Comparative analysis of deformation and failure mechanisms of underground powerhouses on the left and right banks of Baihetan hydropower station

Anchi Shi, Congjing Li, Wnging Hong, Gongd Lu, Jiwen Zhou,c, Hio Li,c,*

a PowerChina Huadong Engineering Corporation Limited, Hangzhou, China

b State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University, Chengdu, 610065, China

c College of Water Resource and Hydropower, Sichuan University, Chengdu, 610065, China

Keywords:Underground powerhouse Stress-controlled failure Structural plane-controlled failure Large deformation Intermediate principal stress

A B S T R A C T

1. Introduction

China’s western region is rich in water resources,but it is located on the periphery of the Qinghai-Tibet Plateau,which is significantly affected by the tectonic stress field.The underground powerhouses of many hydropower stations in this area are often characterized by a high magnitude of in situ stress, a complex geological structure and poor rock mass properties (Wang et al., 2020). These unfavorable geological conditions will lead to the deformation and instability of surrounding rocks,which poses a significant threat to engineering construction and the safety of personnel and equipment (Vazaios et al., 2019). Therefore, it is necessary to study the failure and deformation mechanism of large-scale underground powerhouses to ensure the safety of engineering construction.

Before the excavation of underground powerhouses, the rock mass is in a complex state of stress balance. After excavation, the stress balance around the powerhouse is destroyed,and the stress redistribution will lead to local stress exceeding the strength of the surrounding rocks, or cause excessive deformation of the surrounding rocks,resulting in instability of the surrounding rocks.In addition, the locally distributed soft structural plane (a rock mass system with poor mechanical properties) will also affect the integrity of the rock mass and thus reduce the stability of the surrounding rocks(Bruneau et al.,2003).Undoubtedly,the stability of surrounding rocks is the key to the construction of underground engineering. Over the past few decades, the failure and deformation of underground powerhouses have been of great concern and have attracted extensive attention from many scholars.Through in situ testing, field investigation and numerical simulation, the deformation mechanisms and failure characteristics of surrounding rocks during the excavation of large underground caverns have been studied extensively.

In situ testing is a crucial method for systematically studying the surrounding rock’s deformation and failure mechanisms of underground powerhouses. A large number of in situ tests (such as Atomic Energy of Canada Limited’s(AECL’s)Underground Research Laboratory (URL) (Read, 2004), Äspö Hard Rock Laboratory (HRL)(Stanfors et al., 1999), Yucca Mountain (Hsiung et al., 2005) and Underground Research Test at KAERI (KURT) (Kwon et al., 2006))have been carried out on the brittle rock of deep underground caverns to study the failure phenomena and mechanical mechanism of surrounding rocks,such as excavation damage,spalling and collapse.The research works provide an essential reference for the construction of underground structures. However, due to the limited scale of the testing, the dynamic response of the surrounding rocks cannot fully reflect that of the large-scale caverns with enormous span and high sidewalls.

Through field investigation and numerical simulation, researchers studied the effects of high in situ stress and complex geological conditions on large underground powerhouses. For the influence of in situ stress on the stability of surrounding rocks,Wu et al.(2010)discussed the unloading deformation and failure of the surrounding rock surfaces during the excavation of the Jinping I hydropower station in China by monitoring data analysis and numerical simulation. They indicated that the magnitude of in situ stress,rock properties and the method and sequence of excavation are the main reasons for the deformation and damage of the rock mass. Xiao et al. (2016) performed a study on rock pillars in Houziyan underground powerhouse in China. They pointed out that unloading and stress concentration under high in situ stress can reduce the surrounding rock integrity, resulting in significant rock damage.Kong et al.(2021)conducted a series of tests on a volcanic rock under one- (1D) to three-dimensional (3D) stress states to determine the stress-induced cracking mechanisms of the surrounding rocks. The results indicated that the samples exhibited different failure characteristics under different stress states,and the risk of unstable failure increased with increasing in situ stress.For the influence of complex geological conditions, Jeon et al. (2004)made different types of models to investigate the effects of faults and weak planes on the stability of a tunnel. The results showed that when a fault or weak plane crossed the upper part of the tunnel, the displacement of the crown was more remarkable, and the deformation of the surrounding rocks was closely related to the location of faults and weak planes. Huang et al. (2013) designed physical and numerical model tests to simulate tunnel excavation near an interlayer. The weak interlayer was found to cause asymmetrical stress distribution and increase the failure zones, thus reducing the stability of the surrounding rocks to a specific range.In addition, the distribution of fractures in the surrounding rocks is also closely related to the soft structural plane. Zhao et al. (2018)studied the fracture mechanisms of surrounding rocks with a weak interlayer using in situ microseismic monitoring and found that the fracture was distributed near the weak interlayer, which may harm the safety of the construction. However, many current field investigations and numerical simulations are focused on a single hydropower station, and the influencing factors considered are minimal. The deformation and failure mechanism of surrounding rocks under the control of different vital factors is rarely studied.

