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Assessment of seismic behavior of reinforced geostructures

Tzavara Ioanna

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Assessment of seismic behavior of reinforced geostructures By Ioanna TzavaraAbstractSeismic design of reinforced and unreinforced embankments is of extreme importance, due to the environmental and economical consequences related to a potential failure. The main factors that can cause their instability are those which tend: (i) to increase the shear stresses that developed in the soil and (ii) to decrease the shear resistance. For this purpose, the present doctoral dissertation investigated the seismic distress of reinforced soil structures caused not only due to permanent displacements accumulated during the seismic wave propagation, but also due the development of permanent ground deformation resulting from an abrupt fault rupture.The design of geosynthetic reinforced slopes is based on modified versions of standard limit equilibrium slope stability methods. Kinematically, the potential failure surface in a reinforced homogenous slope is assumed typically to be defined by the same idealized geometry (but not location) as in the unreinforced case (for example circular, log spiral, bilinear wedge). Statically, the inclination and distribution of the reinforcement tensile force along the failure surface need to be assumed. The capacity of reinforcement is taken as either the allowable pull-out resistance behind the potential failure surface, or as its allowable design strength, whichever is less. The target factor of safety for a reinforced slope is the same as for an unreinforced slope. The most common approach for the analysis of seismic stability of geosynthetically reinforced earth structures is based on pseudostatic approach, in which the seismic forces are derived by multiplying the seismic coefficient and the weight of the sliding mass.Nevertheless, soil slopes are flexible systems characterized by a relatively large fundamental period. Therefore, design procedures which are used to evaluate earthquake-induced sliding displacements typically refer to three different approaches: (a) simplified dynamic analysis by means of the conventional Newmark rigid block model that slides on an inclined plane, (b) dynamic analysis accounting for the flexibility of the oscillating mass, where the dynamic response and the sliding block displacements are computed separately, this is commonly referred as decoupled approach, and (c) dynamic analysis where the dynamic response and slip displacement accumulation are considered simultaneously, commonly referred to as coupled approach.For this purpose, flexible one (SDOF) and two lumped (MDOF) mass soil models were developed in the current research, in which sliding has been considered on horizontal or inclined base planes, while the effect of the most important parameters involved: such as flexibility, yield acceleration, dynamic loading, reinforcement, mechanical properties of the soil and the geosynthetics, the frequency content of the excitation and the interface shear strength have been considered. The parametric investigation of decoupled and coupled approaches contributes towards a better understanding of the development of the permanent slip displacements. The characteristics of the failure type are strongly related to the eigenperiod of the geostructure, the period of the excitation, the shear strength of the interface and the maximum applied acceleration.Additionally, numerical finite element analyses were performed considering the impact of several important parameters related to soil and geosynthetics materials, the geometry of the model and the characteristics of seismic excitations. For the verification of the numerical results, available data and results of centrifuge tests were utilized.Finally, parametric numerical analyses were performed in order to assess the propagation of a fault rupture within a geosynthetically reinforced earth structure. A detailed parametric analysis of typical models os slopes and embankments was conducted and it was derived that the permanent deformation of the reinforced geostructures depends both on the characteristics of the fault rupture, the geometry of the embankment as well as the properties and the configuration of the geosynthetics.

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