4-1. Construction with Portland Cement.
cement. On mixed-in-place construction using trav-
a. General construction steps. In soil-cement con-
eling mixing machines, bulk cement is spread on
struction the objective is to thoroughly mix a
the area to be processed in required amounts by
pulverized soil material and cement in correct
mechanical bulk cement spreaders (fig 4-7). Bag
cement is sometimes used on small jobs. Cement
maximum compaction. Construction methods are
spreaders for mixed-in-place construction are of
simple and follow a definite procedure:
two general types: those that spread cement over
(1) Initial preparation
the soil material in a blanket (fig 4-8) and those
(a) Shape the area to crown and grade.
that deposit cement on top of a partially flattened
(b) If necessary, scarify, pulverize, and pre-
or slightly trenched windrow of soil material (fig
wet the soil.
4-9). Cement meters on continuous-flow central
(c) Reshape to crown and grade.
mixing plants are of three types: the belt with
strikeoff, screw, or vane. Cement for batch-type-
(a) Spread portland cement and mix.
pugmill mixers and rotary drum-mixers is batch-
(b) Apply water and mix.
d. Construction. Construction with soil cement
involves two steps-preparation and processing.
Variations in these steps, dictated by the type of
b. Mixing equipment. Soil, cement, and water
mixing equipment used, are discussed in this
can be mixed in place using traveling mixing
chapter. Regardless of the equipment and methods
machines or mixed in a central mixing plant. The
used, it is essential to have an adequately com-
types of mixing equipment are
pacted, thorough mixture of pulverized soil mate-
(1) Traveling mixing machines.
rial and other proper amounts of cement and
(a) Flat-transverse-shaft type:
moisture. The completed soil-cement must be ade-
1 Single-shaft mixer (fig 4-1).
2 Multiple-shaft mixer (fig 4-2).
(1) Preparation. Before construction starts,
(b) Windrow-type pugmill (fig 4-3).
crown and grade should be checked and any fine
(2) Central mixing plants.
grading should be completed. Since there is little
(a) Continuous-flow-type pugmill (fig 4-4).
displacement of material during processing, grade
(b) Batch-type pugmill (fig 4-5).
at the start of construction will determine final
(c) Rotary-drum mixers (fig 4-6).
grade to a major extent. If borrow material is to be
Whatever type of mixing equipment is used, the
used, the subgrade should be compacted and
general principles and objectives are the same.
shaped to proper crown and grade before the
Some soil materials cannot be sufficiently pulver-
borrow is placed. Any soft subgrade areas should
ized and mixed in central mixing plants because of
be corrected. To avoid later costly delays, all
equipment should be carefully checked to ensure it
Almost all types of soil materials, from granular to
is in good operating condition and meets construc-
fine grained, can be adequately pulverized and
tion requirements of the job. Guide stakes should
mixed with transverse-shaft mixers. The exception
be set to control the width and guide the operators
is material containing large amounts of highly
during construction. Arrangements should be
plastic clays. These clays may require more mix-
made to receive, handle, and spread the cement
and water efficiently. The number of cement and
ing effort to obtain pulverization. Revolving-blade
water trucks required depends on length of haul,
central mixing plants and traveling pugmills can
be used for nonplastic to slightly plastic granular
condition of haul roads, and anticipated rate of
soils. For coarse, nonplastic granular materials, a
production. For maximum production, an adequate
cement and water supply is essential. The limits of
rotary-drum mixer can provide a suitable mix;
the different materials and their corresponding
however, if the material includes a small amount
of slightly plastic fines, mixing may not be ade-
cement requirements should be established by the
project engineer. Prewetting by adding moisture
c. Equipment for handling and spreading ce-
before cement is applied often saves time during
ment. There are a number of methods of handling
actual processing. Friable granular materials,