The Importance of Principal Stress Rotation in Transportation Geotechnics and Associated Constitutive Models

Among all the transportation modes worldwide, railways are pivotal in catering to the ever-growing population's increasing demands. The most recurring challenge in the field of railways is track failure. Out of all the track layers, the subgrade is the weakest layer. Therefore it becomes necessary to study the performance of railway track subgrade in detail. The laboratory experiments to study subgrade behaviour under moving loads are primarily conducted on cyclic triaxial equipment. However, the Principal Stress Rotation (PSR) phenomenon affects the actual field condition. PSR is the rotation of principal stresses due to cyclic vertical and shear stresses resulting from a moving load. The axisymmetric loading condition applied in the cyclic triaxial apparatus fails to capture the true stress path in the soil underneath railway tracks and pavements. Therefore, it is imperative to study the effect of PSR to understand soil deformation behaviour when subjected to moving traffic. Several constitutive models have also been developed to understand the soil deformation behaviour under stress paths with PSR. The common approaches to incorporating PSR into classical models include using the Transformed stress method, employing additional strain mechanisms due to PSR, relating plastic modulus and dilatancy to inherent anisotropy, and incorporating fabric evolution law to consider the effect of induced anisotropy, to mention a few. The current paper critically reviews the importance of considering Principal Stress Rotation in Transportation Geotechnics, especially in the case of rail track subgrade. Additionally, it aims to provide an overview of some of the constitutive models studying the effect of PSR on sands and clays. Comparing and analysing different models can help understand the primary mechanism behind PSR and its impact on soil deformation.