Development of a New Nonlinear Unified Strength Theory for Geomaterials Based on the Characteristic Stress Concept

There is a need for a unified strength criterion, which is a variable suitable for describing the different strength properties of different types of geomaterials. There have been efforts to develop unified strength criteria; however, they are usually based on a mechanistic approach with adjustable failure planes and complex expressions. This study presents an alternative mechanistic approach to developing a simple unified strength theory by an adjustable characteristic stress. The characteristic stress is unique for a certain geomaterial. The frictional rule is used to explain the failure mechanism of geomaterials, and by defining the shear strength as a proportion function of normal stress acting on the failure plane, a new nonlinear unified strength theory is developed, which is similar to the Drucker-Prager strength theory. The strength curves of this theory are a series of continuous and smooth conical loci, which are located between Drucker-Prager and Matsuoka-Nakai strength curves in the deviatoric plane in the principal stress space. Another main advantage of the developed strength theory is that its three parameters (sigma 0, phi c, and phi e) have clear physical meanings and can be determined based on simple laboratory tests. Verifications between the developed theory and experimental data from triaxial tests available from the literature show that this theory is able to reasonably reflect the three-dimensional (3D) strength properties of a variety of geomaterials.