Based on global initiatives such as the clean energy transition and the development of renewable energy, the pumped storage power station has become a new and significant way of energy storage and regulation, and its construction environment is more complex than that of a traditional reservoir. In particular, the stability of the rock strata in the underground reservoirs is affected by the seepage pressure and rock stress, which presents some challenges in achieving engineering safety and stability. Using the advantages of the numerical simulation method in dealing deal with nonlinear problems in engineering stability, in this study, the stability of the underground reservoir of the Shidangshan (SDS) pumped storage power station was numerically calculated and quantitatively analyzed based on fluid-structure coupling theory, providing an important reference for the safe operation and management of the underground reservoir. First, using the COMSOL software, a suitable mechanical model was created in accordance with the geological structure and project characteristics of the underground reservoir. Next, the characteristics of the stress field, displacement field, and seepage field after excavation of the underground reservoir were simulated in light of the seepage effect of groundwater on the nearby rock of the underground reservoir. Finally, based on the construction specifications and Molar-Coulomb criterion, a thorough evaluation of the stability of the underground reservoir was performed through simulation of the filling and discharge conditions and anti-seepage strengthening measures. The findings demonstrate that the numerical simulation results have a certain level of reliability and are in accordance with the stress measured in the project area. The underground reservoir excavation resulted in a maximum displacement value of the rock mass around the caverns of 3.56 mm in a typical section, and the safety coefficient of the parts, as determined using the Molar-Coulomb criterion, was higher than 1, indicating that the project as a whole is in a stable state.

Compared with other energy storage facilities (such as Li-ion batteries), pumped storage power stations have the advantages of a low installation cost and smaller environmental impact [

Terzaghi's investigation of land subsidence marked the beginning of the study of the interaction between seepage and stress; Terzaghi developed a one-dimensional (1-D) consolidation model and the effective stress principle, which is the theoretical cornerstone of contemporary soil mechanics and the first model of HM coupling [

The planned Shidangshan (SDS) pumped storage power station located in Zhenjiang City, China, was once an abandoned copper mine. Its construction can both significantly cut costs and address the area’s energy needs. It is of great significance to study the stability of the underground reservoir for the safe construction of the project. Thus, taking the SDS pumped storage power station as the research object, in this study, we established a continuum mechanical model of this area using the HM coupling theory and the numerical simulation method. Then, we simulated the characteristics of the underground seepage field, stress field, and displacement field after excavation of the underground reservoir. Finally, we analyzed the stability and timeliness of the reservoir under filling and discharge conditions, as well as the anti-seepage reinforcement. Our results provide a scientific basis for the safe operation of this future project and also provide an important reference case for the construction of other pumped storage power stations.

The planned SDS pumped storage power station is located between Nanjing City and Zhenjiang City, Jiangsu Province (119°7′16.1″ E–119°9′22.1 E, 32°8′41.4″ N–32°9′ 47.2″ N) (^{2}. The abandoned roadway provides enough underground space for the construction of an underground reservoir for the pumped storage power station. The elevation difference between the upper and lower reservoirs can also be fully utilized to construct a pumped storage power station, addressing the issue of the shortage of hydraulic resources caused by the flat terrain. Furthermore, the project is located in the important power consumption areas of Zhenjiang City and Nanjing City and thus plays a key role in solving the power load problem of the entirety of Jiangsu Province.

According to the drilling data, the rock strata in this area mainly consist of Silurian to Triassic sedimentary rocks and Late Yanshan intrusive magmatic rocks (

As a professional modeling software, Rhino has been widely used in the field of geological engineering. It has the advantages of a high modeling accuracy and strong operability. Therefore, the geological model was established in COMSOL with the help of Rhino software. First, the geometry without rock strata interfaces was produced using COMSOL (

Lithology | Density (kg/m^{3}) |
Cohesive force (MPa) | Internal friction angle (°) | Young’s modulus (GPa) | Poisson ratio | Porosity | Permeability (mD) |
---|---|---|---|---|---|---|---|