The underground powerhouse of Baihetan hydropower station is a large hydropower project under construction.In the process of construction and excavation,the underground powerhouses on the left and right banks showed deformation and failure of surrounding rocks to varying degrees(Zhao et al.,2018;Xia et al.,2020),which poses a severe threat to construction efficiency and safety.Numerous researchers have studied the failure and deformation mechanism of the surrounding rocks of the Baihetan underground powerhouse and achieved remarkable results. For example,through mechanical tests, field investigations and numerical simulations,Meng et al.(2016)and Shi et al.(2020)found that high in situ stress, brittle basalt, and widely distributed structural planes are essential causes of surrounding rock deformation and failure in the Baihetan. An in situ experiment involving microseismic monitoring was carried out to study the stress-and structure-controlled failure in Baihetan by Xiao et al. (2017), and the mechanisms of stress-and structure-controlled collapse were analyzed.Wang et al.(2019) analyzed the large deformation evolution and failure mechanism of the multi-free face surrounding rocks, and found that multi-free face is an essential factor leading to more significant spatial deformation of such a large-scale powerhouse.However,the study on the deformation of Baihetan mainly focuses on the left bank powerhouse, while the right bank is often neglected. The deformation and failure of the left and right banks are different due to the difference in the in situ stress environment and the structural distribution. Therefore, it is necessary to analyze the deformation and failure mechanism of the underground powerhouse on the left and right banks of Baihetan hydropower station.

Based on the geological conditions, geophysical exploration,monitoring data, numerical simulation and on-site investigations,the deformation and failure mechanisms were analyzed by comparing the differences in the deformation and failure characteristics of the surrounding rocks of Baihetan underground powerhouses on the left and right banks.The findings of this study can provide a valuable reference for the prevention and control of surrounding rock failure of large underground caverns under similar high in situ stress or unfavorable geological conditions.

2. Project background

2.1. Overview

The Baihetan hydropower station is located on the mainstream of the Jinsha River at the junction of Ningnan County, Sichuan Province and Qiaojia County, Yunnan Province. The Baihetan hydropower station is the second stage of four-cascade hydropower stations (Wudongde, Baihetan, Xiluodu and Xiangjiaba) in the lower reaches of the Jinsha River (as shown in Fig.1a).

The Baihetan hydropower station consists of main buildings such as barrage, flood discharge and energy dissipation facilities,and a water diversion and power generation system, with a total installed capacity of 1.6×107kW and an average annual generating capacity of 6.244 ×1010kW h. The underground powerhouses on the left and right banks are symmetrically arranged in the mountain on both sides (as shown in Fig.1b). Both underground powerhouses were excavated from top to bottom in ten layers.The total length of the underground powerhouse is 438 m, the height is 88.7 m,the top arch elevation is 624.6 m,the rock beam elevation ranges from 602.3 m to 604.4 m, and the widths below and above the rock beam are 31 m and 34 m, respectively. The Baihetan hydropower station is the world’s largest underground powerhouse project(as shown in Fig.2).The underground powerhouses on the left and right banks of Baihetan are arranged on the upstream side of the arch dam. The underground powerhouse area is characterized by high in situ stress and extremely complex geological conditions.

2.2. Geological conditions

The underground powerhouses on the left and right banks of Baihetan hydropower station are located in the upstream mountain of the dam abutment on both sides of the river valley. The horizontal buried depth of the left bank underground powerhouse is 600-1000 m, and the vertical buried depth is 260-330 m, with a cavern axis of N20°E. The horizontal buried depth of the underground workshop on the right bank is 420-800 m,and the vertical buried depth is 420-540 m,with a cavern axis of N10°W.The rocks of the left bank are composed of P2β23and P2β31layers, where P2β represents the Permian basalt. The lithology of the P2β23and P2β31layers is mainly composed of fresh aphanitic basalt and the second and third category of columnar jointed basalt scatters.The rocks of the right bank are mainly composed of P2β34, P2β41and P2β42layers.The lithology of the P2β34is mostly composed of tuff, which can be easily softened by water. The third category of columnar jointed basalt with a thickness of 15-28 m is developed at the bottom of the P2β41layer of the rock surrounding the right bank.The length of the column is generally 1.5-5 m, and the diameter is 50-250 cm.Based on careful consideration of the rock strength, rock integrity,structural plane condition and in situ stress level, the rocks of the left and right banks were classified into categories II,III1,III2and IV(Fig. 3), and the physico-mechanical parameters are shown in Table 1.

