An Analysis of Two Case Studies on the Construction of Underground Caverns-A Focus on the Tiantai Pumped Storage Station

This paper presents a numerical analysis of two case studies on the construction of underground caverns by FLAC3D, with a specific focus on the Tiantai Pumped Storage Power Station. During the construction period, two main problems, which are crane beam stability and rock support, have been numerically analysed and addressed. The first case study examines the influence of rock wall crane beam support on excavation stability. It has been proven that The reinforcement of connected concrete enhances integrity but may lead to localized cracking. Moreover, over-excavation reinforcement marginally improves anchor safety. The second case study investigates abnormal anchor stress in a powerhouse resulting from excavation. Recommended measures include conducting timely stability evaluations, identifying weakened zones to control rock stress relaxation effects, and establishing effective feedback mechanisms. By enhancing the safety of pumped storage power stations, promoting sustainable energy infrastructure, and improving the design and construction processes of such stations, this research paper aims to contribute to the overall development and success of these projects. In conclusion, the paper has discussed the rock wall crane beam stability and rock support effect, which implement the lack of this type of numerical analysis in underground cavern group construction. Also, it is recommended to accurately determine the rock mass mechanics parameters and timely support the excavated spaces to prevent abnormal increases in anchor stress.


Introduction
Pumped storage power stations (PSPs) play a crucial role in maintaining stable electricity grids, and their rapid development in recent years has further highlighted their importance.[1] During the construction of underground powerhouses, cavern stability is one of the most critical issues because of uncertainties like joint fractures, faults, and fractural rocks, that can affect the safe operation of these stations.[2] To address this problem, it is necessary to determine the rock mass mechanics parameters accurately, although the complexity of geology and the limitations of measurement methods make this task challenging.Feng et al. (2011) and Xu et al. (2019) are actively exploring ways to fully utilize the various mechanical responses of surrounding rocks and employing inverse analysis techniques for material parameter inversion, which has become a current hot and difficult issue.[3][4] Wang et al. have developed a method deriving the complete initial state of stress and Young's modulus from a set of relative displacements measured between adjacent measuring points.[5] Ma H et al (2023) have considered three factors that affect the stability of surrounding rock: excavation sequence, excavation method and support measures.[6] Hu et al. (2022) have applied a method of back analysis to be utilized for the interpretation of field measurements in monitoring the stability of tunnels.[7] However, these researchers have not considered the beam crane and rock support during construction, and these problems have been addressed in this paper.
Improving our understanding of the rock mechanics involved in cavern stability can help optimize the design and construction process, improve the safety of PSPs, and contribute to sustainable energy infrastructure.FLAC3D is a software for analysing underground cavern stability.Engineers can simulate rock mass behaviour by dividing it into small elements and specifying material properties, boundary conditions, and initial stresses to solve governing equations mathematically.FLAC3D also considers geological factors like heterogeneity and fluid flow.It creates 3D models to analyse various aspects of cavern stability such as joint location, rock strength, excavation methods, and supporting system design.This tool provides reliable data on critical stress states or displacements that optimize safety measures, construction processes, and hazard identification.[8]

Engineering background
The proposed Zhejiang Tiantai Pumped Storage Power Station is planned to be built in Tantou Town, Tiantai County, Zhejiang Province.The lower reservoir (dam) is about 18km away from Tiantai County town.The power station hub mainly consists of the upper reservoir (dam), lower reservoir (dam), water delivery system, underground powerhouse, surface switching station, etc.The total installed power generation capacity is 1700 (4×425) MW.
In general, the underground powerhouse cavern group of the Tiantai Pumped Storage Power Station has a large scale.The geological survey results also revealed that the in-situ stress level within the powerhouse area is relatively high-moderate level.It is preliminarily judged that there is a contradiction between the in-situ stress level and the rock mass strength, which is relatively prominent.Therefore, the stability conditions of the cavern group surrounding rock will become a rock mechanics issue requiring particular attention during the construction period.
In the area of the CPD1 exploration tunnel, the rock mass is predominantly sub-block to mosaic structure with relatively developed faults trending mainly in the NWW and NEE direction.Per Chinese specification GB 50218/2014 [9], Class III1, III2, IV, and V rock masses make up 7.8%, 64.7%, 18.4%, and 1.1% respectively in the CPD1 exploration tunnel.Based on geological conditions revealed by drilling and lithology, geological structure, and main structural surface production relationship, the surrounding rock of the hydropower transmission system is mainly Class III, with small areas composed of Class II; the fault fracture zone and joint dense zone are Class IV and Class V. Hydraulic fracturing test [10] results indicate that the underground powerhouse area's maximum principal stress (σ1) ranges between 13.97-15.66MPa,with a direction (θ) of N77~81°E and an inclination angle (α) of 47.5~49.5°.The minimum principal stress (σ3) ranges between 5.96-7.24MPa.

Material model for rock mass
For the purpose of numerical simulation in this study, the Mohr-Coulomb model was employed.This model includes a tension cut-off failure criterion, which is represented by the failure envelope outlined in formulas (1) to (3).The empirical Hoek-Brown system consists of two parts: defining rock mass strength and setting parameter values, particularly with regard to linking the MC strength criterion with the Hoek-Brown system.To establish this connection, the Hoek-Brown system is used to calibrate the rock deformation parameters and determine the internal friction angle, cohesion, and other requirements of the Mohr-Coulomb criterion.To facilitate comprehension of this process, we briefly introduce the relevant basics here.Formula (4) describes the rock mass yield surface in the stress space according to the Hoek-Brown strength criterion, while formulas (5) to (7) detail the H-B parameters. (5)   Where, φ is the friction angle; c is the cohesion; t is the tensile strength; σ1 is maximum principal stress; σ3 is minimum principal stress; σc is uniaxial compression strength.mb, s and a are Hoek-Brown strength parameters.Material parameters are shown in table 1.

