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Ground surface subsidence is an important consideration in connection with foundations in Mexico City. The ground settlement assumes different values throughout the city depending upon the rate of reduction of the piezometric water pressures in the subsoil and different thickness and compressibility of the clay deposit. The problems encountered are many and varied. In places where the water table has been lowered in highly compressible volcanic clay, shrinkage takes place to a high degree and large fissures and cracks develop in the ground surface and in the clay deposits. To solve this foundation problem in these areas, the clay cracks must be treated to restore continuity in the subsoil conditions before constructing foundations. In areas where pile foundations support buildings, the clay around the piles may shrink away, causing the piles to buckle. The occurrence of large differential vertical movements of the ground surface damages public utilities such as sewage, streets, and highways. Maintenance of these services becomes a matter of primary importance. The maximum ground surface subsidence took place in Mexico City around the year 1949, when the rate of settlement of the ground surface amounted to about 35 cm (14 in.) per yr (1), with respect to the second sand stratum.
Professor of Civil Engineering, University of Mexico, Mexico D. F.,