Fig.1. Location and layout of the underground caverns at the Baihetan hydropower station: (a) The cascade hydropower development of the Jinsha River, and (b) Layout of the Baihetan underground caverns.

Fig.2. 3D visualization on the left bank(a)and right bank(b)of the underground caverns and a cross-section of the powerhouse at the Baihetan hydropower station.The elevation is measured in m.

The surrounding rocks of the underground powerhouse has developed unfavorable structures such as sizeable weak interlayer(intralayer) shear bands, minor faults and cracks (as shown in Fig. 3). The geological structure of the underground powerhouses on the left and right banks of Baihetan hydropower station is mainly the primary structure and the faulted structure, and the primary structure mainly includes the columnar joints and the microcracks in the columns.According to the relationship between the fracture structure and rock flow layer occurrence, faulted structures are divided into the scale of faults, interlayer shear bands,intralayer shear bands and cracks.The fracture structures of the left and right banks of the underground powerhouse are dominated by rock debris and rigid structural planes, and the strikes are generally between N40°-70°W.Only a few sizeable weak interlayer (intralayer) shear bands pass through the entire underground powerhouse. A large number of faults and cracks are distributed discontinuously. The left bank underground powerhouse is developed with three main large faults(F719,F720and F721),which are cutting-type steeply dipping faults with widths of 5-25 cm and extension lengths of 300-500 m. The large cracks are mainly composed of T720and T721, two large steep rigid structural planes.The interlayer shear band C2cut through the powerhouse in a lower sidewall, with a thickness of 10-60 cm and an average thickness of ～20 cm. This interlayer shear band C2is mudsandwiched rock debris and can be easily softened when exposed to water.The intralayer shear band LS3152obliquely crosses the roof arch of the powerhouse, mainly in the form of rock debris, with a width of ～2 cm and a length of ～200 m(as shown in Fig.3a).The geological structure of the right bank underground powerhouse is more complex than the left bank, and more parts of the powerhouse are intersected by structural planes.On the right bank,F20is the main steeply dipping fault, and the width of F20is approximately 30-40 cm.The large cracks include T813,with a width of 3-5 cm. The interlayer shear bands (upper section of C3, C3-1) pass diagonally through the sidewall of the powerhouse,and C4cuts the roof arch of the powerhouse. The upper section of C3is of rockdebris type. C3-1is mud-sandwiched rock debris with a thickness of 20 cm.C4is mud-sandwiched rock debris with a thickness of 40-60 cm.Intralayer shear band RS411,which is a group of cracks with a width of approximately 5-8 m (as shown in Fig. 3c), obliquely crosses the sidewall of the powerhouse.The geological features and the strength parameters of the faulted structures are shown in Table 2.

Table 1 Physico-mechanical parameters of the rocks in the Baihetan underground powerhouses.

Table 2 The geological features and strength parameters of the faulted structures.

Fig. 3. The vertical geological (a) and horizontal cross-section (b) of the left bank underground powerhouse and the vertical geological (c) and horizontal cross-section (d) of the right bank underground powerhouse.