General description and numerical model
According to the reinforcement design plan provided, which involves adding a retaining wall to the underground powerhouse's rock wall crane beam, a numerical calculation model was established as shown in figure 1.Additionally, the over-excavation condition resulting from rock anchor beam blasting was taken into account, namely two calculation scenarios were considered: one without overexcavation and one with over-excavation.Considering the current geological information, the exposed structural planes in the factory area are relatively limited, and most of the faults intersect with the factory at high angles (close to 90 degrees) and are located at both ends of the factory.For this reason, the typical tunnel section chosen for the calculation is the 1# machine tunnel section that intersects obliquely with the powerhouse.The F9 fault in this section intersects obliquely with the downstream side, making the surrounding rock on the downstream side more unstable.As shown in figure 2, the deformation growth of the rock wall crane beam decreases during lower plant excavation, with minimal impact on overall stability.Cumulative deformation is close to 20mm without reinforcement and about 15mm with over-excavation reinforcement.Reinforcement measures connect concrete in over-excavated area to crane beam, enhancing integrity.Proper construction quality is needed to ensure concrete integrity.
As shown in figure 3, the rock wall crane beam has slightly increased stress in the lower supporting wall under over-excavation during construction.The maximum compressive stress is less than 3MPa, and the maximum tensile stress is less than 5MPa, which is still lower than the C30 concrete tensile strength.Applying wheel load causes a slight change in stress distribution, and the maximum principal stress increases to some extent.Despite a locally high tensile stress and low safety margin of the supporting wall due to complex geological conditions, both the beam and wall meet structural bearing capacity requirements, posing no significant risks to stability or normal operation.However, there may be potential for local concrete cracking.In figure 4, stress characteristics of the rock wall crane beam anchor under wheel load are shown with and without over-excavation reinforcement schemes -both exhibit tensile anchor rod stress of about 150-200MPa and compressive anchor rod stress of about 50-150MPa.The over-excavation condition results in a more significant decrease of tensile anchor rod stress due to better rock wall beam combination via concrete and steel bars leading to stronger integrity.Though the over-excavation reinforcement scheme has some influence on the stress of the rock wall crane beam anchor, it only reduces the increase of the tensile anchor rod stress to some extent and doesn't significantly improve the safety margin of the anchor.

Problem statement
On September 25, 2022, routine observations were carried out on the monitoring instruments buried in the main powerhouse.Compared with September 22, the variation of anchor rod stress gauge ranges between -0.53 MPa and 149.87 MPa, while the variation of multi-point displacement gauge ranges between -0.01mm and 0.88mm.The measurement values of anchor rod stress gauges and multi-point displacement gauges at the downstream shoulder of pile number 0+000 and 0+042.3showed significant changes.After on-site investigation as shown in figure 6, it was found that there is a newly excavated section of the second-floor wall at 0+040~0+050 on the left side of the plant, and there are obvious signs of slight rock cracking in the surrounding rock, and no supporting measures have been implemented yet.

The principle of anchor stress gauges
Figure 6 provides detailed information on the monitoring principle of anchor stress gauge and various factors that affect its stress.Typically, this gauge is embedded in hydraulic structures, concrete structures, or bedrock to monitor bolt stress or tensile force over an extended period.By doing so, we can effectively monitor the deformation force (tension or compression) of the tested anchor and measure the temperature of the buried point simultaneously.The difference in axial force distribution of bolts between a continuous medium and a rock mass with a structural surface was illustrated using two numerical simulations in figure 7.At the point where the bolt intersects with the structural surface, the recorded axial force was 16.83kN, representing a 53% increase compared to the maximum value of 10.99kN under continuous conditions.

Numerical analysis
A numerical analysis model, shown in figure 8, was established for the section of 0+42.3 on the left side of the powerhouse, and the excavation response was considered under two conditions, which are local weakening with a reduced rock mechanics parameter of 0.8 and 0.6 at the arch shoulder respectively.
The calculation results are shown in figure 9.The deformation and anchor rod axial force distribution characteristics for the left section at 0+42.3 after the middle slot excavation on the second floor of the main transformer chamber are presented in a cumulative deformation contour map.The downstream shoulder of the excavation was monitored to measure the deformation increment and anchor rod stress increment, revealing the following information:(1) At a rock mass weakening parameter of 0.8, the deformation was 8.5 mm, and the anchor rod stress was 121 MPa, measured 2m from the excavation face, while the weakening parameter is 0.6, the deformation was 13.8 mm, and the anchor rod stress was 162 MPa.(2) Local weakening of rock mechanics parameters at the arch shoulder has a greater influence on deformation and anchor rod stress increment at a parameter of 0.6 than reducing it to 0.8, indicating that such local weakening affects the main transformer chamber's deformation and anchor rod stress.

Figure 6 .
Figure 6.Simple sketch of the anchor stress gauge

Figure 7 .
Figure 7.Comparison of axial force distribution under different conditions

Figure 8 .
Figure 8. Illustration of weakening area in the numerical model

Figure 9 .
Figure 9. Stress distribution of anchors and deformation of the powerhouse

Table 1 .
Material parameters of rock masses