Determination of the shear failure process of sediments on the northern slope of the South China Sea with measured electrical resistivity data
Description
This reseach attempts to establish the relationship between resistivity and the shear failure process of shallow sediments in the gas hydrate production test area in the northern part of the South China Sea under saturated and unsaturated conditions. The stress, strain and the resistivity of soil during the shearing process were tested using a self-built, real-time testing system triaxial apparatus. The effects of water content, dry density, confining pressure and other factors on the resistivity variations were examined in detail. The results show that the relationship between resistivity variation and the shear failure process is different for saturated and unsaturated soil during the shear failure process, but both scenarios can be divided into three stages. In the first phase, the resistivity of saturated soil increases and the resistivity of unsaturated soil decreases rapidly. The corresponding soil state is one of initial compression and elastic deformation. In the second phase, the resistivity of the saturated soil and the unsaturated soil decrease slowly. The corresponding soil state is yielding deformation. In the third stage, the resistivity of the saturated soil decreased rapidly, and the resistivity of the unsaturated soil tended to be stable. The corresponding soil state is in failure deformation. The state of the soil can be determined by the variation of resistivity. This work is part of a preliminary study with a long-term objective of establishing a new method for extended observation of in situ marine soil. The initial stage of the project is to analyze the variation of resistivity during the shear failure process of soil and obtain a model that determines the failure state of soil based on resistivity changes. It is hoped that the obtained model will eventually help practicing engineers in the interpretation of resistivity monitoring in marine soil areas.
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Steps to reproduce
A conventional triaxial apparatus was modified to enable measurement of electrical resistivity during the course of the triaxial experiments. This modified triaxial apparatus was then used to investigate the relationship between electrical resistivity and the failure process of the soil. In the modified triaxial apparatus, illustrated in Fig. 1, the electrodes were attached on the top and bottom of the specimen. Two copper electrodes, with a diameter of 39.1 mm and a thickness of 1 mm, were placed on the top and at the bottom of the cylinder specimens during the measurement of the electrical resistivity. Two enameled copper wires, as fine as 1 mm in diameter, were connected to the electrodes. These enameled wires passed through the triaxial cell pressure chamber by a waterproof connector, where it connected to the electrical resistivity measurement apparatus.Electrical resistivity (ER) is an indicator of the electrical conductivity of a substance. Currently two methods, the two-electrode method and the four-electrode method, are mainly in practice for the measurement of soil electrical resistivity in a laboratory (Liu 2006). In the four-electrode method, an electrical probe has to be inserted into the soil specimen. This causes disturbance of the soil specimen, and in the subsequent triaxial tests, it becomes difficult to determine the distance between the electrodes. The two-electrode method, on the other hand, provides a simple means of determining electrical resistivity of soil through two electrode plates placed at the ends of the soil specimen. Therefore, the two-electrode method was used in this study to measure the soil’s electrical resistivity.