2.3. In situ stress

The in situ stress in the underground powerhouse area of the Baihetan hydropower station is dominated by tectonic stress, and the site can be classified as a high in situ stress area.The horizontal in situ stresses of the underground powerhouses on the left and right banks are more remarkable than the vertical ones. The maximum and intermediate principal stresses are almost horizontal, and the minor principal stress is roughly perpendicular to the horizontal plane. According to the field test results, the large horizontal principal stress in the left bank powerhouse area is approximately 19-23 MPa, the azimuth is in the range of N30°-50°W, the inclination is approximately 5°-13°, and the measured maximum horizontal stress can reach as high as 33.39 MPa. However, the large horizontal principal stress of the right bank powerhouse area is approximately 22-26 MPa, the azimuth is in the range of N0°-20°E,the inclination is approximately 2°-11°,and the measured maximum horizontal stress is 30.99 MPa.In contrast,the in situ stresses on the right bank are higher in magnitude.The relationship between the orientation of the in situ stress and the layout of the underground powerhouse shows that the stress distribution of the underground powerhouses on the left and right banks is different. The underground powerhouse on the left bank presents an unfavorable condition where the maximum principal stress intersects the axis of the powerhouse at a large angle of 50°-70°(as shown in Fig. 4a). In comparison, the maximum principal stress of the right bank intersects the axis of the powerhouse at a small angle of 10°-30°(as shown in Fig. 4b). At the same time,there is also a certain degree of difference in the strength to stress ratioRc/σm(whereRcis the saturated uniaxial compressive strength of the rock mass in the underground powerhouse area, and σmis the maximum stress in the surrounding rocks)for the rock mass of the underground powerhouses on the left and right banks.TheRc/σmvalue for the left bank is approximately 3.22-5.89,and that for the right bank is approximately 2.85-5.09.The differences in the in situ stress distribution and the strength to stress ratio of the surrounding rock lay a foundation for the difference in the deformation and cracking characteristics of the underground powerhouses on the left and right banks.

3. Deformation and failure characteristics

The underground powerhouse was excavated using the drilling and blasting method layer by layer,and the overall excavation face was well-formed.Deformation and failure of the surrounding rocks generally occur after a period of excavation in the Baihetan underground powerhouse (as shown in Fig. 5), which can be summarized into the following categories.

3.1. Stress-controlled failure

Under the action of high in situ stress, the excavation of the underground powerhouse will lead to stress redistribution of the surrounding rocks.After stress redistribution,numerous cracks will be created in the surrounding rocks,which will lead to surrounding rock damage,and stress-controlled failure eventually occurs as new fractures extend and coalesce(Cai et al.,2004).In the underground powerhouse of Baihetan,stress-controlled failure occurred on both the left and right banks, and it manifested mainly as spalling,fracture failure, slack collapse and shotcrete cracking.

Spalling is a common macroscopic failure phenomenon in hard,brittle rock masses under high in situ environments and is manifested as flake or plate-like peeling of rock masses. Spalling is distributed mainly in the roof arch near the spandrel of the left and right bank underground powerhouse(Liu et al.,2017)(as shown in Fig. 5a1). In the axial direction of the powerhouse, the spalling developed continuously, with a general depth of 10-30 cm and a local depth of 50-70 cm.The width of the spalling on the left bank is generally 3-5 m in the direction of the vertical powerhouse axis,with the maximum width at 8 m.In contrast,the width of spalling is 3-8 m in the right bank,with the maximum width of 12 m.From the perspective of the damage zone affected by spalling, the damaged area on the left bank is larger, accounting for approximately 21%-36%, while the damaged area of the right bank is slightly smaller, only 6%-7%. From the perspective of distribution,the spalling on the left bank is more continuous in the axial direction, and the total length near the arch section is ～350 m, accounting for 80% of the total length of the powerhouse, while the spalling on the right bank is relatively scattered in the axial direction and develops in some local caves. Generally, spalling is more widely developed in the left bank underground powerhouse than in the right bank.

Fracture failure refers to the structural failure of rocks caused by high in situ stress. The mechanical mechanism is stress concentration caused by excavation, and thus cracks can develop in the surrounding rocks due to insufficient strength. The cracks in the surrounding rocks gradually expanded and connected under high in situ stress, and eventually, the rock mass cracked and even collapsed.The fracture failure of underground powerhouses on the left and right banks occurred mainly at the upstream spandrel and the junction of each layered excavation of the downstream sidewall. As shown in Fig. 5a2, during excavation, fracture failure occurred in the downstream sidewall between sections L0-040 m and L0-047 m of the left bank underground powerhouse, and the depth was generally 10-30 cm, with the maximum depth of 50-150 m. Due to the high in situ stress and stress adjustment after excavation, there was also a large area of collapse in the upstream reaches of the roof arch. However, the fracture failure is more severe on the right bank due to the higher in situ stress.For example,the rock anchor beam is partially missing and poorly formed in the powerhouse on the right bank (as shown in Fig. 5b1), which seriously affects the efficiency of the project.

