Investigation on large-scale 3D seepage characteristics of a pumped-storage power station: a case study in Zhejiang Province, China

Generally speaking, the seepage phenomenon in nature exists in two different media: porous media and fractured media (Du et al. 2000). In practical research, researchers usually adopt different research methods for different rock types located in the project area. Since large-scale projects are generally constructed on hard rock masses, typically contain small cracks that can be considered as fractured media. Therefore, various methods for determining the permeability of fractured rock masses have also been developed (Sun et al. 2006). Under this premise, the equivalent continuum theory provides an alternative method for equating fractured rock masses to continuum, which has lower computational costs and can describe the seepage characteristics of bedrock (Kottwitz et al. 2021). It should be noted that the percolation theory of equivalent continuous media has been developed maturely. Also, it is in good agreement with the actual situation (Du et al. 2000). In other words, the equivalent continuum model enjoys some conveniences in effortless meshing and acceptable computational cost (Azizmohammadi & Sedaghat 2020; Wang et al. 2022), and also it has a strong descriptive ability for the macroscopic seepage characteristics of fractured rock masses. Therefore, it has been widely used in large-scale engineering seepage analysis (Che et al. 2014).

In recent years, as one of the typical projects often deployed in hard rock masses, seepage problems in PSPSs have gradually attracted the attention of relevant researchers. Yang et al. (2021) proposed a simplified analysis method for calculating the leakage of a reservoir basin for a PSPS and deduced a calculation formula for the reservoir leakage volume applicable to different groundwater levels. In recent decades, similar to the calculation of reservoir basin leakage volume, many researchers have proposed seepage analysis methods that are generally applicable to tunnels or rivers (Li et al. 2017; Wang & Li 2018; Ying et al. 2019), which can quickly calculate river leakage volume without complex modelling work. In terms of theoretical research, Zhu (1996) proposed using the improved drainage substructure method to finely simulate the calculation boundaries of drainage holes (DH), drainage corridors (DC) and powerhouses. This solved the numerical treatment problem of many DH crossing the free seepage surface in a project. Tian et al. (2022) summarized the current commonly used drainage hole simulation methods and the finite element method for locating the free surface accurately to provide theoretical support for similar work.

