The design of flexible pavement is based on the principle that the magnitude of stress induced by wheel

load decreases with depth below the surface. Therefore, the stresses induced by a wheel can be

decreased by increasing the thickness of superimposed base and pavement. Figure 2 illustrates this

point. In the diagram to the left, the base (including subbase) is thick. Therefore, the load at the

subgrade is spread out over a wide area and the pressures are small. In the diagram to the right, the base

is thin and the load at the subgrade is confined to smaller area, and the pressures are correspondingly

higher. This pattern of decreasing stresses with increasing depth is the basis of conventional flexible

pavement design in which subgrade materials of low bearing capacity are covered with thick flexible

pavement structures. Thin flexible pavements are adequate for subgrade materials with high bearing

capacities.

axles are beneficial in the case of flexible pavement having high subgrade strength and a thin base

course because the stresses produced by the tires of tandem axles do not overlap appreciably at shallow

depths (Figure 3, Plane A-A).

In the case of flexible pavement, with low subgrade strength and thick base course, the stresses produce

overlap (Figure 3, Plane B-B) and less benefit is gained from the use of tandem axles. Criteria are given

in this subcourse for designing and evaluating tandem axles for both roads and airfields.

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