PROBABILISTIC ESTIMATION IN SOIL DYNAMIC PROPERTIES TO OBTAIN RESPONSE SPECTRA AT CAMPECHE BAY

摘要

In the present study, response spectra for seismic design in the Bay of Campeche for offshore platforms, using probabilistic models in soil dynamic properties (shear modulus "G ", normalized shear modulus "G/Gmax" and damping ratio "D"), to evaluate their influence in the response of the site of interest, are presented. In this study the probabilistic model of the soil, take into account the uncertainty in soil dynamic properties, due to the fact that the soil dynamic properties play an important role on the interpretation of free-field motion. In this work an explicit form of the seismic wave characteristics were supposed and their likely source-site propagation characteristics were performed with two computer programs. In order to obtain seismic design parameters, three types of seismic event records were associated to the soil properties at the site of interest, taking into account local effect conditions. Geothechnical, seimologycal, tectonical and geological information, similar to the area of interest, were used. The maximum acceleration on the surface, the soil dominant period of vibration, the duration and the frequency content due to a dynamic excitation applied in the rigid base of the soil column were considered. We assumed that each one of the three types of earthquakes are representative of strong motions that might be potentially affecting the Bay of Campeche. In the study thirty-one sampling in several sites located at the Bay of Campeche were used as well as field and laboratory tests. The dynamic properties were determined in each test type by different ranges according to their shearing strain. From these results test, we know the G and D variation law in the range of interest. In the same way, the uncertainty level influence of the soil dynamic properties was modeled with the expected value of the soil dynamic properties. Six different sets including the soil modulus plus/minus a standard deviation, coefficient from the dynamic properties-time variant evaluations, for each one of the shearing strain, which are contained between 10~(-4) a 10, were considered. Expected site response spectra for different recurrence periods, may be used to complement available information in order to update the present seismic design spectra for offshore facilities located in this area.