30 June 2001
FLEXIBLE PAVEMENT DESIGN - CBR METHOD
1. REQUIREMENTS. Flexible pavement designs must provide sufficient compaction of the subgrade
and each layer during construction to prevent objectionable settlement under traffic; provide adequate
thickness above the subgrade and above each layer together with adequate quality of base and subbase
materials to prevent detrimental shear deformation under traffic; provide adequate subsurface drainage
control or reduce to acceptable limits the effects of frost heave or permafrost degradation where frost
conditions are a factor; and provide a stable, weather-resistant, wear-resistant, waterproof pavement.
Attention must also be given to providing adequate friction characteristics.
2. BASIS FOR DESIGN. The thickness design procedures included herein for conventional flexible
pavement construction are based on CBR design methods. Design procedures for pavements that
include stabilized layers are based on modifications of the conventional procedures utilizing thickness
equivalencies developed from research and field experience. Design of flexible pavements using the
elastic layer method is covered in Chapter 11.
3. THICKNESS DESIGN CURVES. Figures 10-1 through 10-32 are design curves for use in
determining the required pavement thickness for Army, Navy, Marine Corps, and Air Force airfield
pavements. The individual curves indicate the total thickness of pavement required above a soil layer of
given strength for a given gross aircraft weight and aircraft passes.
4. THICKNESS DESIGN. The thickness design procedure consists of determining the CBR of the
material to be used in a given layer and applying this CBR to design curves (Figures 10-1 through 10-32)
to determine the thickness required above the layer to prevent detrimental shear deformation in that
layer during traffic. The specific steps to follow are:
a. Determine design CBR of subgrade.
b. Determine total thickness above subgrade.
(1) For Army and Navy design and Air Force design for a specific aircraft, enter appropriate
design curve with subgrade design CBR and follow it downward to the intersection with design gross
weight curve, then horizontally to design aircraft passes curve then downward to the required total
thickness above the subgrade.
(2) For Air Force standard designs, enter the appropriate design curve with the design
subgrade and read the thickness required above the subgrade for a given traffic area.
Determine design CBR of subbase.
d. Determine thickness of material required above the subbase by entering the appropriate design
curve with the design subbase CBR and using above procedures to read the required thickness.
e. Determine the minimum thickness of surface and base course from Tables 8-3, 8-4, or 8-5.
When the minimum thickness of surface and base is less than the thickness of surface and base
required above the subbase, the minimum thicknesses would be increased to the actual thickness