Abstract: This study aims to investigate deformation characteristics of natural structural planes with a certain thickness under normal cyclic loading using laboratory tests and theoretical analysis. The studied natural structural planes are flat and slightly rough with a thickness of 2-3 mm. The host rock is limestone and the infillings are primarily rock fragments with a small amount of mud. Firstly, the variation process of normal stress applied on structural planes under engineering effect is summarized. The loading path for experiments is then designed based on the characteristics of normal stress versus closure deformation curve. Then, we present the sampling operation, experimental results of physical and mechanical properties of samples, and the preparation of testing specimens. The specimens are applied to conduct single cycle and multiple cycles of normal loading and unloading experiments. In addition, we summarize the testing curves and the normal deformation mechanisms of structural planes and study the constitutive relation of structural planes under normal cyclic loading conditions. It is found that, when the normal stress is within 20 MPa, the hyperbolic model and the improved one with the consideration of a modification coefficient can be used to well simulate the relation of normal stress versus closure deformation at various loading stages. Meanwhile, when the normal stress increases up to a high-stress level (i.e., the stress magnitude above 20 MPa is defined as high stress, but below that it is recognised as the range of mediate to low stress.), new characteristics were observed from testing data. The discrepancies between predicted values and monitored results become increasingly greater. Finally, the probable reasons for new characteristics are analyzed. It is suggested that it is necessary to use natural samples to investigate normal deformation characteristics of structural planes under high-stress conditions.

Key words: natural structural planes with certain thickness, normal cyclic loading, deformation characteristics, constitutive relation