2750 | 8.77 | 62 | 88.7 | 0.14 | 0.005 | 0.001 | |

_{2} |
2780 | 9.73 | 61.2 | 61.6 | 0.26 | 0.002 | 0.001 |

_{3} |
2630 | 12.5 | 65.9 | 81.6 | 0.06 | 0.005 | 0.0013 |

_{1} |
2770 | 5.66 | 65.9 | 82.9 | 0.19 | 0.0015 | 0.0015 |

_{2}_{3} |
2690 | 2.82 | 49.3 | 85.5 | 0.29 | 0.0115 | 4.71 |

_{1} |
2690 | 5.91 | 48.4 | 73.5 | 0.08 | 0.006 | 0.0071 |

_{1} |
2700 | 11.1 | 60 | 72.7 | 0.25 | 0.002 | 0.008 |

_{2} |
2700 | 11.6 | 60 | 72.7 | 0.25 | 0.002 | 0.008 |

_{1} |
2830 | 5.99 | 62.4 | 82.4 | 0.14 | 0.00226 | 0.0023 |

In the numerical simulation of the stability of the underground reservoir, the simultaneous changes in the rock mass strain and pore pressure due to the bidirectional coupling effect of the stress field and seepage field are taken into account. The corresponding equations of the physical fields are as follows.

The governing equations of stress field:

where ^{−1}MT^{−2}], ^{−1}MT^{−2}], ^{−1}], respectively.

The governing equation of seepage field [

where ^{−1}], ^{2}], ^{−1}MT^{−1}].

According to the HM coupling equations, the seepage field and stress field are coupled bidirectionally with pore pressure and displacement as transfer terms. As a general multi-physics coupling simulation software [

Before the stability analysis of underground reservoir excavation, the crustal stress field and seepage field are in a stable equilibrium state under the action of natural flow field and gravity. Based on the 3-D geological model established above, the steady state simulation of stress field and seepage field in the research area can be obtained with the help of COMSOL software, and it is used as the initial condition for the stability analysis of the underground reservoir excavation. First, the boundary conditions of the 3-D geological model are set. The bottom border of the 3-D geological model (

Hydraulic fracturing testing is one of the recommended methods for measuring rock stress issued by the ISRM in 1987 [

The geometry of the underground reservoir is generated in the 3-D geological model in accordance with its spatial placement and geometric properties, and the difference set is then processed (

Given that the boundary conditions, mesh generation, and other variables may have an influence on the 3-D model’s correctness, in this paper, we further create a 2-D fine model at a typical profile location (x = 1000 m) in the research area and conduct more research. The section of the 3-D model can be used to determine the geometry of the 2-D model, and the rock mass's physical and mechanical properties agree with the 3-D model. The top of the model has a free boundary, the bottom has no displacement, and the two sides have no horizontal displacement. According to the measured value of the groundwater level, the two sides are established as the pressure head boundary, and the bottom is the no-flow border. The numerical simulation results show that in the typical section of the 3-D model, the first principal stress ranges from −6.69 to 5.15 MPa, the third principal stress ranges from −26.1 to 0.311 MPa, and the maximum displacement around the caverns is 3.42 mm (

Currently, because the factors affecting the stability of the underground reservoir are very complex, there has not been a consistent method for evaluating the stability of the surrounding rocks [

where

When

Then:

Based on this, the yield surface equation can be written as:

where

From above, the factor of safety defined by Mohr-Coulomb yield criterion in the stress space can be expressed as:

After the excavation of the underground reservoir, the displacement of the surrounding rock mass around the caverns ranges from 0.1 to 3.56 mm, which are substantially less than the maximum displacement permitted by local national standards (approximately 50.4 mm). Additionally, each component's factor of safety, as determined by the Mohr-Coulomb criterion, is greater than 1, further indicating that the entire underground reservoir is stable and safe after excavation.

