Factors affecting the strength characteristics of calcium-carbonate - cemented soils.

Abstract

The factors which affect the engineering properties of calcium carbonate cemented soil are examined. The influence of calcium carbonate content, molding moisture content, and confining pressure on the strength characteristics of two types of soil is investigated in two distinct phases of the research. Type A soil, obtained from the University of Arizona Campbell Avenue Farm in Tucson, was used for the artificially cemented specimen stage. It is composed of sand and silt-size particles with some clay and is virtually free of calcium carbonate in its natural state. Sierrita soil, obtained from the Twin Buttes Open Pit Mine south of Tucson, was used for the reconstituted sample stage. It is naturally cemented with calcium carbonate and is composed mainly of sand, gravel, a small amount of silt, and occasional large-sized (boulder and cobble) particles. Specimens for triaxial compression testing were compacted for each phase of the study under carefully controlled conditions. Three test series were carried out on Type A soil artificially cemented with calcium carbonate. Three percentages (0%, 15%, and 30%) on a dry weight basis of the soil were used. Two molding water contents, one dry and one wet of optimum moisture content, were established for each test series. Unconsolidated undrained triaxial compression tests were carried out on oven-dried specimens at three different confining pressures to obtain shear strength parameters. The fabric characteristics of selected specimens were then defined by viewing them under a scanning electron microscope. The results indicate that the strength of the calcium carbonate cemented soil depends on the distribution and not necessarily the content of the cementing agent within the soil mass. Visual examination of the various microstructures of the artificially cemented soil confirmed the hypothesis that strength gain occurs when the calcium carbonate particles are concentrated at the points of contact between soil grains. Visual examination of the fabric of the naturally cemented Sierrita soil showed the microstructure to be highly compressed with weathered calcium carbonate particles dominating the soil structure. The calcium carbonate content was found to range from 14 to 23%. Because of sampling difficulties, an in situ cohesion value for the Sierrita soil could not be obtained from conventional laboratory tests. Therefore, the value was obtained by back analysis of the stability of actual slopes existing at Twin Buttes Mine. Slope stability analyses using Bishop's Modified Method with a search routine based on the Simplex Method of Nelder and Mead were performed. Stability analyses were also performed using strength properties of artificially cemented Type A soil. These analyses showed the relationships among cohesion, friction angle, safety factor, and calcium carbonate content for a specified slope geometry.