Shakedown of Unbound Pavements

ABSTRACT The performance of a pavement material beyond the elastic limit has substantial influence on the bearing capacity of a pavement and its serviceability. But in all the extant design methods for unbound pavements, only the elastic strain or stress at the top of the subgrade of the pavements is used as a design parameter. Although experimental data and practical experience have been introduced into the design method through empirical parameters, there is not a simple relationship between the elastic strain and the long-term plastic behaviour of pavement materials. Furthermore, these methods assume that pavements deteriorate indefinitely, whereas in practice pavements are frequently observed to “shakedown” to a steady state. In this paper, we will review progress in developing a new mechanistic approach to unbound pavement design, based on the shakedown theory, which incorporates the plastic as well as elastic properties of the pavement layers, provides predictions of various failure modes, such as rut formation and subsurface slip, all of which are consequences of unlimited accumulation of plastic deformation. The minimum shakedown loads are found by varying the geometry of the proposed failure mechanisms, and this is done for an unbound pavement overlying a clay subgrade.