Q.C. Sun

Experimental study on cascading landslide dam failures by upstream flows

Landslide dams in mountainous areas are quite common. Typically, intense rainfalls can induce upstream flows along the sloping channel, which greatly affects the stability and failure modes of landslide dams. If a series of landslide dams are sequentially collapsed by an incoming mountain torrent (induced by intense rainfall), large debris flows can be formed in a short period of time. This also amplifies the magnitude of the debris flows along the flow direction. The catastrophic debris flows, which occurred in Zhouqu, China on August 8, 2010, were indeed caused by intense rainfall and the upstream cascading failure of landslide dams along the gullies. Experimental tests were conducted in a sloping channel to understand the dynamic process of cascading landslide dam failures and their effect on flow scale amplification. Similar to the Zhouqu conditions, the modeled landslide dams were distributed along a sloping channel and breached by different upstream flows. For each experiment, the front flows were sampled, the entrained grain sizes were analyzed, and the front discharge along the channel was measured. The results of these experiments show that landslide dams occurring along the channel can be destroyed by both high and low discharge flows, although the mechanisms are quite different for the two flow types. Regardless of flow type, the magnitude of the flows significantly increases after a cascading failure of landslide dams, resulting in an increase in both the diameter and the entrained coarse particles percentage.

Studies on Structural and Mechanical Properties under Isostatic Compression with Large-Scale Discrete Element Simulations

Granular systems undergo a jamming transition at point J simply by increasing the packing fraction. A large-scale parallel discrete element code (THDEM: TsingHua Discrete Element Method) was used to obtain a satisfying statistical description of the structural and mechanical properties near point J. The isostatic compressions of 100,000 polydispersed frictionless particles were simulated on high performance computers to clearly observe the sophisticated configurations of force chains. The first peak of the pair correlation function, coordination number, spatial distribution of the packing fraction, and stress were calculated to analyze their variations with increasing packing fraction. The critical packing fraction at point J is determined to be 0.62. The incremental stress and coordination number from point J scale well with the power law, and coincide with previous theoretical predications. The distribution of the packing fraction is a normal distribution around the average value. The standard deviation decreases with increasing packing fraction, indicating the system is more uniform with a denser packing.

Granular materials： Bridging damaged solids and turbulent fluids

Granular materials exhibit abundant dissipations due to fluctuations in both granular motions and con gurations (i.e., granular skeleton) evolutions. Twin granular temperatures T k and T e are introduced accounting for two types of fluctuations, and the so-called twin granular temperatures theory is established as an extension of granular solid hydrodynamics. By using simulations, the nonaffine deformations in a 2D assembly are simulated by using discrete element methods. By analogizing with microdamages in deformed solids, double scalar damage variables, D P and D q , are proposed to describe the deformed granular solid under triaxial compressions. Granular flows are found intrinsically turbulent due to the presence of T k and the Naiver-Stokes equation is obtained for granular flows.

Surge impact behavior of granular flows: effects of water content

Understanding the fundamental dynamics of interaction between multi-phase geophysical flows and engineering structures is crucial for mitigating geophysical hazards. Specifically, liquid phase between particles induces matric suction which could play a significant part in regulating flow dynamics and warrants further consideration. In this study, flume model tests were conducted to investigate the effects of water content (0–30%) on the impact behavior of granular flows. The particle image velocimetry technique was adopted to visualize the impact kinematics and the impact force was measured through a model barrier system. Results revealed that, besides geometric effects (kinetic sieving), mechanical effects (shearing and collision) are also vital for the mechanism of reverse segregation. At higher water contents, 20 and 30% in this study, discrete-surge impact, rather than a progressive impact process, was observed. The discrete surges induce impulses on the barrier. The discrete surges result from self-organization of unsaturated granular flows to overcome the enhanced shear strength induced by matric suction. Finally, a dimensionless index, namely the suction number, is used to quantify the effect of suction on the dynamic behavior of granular flows. Even for large-scale geophysical flows, if the content of fine particles is high, effect of suction should not be neglected.

Study of solids contact shearing and collisions in granular debris flows

Granular debris flows in nature are generally composed of a wide range of solid particles and viscous pore fluids, moving at high traveling velocities down sloping channels. Multiple parameters govern a granular debris flows rheological properties and affect the interactions between the solid and fluid phases of these flows. Study of the solids contact behavior (i.e., contact shearing and collisions) is essential for understanding the high flow mobility of catastrophic geophysical granular flows/avalanches. This study critically reviews two dimensionless numbers with clear physical meanings (e.g., the Savage number and the friction number), then demonstrates a new application of field monitoring data of natural debris flows to obtain specific values of these numbers for classifying flowing regimes of granular debris flows on large scales. These numbers are then used to analyze and illustrate the contact behavior of solid particles in different flow regimes. This study shows that the flow regimes of granular debris flows are governed by the relative dominance of contact shearing and collisions between solid particles, as well as being significantly affected by the flows pore fluid viscosity.