Clay Behavior, Ground Response and Soil-Structure Interaction Studies in Mexico City

This paper focuses on the most relevant results of recent investigations carried out on the behavior of Mexico City clays under dynamic loading, on the effects of soil site conditions and on the evaluation of the dynamic soil-structure interaction phenomenon. The paper shows the impact of these studies upon foundation engineering practice in earthquake-prone regions where clayey deposits exist, and advances simple, yet accurate, procedures to develop site-dependent, building-specific input motions for the design of structures in the Valley of Mexico. It discusses recent seismic observations that clarify the origin of the long coda observed in several records obtained in the lake zone in Mexico City

Settlements Induced by Soft Ground Tunneling

Ground settlements induced by air-pressure shield tunneling throughout the soft clays of Mexico City have been monitored in five test sections by means of settlement points installed at various depths and distances from the tunnel axis. Subsoil conditions and soil characteristics at each site were thoroughly studied in the field and in the laboratory. Measured settlement profiles have been compared to those computed by a simplified analytical procedure showing very good agreement

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

Behavior of a Friction-Piled Box Foundation for an Urban Bridge in Mexico City Clay

Severe seismic effects and regional ground subsidence are well recognized conditions that should be considered for the design of foundations in Mexico City. These phenomena are magnified by lacustrine clayey soft deposits with very high compressibility and very low shear strength, which are found at the extensive Lake Zone of the city. To clarify the soil-structure interaction phenomena, a prototype foundation was instrumented, selecting a friction-piled support of an urban bridge located in the Metropolitan area of Mexico City. Soil-raft contact pressures, loads on selected piles, and pore water pressures in the subsoil below the foundation have been monitored continuously, since the beginning of the foundation construction eight years ago. The long-term performance of the foundation is known in terms of these state variables. The response of the soil-foundation system has been recorded during the occurrence of ten mild to strong earthquakes. Time-records of those geotechnical variables were obtained at the very moment of the earthquakes, additionally to the accelerograms. This case history yielded valuable information about the foundation performance before, during and after seismic events, regarding the analysis, design and regulations for this kind of foundation

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

Seismic Microzonation of the Texcoco Lake Area, Mexico

This paper presents some of the most relevant results obtained from field, laboratory and analytical investigations aimed at characterizing the seismic environment prevailing at the Texcoco lake region, in the Valley of Mexico, with the goal of developing a microzonation. In particular, this study focuses on an area of 19.0 by 5.5 km2, which has been instrumented with four seismological stations: TXSO, TXS1, TXS2 and TXCH that have recorded ground motions for at least 15 years. Field investigations were conducted to define the subsoil conditions underneath each station. Dynamic testing in these high plasticity clays was carried out to establish the variation of shear stiffness and damping with strain level. Ground motion definition was achieved through empirically derived response spectra obtained from sets of earthquake ground motions recorded at a nearby station located in soft soil, which were deconvolved to the base rock. An statistical analysis using random shear wave velocity profiles and an stochastic site response analysis was used to developed sets of response spectra to reduce uncertainties associated with soil properties determination and seismic environment characterization. The final proposed response spectra for each studied point were developed from the envelope plus one standard deviation computed at each ground motion station. These spectra show good agreement both in frequency content and spectral ordinates with those obtained directly from measurements taken at these seismological stations. Finally equations to construct the proposed response spectra were proposed

Evaluation of Mexico City Clay Dynamic Properties Using a Parameter Identification Approach

Laboratory-determined soil dynamic properties are always (to different degrees) affected by sample disturbance, scale effects, deficient modeling of in situ conditions, and so on. The installation of vertical arrays of strong motion instruments and the ensuing records obtained during various seismic events, have opened the opportunity to explore other alternatives to evaluate soil dynamic properties by solving the inverse problem. In this paper, an analytical procedure that allows the solution of this problem in a simple way is presented and applied to a case history in Mexico City. The model assumes 1-D propagation of shear waves throughout homogeneous viscoelastic soil deposits. The results obtained here are compared with the velocities measured by means of field studies at Central de Abasto Oficinas (CAO) site with a P-S logging system. These comparisons show the potential of this procedure

Numerical Study of the Seismic Response of an Urban Overpass Support System

A strategic urban overpass is to be built in the so-called transition and hill zones in Mexico City. The subsoil conditions at these zones typically consist on soft to stiff clay and medium to dense sand deposits, randomly interbedded by loose sand lenses, and underlain by rock formations that may outcrop in some areas. Several critical supports of this overpass are going to be instrumented with accelerometers, inclinometers and extensometers to assess their seismic performance during future earthquakes and to generate a database to calibrate soil-structure-interaction numerical models. This paper presents the seismic performance evaluation of one of these supports. The support foundation is a 3.6 by 4.6 m mat, structurally connected to four cast-in-place 0.80 m diameter piles. A finite elements model of the soil-foundation-structure system was developed. Initially, the model was calibrated analyzing the seismic response that an instrumented bridge support exhibited during the June 15th, 1999 Tehuacan (Mw=7) Earthquake. This bridge is located also within the surroundings of Mexico City, but at the lake zone, where highly compressible clays are found. The computed response was compared with the measured response in the free field, box foundation, and structure. Once the model prediction capabilities were established, the seismic response of the critical support of the urban overpass was evaluated for the design earthquake in terms of transfer functions and displacement time histories