Lesson 1/Learning Event 2
Careful attention should be given to the selection of materials for base courses and to their construction.
The materials should be dense and uniformly compacted so no differential settlement occurs in adjacent
areas. For continuous stability, all base courses should meet the requirements listed below.
Gradation of particle size must, whenever feasible, be within specified limits as determined by
mechanical analysis. However, in construction in forward areas it may not be practicable to hold to
close gradation requirements.
For deliberate construction base course material should contain no more than 15 percent passing
a No. 200 sieve.
Material passing the No. 40 sieve which acts as a binder in a basecourse material must have
desirable properties. No material which has a liquid limit greater than 25 or a plasticity index greater
than 5 should be used for a base course in deliberate construction.
Compaction and strength requirements.
Thickness of layers in constructing base courses must be within the limits which will insure
proper compaction. Thickness of layers depends upon type of material, equipment used, and method of
All base courses must be compacted. Compaction should meet the requirements given in Figure
The CBR of the finished base course must conform to that used in the design, and the total
compacted thickness must equal that obtained from the design curves, as previously discussed. Table 4
lists nine types of materials and/or processes that may be used as base courses for roads and airfields. A
design CBR is given for each type.
Natural materials. A wide variety of gravels, sands, gravelly and sandy soils, and other natural
materials such as limerock, coral, shells, and some caliches can be used alone or blended to provide
satisfactory base courses. In some instances, natural materials will require crushing or removal of the
oversize fraction to maintain gradation limits. Other natural materials may be controlled by mixing
crushed and pit-run materials to form a satisfactory base-course material.
Gravel and sand. Many natural deposits of sandy and gravelly materials make satisfactory base
materials. Gravel deposits vary widely in the relative proportions of coarse and fine material and in the
character of the rock frag-