Engineering Features Of Unsaturated Net-Like Red Soils In Southern Anhui Province

The net-like red soil is a widely distributed paleosol unit in southern China. It is exhibited typically unsaturated characteristics. At present, numerous studies are mainly focus on age, particle size distribution and the origin of net-like red soils, while few reports on engineering features. Unsaturated mechanics and microstructure characteristics of net-like red soil are still in exploring stage. However, unsaturated features of net-like red soil have a great significance and engineering apply value for ensuring the safety of the foundation, subgrade construction and slope support.The study on net-like soil of this paper is taken from the Chenshan section. The basic mechanical properties are achieved by laboratory test. And GDS stress path testing system is adopted to research the soil-water characteristic curve and unsaturated strength of net-like red soil. Fredlund model, Van Genuchten model and Gardner model are used to conduct the numerical inverse analysis of soil-water characteristic curve. Finally the qualitative and quantitative analyses of microstructure photos are made by means of scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), digital image processing (DIP). And inner mechanism of microstructure and macro-mechanical properties is preliminary discussed. Some main conclusions are researched as follows:(1) Some laboratory tests such as compaction test and unconfined compression test are made. The liquid limits, plastic limits, optimum moisture content, plasticity index of net-like red soil are48.5%,28.2%,17.9%and20.0, respectively. Peak unconfined compressive strength of remolded net-like red soil is about4.65MPa.(2) The S WCC of net-like red soil has been gotten from the soil water characteristic test. Result is shown that the air entry value is about200kPa under the load condition. It behaves obvious hysteresis characteristics during wetting process in reference to the dry process. Based on Van Genuchten, Gardner and Fredlund-Xing model, the result of numerical simulation indicated that Fredlund-Xing model does a better agreement with the measured value.(3) Failure features of undisturbed net-like red soil appear strain softening in general and the peak values of shear strength appear within the axial strain1.5%. For the cases of the same suction and different confining pressure, the peak of deviatric stress and the mean effective stress increased with increasing effective confining pressure and volumetric strain decreased with increasing effective confining pressure. For the cases of the same confining pressure and different suction, the peak of deviatric stress, the mean effective stress and volumetric strain increased with the increasing matrix suction.(4) The result from EDS tests reveals that the main element of net-like red soils are oxygen, silicon, aluminum and iron. The chemical components are quite different for red matrix and white vein. It is shown that more quartz accumulated and more intense depletion of Fe oxide in the white vein than that of red matrix. In general, Fe is lower in the white stripes than in the red matrix.(5) White veins exhibited scattered texture of micro-pores and micro-particle, and red matrix exhibited aggregate fabric texture of layered structure and aggregate pore. This microstructure characteristics show net-like red soil has obvious anisotropy. Pore characters such as area, diameter and perimeter are reduced during the shear test. The pore area is mainly1-5μm, and there is a lower percentage of area with10-50μm and diameter with10-30um in shearing soil relative to undisturbed soil.Research achievements of unsaturated net-like red soils in southern Anhui province will make a practical significance and theoretical foundation, and also provide some reference value of geotechnical problem in this area.