Yu-long Luo

Hydro-mechanical experiments on suffusion under long-term large hydraulic heads

The long-term large hydraulic head in flood season is the main cause of suffusion failure or dam break, while the suffusion failure mechanism in this case has not been specially studied. First, a newly designed hydro-mechanical coupling suffusion apparatus was introduced. Second, two short-term suffusion experiments under different confining pressures were carried out to determine the suitable large hydraulic heads. Finally, four experiments on suffusion under long-term large hydraulic heads and different confining pressures were performed to investigate the influences of the long-term large hydraulic heads and the confining pressures on the evolution of suffusion. The results indicate that: (1) The signs of suffusion failure in the short-term experiments are the sudden decrease in hydraulic gradient and the sharp increase in eroded mass. While the flow rate in the long-term experiments appearing suffusion failures increases rapidly with the decrease in hydraulic gradient. (2) The suffusion failure in the long-term large hydraulic head experiment is more likely to happen and much more serious than that in the short-term experiment. The long-term large hydraulic head can reduce the suffusion failure hydraulic gradient significantly and increase the eroded mass dramatically. The results gained here provide a deeper understanding on the suffusion failure caused by the rapid increase in the high water level upstream the hydraulic earth structures in flood season.

A New Apparatus for Evaluation of Contact Erosion at the Soil–Structure Interface

The interface between clay core-wall and concrete cut-off wall is one of the weakest parts of high earth and rockfill dams. An unsuitable design can cause contact erosion at the interface or shear failure surfaces, and eventually this will harm the safety of the dam. A new soil–structure contact erosion apparatus was developed for the evaluation of contact erosion at the interface under high stress, high hydraulic head, and large shear deformation. It consists of a soil–structure model base, a seepage pressure system, a confining pressure system, an axial pressure system, and a data acquisition system. The seepage pressure system simulates the seepage erosion effect at the interface, and the maximum seepage pressure is 2.0 MPa. The confining and axial pressure systems simulate the triaxial stress state of the interface and the large shear deformation of the soil structure; the maximum confining and axial pressures are 2.0 MPa and 4.0 MPa, respectively. The data acquisition system can monitor pore pressure dissipation and settlement. Two repeatable experiments on the interface between a kind of highly plastic clay and a concrete cut-off wall were conducted to verify the scientific utility of the new apparatus. The new apparatus was found to be capable of yielding fairly consistent results in repeatable experiments. The new apparatus will prove an effective tool for studying a reasonable connection form of concrete cut-off wall and core wall in high earth and rockfill dams.

Hydro-mechanical coupling experiments on suffusion in sandy gravel foundations containing a partially penetrating cut-off wall

The influence of the stress state on the evolution of suffusion failure is often neglected in the design of sandy gravel foundations containing a partially penetrating cut-off wall. A series of hydro-mechanical coupling experiments on such structures was carried out to investigate the influence using a newly designed apparatus. The results indicate that: (1) The stress state has a significant influence on the evolution of suffusion, which increases the critical suffusion hydraulic gradient dramatically. (2) The critical suffusion hydraulic gradient is linearly related to the confining pressure and to the penetration ratio, and a simple interpolation function for the critical hydraulic gradient was fitted based on the experimental results. (3) The eroded mass in the development of suffusion is nonuniform and intermittent. The evolution of suffusion is a complicated and iterative process involving fine particle migration, pores being clogged, flushing out of the clogged pores, and fine particle remigration. (4) Flow along the interface of sandy gravel and cut-off wall conforms to the piecewise-linear Darcy flow rule. The results will enhance the understanding of the influence of the stress state on the suffusion failure in sandy gravel foundations containing partially penetrating cut-off walls.

