2. BASE DRAINAGE.
a. Conditions Requiring Drainage. Consider the use of a base course
drainage system wherever the following conditions exist:
(1) Where ground water levels approach the bottom of the base.
(2) Where frost action penetrates the subgrade.
(3) In sag vertical curves where the subgrade soil has low permeability.
b. Design Details. For typical basic drainage details, see Figure 6. For
design criteria for subdrains filter materials see NAVFAC DM-7. For drainage
computation procedures, see NAVFAC DM-21.
3. BASES FOR RIGID PAVEMENT.
a. Materials. Materials used as a subbase in flexible pavements are
suitable for base courses in rigid pavements. Refer to Section 5.
b. Gradation and Plasticity. Base materials should be well graded,
conforming to AASHTO Specification M-147. The LL should be no greater than 25
and the PI no greater than 5.
c. CBR. The minimum required CBR is 30.
d. Thickness. The minimum thickness of granular base should be 6 inches.
Thicker bases may be required in order to reduce frost penetration into the
subgrade or to increase K modulus value in weak subgrade soils. For thickness
design procedures for base courses, see NAVFAC DM-21.
e. Compaction. Base courses under rigid pavements should be compacted to at
least 95 percent of maximum density.
4. BASES FOR FLEXIBLE PAVEMENTS.
a. Materials. The selection of a base course material for flexible pave-
ments should be based on the economic availability. Generally the following
natural and processed materials can be used as base:
Crushed or uncrushed gravel
Graded crushed rock
Waterbound macadam
Drybound macadam
Limerock
Coral
Sand-shell
Blast furnace slag
Cement, bitumen, or lime stabilized soil mixtures
b. Gradation and Plasticity. Recommended gradations for drybound and
waterbound macadam bases are given in Graded Aggregate Base Course for
Flexible Pavement, TS-02686. For other local materials including limerock,
sand-shell, and coral, use locally available and proven gradations. For all
base types for flexible pavements the LL should be no greater than 25 and the
PI no greater than 5.
c. Design CBR. For flexible pavement thickness design, use the following
CBR values, without testing, for the following materials:
Base Type
Design CBR
----------------------------------------------------------------
Graded, Crushed Aggregate
100
Waterbound Macadam
100
Drybound Macadam
100
Cement- and Asphalt-treated Aggregate
100
Limerock
80
Sand-shell
80
Coral
80
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