Based on these results, recommendations are provided as related to the most appropriate test methods for determining the strength properties of residual soil for use in geotechnical design.Ĭitation: Richard E. In both excavations, failure was observed along a similar plane, which began at the bottom of the excavation, and propagated to the surface (approximately 2 m behind the face of the excavation) along existing tension cracks that developed during the construction of the excavation. In addition to common laboratory tests, unsaturated triaxial tests were also conducted, and soil-water characteristic curves were measured to further define the unsaturated properties of the soil. Undisturbed soil samples were taken during the subsurface exploration and used in conjunction with previous soil test results to accurately define the material properties and layering based on common laboratory test methods. Time-lapse cameras were used to identify when failures occurred, and the approximate geometry of the failure planes. The instrumentation plan was designed to monitor real-time pore water pressures (positive and negative) surrounding the excavation throughout the course of each 1-year test period. In doing so, conclusions were drawn regarding the accuracy of common laboratory test methods for estimating the strength properties of residual soil. The primary goal of this experiment was to determine the boundary conditions that resulted in failure of the excavation. The excavations were constructed approximately 6 m deep × 30 m long with a vertical face. It is concluded that significant genetic relationships do exist between the residual soils and underlying bedrock in each of the three cases studied, and these relationships exist in a rational and predictable manner.Two excavations were instrumented at the Auburn National Geotechnical Experimentation Site (NGES) in Opelika, AL. The W.P.I, values of the Lloyd soil indicate a high level of maturity and stability approaching laterite in character. The W.P.I, values for the Iredell soil show that the saprolite developed in the C horizon is chemically similar to the unaltered bedrock. Weathering Potential Index values also reflect (a) the homogeneous nature of the Frederick soil, and (b) the progressive increase in weathering and soil stability up the Iredell and Lloyd profiles. Somewhat surprising were the relative mobilities of iron (high) and magnesium (low) in the C horizon of the Iredell soil. Variations in major element geochemistry are also reflected in the various horizons. Illite is the major clay mineral in the Frederick series kaolinite, in the Lloyd series and vermiculite, illite, and montmorillonite characterized different genetic layers of the Iredell series. Both the Frederick and the Lloyd series are characterized by a single major clay species, but the Iredell mineralogy shows significant vertical changes. The clay fraction of each soil contains significant amounts of quartz as well as several species of clay minerals. This absence of an A horizon is attributed to improper farming methods existing over much of the Virginia Piedmont during past decades. The Lloyd soil, like the Iredell, exhibits a significant grain size variation down the profile, but surprisingly has no A horizon. The Iredell profile is relatively shallow at less than 6 ft in depth, but displays a well-developed eluvial or A horizon and a very fine, clay-rich illuvial or B horizon overlying a sandy gabbroic saprolite. Texturally, the limestone-derived Frederick profile is the most uniform and is very fine grained, containing approximately 40 percent clay-sized materials. The studies reported here indicate that where factors, such as climate, slope, and organisms are not radically different, the bedrock plays a very important role in determining the characteristic properties of the overlying soil profile. These residual soils, developed in a temperate nonglaciated area, are quite old in a time sense, and normally show classical A, B, C, and D profile development. The soils, in pedological terms, are from: (1) the Frederick series developed over the Chepultepec limestone (2) the Iredell series developed over an unnamed hornblende metagabbro and (3) the Lloyd series developed over the Robertson River granitic gneiss. In order to substantiate this hypothesis three distinctly different profiles were chosen for textural, mineralogical, and geochemical investigations. It was suspected that these soils would also show significant genetic relationships to the underlying bedrock. The diversity and complexity of the geology in central Virginia are accompanied by an equally diverse and complex mantle of residual soils which characteristically show strong profile development.
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