TRANSPORTATION ENGINEERING IIResilient Behavior of Unbound Granular Material

Proper characterization of the mechanical response of unbound aggregate materials is a crucial factor in the design and rehabilitation of pavement structures.Among the many parameters that exist today for characterization, the resilient modulus appears to be the most suitable.

Due to the non-linear behavior of granular materials, their resilient moduli appear to be influenced by several factors. Some of them areDeviator stressConfining PressureSoil Dry DensitySoil Moisture Content

(3 & 3) Confining Pressure(1) Deviator Stress

Major Principle StressMajor Principle Stress = 1 1 = c + aWhere c = Confining Pressure a = Axial Stress

Deviator StressDeviator Stress = qq = 1- cWhere a = q , i.e.,Axial Stress is equal to the deviator stress.

Mean Normal StressMean Normal Stress = P

P = (1+ 2+ 3) / 3Where 3 = cPut 2= 3

P=(1+ 23) / 3

The new 2002 AASHTO Guide for Pavement Design advocates the use of the resilient modulus parameter for the characterization of granular materials used as base and sub-base layers in flexible pavements. This mechanistic parameter is known to be influenced by the material physical conditions and the state of stress the material is subjected to.

The resilient modulus is a key element in the design and rehabilitation of pavement structures.It was introduced by the American Association of State Highway and Transportation Officials (AASHTO) in the design of pavements in 1986. It closely simulates the in-situ conditions in terms of traffic and environmental loading.

Suitability of resilient modulus is due to the ability to closely simulate the material in-situ conditions under traffic loading.The resilient modulus, Mr, is a mechanical property of the material that describes its stress strain relationship under dynamic loading and specified physical conditions.

In repeated tri-axial load tests, Mr, is defined as the ratio of the peak cyclic deviator stress to the recoverable measured axial strain as given in equation below.

Mr = d / r

Under cyclic loading, unbound granular materials exhibit elastoplastic behavior, characterized by increases in stiffness and permanent deformation with load repetitions.The repeated load tri-axial apparatus was developed in order to investigate the mechanical behavior of unbound pavement materials.

The apparatus aims to simulate traffic loads by subjecting a cylindrical specimen to repeated cyclic stresses.