An experimental study of the mechanical features of layered structures in dam tailings from macroscopic and microscopic points of view

•Layered structures have weaker mechanical features compared with coarse tailings.•The motion of particles near the interface of coarse and fine tailings is obvious.•The local stresses have nonlinearity increase rule of in tailings layers.•The micro-mechanical mechanism of layered structure owns poor strength.

Layered structures of coarse and fine particles (usually called lenticles), which are often found in dam tailings that were built using the up-stream method, significantly impact the stability of a dam. A systematic experimental study of the mechanical features of these layered structures was conducted to determine the shear strength, rules of particle motion and local stresses present in these coarse and fine particle layers. The study was performed using a tri-axial soil mechanics apparatus and a self-designed “observation apparatus for micromechanics and deformation of tailings” from the macroscopic and microscopic points of view. The results demonstrated the following four main points. 1) The shear strength of the layered structure was significantly lower than the shear strength of the coarse tailings. 2) The micro-deformation of the layer with an interface of coarse and fine tailings was greater than the micro-deformation of the other layers in the compression test. 3) Under a constant load, the local stress of the tailings gradually increased in a wave line and the fluctuation in the local stress of the tailings' upper layer increased more rapidly. 4) As the load increased, the motion of the particles near the interface of the coarse and fine tailings was more obvious than the motion of the other particles, which is supported by the observation that the fine particles moved into the pores of the coarse tailings. In general, the poor mechanical features and corresponding micro-mechanical mechanisms associated with layered structures are described in this paper.