DEFORMABILITY OF VOLCANO-LACUSTRINE SEDIMENTS DURING SUBSIDENCE BY AQUITARD CONSOLIDATION

Subsidence of the ground surface can be produced by several causes. Now a day, the subsidence caused by aquitard consolidation (vertical deformation) is an environmental problem which has been increased in several cities of the world because of groundwater extraction. According with Terzaghi’s theory, with the groundwater withdrawal the effective stress in the soil mass (pressure supported by solid particles) increases, while pore pressure (supported by interstitial water) decreases. Aquitards often overlay aquifer units and are mainly formed by clayey and silty sediments, although they also present hydrogeological heterogeneities such as sand and gravel lenses, or mineralogical differences within clayey sequences whit different water retention capacity. In this work, we analyzed a volcano-lacustrine sequence from the Chalco lacustrine plain, in the south of the Mexico Basin. Each silty and clayey layer of a 15 m depth sedimentary sequence was tested in a computer controlled consolidation apparatus, with pore pressure control capacity. We report the obtained results of compressibility for different layers and compare them with previous reported data for similar clayey sediments. Also, we calculate the potential subsidence using: 1) an average value of compressibility for the whole sequence; and 2) using values representative from each layer of the sequence. Results show that using the average value, the total subsidence of the sequence is overestimated in about 40% when compared with the inter layer analysis. This result implies that during subsidence deformation accomodates differentially in each layer. We propose that a parametrical analysis of the variation of the compressibility, the consolidation coefficient, and the permeability (that involves microfracturing of the clayey strata) is necessary to obtain real and accurate estimations of subsidence.