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Variability of the coefficient of consolidation of the Mexico city clayey sediments on spatial and time scales

  • Ortega A. G. 
Article
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Abstract

This study pertains the two of the six main lacustrine plains of the Basin of Mexico within which Mexico City is situated. These lacustrine sediments are referred to the Mexico City clay in the geotechnical literature and overlie a regional productive aquifer which provides about 50 m3/s of water for the city. Consolidation settlement of the lacustrine sediments in Mexico City is due mainly to pumping of this regional aquifer and by construction of buildings. Although the coefficient of consolidation has much geotechnical importance, little is known about its magnitude or geological controls at various spatial and time scales relevant to consolidation settlement. Different methods are used to evaluates the coefficient of consolidation at four different scale volumes of sediment: the traditional oedometer tests, piezometer response tests, surface loading tests and tests and modelling of long-term transient land subsidence due to aquifer pumping in an area where the present subsidence rate is 0.40 m/year. The spatial scale of the measurements range between 0.02 m to 300 m and the time scale between 24 hours to 30 years. Results show that the coefficient of consolidation depends on the scale and time encompassed by each type of measurement. As the scale of the test increases the coefficient of consolidation also increases, showing the increasing effect of discontinuities within the lacustrine sequence. When laboratory values are used in larger scale subsidence models, results are unrealistic. The bulk coefficient of consolidation is two orders of magnitude higher than the upper limit of the laboratorys measurements. Therefore, the bulk coefficient of consolidation cannot be approximated from oedometer tests and a ranges of this coefficients has to be obtained based on the scale of application.

Keywords

Hydraulic Head Mexico City Land Subsidence Lacustrine Sediment Excess Pore Pressure 

Variations dans le temps et L'espace du coefficient de consolidation des Sédiments argileux de la ville de Mexico

Résumé

Cette étude concerne les deux plaines lacustres, parmi les six du Bassin de Mexico, où est situées la ville de Mexico. Ces sédiments lacustres sont appelés dans la littérature géotechnique argile de Mexico et ils recouvrent un aquifère qui produit environ 50 m3 d'eau par seconde pour la ville. La consolidation par tassement des sédiments lacustres à Mexico est essentiellement due aux pompages dans cet aquifère et à la construction d'immeubles. Bien que le coefficient de consolidations ait une grande importance sur le plan géotechnique, on sait peu de choses sur les facteurs géologiques qui dans l'espace et dans le temps influent sur la consolidation par tassement. Différentes méthodes sont utilisées pour évaluer le coefficient de consolidation à quatre échelles différentes de volume de sédiment: les essais traditionnels à l'œdomètre, les essais piézométriques, les essais de chargement en surface et enfin la modélisation du tassement à long terme dû au pompage de l'aquifère dans un secters où la vitesse d'enfoncement du sol est actuellement de 0,40 m par an. L'échelle spatiale des mesures s'étale de 0,02 m à 300 m et l'échelle de temps de 24 heures à 30 ans. Les résultats montrent que le coefficient de consolidation dépend du volume et du temps pris en compte dans chaque type de mesure. Quand le volume concerné par l'essai augmente, le coefficient de consolidation augmente également, montrant ainsi l'effet croissant des discontinuités de la séquence lacustre. Quand les valeurs obtenues en laboratoire sont utilisées dans les modèles d'affaissement à plus grande échelle, les résultats ne sont pas réalistes. Le coefficient de consolidation apparent est supérieur de deux ordres de grandeur à la limite supérieure des mesures de laboratoire. Donc, le coefficient de consolidation apparent ne peut pas être approché à partir des essais œdométriques et il doit être caractérisé par une série de valeurs dépendant de l'échelle d'application.

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Copyright information

© International Association of Engineering Geology 1996

Authors and Affiliations

  • Ortega A. G. 
    • 1
  1. 1.Institute of GeologyNational Autonomous University of Mexico, Ciudad UniversitariaMexico City

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