A Digital Image-Based Deformation Measurement System for Triaxial Tests

Abstract

This paper describes the development of a new digital image-based deformation measurement system for triaxial tests on soils. Three digital cameras placed on radii at intervals of 120° viewed on plan outside a transparent triaxial cell were used to capture images of a deforming cylindrical soil specimen at various instants. A digital image correlation program was written to analyze the captured images, using ray tracing to take account of image distortion due to refraction at the interfaces between the cell fluid, the cell wall, and the atmosphere. The technique is validated with reference to a sand specimen tested in drained triaxial compression. Typical implementations of the system to study surface deformation characteristics (e.g., barreling, onset of localization, and shear band evolution) of deforming soil specimens are illustrated.

Micromechanical Behavior of Granular Soils Characterized by Acoustic Emission

“…The results suggest that the incipient failure of strain localization is not always synchronous to but fluctuating at around the peak stress in drained triaxial shearing. A similar argument has also been proved by previous experimental studies [9][10][11][12][13][14][15][16]. For example, strain localization had been mostly reported initiating at the strain softening regime ([9, 10, 16]), while there also existed evidence to suggest that strain localization coincided well with the peak stress [11].…”

“…For example, strain localization had been mostly reported initiating at the strain softening regime ([9, 10, 16]), while there also existed evidence to suggest that strain localization coincided well with the peak stress [11]. Researchers (e.g., [12][13][14][15]) had even reported shear band(s) formedas a prepeak phenomenon.…”

“…However, our current knowledge toward strain localization and shear banding and their implications to the soil mechanical behavior are still limited. Particularly, the moment regarding the initiation of strain localization, which was normally interpreted as indicative of incipient failure warning soil instability, remains a debated issue [9][10][11][12][13][14][15][16][17]. This is because the technical constraints and the inherent opacity of soil specimens have limited measurements and analysis to be conducted either through specimen boundary or internal observations over widely spaced strain increments, while strain localization in a soil specimen usually initiates instantaneously, with its evolution appears progressively.…”

Study of Dynamic Evolution of the Shear Band in Triaxial Soil Samples Using Photogrammetry Technology

“…The propagation path of light in the triaxial test is: water → organic pressure chamber → air, during which the refraction effect will distort the obtained image of the soil sample [8,33]. A method to correct this is using the three-dimensional refraction correction model proposed by Xia et al [34,35], which adopts the ray tracing method to correct the error of the image.…”

“…The traditional triaxial test that is usually used to test the soil mass performance is known to have limitations. Specifically, the assumption that the soil sample is isotropic and uniform deformation has to be made in the test [4][5][6][7][8]. Therefore, the stressstrain curve obtained from the test reflects the change of the soil sample as a whole unit and cannot monitor the local deformation of the soil sample at each stage, making it difficult to judge the shear band information of the soil sample.…”