Mineralogical and geotechnical properties of a latosol from Minas Gerais, Brazil
Abstract
The mineralogical, chemical, and geotechnical properties of a latosol from the uplands of Minas Gerais, Brazil were studied to identify its collapsibility characteristics, which are little known. Near surface subsidence is a serious problem in the area and civil engineers consider soils with similar characteristics to cover 65% of the Brazilian land surface. This soil is classified as a lateritic or residual soil from a civil engineering standpoint, and as an Oxisol in the U.S. Soil Taxonomy. The parent material is believed to be Upper Pre-Cambrian limestone deposited about 500 million years B.P. Three hand-cut blocks from the horizons, the AB (20-70 cm), the B (70-120 cm) and the C (120-170 cm), were sampled at a well-drained upland site. All three are similar chemically and mineralogically. All are clays, averaging 11% sand, 28% silt, 61% clay. The silt fraction consists of aggregates cemented by iron oxides because clay increases and silt decreases after reductive dissolution of the iron oxide minerals. Kaolinite is the dominant clay mineral, with gibbsite, mica, hydroxy-interlayered vermiculite, rutile, anatase, goethite, and hematite also present. The pH (in 0.004 M CaCl$\sb2$) ranges from 5.2 in the AB to 4.30 in the B and C horizons. The point of zero charge ranges from 5.2 to 3.9 and is within 0.4 pH units of the soil pH. The soil is very porous (60% porosity) and has a very low dry density of 1000 kg m$\sp{-3}$. At normal overburden pressures the soil is moderately collapsible. The collapse potential index (CPI) is a function of the soil pH, the load at which water inundation is applied, and the initial degree of saturation. At moisture contents $<$25%, CPI decreased from 4.05% to 2.34% when the pH of undisturbed specimens was increased from 4.7 to 6.8 using CaCO$\sb3$. At moisture contents $>$25%, soil pH does not affect the CPI. At a constant soil pH of 5.61, when the initial moisture content increased from 9.4% to 25%, CPI decreased from 3% to 0.7%. When the soil pH increased the CPI decreased, in the range of moisture content below 25%, afterwards the soil pH seems not to affect the CPI. When the load under which the specimen is inundated with water increased, the CPI increased.
Degree
Ph.D.
Advisors
Schulze, Purdue University.
Subject Area
Civil engineering
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