coral, caliche, tuff, rubble, lime rock, shells, cinders,
iron ore, and other select materials. Some of these are
primarily soft rock and are crushed or degraded under
construction traffic to produce composite base
materials. Others develop a cementing action, which
results in a satisfactory base. The following text
describes the characteristics and usage of some of these
materials:
1. CORAL. Uncompacted and poorly drained
coral often results in an excessive moisture content and
loss of stability. The bonding properties of coral, which
are its greatest asset as a construction material, vary with
the amount of volcanic impurities, the proportion of fine
and coarse material, age, length of exposure to the
elements, climate, traffic, sprinkling, and method of
compaction. Proper moisture control, drainage, and
compaction are essential to obtain satisfactory results.
2 CALICHE. A variable material that consists of
sand, silt, or even gravel, that when saturated with water,
compacted, and allowed to settle, can be made into
high-quality base courses, especially caliches that are
cemented with lime, iron oxide, or salt. Caliches vary,
however, in content (limestone, silt, and clay) and in
degree of cementation; therefore, it is important that
caliche of good uniform quality be obtained from
deposits and that it be compacted at optimum moisture.
3. TUFF. A porous rock usually stratified, formed
by consolidation of volcanic ashes, dust, and so forth,
and other cementitious materials of volcanic origin, may
be used for base courses. Tuff bases are constructed the
same as other base courses except that after the tuff is
dumped and spread, the oversize pieces are broken and
the base compacted with sheepsfoot rollers. The surface
is then graded, compacted, and finished.
4. RUBBLE. It may be advantageous to use the
debris or rubble of destroyed buildings in constructing
base courses. If so, jagged pieces of metal and similar
objects are removed.
Bituminous Base
Bituminous mixtures are frequently used as base
courses beneath high-type bituminous pavements,
particularly for rear-area Wields which carry heavy
traffic. Such base courses may be used to advantage
when locally available aggregates are relatively soft and
otherwise of relatively poor quality, when mixing plant
and bituminous materials are readily available, and
when a relatively thick surface course is required for the
traffic. In general, a bituminous base course may be
considered equal on an inch-for-inch basis to other types
of high-quality base courses. When a bituminous base
course is used, it will be placed in lifts not exceeding 3
1/2 inches in thickness. If a bituminous base is used the
binder course may be omitted, and the surface course
may be laid directly on the base course.
QUESTIONS
Q1. What is the correct nomenclature for each of the
items labeled in figure 3-18?
Q2. What feature is normally provided in a
horizontal curve to counteract the effect of
centrifugal force?
Q3. What type of section is used to set slope stakes
and to show as-built conditions?
Figure 3-18.-Typical section.
3-19
