Assessment of reinforced slopes instability in view of semi-analytical methods

The stabilization of natural or man-made earth slopes is usually achieved via proper reinforcement, which usually comprises of geosynthetic geogrids. On the other hand, earthquakes impose in such geostructures dynamic stresses, which may be excessive and can lead to accumulation of slip displacements along discrete failure surfaces. Under this perspective, in the dynamic response of reinforced soil structures and the potential of the geosynthetics to prevent the seismic induced instabilities are being assessed the current study. The method employed is based on a modified procedure of the wellknown Newmark’s sliding block approach. The shortcomings, that are associated with the inherent assumptions in the original formulation of the aforementioned model, are overcome through efficient semi-analytical models. In particular, lumped mass SDOF shear beam models have been developed, which account for the flexibility of the sliding system of the mechanical properties of soil and geosynthetic material, the depth of the sliding mass, and the interface shear strength. The results of the present investigation provide a valuable insight into the seismic response of the sliding displacements in reinforced soil structures. Two approaches were adopted for the analysis of the dynamic stability in order to analyze the conservativeness of the employed method. Firstly, the dynamic response of the sliding soil mass and the development of the seismic accumulated slippage are taken into account simultaneously in a so-called coupled analysis, while a two-step (decoupled) procedure is also followed. Parametric analyses were performed, taking into account the most crucial parameters of the examined problem, i.e., the flexibility of the examined system, the amplitude and frequency content of the excitation, the reinforcement
stiffness and the shear strength of the interface. Permanent displacements are shown to decrease with the increase of the stiffness of the reinforcement, while the decoupling approximation appears to be conservative for the majority of the examined cases. Simplified design charts demonstrate also that the increase of the hysteretic damping increases the ratio of the displacements calculated after the two approaches. It was observed that permanent slip displacements depend on the tanφ*g/a_max ratio and the so-called tuning ratio (β) of the eigenperiod of the structure to the period of the excitation (T_str/T). The increase of the ratio tanφ*g/a_max as well as the contribution of the geosynthetic reinforcement, were
found to reduce the conservativeness of the decoupling approximation. This may be attributed to the decrease of the absolute value of the permanent seismic displacements under these conditions.