern is finding an
   adequate, rather than minimum,
   cement content, a sample is often
   taken of only the poorest soil on
   the site. The cement content found
   for this soil is used throughout.
     Soil Identification-The soil
   samples taken are then identified
   and the group or series to which
   they belong are determined. There
   are two principal systems of soil
   classification the AASHO classifica-
   tion and the U.S. Department of
   Agriculture classification.
     The primary objectives of the
   soil identification tests are the de-
   termination of:
     1. Grain size
     2. Liquid limit
     3. Plastic limit
     Accurate soil identification is im-
   portant because it can be used to
   determine the cement content re-
   quirements for a particular soil.
   Soils of the same soil series and
   horizons have been found to re-
   quire the same cement content for
   hardening  wherever they      are
   found. Therefore, the engineer can
   determine the cement content re-
   quired for a certain job from the
   soil identification, and considerable
   laboratory testing can be avoided.
THE WISCONSIN ENGINEER


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