During the operation of the pumped storage power station, the frequent filling and discharge of the underground reservoir with water may affect the local stress field, resulting in deformation and destruction of the surrounding rock mass. Therefore, the stability of the underground reservoir under filling and discharge conditions is numerically simulated under the condition when excavation anti-seepage measures are considered in the actual project; that is, the boundaries of the underground reservoir are set as impervious. The draining and filling of the underground reservoir are assumed to be possible working conditions, the bottom of the reservoir is regarded as the dead water level (−300 m), and the arch of the reservoir is regarded as the maximum storage water level (−271 m); that is, the water level fluctuates between −300 and −271 m under the operation conditions of the underground reservoir (

Considering the connection between the underground reservoir and the atmosphere, as well as the timely implementation of anti-seepage measures during the excavation process, there is no hydraulic exchange at the boundaries of the underground reservoir in the numerical simulation under the filling and discharge conditions. In addition, the pressure head can be transformed into the boundary load and can change over time. When the groundwater depth is 29 m, the maximum load generated is 0.29 MPa. According to the operating conditions shown in

For unsafe phenomena such as rock bursts that may occur in underground engineering, the common treatment measures include bolt support and concrete lining, among others. In this paper, a straightforward anti-seepage anchorage measure was simulated. The steel bars used for anchoring have a prestressing force of 1000 KN, a diameter of 28 mm, a length of 6.5 m, and a bar spacing of 2 m. In

Arch crown | Side wall | Bottom | |
---|---|---|---|

After the excavation | 2.18 | 1.123 | 1.2 |

Filling and discharge conditions | 2.52 | 1.118 | 1.77 |

Anti-seepage and anchoring measures | 3.31 | 1.46 | 1.3 |

Note: The value of

The chosen rock mechanics parameters frequently have an influence on the effectiveness of the numerical simulation method in the stability analysis of underground engineering. The rock mass characteristics measured via laboratory and field experiments may be slightly different from the actual rock mass because the surrounding rock is affected by the mineral structure and geological structure, which results in a great deal of spatiotemporal variability in its mechanical behavior. Considering that Young’s modulus and Poisson ratio are two important parameters that reflect the deformation degree of a rock mass under stress, in this paper, we examine the sensitivity of Young’s modulus and Poisson ratio to the surrounding rock by using the caverns in the _{2}

Taking into account the bidirectional coupling effect of the stress and seepage of porous media, in this study, we used the finite element method in COMSOL to conduct numerical simulations of the stability of the rock surrounding an underground reservoir after excavation and under the operating conditions, and we analyzed the stability and efficiency of the underground reservoir. Due to the good geological conditions of the site, the deformation of the surrounding rock of the underground reservoir is small, and no damage has occurred after the excavation. Even when only anti-seepage measures are considered, the cavern remains stable after 10 years under the filling and discharge conditions. In addition, it was found that the anti-seepage anchorage measures are very effective in reducing the tension stress around the cavern and the deformation of the surrounding rock. Inevitably, in the numerical simulation, the same rock strata are regarded as a homogeneous isotropic material in the stability analysis of the surrounding rock of the cavern, which may lead to some deviation of the simulation results from the actual situation. In addition, because the distribution of the faults and cracks in the project region is quite sparse, their influence on the outcomes was ignored in this numerical simulation. However, the findings of this study can still be used as a general guide for managing and operating projects safely in the future.

Longitude | Latitude | |
---|---|---|

Southwest corner | 119.12 | 32.1516 |

Northwest corner | 119.12 | 32.1624 |

Northeast corner | 119.152 | 32.1624 |

Southeast corner | 119.152 | 32.1516 |

Thanks to Dr. Mingqian Li for his careful guidance on model construction in this study. And thanks to Dr. Baoming Chi for providing relevant geological data and guiding the paper in the revision process.

This study is funded by the Beijing Natural Science Foundation of China (8222003) and National Natural Science Foundation of China (41807180).

The authors confirm contribution to the paper as follows: study conception and design: Hongbiao Gu; data collection: Zhengtan Mao; analysis and interpretation of results: Peng Qiao; draft manuscript preparation: Peng Qiao and Shuangshuang Lan. All authors reviewed the results and approved the final version of the manuscript.

All relevant data are within the paper and its Supplementary Materials files.

The authors declare that they have no conflicts of interest to report regarding the present study.