Undrained monotonic and cyclic behavior of a stiff fissured overconsolidated clay

Due to their geological history, highly overconsolidated stiff clays often contain fissures which can induce strength degradation and structural anisotropy. The aim of this paper is to deepen our knowledge on the monotonic and cyclic behavior of this type of clay. For this purpose, a series of laboratory tests on overconsolidated Merville clay samples were carried out to analyze the influence of the fissure orientation and amplitude on its behavior. Clay samples were scanned by X-ray tomography (CT) and scanning electron microscope (SEM) to obtain a representation of the structure at the micro scale completed by observations of fissure networks within intact specimens. Specimens tested at the same effective confining pressure had different shear strengths due to the influence of the amount and orientation of pre-existing fissures. The value of the shear strength of horizontally drilled specimens was found higher than that of vertically drilled specimens. This anisotropic behavior was mainly the result of preferential orientation of the fissures within the clayey material. The results of one-way and two-way cyclic tests demonstrated also the strong influence of pre-existing fissures on the cyclic behavior. As for the monotonic shear strength, the cyclic resistance of a fissured clay is mainly governed by the intensity and the orientation of pre-existing fissures within the specimens.