Studies On The Digital Photograph Measuring Method Of Volume Strain In Triaxial Test Of Soil

Triaxial test of soil is an important experimental method in the study of soil mechanics. The reliability of the data, by which the strength, stress-strain relation and other mechanic properties of soil are affected, will to a great extent depend on the validity of the basic principle and the accuracy of test method. In the conventional triaxial tests of soil, the volume change of saturated soil sample is measured by using the volume of drainage from a drainpipe in triaxial apparatus. Because of the limitations of this method, it's measuring errors are more larger and the measured value of the drainage volume is only the mean value of volume change of soil sample. Especially, it is impossible to measure the volume strain for unsaturated soils by using the method.For the reason, a new method of non-contact measurement of soil volume deformation based on a combine of the digital camera and the computer processing of digital image is proposed in this paper. This approach has some advantages : non-contact, high speed sampling , intuitively showing the whole configuration of whole deformation process.The main works in this paper are as follows:(1) Limitations of volume strain measurement in the traditional conventional triaxial tests of soil are pointed out.(2) A new method of measurement of soil volume deformation based on a combine of digital camera and computer processing of digital image is proposed, in which many technical problems have been solved. Also the accuracy and errors in the measurement are analysised.(3) In comparison with the measurement results of traditional test method, the superiority and effectiveness of the new method have been shown.(4) According to the test results of the new method, an elastoplastic model of constitutive relation for sand obtained by using the numerical method for modeling the constitutive law for soil, and the three-dimension plot of stress-strain relation has been drawn through visualization.It is shown from the test results that the new approach is a simple, effective one with broad prospect, which has successfully overcome the limitations in the traditional method.