Meanwhile, optimizing an anti-seepage system is also a key issue in engineering. For example, Zhang et al. (2021) proposed an anti-seepage optimization scheme for a group anti-seepage curtain at a reservoir dam site. Their research results provide support and reference for anti-seepage optimization designs of reservoir dams constructed in karst areas. Wang et al. (2015) examined the Huilong PSPS, and a numerical simulation calculation was carried out by using a 3D seepage model of a double-crack system. By comparing the corresponding effects of four anti-seepage schemes, the leakage volume of the fractured rock mass under each scheme was obtained. At the same time, an optimal anti-seepage scheme was selected from multiple factors such as the anti-seepage effect, engineering difficulty and engineering cost, to provide reference value for similar projects with similar geological conditions. Furthermore, Zhang et al. (2020) based on Mopanshan Reservoir, proposed an optimized reservoir seepage control system to ensure the normal operation of the dam under this optimized scheme while ensuring that the leakage volume met the seepage control requirements. For seepage control measures, Miao et al. (2022) proposed a new control inversion method suitable for the structural plane of a fractured rock mass and simulated the seepage field in the study area using the corrected model. The change in leakage volume in each part of a project area under partial and total damage of the anti-seepage system of the Jurong PSPS was also assessed to explore the effect of the anti-seepage system and put forward optimization suggestions. Based on the Hongping PSPS, an analysis method combining drainage hole secondary dissection technology and the modified node virtual flow method was adopted by Zhou et al. (2015), which could easily solve a seepage field problem containing complex seepage control measures. They also made a corresponding evaluation on the seepage control measures of the Hongping PSPS, which has guiding significance for the seepage control optimization of similar projects. Lin & Xu (2022) studied the seepage characteristics of a water conveyance and power generation system of a PSPS under normal conditions. Meanwhile, they assessed the role of seepage control measures such as multilayer drainage galleries and drainage hole curtains in the actual project. The results showed that the seepage control measures of the project had a good effect on water diversion and drainage. Zhang et al. (2018) established a large-scale 3D seepage model and analysed the characteristics of the seepage field and rationality of seepage control measures in a PSPS under normal operation. Meanwhile, a reinforcement scheme of the dam abutment position by grouting was proposed, and the original seepage control scheme was optimized. Xu et al. (2019), examining the Qingyuan PSPS, proposed a new drainage hole array simulation method using an existing theory. Additionally, a 3D equivalent continuous seepage finite element simulation model was established to simulate the seepage field in an underground powerhouse (UP). Shen et al. (2015) compared the effects of overall and local seepage control of the upper reservoir of the Zhenan PSPS. As a result, the local seepage control scheme considering both safety and economic benefits was obtained, which provided reference value for similar projects. In addition, based on an existing theory for water conveyance and power generation system, underground powerhouses, etc. Wu (2007), Zhou et al. (2009) and Yao & Gao (2017) estimated the seepage prevention effects of various seepage control measures in actual project operations. The seepage field in the local area was also analysed and calculated. It should be noted that seepage in fractured rock masses, which is difficult to avoid in actual engineering construction, has interested many researchers. Wen et al. (2020) focused on seepage control in underground chamber engineering. Based on the Wunonglong Hydropower Station Project, the long-term seepage control effect of a complex seepage control system on the fracture surrounding rock of the Wunonglong UP was studied by using a finite element numerical model. The rationality of the seepage control system for the fractured rock of the Wunonglong UP and the potential for further optimization were assessed by combining a global equivalent model with a refined submodel. The inversion method and basic theory of the permeability coefficient of fractured rock masses are also being continuously developed (Chen et al. 2015a, 2015b; Gan et al. 2020). In conclusion, few studies have focused on the large-scale seepage field distribution characteristics of a PSPS project area. In particular, this cannot be duplicated between different projects. Due to the high anti-seepage requirements and special geological conditions of a PSPS, investigating the distribution characteristics of large-scale seepage fields is one of the challenges brought by current engineering design. More importantly, under the different layout schemes of seepage control measures, the seepage characteristics and leakage volume will be changed. In other words, whether the abovementioned schemes will affect the normal operation of a power station is still a scientific problem to be solved urgently in PSPSs.

In practice, an intact rock mass can be regarded as a continuous porous medium, but rock often shows anisotropic seepage characteristics under the influence of structural planes such as fractures, joints and faults. However, since the sizes of rock fractures are much smaller than those of structures and buildings, the effect of the fractures can be averaged into the surrounding rock of the structures. In other words, the surrounding rock is still assumed to be a continuous porous medium that is in accordance with Darcy's law, and the groundwater within the rock is regarded as a steady flow. The analysis method based on steady flow theory usually obeys the following assumptions: (1) the surrounding rock is an isotropic continuous porous medium; (2) the seepage meets Darcy's law; and (3) the stress field of the surrounding rock and coupling effect between the seepage field and the stress field are not considered (Fang et al. 2007).

In the project area of the Qingyuan PSPS, the lithology is intact tuff, which mainly shows the characteristics of slight weathering and low permeability. The overall calculation area consists of three parts: the upper reservoir area, the water transmission and power generation system and the lower reservoir area. According to the results of the field water pressure test, the material parameters are given in Table 2. In addition, the permeability coefficient of anti-seepage curtains is 1 × 10⁻⁸ m/s, the permeability coefficient of the concrete structure is 1 × 10⁻⁹ m/s and the permeability coefficient of the steel plate lining structure is 1 × 10⁻¹³ m/s.

