Analysis of Surrounding Rock Stability and Supporting Structure Effect Based on a Hydropower Station Underground Cavern

The surrounding rock stability is a main difficult problem in the hydropower station construction and operation. Taking one hydropower station as object, FLAC 3D numerical simulation software was used to calculate the changes in surrounding rock failure zone distribution, surrounding rock strain, and block safety factor before and after supporting. The stability of the surrounding rock after excavation was analyzed and effect of the supporting structure was also evaluated. Finding that: the supporting structure can effectively improve the stability of surrounding rock. After supporting, the failure zone range, the deformation of surrounding rock, and the internal force of the supporting structure all reduced, while the safety factor of the block improved. Due to the effect of bolt, the deformation of the surrounding rock transformed from plastic to elastic, and the rebound zone of the surrounding rock increased, while the plastic and failure zone reduced. The depth of the failure zone on the downstream side is greater than that on the upstream side, and a large number of cracking zones are distributed on the downstream side. At the same time, due to the influence of cracks and geological fracture, the failure of surrounding rock around the main powerhouse is higher than that around the tailgate chamber higher than that in the main transformer chamber. The safety factor of the key blocks distributed around the main transformer chamber and the main power house improved, the sliding form of some blocks transformed from collapse to single-surface sliding.


Introduction
The stability of the surrounding rock is a frequent issue due to factors such as varying depths, high levels of stress, and complex geological conditions and structures [1].These challenges pose a significant threat to the safe construction and stable operation of the power plant.
Due to the reinforcing effect of support measures such as rock bolts and anchor cables, the surrounding rock can be strengthened in terms of interlocking, embedding, and clamping.In hard fractured rock masses, the tensile force of rock bolts or anchor cables can directly provide anti-slip force to unstable blocks and improve the cohesive force and frictional force along weak structural surfaces.Therefore, they are widely used in the support of underground tunnel surrounding rock [1,2].However, there are currently no clear design guidelines for the support of underground water conservancy tunnels, and most projects still follow the principle of "empirical design and construction".With the development of numerical simulation technology, many scholars have used numerical simulation software to calculate and review the design of surrounding rock support structures in hydropower station underground tunnels.Su et al. [3] used ABAQUS software to simulate the support timing with different ratios of equivalent node forces and analyzed the influence of support timing on the stability of surrounding rock.Wang et al. [4] studied the improvement effect of anchor support on different rock structure deformations, stress states, and damage zones based on the underground tunnel group of Dagangshan hydropower station project.Deng et al. [5] compared different support structures by analyzing the differences in parameters such as surrounding rock damage, stress and strain around the tunnel, and internal forces of the support structure under three support schemes.Chen et al. [6] studied the propagation law of blasting waves in surrounding rock and analyzed the dynamic response characteristics of three forms of anchor support.Liang [7], Zhang [8], and others used numerical simulation methods to analyze the failure characteristics of different types of surrounding rock under high stress conditions and proposed corresponding support measures.Zheng et al. [9] used FLAC 3D to analyze the influence of different anchor layout methods on the stress of surrounding rock and anchor force, and determined the optimal anchor layout scheme for large-section tunnels in soft rocks.Zhao et al. [10] provided a method for calculating the impact range factor of anchor reinforcement and determined the length of anchors based on the minimum thickness of the pressure arch that must be formed for surrounding rock stability, the impact range factor of anchor reinforcement, and the spacing of anchors.Hu et al. [11] optimized the optimal length, transverse spacing, and longitudinal spacing of anchor bolts in the Wuqiaoling Tunnel.Xu [12] preliminarily determined the range of anchoring parameters based on engineering analogy, optimized the selected parameters through orthogonal experiments, and calculated the optimal support scheme.
Many researchers have studied the design and parameter selection of underground tunnel support structures and have achieved fruitful results.However, due to the complexity of the structure and geology of underground tunnels in hydropower stations, the layout of support structures for different projects varies and requires specific analysis.In this regard, this paper relies on actual engineering, selects an underground tunnel in a certain hydropower station, and uses FLAC 3D numerical simulation software to analyze the stability of surrounding rock and the support effect of support structures after excavation by comparing parameters such as the distribution of damaged zones in surrounding rock before and after support, surrounding rock deformation, internal forces of support structures, and block stability.

