Lesson 1/Learning Event 1
FIGURE 2. DISTRIBUTION OF PRESSURE UNDER SINGLE WHEEL LOADS
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.
Effect of Tandem Axles and Tire Pressures
Tandem Axles: Figure 2 illustrates the distribution of pressure under a single wheel load. Tandem
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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