The storage capacity of the upper and lower reservoirs of Qingyuan PSPS is 12.07 million m³ and 10.8 million m³, respectively. The main surface runoff in the project area comes from atmospheric precipitation and surface water. In addition, groundwater is often used to maintain runoff during the dry season. In fact, the reservoir storage capacity of the Qingyuan PSPS is small and the recharge from surface runoff is relatively limited. This is a very practical demonstration that a reasonable seepage control scheme needs to be laid out in both the upper and lower reservoir areas to reduce the leakage of water stored in the reservoir basin. In addition, since the electromechanical facilities are all arranged underground, it is necessary to ensure that the seepage control measures around the UP can undertake the anti-seepage task to ensure the safety of the power station.

To investigate the changes in groundwater level during the operation of the Qingyuan PSPS and the effect of seepage control measures on inducing and discharging seepage water, the overall seepage field of the project area is simulated and analysed by a numerical simulation method. Firstly, the initial seepage field is calculated for the unconstructed state of the power station. On this basis, the distribution form of the seepage field is considered under the normal operation state after completion. Additionally, extreme conditions such as the failure of the seepage control measures during the operation process are considered, and a simulation analysis of different seepage control measure layout schemes is carried out. The specific scheme settings are demonstrated in Table 3 (‘√’ represents that the seepage control measures operate normally, ‘ × ’ represents that the seepage control measure is in the failure state). The storage level of the upper reservoir is set as the normal water level (1,185.00 m), while the lower reservoir storage level is set as the dead water level (639.00 m). The upper and lower reservoir anti-seepage curtain is laid at the bottom of the dam base, and the anti-seepage curtain of the UP is located at the tail of the powerhouse. Notably, the drainage measures of the UP include three-layer DC and DH.

In the case of normal operation, the groundwater level equipotential line bends upstream at the starting position near both the middle and lower flat sections of the diversion pipelines (DP). The reason is the fact that the middle and lower flat sections of the DP are equipped with DC. In other words, due to the effect of DC on draining water, the distribution of the surrounding groundwater level changes, which is reflected as a bent equipotential line.

By comparing the groundwater level equipotential distribution under the two schemes, it can be seen that the groundwater level around the UP in the initial state is 600–650 m, approximately 170 m higher than that under normal operation. The calculation results of S01 reveal that the groundwater diving surface shows a rapid downwards trend on the upstream side due to drainage measures near the powerhouse. At this juncture, the groundwater level at the bottom of the powerhouse is similar to the bottom elevation, indicating that the area of the powerhouse is basically dried out and that there is no seepage. One also should note that the drainage system of the UP can quickly drain the seepage water, ensuring the normal operation of the electromechanical equipment in the powerhouse.

Note: When the impermeability standard is 3Lu, the allowable infiltration slope drop of the impermeable curtain is 10 (Sun 2004).

For the UP, when all the seepage control measures are in normal operation, the total leakage volume of the powerhouses is 447.85 m³/day. When the UP anti-seepage curtain fails, its total leakage volume increases to 1,337.18 m³/day, which is about three times that of the normal operating condition. Correspondingly, when the drainage measures around the UP fail, its total leakage volume increases to 3,472.41 m³/day, which is about 7.7 times that of the normal operating condition. This result indicates that the seepage control measures around the UP can have a positive effect in blocking and discharging seepage water. Meanwhile, they also reduce the possible seepage water of the UP effectively. In contrast, drainage measures have a greater impact and play a vital role in the normal operation of the electrical facilities of the powerhouse. At this juncture, the leakage volume of the upper and lower reservoir basins will also change with the state of the seepage control measures around the powerhouse. When the ASCUP fails, the leakage volume of the upper and lower reservoir basins increases to 101.5 and 112% of the normal operation condition, respectively. Correspondingly, in the case of the drainage measures of the powerhouse failure, the above values increase to 102 and 114.9%, respectively. In conclusion, the total leakage volume of the UP is mainly related to the seepage control measures around the powerhouse. The impact caused by the failure of drainage measures is greater than that of the anti-seepage curtain. Notably, the working state of the seepage control measures around the UP can also be reflected in the variation in the leakage volume in the upper and lower reservoir basins. Due to the influence of distance, the impact on the leakage volume of the lower basin is greater than that of the upper basin.

