Seepage and Deformation Analysis of Tailing Dam regarding the Upper Reservoir of Tiantai Pumped Storage Station

This paper has studied on the seepage and deformation analysis of a tailing dam in the upper reservoir of the Tiantai Pumped Storage Station. The analysis of seepage and deformation is crucial for evaluating the stability and safety of the dam and its impact on the surrounding environment. The study uses numerical simulation software (Phase2 and FLAC3D) to analyse the behaviour of the dam under different drainage conditions and construction phase. The results indicate that two drainage schemes in different filling stages are effective. In the same cross-section of the rock and soil basement, the flow rate of the no dumped and dumped slope is 0.0315 L/s and 0.0319 L/s respectively. The filling of the tailing dam has no predominant impact on the internal deformation magnitude of the auxiliary dam, but it changes the direction of the slope deformation from being squeezed out towards the lower slope to being towards the near-vertical direction, achieving a stable slope. Therefore, it is necessary to ensure the drainage function of the anti-filter layer and the deep interceptive drainage ditch and strengthen the monitoring of slope deformation during the construction phase.


Introduction
Seepage and deformation are two important aspects of tailing dam behaviour, which affect the stability and safety of the dam and the surrounding environment.[1] Tailing dams are artificial structures that store tailings, which are often subjected to various external conditions, such as water level fluctuations, rainfall infiltration, earthquake loading, etc., which may induce seepage and deformation in the dam body and foundation.[2] Therefore, it is necessary to analyse the seepage and deformation characteristics of tailing dams under different scenarios, and to evaluate the potential risks and hazards of the dam failure.
Numerical simulation is a common and effective method for seepage and deformation analysis of tailing dams, which can provide detailed and comprehensive information on the physical and mechanical processes of the dam.Numerical simulation can also help to optimize the design, construction, and management of tailing dams, and to provide guidance for the monitoring and control of the dam.However, numerical simulation of tailing dams also faces some challenges, such as the complexity of the dam geometry and material properties, the uncertainty of the boundary and initial conditions, the coupling of the hydro-mechanical process, the nonlinearity of the constitutive models, etc. [3] The tailings disposal site (No. 3 disposal site) of the auxiliary dam is located on the gentle slope downstream of the upper reservoir auxiliary dam, with a design tailings elevation of 858.5~955m, a planned tailings capacity of about 2.2 million m 3 , a tailings slope ratio of 1:2.8, and a maximum tailings height of about 96.5m.

Problem definition
The cover layer and the fully weathered layer of the tailings disposal site behind the auxiliary dam of the upper reservoir are thick, with a total thickness of 12~25m, while the groundwater level is high, and the groundwater level is 4~5m from the ground surface.The construction of the auxiliary dam and the tailings dam may affect the groundwater level during and after the water storage process of the upper reservoir, which in turn affects the deformation of the disposal site.The change of the reservoir water level has a great impact on the stability of the slope.The change of the water level causes the pore water pressure of the soil to change, and the permeability coefficient of the soil also seriously affects the change of the water pressure inside the soil.Therefore, it is very important to study the change of the reservoir water level and the permeability coefficient of the soil on the stability of the slope.
In this paper, we reviewed some of the recent studies on the numerical simulation of seepage and deformation of tailing dams, and present a case study of a tailing dam that serves as the upper reservoir of a pumped storage station in China.We use the finite element software Phase 2 [4] and finite difference element FLAC3D [5] to model the coupled hydro-mechanical process of the tailing dam under different water levels and rainfall conditions.We analyse the distribution of pore water pressure, seepage velocity, displacement, and stress in the tailing dam, and evaluate the stability and safety of the dam.
Yang et al. ( 2022) presented that the tailing dam was stable and safe under normal water level and rainfall conditions, but the stability and safety of the dam decreased significantly under high water level and rainfall conditions, and the risk of seepage failure increased.[6] Zhang et al. ( 2021) concluded that the permeability coefficient ratio is an important parameter for the seepage stability analysis of tailings dam, which should be determined by the field test and adjusted according to the actual situation.[7] Fetisov et al. ( 2020) believed that seepage evaluation should be carried out when designing of dams' height increase in a framework of water balance prediction.[8]

Method and material properties
The seepage analysis uses the finite element software Phase2, and after determining the infiltration surface, the plastic-hardening constitutive model built in the finite difference software FLAC3D is applied to perform the deformation analysis of the auxiliary dam and the tailings construction.

