## 1 Introduction  
During the construction of unsaturated subgrades, the mechanical properties of the unsaturated subgrade fill have become key factors influencing its stability and safety. The triaxial test, widely used to characterize unsaturated soils, has been extensively applied. However, since the pores contain both water and air, the volume change of the soil sample no longer corresponds solely to the exchange of water, rendering conventional triaxial test equipment for saturated soils unsuitable for testing unsaturated soils. Recently, with the growing demand for measuring the volume change of unsaturated soils, various innovative methods and equipment have been developed. [PERSON] and [PERSON] (2023) [1] proposed a novel method based on the stress path approach, which independently quantifies the effects of net total stress and suction on the volume change of unsaturated soils, providing a more refined framework for precise analysis. [PERSON] and [PERSON] (2021) [2] used a high-capacity tensiometer (HCT) to directly measure the soil water retention curve (SWRC) and combined it with photometric methods to measure volume changes. Their findings indicated that this procedure is more precise for high-suction soils, as it is able to define hydraulic and volumetric behavior. [PERSON] et al. (2024) [3] created 3D scanning technology to measure residual soils' volume change, eliminating the limitations of the historical drying method and offering better volume change data. [PERSON] et al. (2021) [4] performed compression experiments on soil samples while controlling suction and porosity to understand the behavior of unsaturated soils' volume change. Their findings highlighted that their procedure was able to sufficiently control saturation variation for soil samplesand sufficiently define hydraulic and mechanical responses. [PERSON] et al. (2020) [5] used tensiometers, filter paper methods, and dew point moisture meters to measure the water retention curves of several unsaturated soil samples, investigate the volume change of soils under different moisture conditions, and showed that these results were sufficient and capable of jointly mapping the water retention curve of unsaturated soils, particularly tropical clay. [PERSON] and [PERSON] (2021) [6] employed the balloon method to measure the volume shrinkage curve (VSC) using tensiometers and dew point moisture meters. They noted the significance of the VSC in deriving the water retention curve (WRC) in low plasticity soils and argued that ignoring volume changes will result in measurement errors. [PERSON] et al. (2021) [7] suggested a straightforward method to measure volume change to derive effective soil water retention curves (SWRC) and soil shrinkage curves (SSC) for soft soils, using a simple balloon method to examine earth structure while also utilizing high-capacity polymer tensiometers. In recent years, there has been a shift in the demand for research directed at investigating the mechanical behavior of unsaturated soils, which has led to the construction of new suction-controlled triaxial testing devices that provide improved measurement of unsaturated soil behavior. For example, some suction-controlled triaxial testing devices consist of a double chamber, with the inner chamber designed to monitor the soil sample's volume change [8]. In some device designs, the inner chamber features an air inlet disk located at the bottom of the test chamber, supplemented with modern axial conversion technology to allow for proper matric suction control. More established designs of triaxial testing devices also contain an inner chamber filled with mercury, where a soil sample's volume change is measured based on the change in the mercury meniscus [9]. The condensed communication pathway between the inner and outer chambers assists in mitigating any potential influence that confining pressure changes may have on measured data reliability.