7. Conclusions
Soil effective stresses changes during the economic life of a structure built in highly compressible clays leads to important ground deformations, and changes in the dynamic properties (i.e. shear wave velocity distribution and modulus degradation and damping curves). These effects, which are often ignored in practice, can substantially modify the frequency content and spectral accelerations in both free field and in the soil-structure system. Pore pressure variations over the project economic life is due to both regional subsidence as well as dissipation of excess pore pressure caused by the structure weight. For the cases studied herein, a complex interplay between soil nonlinearities, which tend to elongate the fundamental period of the soil deposit, Tp, and the overall tendency of ground consolidation to shorten it, led to variations in the spectral ordinates depending on how close Tp is of the predominant period of the excitation, Tpe. For subduction events recorded at Mexico City firm soils Tpe ranges between 0.8 and 1.8 s. A similar trend was observed in the soil-structure system response for the cases analyzed inhere. Based on the analyses presented herein, the effect of the changes in dynamic soil properties, and layering configuration should be taken into account in high compressible clays, subjected to rapid regional consolidation rates (i.e. average consolidation rates larger than 10 cm/year), or where the expected primary consolidation settlements over the economic life of the structure exceeds 2.5 m. The key issue is the compressibility of the Mexico City clay, rather than the plasticity. For clays with lower plasticity, or sensitive, but a similar high compressibility, it will be necessary to account for changes in dynamic properties as well as layering configuration, considering that volumetric strain changes in the soil will be associated to changes in the mean consolidation effective stresses. The effect of the lower plasticity will increase the amount of soil nonlinearities developed in the soil and, thus, the amount of pore pressure generated during the cyclic loading. As previously mentioned, this pore pressure will dissipate afterwards, leading to post earthquake-induced settlements. This phenomenon has been studied by several researches, and will eventually also led to soil layer distortion as well as an increase of the dynamic soil properties. Since the effect on the response of a specific soil deposit is also a function of the input motion frequency content, the coupled influence soil properties-input motion characteristics should be evaluated for each condition. Needless to say, this should be taken into account during the design process.