Constitutive models
The bottom of the auxiliary dam body has two layers of 40mm anti-filter layer sandwiched with 80mm transition layer.To simplify the calculation, the three layers of material are assumed to be 160mm thick anti-filter layer.For the deformation calculation of the tailings disposal site behind the auxiliary dam, the main focus is on the deformation of the auxiliary dam and the tailings pile.Therefore, the anti-filter layer, the fully weathered mixture, and the tailings body all adopt the plastic-hardening constitutive model.The rock filling area adopts the Mohr-Coulomb model, while the base layer adopts the linear elastic model.At present, the Duncan-Chang model parameters of the anti-filter layer, the fullyweathered mixture, and the tailings body material are obtained by experiments.Due to the lack of the Duncan-Chang constitutive model in the mainstream numerical analysis software (such as Plaxis, RS2, etc.), the FLAC3D software of ITASCA group is used and the hyperbolic stress-strain relationship is expressed by the degraded plastic-hardening (PH) constitutive model.
The PH model simulates how soil behaves by using a constitutive model that accounts for both shear and volumetric hardening.When soil is loaded in a way that causes deviatoric stresses, it usually becomes less stiff and deforms permanently.[5] The shear yield function determining the onset and development of shear hardening is defined as formula (1).0 ( ) 2 Where, ur E is unloading-reloading stiffness modulus; i E is initial stiffness modulus; p  is a shear hardening parameter.

Material parameters and calibration
The engineering properties and parameters of the materials in each zone of the dam body are obtained by tests as shown in table 1.The permeability parameters are shown in table 2. To calibrate the parameters from the Duncan-Chang to the PH plastic-hardening constitutive model, the method is to use the parameters of the Duncan-Chang model (c, φ, R f , K, n, Pa, Kb, m) to perform the constitutive relationship analysis, obtain the stress-strain curve again, and then calibrate the PH model parameters (E50, m) according to the curve.The calibrated example has been shown in figure 1, which indicates that the stress-strain relationships of two models are fitting well.[

Seepage analysis
The seepage calculation uses the slope model established by the software Phase2.When the tailing body is not filled, the drainage surface is the anti-filter layer + slope surface, and when the tailing body is filled, the drainage surface is the anti-filter layer + deep drainage ditch.The seepage analysis results of the tailings disposal site behind the auxiliary dam of the upper reservoir with or without the tailing body are shown in figures 3 and figure 4. The calculation shows that, with or without the tailing slope, the flow rates of the same cross-section are not much different after equilibrium.In the cross-section shown in the figure, the flow rate of the tailings slope is 0.0315L/s, while the flow rate of the slope with tailings is 0.0319L/s, and the flow rate of the cross-section of the slope with pressure is 0.027cm 3 /s.

Deformation analysis
The deformation during construction of tailing dam and auxiliary dam are shown in figure 5 and figure 6, while the deformation direction of both model is shown in figure 7 and figure 8.
The deformation analysis results show that the water level of the upper reservoir only affects the upstream side of both schemes, with or without the filling body, and causes a deformation of about 3.8cm.The lower part of the auxiliary dam deforms the most during its construction, due to the filling material's self-weight, and the deformation increases to 10.4cm as the construction goes on.The deformation mainly occurs downward inclined toward the slope.For the tailings material filling, the filling body's maximum deformation is about 16cm on the filling surface, when the first and second layers are filled.The deformation in the lower-middle part grows gradually as the filling continues, due to the filling material's self-weight, and reaches 27cm at the end.The deformation mainly occurs nearly downward.

Conclusion and discussion
This paper adopts the finite element software Phase 2 and the finite difference software FLAC3D with a plastic hardening model to analyse the seepage and deformation during the construction of the auxiliary dam and tailing dam.The main conclusions are as follows: Before the auxiliary dam is constructed, the drainage surface is the anti-filter layer and slope surface.After construction, the drainage surface is the anti-filter layer and deep interceptive drainage ditch.The drainage effects of the two schemes are basically the same.In the same cross-section of the rock and soil, the flow rate of the no dumped slope is 0.0315 L/s, and the flow rate of tailing slope body is 0.0319 L/s.Therefore, it can be believed that the two drainage schemes in different filling stages are effective.
During the construction process of the auxiliary dam, the lower part of the dam deforms the most due to the gravity of the filling material, with a maximum deformation of 10.4 cm, and the deformation mainly squeezes out towards the lower slope.After the auxiliary dam is backfilled, when filling the first and second layers of tailing dam, the maximum deformation of the filling body is on the filling surface, about 16 cm.As the filling process continues, the deformation at the middle and lower part gradually increases, and eventually reaches 27 cm.The deformation mainly tends to be towards the lower part.The filling of the tailing dam has no predominant impact on the internal deformation magnitude of the auxiliary dam, but it changes the direction of the slope deformation from being squeezed out towards the lower slope to being towards the near-vertical direction, achieving a stable slope.
Overall, both drainage schemes are effective, and the deformation of the tailings is not significant, while deformation direction is conducive to the stability of the auxiliary dam.However, during construction, it is necessary to ensure the drainage function of the anti-filter layer and the deep interceptive drainage ditch and strengthen the monitoring of slope deformation.

Figure 1 .
Figure 1.PH model material parameters calibration Duncan-Chang model

Figure 3 .
Figure 3. Seepage calculation results of the auxiliary dam in Phase2

Figure 4 .
Figure 4. Seepage calculation results of the tailing dam in Phase2

Table 2 .
Permeability parameters of different materials.