WU Ziyu，CHI Zhipeng，LI Ran，YANG Hui

School of Optical-Electrical and Computer Engineering， University of Shanghai for Science and Technology， Shanghai 200093， China

Abstract

Objective Landslide and debris flow are significantly influenced by precipitation distribution, mountainous topography, and soil granular properties. This study aims to explore the effects of various factors, such as moisture content and soil properties, on disaster behavior. A wet granular column collapse model is constructed to analyze how different moisture levels impact collapse modes and behavior, providing a theoretical basis for disaster prevention.

Methods This study investigated how moisture content affects the collapse modes and behavior of granular columns by systematically varying moisture levels from dry to fully saturated conditions. The experiments employed a custom horizontal transparent channel. The glass beads with diameters of 1. 0 mm and 2. 0 mm and a density of about 2，500 kg/m³ were used to form the granular columns. The initial aspect ratio, defined as the height-to-width ratio of the columns, varied between 1 and 3. Moisture content was controlled as the mass ratio of water to particles. The centroid vector displacement method was applied to analyze the role of liquid bridges in collapse dynamics across different saturation levels. A high-speed camera captured the collapse process at 3-millisecond intervals to ensure precise measurements.

Results and Discussion The collapse mode of granular columns with a particle diameter of 1 mm and an initial aspect ratio of 1 exhibited a non-monotonic transition under varying water content: initiating from continuous collapse in the dry state, progressing to blocked collapse at a water content of 2%, achieving static stability at 6%, reverting to blocked collapse at 8%, and ultimately returning to continuous collapse under over-saturated conditions with water content approaching 30%. This indicated that moisture content, along with aspect ratio and particle diameter, played a key role in determining collapse mode. The transition between collapse modes was driven by changes in liquid bridge forces. At low moisture content, liquid bridges enhanced particle cohesion, leading to blocked collapse. As moisture content increased, cohesion peaked at 6%, resulting in stability. Further increases in moisture content reduced cohesion, leading to blocked or continuous collapse. Calculation results from the centroid vector displacement method showed that moisture suppressed the vertical displacement of centroid and reduced the maximum kinetic energy during collapse compared to dry granular columns. Gravitational potential energy loss and kinetic energy changes were analyzed. For a granular column with a particle size of 1 mm and an initial aspect ratio of 1, when the water content is 2%, the gravitational potential energy loss rate is 27. 2%; when the water content is 6%, the granular column remains stationary; and when the water content increases to 8%, the column exhibits "slight and slow" block collapse， with a gravitational potential energy loss rate of less than 15%. By comparing the gravitational potential energy loss rates at different water contents, the intensity of block collapse can be quantitatively assessed. For 2 mm particles, the energy loss rate decreased with increasing moisture content. This study found that the influence of interstitial liquids on granular column collapse followed an increasing-stable-slightly decreasing trend as moisture content increased, with the maximum influence occurring at 6% moisture content. The ratio (S_r ) of the centroid displacement magnitude of wet columns to dry columns was defined, with larger Sr values indicating smaller reductions in centroid displacement due to moisture. Moisture content had a more significant impact on columns with smaller aspect ratios and particle diameters.

Conclusion Moisture content is a critical factor influencing granular column collapse. For sample S1, collapse modes transitioned sequentially with increasing moisture: continuous→blocked→stable→blocked→continuous collapse, proving that moisture determines collapse patterns. Analyzing collapse modes and gravitational potential energy loss revealed that 6% moisture maximizes liquid-induced cohesion. This identifies an optimal moisture range for stability, offering key insights into wet granular mechanics. A novel centroid displacement vector ratio was proposed to quantify energy conversion and liquid effects. Moisture not only alters the energy dissipation rates but also significantly affects the displacement ratio between wet and dry granular columns, with its influence inversely correlated to both the initial aspect ratio and particle diameter.

Keywords：wet particle; scaling law; centroid displacement; gravitational potential energy