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This paper investigates the validity of the interpretation of results from testing saturated axisymmetric triaxial compression (ATC) sand specimens utilising three-dimensional (3D) synchrotron micro-computed tomography (SMT) to probe localised events that are completely missed or misinterpreted when analysing ATC measurements based on global standard measurements. Drained and undrained experiments were conducted at low and high back-pressures (BPs) coupled with multiple in situ 3D SMT to acquire high-resolution scans of the specimens at different axial strains. Specimens tested under low BP exhibited a large pore air volume change, which was not detected by the pump system that represents standard volume measurement. The increase in air volume caused a significant reduction in the degree of saturation leading to a possible transition from saturated to partially saturated constitutive behaviour. Undrained experiments exhibited a significant volume change contrary to the assumption of negligible volumetric strain for saturated undrained experiments. Air bubbles within the shear band for drained and undrained low-BP specimens showed opposite capillary pressure responses, increased for drained and decreased for undrained cases, due to the variation in the mechanism by which each of the two experiments predominantly counters the volume expansion within the shear band.
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This content will become publicly available on July 24, 2026
Influence of Back Pressure on Degree of Saturation of Sand Triaxial Experiments
Back pressure (BP) is used to saturate triaxial specimens before the shearing phase. A high BP is commonly used in the laboratory to ensure a high degree of saturation. Most soil deposits in the field for Civil Engineering applications have a low pore water pressure in the range of 30 to 50 kPa. The principles of soil mechanics postulate that the value of BP does not affect the behavior of triaxial specimens as long as the effective confining pressure is kept constant. This paper investigates the influence of BP on the behavior of uniform saturated sand tested using axisymmetric triaxial compression (ATC). 3D synchrotron micro-computed tomography (SMT) technique was used to acquire 3D scans while shearing the specimens to probe localized events that are completely missed or misinterpreted when analyzing ATC measurements based on global standard measurements. Specimens tested under low BP exhibited a large pore air volume change, which was not detected by ASTM standard measurements for triaxial cells. The paper discusses the influence of BP on the deformation mode of the specimens using rich SMT images and sheds light on the change of degree of saturation for specimens tested at low BP, and compares the behavior of specimens tested at high BP.
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- Award ID(s):
- 2016392
- PAR ID:
- 10617931
- Publisher / Repository:
- Proceedings of the 21st International Conference on Soil Mechanics and Geotechnical Engin
- Date Published:
- Format(s):
- Medium: X
- Location:
- Vienna, Austria
- Sponsoring Org:
- National Science Foundation
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