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

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 evaluate the coefficient of consolidation at four different scale volumes of sediment: the traditional oedometer tests, piezometer response tests, surface loading 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 laboratory measurements. Therefore, the bulk coefficient of consolidation cannot be approximated from oedometer tests and a range of this coefficient has to be obtained based on the scale of application.