An Experimental Study to Assess the Shear Modulus Degradation by Fatigue of Mexico City Clay

In this research, the degradation by fatigue of Mexico City clay is studied using a triaxial equipment where the cyclic stress amplitude was maintained constant during the experiment. The variables considered in the study were the following: state of the soil, effective mean confining stress, magnitude of cyclic stress and number of loading cycles. Undisturbed samples, anisotropically and isotropically consolidated, were subjected to cyclic loading for this purpose. When analyzing the cyclic stress-strain response with the number of cycles a threshold of permanent deformation in function of the cyclic deviator stress and axial strain was found. When the cyclic strain exceeds this distinctive value the rate of permanent (plastic) deformations accumulate faster. For practical applications of computing permanent deformations in Mexico City a simplified method is proposed. This method considers the above threshold and a hyperbolic model to represent the cyclic response in Mexico City clays

Analysis of Some Downhole Acceleration Records from Central De Abasto Oficinas Site at Mexico City

The surface and downhole accelerations records of “Central de Abasto Oficinas (CAO)” array at Mexico City, have been analyzed to determine the soil stiffness as a function of shear strain amplitude. The 09/10/95 seismic event (NS component) has been used for this purpose. The shear stress-strain histories have been evaluated directly from the field downhole acceleration records, employing a technique of system identification, and used to obtain the variation of shear modulus with shear strain amplitude. A shear-beam model, calibrated by the identified properties, is found to represent the site dynamic response characteristics. The results have been compared with values obtained in previous investigations from field and laboratory tests

Prediction of Lateral Displacements Induced by Liquefaction in the Port of Manzanillo, Mexico During the Earthquake of October 9, 1995

This paper presents the prediction of the lateral displacements due to liquefaction in the yards of the Container Terminal at San Pedrito in the Mexican port of Manzanillo during the earthquake of October 9, 1995 (Ms=7.3, Mw=8.0). It describes briefly the results of field and laboratory tests carried out after the earthquake in order to obtain the required parameters to compute lateral displacements using the Newmark model and a modified Newmark block analysis which takes account of dilatant behavior; the models were calibrated with results of centrifuge experiments. Field data and simplified liquefaction analysis showed that the liquefied fill had a thickness of 13 m and a gentle slope of one degree. From the laboratory tests a yielding shear stress of 3.3 kPa was estimated, and from seismic risk analyses the input used for the prediction of the lateral displacements was a sinusoidal wave of twenty two cycles of constants amplitude of 3.25 m/s2 at a frequency of 1.4 Hz. Since in the case of Manzanillo the laboratory results did not show evidence of dilatant behavior, the original Newmark sliding block analysis was used and predicted a lateral displacements of 1.95 m, which is in good agreement with the actual movements (slightly higher than 2 m) observed after the earthquake