Study on the Stability Evolution Mechanism of a Red Mud Dam During Construction and Safety Under Earthquake During Operation

Abstract

Instability in red mud dam bodies is not uncommon. In order to study the stability evolution mechanism during the process of red mud landfill and the deformation characteristics under earthquake action when the landfill site is closed, the deformation law and potential sliding surface motion characteristics of the landfill site were explored based on the finite difference method, revealing the influence of peak ground acceleration (PGA) on red mud deformation. The results showed that: (1) As the height of the red mud landfill increases, the shear force of the red mud landfill gradually increases. Meanwhile, the maximum shear force always occurs near the initial dam, indicating that under the action of gravity, the possibility of shear slip occurring near the initial dam is the highest. (2) The distribution pattern of the plastic zone in the red mud pile during the filling process is relatively complex, and continuous monitoring of the filling process should be carried out to ensure the safety of the filling project. (3) With the increase in earthquake acceleration, the shear force of red mud piles gradually increases. Meanwhile, as the acceleration increases, the maximum shear stress always occurs at the bottom of the initial dam body. Under the action of power, special attention should be paid to the stability of the pile near the initial dam.

1. Introduction

With the acceleration of industrialization, the safe storage and disposal of industrial solid waste has become an urgent problem to be solved. According to statistics, for every ton of alumina produced, approximately 1.5 tons of red mud are produced. At present, the cumulative stock of red mud worldwide has reached 4 billion tons, and it continues to grow at a rate of 175.5 million tons per year. In China, the discharge of red mud has exceeded 70 million tons, with a cumulative stock of up to 500 million tons. In 2022, Guangxi Zhuang Autonomous Region issued the “Regulations on the Prevention and Control of Solid Waste Pollution”, which clearly require alumina production enterprises to strengthen scientific and technological innovation and application and improve the comprehensive utilization rate of red mud and tailings. Due to the coupling effect of its strong alkalinity and multivalent cation properties, the comprehensive utilization of red mud in Guangxi region is still limited, and it is still disposed of by stacking. Red mud is a strongly alkaline industrial waste discharged during the process of extracting alumina from bauxite. It is called red mud because it is rich in iron oxide and has a reddish-brown color. Its main properties are extremely fine particles, high water holding capacity, and strong alkalinity [1,2,3,4]. Due to the low utilization rate of red mud and the lack of good treatment methods, most red mud is stored in the form of dams in the open air [5,6].

References [7,8,9] analyzed the physical properties, chemical composition, microstructure, leaching toxicity, and other technical indicators of red mud raw materials. References [10,11,12] investigated the applicability of red mud as a building material, reducing storage capacity and effectively protecting the surrounding environment. Reference [13] proposed a stability calculation method for dam bodies based on the generalized probability density evolution method and an efficient and high-precision seismic stability evaluation method. Reference [14] derived the stepwise seepage differential equation of the saturation line of a dry tailings dam using Darcy’s law and energy equation. Combining downstream boundary conditions and rainfall boundary conditions, an analytical solution for the saturation line of a dry tailings dam under multi-year rainfall conditions was derived. Reference [15] evaluated the mechanical responses of three types of tailings in a tin mine tailings storage facility. The experimental results were explained using the critical state soil mechanics framework and different instability standards. Reference [16] studied the stress and deformation mechanisms of soil dam slopes under dynamic conditions. Reference [17] took the interface between sandstone and mudstone on the slope of Yunnan sandbar as the research object, and the mechanism of shear strength weakening of discontinuous surfaces of different wall materials under cyclic loading was analyzed. Reference [18] introduced a finite element limit equilibrium framework suitable for the stability analysis of high rockfill dams and proposed an optimized angle three-dimensional failure mechanism. Reference [19] analyzed the stability and consolidation characteristics of saturated and unsaturated zone landfills, and explored the impact mechanism of unsaturated bodies on dam stability. Reference [20] proposed a dynamic reliability analysis method for the three-dimensional slope stability of surrounding rock dams based on a generalized probability density evolution method, revealing the influence of different node distribution models on the dynamic reliability of the slope stability of rockfill dams. References [21,22,23] constructed a calculation model for the permanent displacement of slopes and analyzed the stress characteristics and deformation mechanism of slopes under dynamic conditions. References [24,25] tested the mechanical properties of red mud piles under rainfall conditions, and based on this, explored the slip mechanism and deformation characteristics of red mud piles under rainfall conditions. Reference [26] proposed a real-time ASSAD analysis method based on the Stacking BSRG-PLS model, which integrates bidirectional long short-term memory networks, residual neural networks, support vector machines, Gaussian process regression models, and partial model least squares regression. References [27,28] studied the theoretical solution of the infiltration line of tailings dams under complex conditions and the stability evolution mechanism of tailings dams. The research results can serve as the basis for the drainage design and slope stability analysis of dry tailings dams, which is crucial for designing safer and more efficient mining and construction waste management practices.

