Characterization of Pavement Subgrades and Bases
3.1 Introduction
The properties of the base/subbase and subgrade layers play a vital role in the structural integrity and performance of pavements. In flexible pavements, the base and subbase layers are structural components that need to provide sufficient strength, while reduc-ing stresses to levels that can be sustained by the subgrade. In rigid pavements, the base layer is used for leveling and structural strengthening of weak subgrades. Furthermore, properly constructed base/subbase layers can provide internal drainage, while preventing water ingress into the subgrade. The properties of the subgrade and base layers can be improved through compaction or chemical stabilization under controlled moisture conditions.
3.2 Mechanical Behavior
Granular base/subbase layers exhibit an elastoplastic behavior in response to the loading and unloading conditions imposed by traffic loads. Upon unloading, this entails an elastic (i.e., recoverable) and a plastic (i.e., permanent) deformation components. This behavior can be described with the aid of the “shakedown” theory, as modified by Werkmeister et al. and illustrated in Figure 3.1.28 At small stresses, the behavior can be purely elastic, whereby no plastic strain develops upon unloading. In this purely elastic response,the loading and unloading paths are the same, and there is no shift in the horizontal direction, indicating that the energy input
in deforming the solid grains is released upon unloading. However, if the applied load increases, the material begins to develop small levels of permanent strain over a few cycles. Nevertheless,subsequent cycles at the same strain level yield no additional plastic deformation (Figure 3.1). This response is referred to as elastic shakedown. The permanent strain under these conditions is small,which is attributed to limited slipping of particles and changes in density, while the material adjusts to the applied loads. In a laboratory experiment, this permanent strain can take place as a result of speciman conditioning and adjusting under applied loads. The elastic shakedown is shown in Figure 3.1 by a linear stress-strain relationship in which the loading and unloading paths coincide.
Figure 3.1
Illustration of the Granular Material Response under Cyclic Loading, Based on Shake-down Theory (Ref. 28)
Further increases in the applied loads cause a plastic shakedown behavior. In this behavior, the aggregate develops plastic strain higher than that in the elastic shakedown region. After a certain number of cycles, the plastic strain deformation development ceases. The stress level at which this condition is achieved is referred
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