Project Overview
This hydroelectric station is located in Sichuan Province.The underground powerhouse, main transformer chamber, and tailgate chamber are arranged parallel to each other, with the axis direction in a north-south direction and an angle of 30°-40°with the rock formation.The rock classification and mechanical parameters, and the mechanical indicators of structural surfaces are shown in table 1.The three-dimensional finite element calculation model of the underground tunnel group is shown in figure 1, and the excavation unit of the underground tunnel is shown in figure 2. The finite element model used in this calculation adopts a hexahedral eight-node element.The bottom of the model is fully constrained, and the side is normal constrained.

Distribution Pattern of Surrounding Rock Failure Zones
Selecting a typical section for analysis.Figure 3 shows the distribution of the surrounding rock failure zone before and after support.As shown in figure 3, the support structure has a significant effect on supporting the surrounding rock.Due to the support effect of anchor rods, the surrounding rock is transformed from irreversible plastic deformation to reversible elastic deformation.After support, the rebound zone of the surrounding rock increased by about 139%, while the plastic zone decreased by 63%.The total damage and dissipation energy after support were greatly reduced.However, it should be noted that in the downstream side wall of the main powerhouse, due to the complex stress conditions caused by multiple tunnel intersections, the damage is more serious than in other parts, and there are many cracking zones distributed.Therefore, appropriate reinforcement support should be applied to this area of surrounding rock.
The depth of the failure zone on the downstream side of the hydropower station is greater than that on the upstream side.This may be due to the fact that there are more bus duct holes and tailwater holes connected on the downstream side of the underground powerhouse, which leads to complex stress conditions and severe stress concentration at the intersection of the downstream side of the cavern.
The damage to the surrounding rock of the cavern around the main powerhouse is greater than that of the tailwater regulating chamber and the main transformer chamber.This is mainly due to the fact that there are more developed cracks around the main powerhouse, and two cracks, f20 and f31, penetrate the main powerhouse cavern.In addition, the large excavation area of the main powerhouse cavern leads to poor stability of the surrounding rock.The ph band is the main factor causing instability of the surrounding rock around the tailwater regulating chamber.
After support, the depth of the surrounding rock failure zone around the cavern has significantly decreased.However, the depth of the failure zone at the downstream wall of the main transformer chamber remains relatively large, indicating the need for corresponding reinforcement support in that area.The effect of support on the depth of the failure zone at the upstream and downstream walls of the tailgate chamber is minimal, with only a 1.0m reduction in depth after support.The maximum depth of the surrounding rock failure zone at the downstream wall of the main transformer chamber is 6.84m, and it does not intersect with the plastic zone of the tailwater pipe

Distribution Pattern of Surrounding Rock Deformation
The displacement changes of the surrounding rock before and after support of the cavern are shown in figure 4. It can be seen from figure 4 that the deformation of the surrounding rock after support is significantly reduced compared to before support, especially at the high side wall.After the excavation and support of the cavern were completed, the surrounding rock of the main powerhouse deformed towards the excavation face, and the underground powerhouse cavern was generally stable under the support conditions.Due to the influence of the deformation of the two side walls, there was a slight arching phenomenon in the vault area, with arching values of 1.1mm and 2.3mm before and after support, respectively.The deformation of both side walls and rock anchor beams gradually increases from top to bottom.The deformation law of the surrounding rock in the main transformer chamber and tailgate chamber is basically consistent with that of the main powerhouse, mainly deforming towards the inner excavation face.Among them, the deformation of the lower part and bottom plate of the downstream wall of the main transformer chamber and the downstream wall of the tailgate chamber cavern is relatively large, with deformation values of 40mm and 20mm before and after support, respectively.