There are certain safety and stability issues in the landfill process of red mud, and seismic effects exist under closed storage conditions [29]. Based on the finite difference method, this paper studies the stability evolution mechanism of red mud landfill process and red mud landfill under different peak ground accelerations, and reveals the deformation characteristics during red mud landfill. This study can provide guidance for the safety analysis of the red mud storage construction process and the stability evolution mechanism under earthquake action during operation.

2. Engineering Background

The site of the engineering red mud storage yard is located in Ma’ai Township, Debao County. On the north side of the red mud storage yard and Dayu Village, there is a high mountain as a screen, and 500 m downstream of the west side pass of the storage yard is the alumina plant area. The site belongs to a large karst valley in terms of topography, with a nearly east–west distribution. The site is a peak cluster valley landform, with undeveloped vegetation and exposed limestone peaks. The site is narrow and flat, surrounded by mountains on all sides, with a length of about 1200 m and an average width of about 300 m. The elevation of the site is 685–800 m. The storage yard is composed of a large karst valley, with the geomorphic unit belonging to the peak cluster dissolution valley, and the sloping valley extending in the SE-NW direction, belonging to a dissolution erosion landform. The site is generally high in the northeast and low in the southwest, with a valley floor elevation ranging from 682.57 to 734.39 m. Most of the valley is covered by Quaternary soil layers, with occasional outcrops of bedrock. The thickness of the covering layer varies greatly, generally ranging from 0.30 to 10.00 m, and locally up to 20.00 m thick, as shown in Figure 1. The site is surrounded by peak clusters, with exposed bedrock and generally developed vegetation. The slope is steep, especially on the north side of the mountain, where cliffs are often seen. The top elevation of the peak clusters ranges from 729.23 to 898.35 m, as shown in Figure 2. Earthquakes in Guangxi are mainly distributed on the fault zones in northwest and southeast Guangxi. These fault zones include Fangcheng Lingshan fault zone, Hepu Beiliu fault zone, Bama Bobai fault zone, etc. The activity of these fault zones has a significant impact on seismic activity in the Guangxi region.

3. Numerical Model of the Red Mud Landfill

3.1. Numerical Model and Boundary Conditions

The finite difference method transforms continuous problems into discrete problems by discretizing a continuous region into a finite number of grid nodes and using differential approximation formulas to calculate the function values on the nodes. It has good numerical stability and convergence. By selecting appropriate difference schemes and grid arrangements, stable numerical solutions can be obtained. It can handle various irregular boundary conditions and conveniently handle multi-physics field coupling problems. By coupling the equations of multiple physics fields, more realistic physical phenomena can be simulated. We built a three-dimensional numerical calculation model based on a red mud landfill project in Guangxi. The model includes bedrock, clay, dam body, and red mud. The initial dam and auxiliary dam are earth–rock mixed dams. The initial dam has a bottom width of 145 m, a top width of 10 m, an outer slope ratio of 1:2.5, and an inner slope ratio of 1:2. The width of the auxiliary dam bottom is 35 m, the width of the dam crest is 5 m, and the inner and outer slope ratio is 1:3. There are 15,423 units in the computational model.

3.2. Material Strength Parameter

An elasto-plastic model with the Mohr–Coulomb criterion was used for numerical calculations and each soil unit was considered as an isotropic material. The values of various material parameters in the numerical model were taken from local survey data and experience, and the strength parameters of the materials are shown in

Table 1. Material strength parameters of red mud landfill.

	Bulk Modulus (Mpa)	Shear Modulus (Mpa)	c (kPa)	φ (°)	γ (kN/m3)
Bedrock	2 × 104	1 × 104	400	51	26.5
Clay	30	9.5	35	17	18.5
Initial Dam	75	34.6	25	18.5	18
Sub-Dam	75	34.6	25	18.5	18
Soft Plastic Red Mud	20	8	5.6	16.4	20
Plastic Red Mud	20	8	10	18.5	18.5

.

3.3. Seismic Wave Parameter

To simplify the analysis, a typical EI-Centro seismic transverse wave was selected to simulate the ground motion, with a duration of 10 s (modeling time step of 0.02 s, with a total of 500 interval steps) and a peak ground acceleration (PGA) of 3.12 m/s². The seismic wave acceleration time history is shown in Figure 3. In order to investigate the seismic response mechanism of the red mud landfill slopes under different seismic intensities, the EI-Centro seismic waves were appropriately scaled to obtain seismic waves with PGAs of 0.2 g, 0.4 g, and 0.5 g, respectively.

4. Stability Evolution Mechanism of Red Mud Landfill During the Filling Period

4.1. Shear Force Analysis of the Landfill Site During the Red Mud Landfill Process

The whole red mud landfill is divided into six landfills, within which the red mud at the initial dam is divided into two landfills, and the red mud at each sub-dam is in one landfill. The evolution law of shear force during the process of red mud landfilling is shown in Figure 4.

