Investigation Of Seismic Behavior Of Underground Structures In Different Soil Media

Underground structures like tunnels and piles are main importance for human technological life.They maybe use for many purposes including subways,roadway,water transportation,and deep foundations to transfer loads to the stronger lower layers.By considering the increasing necessity of underground structures for different purposes in addition to earthquake potential in many countries,designing these structures are of main importance.It’s been proved that surficial structures are more prone to damage in comparison with underground structures but there are many instances for these structures to be destructed due to earthquakes like 1995 Japan Kobe earthquake,and 1999 Tai Chi earthquake in Taiwan.Seismic Waves propagate from bedrock through the soil layers and during this propagation they pass different layers of soil and rock until they get to the soil surface.These waves can be amplified or damped by the soil layers.Underground structures response is related to number of factors including,dimensions of structures,frequency of earthquake,soil deposit above the structures,type of soil,number and thickness of soil layers.In the current study,the influence of these parameters on seismic behavior of underground structures are investigated.Using numerical methods to investigate the dynamic behavior of underground structures requires a suitable constitutive model for the materials involved.In this study,advanced constitutive models including both multilinear and nonlinear kinematic hardening models with the Von Mises failure criterion which can consider the stiffness degradation caused by imposing cyclic loading and unloading to the soil were used to investigate the seismic behavior of structures constructed in clay soils.Also,these constitutive models were extended using subroutine USDFLD for use them in sand soils.The parameters of these constitutive models are calibrated based on the results of experimental cyclic tests and finite element simulation.Here,there new methods using cyclic shear and undrained cyclic triaxial tests are used to calibrate the parameters of constitutive models.In this research,the seismic performance of circular and square tunnels constructed in different depths of soils with different thicknesses as well as single piles and pile groups have been investigated.It is worth mentioning that the seismic behavior of a batter pile group constructed in saturated sand soil using both centrifuge test and numerical analysis methods and also,a vertical pile group with conditions similar to the batter pile group using numerical simulation method was investigated.In this thesis,modeling of five soil-structural models including tunnels and piles in different soils has been done.The results of four simulation models have been compared with the results of experimental tests(shaking table test or centrifuge models),and the results of one of the modeling presented in this study(soil-tunnel model)were compared with the modeling results of a published paper.It is worth mentioning that in the published paper,another constitutive model and software have been used to model the soil-tunnel system.In general,the results of these studies showed that the maximum bending moment generated on the perimeter of the square tunnels is more than that generated on the perimeter of circular tunnel,and also the corners of the square tunnel have maximum values,proving that the circular tunnel performs better than the square tunnel under the applied seismic loads.In the case of piles the results showed that the variation of earthquake frequency and intensity affects the bending moment created along the pile,and dynamic lateral stiffness of piles is considerably increased by seismic loadings.Also,maximum bending moments and maximum axial forces of inclined piles are less than those of the vertical ones.