Analysis of Local Block Stability
Based on the geological profile of the underground powerhouse of the hydropower station and the geometric parameters of the structural planes, the shape and exposure conditions of each block were analyzed as shown in figure 5.
Through safety factor calculations, B2, B3, B4, B7, B8, B9, B13, B17, B18, B19, B20, B21, B22, B23, B24, and B27 were identified as the critical blocks.The sliding failure modes of the critical blocks include direct collapse failure, single face sliding failure, and double face sliding failure.The safety factors of the critical blocks before and after support are shown in table 2. The safety factor of the block with a collapse failure mode is regarded as 0.  As shown in table 2, the blocks are mainly exposed at the arch and sidewall positions of the main powerhouse and the main transformer cavern, and the block exposure locations coincide with the intensity of fault distribution.The volumes of the critical blocks range from 0.18 to 43.77m 3 , with 9 blocks having a volume greater than 10m 3 .Table 3 shows the minimum safety factor requirements for block stability in the underground powerhouse [13].
From table 2, it can be observed that the safety factors of the critical blocks in the main powerhouse area are relatively low, indicating relatively poor local stability.Prior to support, there are a total of 12 critical blocks in the main powerhouse, with all three types of sliding failure modes present.The safety factors range from 0 to 5.77, with 7 blocks falling below the minimum safety factor requirement.The local stability of the surrounding rock in the main transformer cavern is relatively good.There are 4 critical blocks in the main transformer cavern area, and the failure mode for these blocks is sliding along the sliding surface.The safety factors range from 1.25 to 14.53, with 2 blocks falling below the minimum safety factor requirement After support, the safety factors of the critical blocks in the main powerhouse surrounding rock range from 2.03 to 7.99.Compared to before support, the resistance to sliding and safety factors of the cavern surrounding rock blocks have been improved to varying degrees.For example, block B13 has transitioned from a collapse failure mode to a single sliding surface mode.The large-volume block B22 has seen its safety factor increase from 0 to 4.86 after support.The safety factors of the 4 critical blocks in the main transformer cavern surrounding rock range from 4.90 to 20.01, showing a significant improvement compared to before support.

Conclusion
Based on the actual excavation and support situation of an underground powerhouse at a hydropower station, numerical simulation software was used to calculate the distribution of damaged areas in the surrounding rock, depth of damage, deformation of the surrounding rock, and safety factors of the block structure.After analyzing the stability of the surrounding rock after excavation, the support effect of the support structure was reviewed.It is found that： (1) After support, the overall range of the damaged area in the surrounding rock shows a decreasing trend, with an increase in the elastic failure zone and a decrease in the plastic and cracking zones.After support, the deformation of the surrounding rock and internal forces in the support structure both decreased, and the safety factors of the block structure increased.The support structure of the cavern is reasonably arranged, and the support measures can effectively improve the stability of the surrounding rock in the cavern.
(2) Due to the presence of multiple bus holes and tailwater holes on the downstream side of the underground plant, the stress conditions at the intersection of the downstream chambers are complex and there is severe stress concentration.The depth of the damaged rock mass is greater on the downstream side than on the upstream side, and there are numerous cracked areas distributed.It is recommended to reinforce the surrounding rock at this location appropriately.
Due to the influence of faults and geological fractured zones, the damage to the surrounding rock of the main plant chambers is greater than that of the tailwater pressure regulating chamber, and the damage to the surrounding rock of the main transformer chamber.
(3) Before support, the sliding failure modes of the key blocks in the main plant and main transformer chambers include direct collapse failure, single sliding surface failure, and double sliding surface failure.Compared with the main transformer chamber, the safety factor of the key blocks

Figure 1 .Figure 2 .
Figure 1.The 3D finite element calculation model of the underground tunnel group

Figure 3 .
Figure 3. Distribution of the surrounding rock failure zone before and after support

Figure 4
Figure 4 Displacement distribution of surrounding rock before and after support, unit, mm

Figure 5 .
Figure 5. Overall distribution of potential instability blocks

Table 1 .
Mechanical parameters of surrounding rock and structural plane

Table 2 .
Stability calculation results of key blocks before and after support

Table 3 .
Minimum safety factor of block stability of underground powerhouse of hydropower station