Increasing the accuracy of two-dimensional image processing techniques to estimate stress-strain curves in triaxial tests

Document Type : Article


I‌n‌t‌e‌r‌n‌a‌t‌i‌o‌n‌a‌l I‌n‌s‌t‌i‌t‌u‌t‌e E‌a‌r‌t‌h‌q‌u‌a‌k‌e E‌n‌g‌i‌n‌e‌e‌r‌i‌n‌g a‌n‌d S‌e‌i‌s‌m‌o‌l‌o‌g‌y


The triaxial test is one of the most versatile and widely performed geotechnical laboratory tests, allowing the shear strength and stiffness of soil and rock to be determined for use in geotechnical design. In saturated soils, due to the presence of water and soil, the triaxial device can measure changes in sample volume by the amount of water expelled out of the sample during loading. However, in unsaturated soils, in addition to soil and water parameters, the air parameter will also be effective in obtaining soil characteristics. To avoid high demand related to the time and cost of the test on unsaturated specimens by advanced related types of equipment, a simpler method based on 2D image processing is proposed in this study.

Firoozkooh sand No.161 was used as the material, and consolidated drained triaxial tests were performed. Six saturated specimens (3 with a relative density of 85% and 3 with 50%) were loaded with a strain rate of 1 mm/min. Before 15% axial strain, the error calculated in measuring the average diameter of specimens was in the range of 0.001 to 0.002. From the strain of 15% to 20%, the error in some tests increased and reached a maximum of 0.01.

The first achievement of this research is the accuracy of determining volume changes. Though the method is based on two-dimensional image processing, the results are as precise as a three-dimensional processing.

The second achievement is the elimination of the complex calibration process from the image processing steps, making it possible to implement the approach at conventional laboratories. Finally, the third achievement is the possibility of probing diameter changes along the specimen’s height. This feature facilitates better interpretation of the tests’ results. These features make the proposed method a proper framework to measure results of triaxial test on any type of soil.


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