Mineralogical properties of a collapsible lateritic soil from Minas Gerais, Brazil

Soils weathered from limestone and sediments of the Bambui Formation in parts of Minas Gerais, Brazil, are prone to near-surface subsidence, resulting in damage to buildings and roads, and loss of life. Engineers consider them ''collapsible soils'' because they exhibit large decreases in volume and associated settlements with no increase in applied stress when wetting occurs. The objective of this study was to determine a possible mineralogical or chemical basis for this behavior. Three horizons to a depth of 170 cm from a Rhodic Hapludox near Sete Lagoas were analyzed by x-ray diffraction, infrared absorption, electron microscopy, and wet chemical methods. All horizons were mineralogically very homogeneous, with kaolinite, gibbsite, and hematite dominant, but (dioctahedral) mica, hydroxy-interlayered vermiculite, goethite, anatase, rutile, quartz, and perhaps cristobalite also present. More than 850 g kg(-1) of each horizon consists of minerals with particles <2 mu m, but aggregation of particles by Fe oxides results in a silt content of about 300 g kg(-1). The pH of each horizon is at or very near to its zero point of charge. The physical properties of this soil are apparently dominated by: (i) tbe residual nature of the soil, which has weathered in place a very long time, resulting in high porosity and low bulk density, (ii) the aggregation of kaolinite and gibbsite by Fe oxides to form silt-sized aggregates, and (iii) the variable surface charge of the dominant minerals. Future soil mechanics research on these soils should consider the consequences of chemical changes that might alter interparticle forces important in soil stability.