Figure 4 shows that as the height of the landfill increases, the shear force of the red mud landfill gradually increases. The maximum shear force increased from 0.088 Mpa in the initial Dam Layer 1 to 0.1 Mpa in the initial Dam Layer 2. After the construction of the first stage sub-dam, the maximum shear force was 0.108 Mpa. After the construction of the second level sub-dam, the maximum shear force was 0.124 MPa. After the construction of the third level sub-dam, the maximum shear force was 0.13 MPa. After the construction of the last level sub-dam, the maximum shear force was 0.137 MPa. The maximum value gradually increased from the initial 0.088 Mpa to 0.137 Mpa, with a growth rate of 56%. At the same time, the maximum shear force always appeared near the initial dam, indicating that under the action of gravity, the possibility of shear slip occurring near the initial dam is the highest. During the filling process of the landfill, it is necessary to constantly monitor the stress characteristics of the initial dam.

4.2. Analysis of the Plastic Zone in the Landfill Site During the Red Mud Landfill Process

The plastic zone is the area formed by the plastic deformation that occurs in certain regions of a structure under load. This deformation is permanent and does not disappear with the removal of the load. Plastic strain assessment is mainly used to evaluate the safety of structures. To further analyze the safety of the structure during the filling process of the red mud landfill and to study the evolution law of the plastic zone during the filling process, a curve is drawn as shown in Figure 5.

Figure 5 shows that during the red mud landfill process, the area of the plastic zone did not gradually increase. The plastic zone only appeared after the completion of the initial dam and the first stage auxiliary dam construction. And only plastic zones appeared near the dam body. As the height of the dam increased, the increase in storage capacity slowed down and the dam became more stable. The distribution pattern of the plastic zone in the filling process of a red mud dam is relatively complex, and the filling process should be continuously monitored to ensure the safety of the filling project.

4.3. Evolution Law of the Safety Factor During the Filling Process of Red Mud Landfill

The safety factor can directly reflect the stability of the dam body during the red mud filling process. To further study the safety of the dam body during the red mud filling process, calculate the safety factors of the dam body at different stages and draw curves, as shown in Figure 6.

Figure 6 shows that as the height of the red mud landfill increases, the safety factor of the dam body shows a non-linear decreasing trend in a stepped manner. Among them, the safety factor is relatively high when the initial dam and the first stage auxiliary dam are constructed separately. With the increase in auxiliary dams, the safety factor decreases. During construction, special attention should be paid to the impact of auxiliary dam construction on the stability of the dam body.

5. Stability Analysis of the Dam Body Under Dynamic Conditions

In FLAC3D6.0, for different dynamic boundary conditions, different methods are needed to apply the dynamic loads. The free-field boundary can directly apply the dynamic loads through the acceleration or velocity time history of the seismic wave, while the dynamic loads of the static boundary need to be applied through the stress time history of the seismic wave. EI-Centro seismic transverse waves with different peak ground accelerations of 10 s duration were applied to the bottom of the red mud landfill in the closed condition, that is, at the end of the Fourth Sub-Dam filling, to reach the stable state. Figure 7 shows the shear response of a red mud landfill site under seismic waves with different peak ground accelerations.

Figure 7 shows that as the seismic acceleration increases, the shear force of the red mud pile gradually increases. When the acceleration is 0.2 g, the maximum shear stress is 0.141 MPa. When the acceleration is 0.3 g, the maximum shear stress is 0.148 MPa. When the acceleration is 0.4 g, the maximum shear stress is 0.156 MPa. When the acceleration is 0.5 g, the maximum shear stress is 0.174 MPa. As the acceleration increases, the increase in shear force is 4.7%, 10.6%, and 23.4%, respectively. Meanwhile, as the acceleration increases, the maximum shear stress always appears at the bottom of the initial dam.

6. Conclusions

In order to investigate the stability of the red mud landfill during the filling process and under seismic action after closure, in this study, we explored the evolution mechanism of the stability of the red mud landfill in the process of filling and the deformation characteristics of red mud landfill under seismic action when the landfill is closed. The conclusions obtained herein are as follows:

(1) As the height of the red mud landfill increases, the shear force of the red mud landfill gradually increases. Meanwhile, the maximum shear force always occurs near the initial dam, indicating that under the action of gravity, the possibility of shear slip occurring near the initial dam is the highest.

(2) The distribution pattern of the plastic zone in the red mud pile during the filling process is relatively complex, and continuous monitoring of the filling process should be carried out to ensure the safety of the filling project.

(3) With the increase in earthquake acceleration, the shear force of red mud piles gradually increases. Meanwhile, as the acceleration increases, the maximum shear stress always occurs at the bottom of the initial dam body. Considering the complexity of seismic forces, model experiments or references to existing engineering should be added to the design of dam bodies in earthquake active areas.