X-Ray CT Investigation on Fractal Characteristics of Fine-Grained Tailing Sand in Fujian’s Makeng: Insight into the Mesoscopic Seepage Failure

This work is aimed at revealing the mesophysical process and mechanical behaviors of fine-grained tailing sand during seepage failure. The macroscopic seepage tests combined with posttest X-ray computed tomography (CT) were employed to study the fractal characteristics of mesostructure. Results show that before and after the seepage failure of fine-grained tailing sand, fractal of pore distribution ( D pd ) and fractal dimension of pore size ( D ps ) show a relatively obvious positive correlation with porosity. Tailing particles migrate along the seepage direction during the seepage process, resulting in the gradual decrease of D pd and pore distribution area. The D ps reflects the variation characteristics of pore number distribution with different pore sizes. The increase in D ps leads to a decrease in the uniformity of pore size and an increase in the size difference between pores. The mass fractal dimension ( D m ) of fine-grained tailing sand samples ranges from 1.6472 to 1.8256. With the increase of D m , the coefficient of uniformity ( C u ) of tailing sand tends to increase. The D m method can discern the seepage failure type of fine-grained tailing sand, and it is more accurate than the traditional method. This study provides a reference for the prevention and control of the seepage failure of tailing dam.

Seepage behavior assessment of earth-rock dams based on Bayesian network

Seepage behavior assessment is an important part of the safety operation assessment of earth-rock dams, because of insufficient intelligent analysis of monitoring information, abnormal phenomena or measured values are often ignored or improperly processed. To improve the intelligent performance of the monitoring system, this article has established an assessment framework covering project quality, maintenance status, monitoring data analysis, and on-site inspection based on the relevant norms of seepage safety assessment of earth-rock dams and the expert survey scoring method, and the Leaky Noisy-OR Gate extended model were used to determine the probability of events, and the dynamic and static Bayesian networks used to assess the possibility of seepage failure of earth-rock dams and diagnose the most likely cause of failure. The function of static and dynamic Bayesian networks to assess the seepage behavior of earth-rock dams, abnormal measured values, and causes of anomalies can make up for the limitations of reservoir management personnel and monitoring system in seepage failure experience and seepage knowledge of earth-rock dams and enable better handling of abnormal phenomena and monitoring information, making the monitoring system more intelligent.

Research Status and Prospect of Seepage Erosion in Foundation Pit Engineering

In water-rich strata, the distribution of groundwater is complicated. Construction projects related to foundation pit engineering in such areas are prone to cause piping failure, fluid soil failure, and other seepage-related failures. Seepage erosion is a kind of seepage failure that occurs inside the soil. Because the process is difficult to observe and measure, seepage erosion is rarely considered in the current foundation pit engineering. This article analyzes the relevant research on seepage erosion, and further clarifies the influence of seepage erosion on soil characteristics. In addition, this article puts forward some suggestions for the shortcomings of the current research on erosion, and made some prospects for future research on erosion in foundation pit engineering, hoping to provide theoretical guidance and thinking inspiration for future research.

ANTI-EROSION CAPACITY OF A GRANULAR FILTER SUBJECTED TO A PERIODICALLY VARIABLE HYDRAULIC GRADIENT

The instability of a filter system is a significant cause of seepage failure in embankment projects. The filter system in the earth-rock embankment is mainly composed of graded cohesionless soil. To uncover the performance of the granular filter in resisting the internal erosion, a set of experiments was carried out with an improved experimental apparatus, considering different hydraulic loading scenarios. The movement of graded cohesionless soil, the seepage velocity and the hydraulic gradient were monitored in the experiments. It was found that during the process of increasing the hydraulic gradient, the failure of the granular filter mainly experienced three stages: the first one was the dynamic equilibrium stage; the second was the critical start stage; and the third was the failure stage, in which a sudden change in the seepage velocity was the precursor of seepage failure. The critical hydraulic gradient and destructive hydraulic gradient decreased with the water level amplitude. Moreover, the experiments revealed that the loading modes of the hydraulic gradient significantly influenced the anti-erosion capacity of the granular filter. Compared with the stepwise loading mode, the cyclic reciprocating loading mode greatly weakened the anti-erosion capacity of the granular filter under the same water level amplitude. The destructive hydraulic gradient of the latter was only 71.8 % of the former under a higher water level amplitude, indicating that the corresponding measures should be considered to avoid the occurrence of a periodically variable hydraulic gradient.

Study on Critical Hydraulic Gradient Theory of Flow Soil Failure in Cohesive Soil Foundation

The current formula of critical hydraulic gradient is not adapted to solve critical hydraulic gradient of cohesive soil. Assume that the seepage failure mode of the cohesive soil foundation was cylindrical or inverted circular truncated cone, based on the calculation formula of the critical hydraulic gradient of Terzaghi, the analytical formula of the critical hydraulic gradient considering the influence of the shear strength of the soil was derived. Then, the seepage failure process of the clay layer was simulated numerically, and the effects of the clay layer thickness, failure radius, and shear strength indexes on the critical hydraulic slope were analyzed. The comparison results show that the numerical test results are in good agreement with the calculated results of the new formula. In addition, the critical hydraulic gradient of sandy loam and loess under different working conditions was studied severally by a self-made permeation failure instrument. The results show that the critical hydraulic gradient decreases with the increase of soil thickness and failure radius, and the maximum error between the test and the corresponding formula results is no more than 16%.