This study examines a triaxial testing system for unsaturated subgrade fillers, utilizing a high-suction tensiometer and photogrammetry to more accurately simulate and analyze their mechanical behavior. Digital image correlation (DIC) technology is combined with non-contact photogrammetry, employing a multi-ray tracing method to reconstruct the 3D model of the sample and monitor its volume changes. Real-time matric suction is measured using a high-suction tensiometer, avoiding traditional suction control methods and enabling a more accurate reproduction of deformation and suction changes in unsaturated soil samples under natural conditions. This study further analyzes key parameters, such as specific volume change, suction change, and shear failure state, under varying moisture content and stress conditions, with parameter calibration for mechanical behavior performed using the BBM model. This system significantly reduces traditional experimental time, offering a new tool for studying the mechanical behavior of unsaturated subgrade fillers, with substantial theoretical value and practical application potential.

The formation layers of railway embankments are often unsaturated and subjected to coupled cyclic traffic-induced and hydraulic loading. Understanding this coupled response requires the development of a testing protocol capable of subjecting soil samples to cyclic loading while continuously monitoring water retention response of the soil. An accurate measurement of the suction variation for the case of repeated cyclic loading is crucial for interpreting the response of the soil considering the principles of unsaturated soil mechanics that are commonly neglected during the design of this infrastructure. In this paper, we present the use of a high-capacity tensiometer of capacity 2MPa and resolution 0.5 kPa developed at Durham University, capable of measuring suction on the body of soil samples. The setup allowed continuous monitoring of suction at the mid-height of the unsaturated soil sample during cyclic triaxial testing while continuously measuring the volumetric deformations with the help of local displacement transducers. The obtained results indicated that the volumetric compression during cyclic loading reduced the voids ratio leading to an increase in the degree of saturation under constant water content conditions that reduced the soil suction. The obtained results were then interpreted by using mean Bishop's stress where the permanent strain was consistently found to increase with an increase in the Bishop's stress ratio. The resilient modulus was also found to be correlated to Bishop's stress ratio.