As can be seen from Figure 12, the DC and the DH around the powerhouse both undertake important drainage tasks under different operating conditions. This means that they can effectively drain water around and inside the powerhouse. It is noteworthy that the leakage volume of the channel system is relatively stable under different cases, while the leakage of TT is greater than that of DP. The maximum leakage volume of the channel system occurs under scheme S03. Compared with the normal operation condition, the leakage volume of the DP and the TT increase by 10 and 13%, respectively.

In summary, the leakage volume of the UP is mainly affected by the seepage control measures around the powerhouse. Simultaneously, the state of the upper and lower reservoir areas will cause corresponding changes in the UP. Among them, the effect caused by the upper reservoir such as the change in water level and the failure of the anti-seepage curtain is the weakest. In contrast, the operation status of the drainage measures around the powerhouse has the most obvious impact.

Notably, it can also be demonstrated that when the anti-seepage curtains of the upper and lower reservoirs fail, the influence on the leakage volume of the UP, DC and DH is very limited. Additionally, the effect caused by the lower reservoir anti-seepage curtain on the above positions is slightly greater than that of the upper reservoir area. To summarize, in terms of this engineering condition, different areas at far distances have less interaction with each other. Meanwhile, the anti-seepage curtains at different locations play an obvious role in reducing the leakage volume at the corresponding sites. As a result, during the construction and operation of PSPSs, it is recommended to strengthen the patrol inspection of the project area to ensure that the failure of the anti-seepage curtains is found and reinforced in time, so as to reduce leakage in the project area and improve the operational efficiency of the power station.

By establishing a 3D finite element seepage calculation model of the Qingyuan PSPS, a 3D numerical simulation of the large-scale seepage field was conducted, and the effects of various seepage control measures in the project area were assessed. Simultaneously, the distribution of the seepage field, leakage volume and external water pressure under different deployment schemes of seepage control measures are analysed. More importantly, we evaluated the seepage characteristics of the Qingyuan PSPS and obtained the following conclusions:

• (1)

  For normal operating conditions, the underground water level of the powerhouse is in accordance with the bottom elevation, which proves that the drainage measures are conducive to rapidly draining the seepage water from the powerhouse. In terms of different calculation conditions, the hydraulic gradient at anti-seepage curtains is less than the allowable hydraulic gradient, which can ensure the seepage stability of the structure.

• (2)

  In the case of damage to seepage control measures in the study area, the leakage volume at the damage locations will increase significantly, but the interactions among them are very limited. For example, when the ASCUR fails, the leakage volume of the URB will increase to 2.3 times that of the normal operation, while the leakage volume of other typical locations will not be affected. One should also note that the leakage volume of the UP is mainly affected by the seepage control measures nearby. Especially the influence brought by the availability of drainage measures.

• (3)

  Most of the channel system is located below the free face of the groundwater. That means the tunnels need to undertake the external water pressure to a certain degree. From the results of longitudinal profiles at different locations, it can be highlighted that the external water pressure at the lining location is significantly reduced under the protection of the grouting ring, which can ensure the normal function of the lining.

• (4)

  Considering the partial failure of anti-seepage curtains, the percentage of curtain failure is set as 0, 30, 50, 80 and 100%. When the failure rate of the anti-seepage curtain at each part is increasing, the corresponding leakage volume of the URB will add up to 22.1, 49.01, 89 and 130%, respectively; the leakage volume of the LRB will increase to 3.53, 92.07, 148.71 and 242.99%. On the other hand, the failure of the powerhouse curtain will cause a significant increase in the leakage of DH, DC and caverns. In summary, the findings of this study highlight the significance of anti-seepage curtains to prevent water seepage, which means that the failure of curtains should be found and handled in a timely manner to guarantee the safety of the power station.

The authors gratefully acknowledge the support of the Natural Science Foundation of Tianjin (21JCYBJC00410).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.