Composition and Compressibility of Typical Samples of Mexico City Clay

Composition and compressibility characteristics of Mexico City clay were investigated using samples with natural water contents in the range of 421574%. The composition study included scanning electron microscopy, X-ray diffraction and chemical analyses, as well as measurements of physical properties. Compressibility characteristics were investigated by means of one-dimensional consolidation tests run on specimens in the following states: natural undisturbed, artificially sedimented, remoulded at natural water content, remoulded-aged, and hydrogen peroxide treated. The primary objective was to study the secondary compressibility of Mexico City clay with special reference to the effects of previous secondary compression and thixotropic hardening. Compositional analysis indicated 5-10% sand-sized particles, most of which are calcareous ooliths (the remainder of this size fraction is a suite of heavy minerals); 55-65% of the whole sample is composed of silt-sized siliceous microfossils, mostly diatoms; 20-30% is composed of clay-sized particles, of which an estimated 10% are smectite and the rest biogenic and volcanogenic silica; the remaining 5-10% is organic matter. Based on test results it was hypothesized that the smectite in Mexico City clay is interlayered with hydroxide complexes of aluminium, iron and possibly magnesium. Primary and secondary compressibilities are interrelated. During sustained loading secondary compression and thixotropic hardening take place simultaneously. After sustained secondary compression there is a decrease in primary as well as secondary compressibility for pressure increments less than the reserve consolidation pressure. Thixotropic hardening, however, increases the viscous resistance, and thereby decreases the amount of compression during primary consolidation, but increases the rate of secondary compression. The structural resistance which develops during sustained secondary compression decreases the rate of primary consolidation during the subsequent increment and results in computed permeabilities which are less than the true permeability of the soil fabric.