A physical model study was conducted to examine the development of submerged sand erosion through a slot on a defective sewer pipe. Various parameters that affect the erosion process were studied, including particle size, defect size, position on the pipe, and water depth. This experimental study used particle image velocimetry analysis to provide a deeper understanding of the mechanism of sand erosion due to defective sewer pipes. The sand velocities were determined, and the sand and water flow rates were measured. After the erosion process was initiated, the sand and water outflow rates quickly reached the steady state until the eroded zone reached the slot opening. During the steady erosion process, the region of sand motion was restricted to a narrow zone above the opening, with sand particles sliding into the newly formed cavity at the angle of repose. An increase in slot width significantly increased the sand and water flow rates. A decrease in sand particle size from medium to fine resulted in a decreased sand flow rate mainly due to decreased water flow. An increase in sand particle size from medium to coarse resulted in a decreased sand flow rate due to the arching resistance close to the slot for larger sand particles. In this study, the sand flow ceased when the ratio between the slot width and particle size was less than 3.1. The ratio between the sand and water flow rates was found to be a function of the ratio of slot width to particle size. The water level above the pipe and the slot position affected the sand and water flow rates. A simple analytical model was developed to estimate the sand and water flow rates in the erosion process.

This paper presents a critical state constitutive model for cemented sand. The model uses a single capped yield surface as a function of the void ratio, confining pressure, preconsolidation pressure, and stress ratio at the peak of the undrained effective stress path. To model the cemented materials, the formulation of the yield function, elastic moduli, plastic modulus, flow rule, and other components of the model have been modified. Having incorporated the tensile strength and cohesion, the radial-mapping formulation of the bounding surface plasticity is incorporated in the model. The modified model has been calibrated and verified based on experimental results.

Debris flows in a burned area, post-fire debris flows, are considered as one of the most dangerous geo-hazards due to their high velocity, long run-out distance, and huge destruction to infrastructures. The rainfall threshold to trigger such hazards is often reduced compared with normal debris flow because ashes generated by mountain fires reduce the permeability of the top soil layer, thus increasing surface runoff. At the same time, burnt material and residual debris have very poor geo-mechanical characteristics, e.g., their internal friction angle and cohesion are typically low, and thus an intense rainfall can easily trigger some debris flows. Studying post-fire debris flow enables us to get a deeper understanding of disaster management. In this paper, the debris flow that occurred in Montecito, California, USA, and was affected by the Thomas Fire was used as a case study. Five major watersheds were extracted based on the digital elevation model (DEM). Remote sensing images were used to analyze the wildfire process, the extent of the burned areas, and the burn severity. The hypsometric integral (HI) and short-duration rainfall records of the watersheds around Montecito when the post-fire debris flows occurred were analyzed. Steep terrain, loose and abundant deposits, and sufficient water supply are the important conditions affecting the formation of debris flows. Taking watersheds as the research objects, HI was used to describe the geomorphic and topographic features, open-access rainfall data was used to represent the water supply, and burn severity represented the abundance of material sources. An occurrence probability model of post-fire debris flow based on HI, short-duration heavy rainfall, and burn severity was developed by using a logistic regression model in post-fire areas. By using this model, the occurrence probability of the post-fire debris flow in different watersheds around Montecito was analyzed based on the precipitation with time. Especially, the change characteristics of occurrence probability of debris flows over time based on the model bring a new perspective to observe the obvious change of the danger of post-fire debris flows and it is very useful for early warning of post-fire debris flows.

This paper presents a numerical scheme for a solid–fluid coupled discrete-element method (DEM) that takes into consideration solid deformation and pore pressure generation and dissipation. Analytical solutions of conventional soil mechanics examples obtained by oedometer testing are used to quantitatively validate the proposed algorithm. The numerical results show good agreement with the analytical solutions. The proposed method advances the current capability of solid–fluid coupled DEM analysis to simulate the mechanical behavior of saturated granular materials.

An analytical solution for groundwater flow through a circular tunnel in anisotropic soils is derived based on conformal mapping and can be used to calculate the water flow rate into the tunnel and predict the pore water pressure distribution in surrounding soils. If the vertical and horizontal permeabilities are equal, this analytical solution is reduced to the solution for the isotropic flow condition. This proposed analytical solution is verified by numerical simulations. The water flow rate into the tunnel decreases with a decreasing value of soil permeability in the horizontal direction, and the pore water pressure distribution is distorted due to the anisotropy of soil. This proposed analytical solution provides an efficient approach for estimating the groundwater inflow into a circular tunnel accounting for anisotropic soils.