Study Of Stress-Strain Characteristics Of Unsaturated Compacted Soil Based On Digital Image Measurement

Digital image measurement method, which is based on sub-pixel corner detection, is a new technique of triaxial specimen deformation measurement. This technique measures the specimen deformation through the tracking of corner detection displacement, and the result precision can be up to sub-pixel level (0.02 pixel), and the strain measurement precision can up to 10E-4. This new technique can measure the triaxial specimen deformation of both saturated and unsaturated soil more exactly, and can deeply analyze the soil stress-strain property under triaxial stress state. Due to the peculiar measurement mechanism, this technique obtains the deformation test data that discretize the specimen surface, which can be used to calculate the surface deformation field of the test specimen, and then analyze the surface deformation property.The application of sub-pixel corner detection measurement based digital image measurement technique in unsaturated and compacted soil triaxial test under the condition of suction control, makes it convenient to measure the local deformation of the soil specimen, overcomes such shortcomings as radial constraint caused by contact sensor and small measurement section, and improves the precision of local measurement. Because of the influence of end restraint, the integral measurement of unsaturated and compacted soil specimen overrates the specimen strength and underrates the deformation compared to the real situation; hence the test data cannot stand for the real strength and deformation property of unsaturated and compacted soil. The comparison of test result of the overall specimen and the middle 1/3 part of unsaturated and compacted soil specimen under suction control, reflects that end restraint has little influence on the middle 1/3 part, and the test result of this part can stand for the principal deformation property of unsaturated soil, and provides credible test data for the research of unsaturated soil stress strain relationship. The end restraint has great influence on parameter of the shear resistant strength formula, when using two stress state variable strength theory systems to analyze the shear resistant strength. This paper uses unsaturated soil Alonso elasto-plastic model to analyze the influence of model parameter value on numerical calculation. And the result shows that Finite Element numerical calculation which adopts the specimen integral deformation model parameter will generate errors, and overestimates unsaturated soil strength. Under the condition of isotropic compression condition with suction control, the measurement result of the integral specimen and the middle 1/3 part is consistent, and the deformation of the former is less than that of the latter under the critical net stress; the yield stress of the former is greater than that of the latter, and they both increases with the suction increase. But there is obvious difference between LC yield curve model parameters that are obtained from the integral specimen and the middle 1/3 part. And the LC yield curve formats in p - s plane in also different between experiment and model calculation, and the elastic zone of the former is greater than that of the latter. Due to this great error of Alonso model LC yield equation between calculation and experiment, and the uncommon assumption, the reasonable LC yield locus equation is analyzed based on the result of local deformation in this paper.In the unsaturated and compacted soil triaxial shear test, the strength and deformation measurement results of the specimen middle 1/3 part show that the suction greatly affects unsaturated soil strength, which increases with the increase of the suction. The unsaturated and compacted soil triaxial test result under constant suction describes the unsaturated soil critical state surface in p-q-s three dimensional space: in p - q plane, the slope M of critical state line is independent of suction; the cross line of LC yield surface and critical state surface is a curve with extremum in three dimensional stress space, and its projection in p-s plane is located in quadrant 1 in triaxial compression state; its projection in q-s plane has asymptote, and there should also be asymptote in respect of the curve that stands for the function relationship between cohesive and suction. This paper improves the expression format of the critical state line in Alonso model, and verifies its rationality.According to the drip type plastic volume strain isoline obtained by local deformation test result, and due to the defect of unsaturated soil elsato-plastic Alonso model, the drip type yield surface is expanded to unsaturated soil. This paper proposes unsaturated soil drip type surface function, adopts associated flow rule and reasonable LC yield lotus equation and critical state line expression, and improves the model with dual stress hardening principle. The comparison of the calculation result of the improved model and the original model shows that the improved model can better simulate the unsaturated soil triaxial compression test under suction controlled, and the result is better than that of the original model, which further demonstrates that improved model is more reasonable.