Manufacture Of New Triaxial Compression Test Apparatus For Unsaturated Soil

A large part of earth surface is under the water level, therefore many of the soil we meet in engineering field is unsaturated, so it is very important to research the mechanical properties of unsaturated soil. Triaxial compression test is the most important test for researching the mechanical properties of unsaturated soil, which loads stress on soil sample from three directions, then gets its mechanical properties by measuring the relationship between its stress and strain. The triaxial compression test apparatus for saturated soil is well developed and widely used, but there is no mature apparatus for unsaturated soil which due to the imperfect theoretical system for unsaturated soil and the more complex testing technology.Many areas of the existing triaxial compression test apparatus for unsaturated soil in China and abroad can be improved. For example, the measurement of strain is contact measurement, which will cause great disturbance to the sample, and it can’t accurate measure the volume change of the sample in unsaturated soil test. The pore pressure loading system can’t provide enough gas at a high pressure, and it needs to use the external air source to increase the initial pressure. The bottom-up axial loading will led the pressure chamber, this will influence the stability of the pressure inside the pressure chamber and the pipe connected to it. The apparatus needs an extra water container to fill the pressure chamber, and its structure is not compact and so on.We developed a triaxial compression test apparatus for unsaturated soil to solve the above problems, and largely improved the accuracy of triaxial compression test for unsaturated soil. The apparatus has many features. For example, the measurement of strain uses digital image measurement system, it can record the spatial location of calibration point on the soil sample in real time, then get the change of sample various parameters by the post-processing program. We use top-down axial loading mode and built a high-precision underwater sensor into the pressure chamber to measure the principal stress. Using this apparatus can complete various types of triaxial compression tests for not only saturated soil but also unsaturated soil, such as the strain or stress path control test and the loops and unloading test. An electric proportional valve was used to control the pore air pressure which have fast response and high precision and stable pressure. We optimized the structure of the principal stress loading system and the confining pressure stress loading system and the suction control system, in order to achieve the result of rational layout, optimized structure, simple operation, multi-usage, etc.In this paper, by integrating the process of unsaturated soil triaxial compression test and the proper function of the apparatus, we completed the following work, we completed the design of the loading system and rational layout of the whole apparatus, and formulated the plan of the matrix suction control of the sample, and designed the control process, selected the components and parts, connected the electric control board, then introduced the whole surface digital image measurement system, at last debugged the apparatus and did some tests.