Shakedown Analysis of Granular Sub-base Behaviour

Granular sub-base (GSB), used as a pavement layer, comprises of aggregates of different gradation bounded with non-plastic fines at an optimum moisture content. In the mechanistic pavement design, resilient modulus of pavement material is considered. These materials when subjected to vehicular loadings undergo elastic and plastic deformations, which will eventually lead to pavement failure. Shake down method correlates elastic strain and plastic strain rate to mechanical behaviour of these materials. In the present study, shakedown approach is used to assess the mechanical behaviour of granular sub-base with respect to increase of confining pressure, increase of cyclic deviator stress, and reinforcing action of geosynthetics. For the experimental studies, cyclic load is applied to consolidated undrained triaxial specimens of 100 mm diameter × 200 mm height. Shakedown limits as prescribed by literature is used to assess the mechanical behaviour of granular sub-base by considering elastic strain and plastic strain rates. From the study, it is observed that, increase of confining pressure and cyclic deviator stress causes a surge in shakedown and the material exhibit an incremental collapse. It is also observed that with the use of geosynthetics, the plastic strain developed is 1/10 times that of unreinforced specimens. At a q/p ratio of 1.5, geosynthetic reinforced specimens moved to plastic shakedown range whereas for unreinforced specimen, this happens at a stress ratio of 1:1. The analysis also revealed that for any load application, the maximum shakedown occurs in 0–1000 cycles. Further, a three-layered pavement system is modelled and decrease in rutting life/increase in damage ratio with reduction in resilient modulus associated with shakedown of granular sub-base is estimated.