Experiments on internal erosion in sandy gravel foundations containing a suspended cutoff wall under complex stress states

Abstract Internal erosion is one of the most common failure modes of embankment dams or foundations, and the simplest and most effective preventive measure is to build a cutoff wall. The soil at the bottom of the cutoff wall is usually under complex stress states. The deeper the cutoff wall, the higher is the stress. In this study, the effects of stress conditions on the evolution of internal erosion were investigated in sandy gravel foundations containing a suspended cutoff wall using a newly developed stress-controlled erosion apparatus. Three series of erosion tests were conducted on gap-graded soil under different confining stresses, different deviatoric stresses, and different confining and deviatoric stresses. The results of these tests are as follows: (1) The discharge and permeability decrease with an increase in the confining stress, but the critical hydraulic gradient increases. (2) In the second series of erosion tests, the specimen is compressed under low deviatoric stress; the specimen undergoes shear expansion under high deviatoric stress. (3) In the third series of erosion tests, the confining and deviatoric stresses synchronously change, and therefore, their combined effect on the evolution of internal erosion is complicated. Under low stress, the soil is compressed in the early stage of the experiment, and its structure may change during internal erosion. When the stress level is high, the specimen also undergoes shear expansion, and the degree of expansion is controlled by both confining and deviatoric stresses.

Investigation of fluid flow-induced particle migration in granular filters using a DEM-CFD method

In embankments and earth dams, the granular filter used to protect the base soil from being eroded by the fluid flow is a major safety device. In this paper, the migration mechanism of the base soil through this type of filters with a fluid flow in the base soil-filter system is studied by using the coupled distinct element method and computational fluid dynamics (DEM-CFD) model. The time-dependent variations of the system parameters such as the total eroded base soil mass, the distribution of the eroded particles within the filter, the porosity, the pore water pressure, and the flow discharge are obtained and analyzed. The conceptions of the trapped particle and the trapped ratio are proposed in order to evaluate the trapped condition of the base soil particles in the filter. The variation of the trapped ratio with time is also analyzed. The results show that the time evolutions of the parameters mentioned above are directly related to the gradation of the filter, which is defined as the representative particle size ratio of the base soil to the filter using an empirical filter design criterion. The feasibility of the model is validated by comparing the numerical results with some experimental and numerical results.

Experimental Investigation of the Erosion Mechanisms of Piping

Piping, or backward erosion, resulting in tunneling under an embankment or foundation, is an important phenomenon that can lead to the failure of the embankment or foundation. We have experimentally investigated the erosion mechanism of this piping phenomenon using a laboratory-scale model. We discovered two types of coupled erosion in the evolution of piping through an analysis of the grain size distribution of the eroded mass. Overall, grain sizes in the eroded mass ranged from 0.1 to 0.5 mm. The 0.1-0.25 mm grains were found to dominate when the erosion was induced by stress release, whereas in the case of clogging, the dominant grain sizes were 0.25-0.5 mm. The first form of erosion caused the tunnel to advance upstream, whereas the second increased the depth of the tunnel; otherwise, the two types of erosion were coupled. Furthermore, the eroded mass throughout the evolution of piping was non-uniform and intermittent.

Stochastic simulation of fluid flow in porous media by the complex variable expression method

A stochastic simulation of fluid flow in porous media using a complex variable expression method (SFCM) is presented in this paper. Hydraulic conductivity is considered as a random variable and is then expressed in complex variable form, the real part of which is a deterministic value and the imaginary part is a variable value. The stochastic seepage flow is simulated with the SFCM and is compared with the results calculated with the Monte Carlo stochastic finite element method. In using the Monte Carlo method to simulate the stochastic seepage flow field, the hydraulic conductivity is assumed in three different probability distributions using random sampling method. The obtained seepage flow field is examined through skewness analysis, and the skewed distribution probability density function is given. The head mode value and the head comprehensive standard deviation are used to represent the statistics of calculation results obtained by the Monte Carlo method. The stochastic seepage flow field simulated by the SFCM is confirmed to be similar to that given by the Monte Carlo method from numerical aspects. The range of coefficient of variation of hydraulic conductivity in SFCM is larger than used previously in stochastic seepage flow field simulations, and the computation time is short. The results proved that the SFCM is a convenient calculating method for solving the complex problems.