When support is not implemented promptly, the rock mass slack collapse phenomenon will occur with the continuous development of relaxation. The slack collapse area of the Baihetan underground powerhouse generally has a length of 4-15 m,a height of approximately 1-3 m, and a development depth of generally 20-50 cm. The slack collapse of the left bank is more pronounced than the right bank under unfavorable in situ stress environments.Nearly 30 large-scale slack collapses occurred in the underground powerhouse on the left bank, which seriously threatened the construction and personnel safety. For example, fractures were developed in the downstream sidewall between sections L0-005 m and L0+016 m of the left bank, the surface rock mass was relaxed,and slack collapse occurred without support after excavation (as shown in Fig. 5a3). However, the slack collapse of the right bank mainly occurred in the vicinity of the fracture failure.The collapse depth is 20-40 cm and is mainly controlled by the joints in the surrounding rocks.

During the powerhouse excavation, due to the adjustment of surrounding rock stress, shotcrete cracking occurred in the roof arch, downstream sidewall and surrounding auxiliary caverns of the powerhouse on the left and right banks after the shotcrete-bolt support was completed.This failure is similar on the left and right banks.Shotcrete cracking in the left bank occurred mainly between sections L0-71.6 m and L0+350 m, which were distributed along the axial direction of the powerhouse. The shotcrete cracking was concentrated mainly upstream of the roof arch and the spandrel(as shown in Fig. 5a4), and only a tiny amount was distributed on the downstream side. The length of the cracking upstream was～390 m,accounting for 86%of the total length of the powerhouse.Cracking was approximately distributed along the boundary of the excavation.The failure is manifested mainly as cracking and falling of shotcrete,the width of which is generally 3-6 cm,and the local maximum width is 16 cm.In comparison,the shotcrete cracking on the right bank was concentrated mainly in the upstream spandrel between sections R0-040 m and R0+149 m (as shown in Fig.5b4).The cracking length was ～253 m, accounting for 56% of the total length of the powerhouse.

Fig. 4. The relationship between the direction of principal stresses and the axis of the Baihetan powerhouse on the left (a) and right (b) banks.

3.2. Structural plane-controlled failure

The soft structural plane is a rock mass system with poor mechanical properties,which will affect the integrity of the rock mass and thus reduce the stability of the surrounding rocks. When the surrounding rock loses its support under gravity during excavation,it will fall into the tunnel or slide along the existing structural plane due to the disintegration of the structure and weak structural planes (Fraldi and Guarracino, 2009), which can be treated as structural plane-controlled failures. Block instability phenomena such as falling, collapse and sliding along the structural plane are considered a structural plane-controlled failure. In the underground powerhouse of the Baihetan hydropower station, this failure occurred mainly on the right bank.

The unstable rock block was formed as the joint plane, and the free surface cut through. Under the influence of disturbance, the block may fall or slide down along the structure plane.The unstable rock blocks exposed in the right bank underground powerhouse of the Baihetan hydropower station pose a greater threat to construction. Thirteen small rock blocks were found on the left bank sidewall,with a volume of 2-80 m3each.However,the rock blocks were relatively stable and did not collapse.In contrast,three small blocks were exposed on the right bank sidewall, and all had a certain degree of block collapse. For example, rock block falling occurred at the downstream sidewall of layer I between sections R0+290 m and R0+295 m of the right bank powerhouse. The collapsed length, height, depth and volume were 4 m, 4 m, 0.6 m and 10 m3, respectively(as shown in Fig. 5b2).

The structural collapse of the underground powerhouses on the left and right banks is closely related to the intralayer and interlayer shear bands distributed in the region. This failure is even more evident in the right bank underground powerhouse.In the process of excavation,the structural collapse will occur in the bottom rock mass when the shear band obliquely crosses the powerhouse. The phenomena of collapse and block falling in the left bank occurred mainly in the affected area of the intralayer shear band LS3152and interlayer shear band C2.However,the scale is generally small,and the depth is 0.3-2 m with little impact on the project.This situation is opposite to the case in the right bank underground powerhouse.There are serious safety problems in the underground powerhouse on the right bank under the influences of interlayer shear band C4,intralayer shear band RS411and fault F20. Under the influence of RS411,a large area of collapse occurred at the downstream spandrel between sections R0+280 m and R0+304 m of the right bank powerhouse.The scope of the collapse has a length of 24 m,a width of 5-7 m,an average depth of 2 m,a maximum depth of 3.1 m,and a volume of 120-150 m3, as shown in Fig. 5b3.

3.3. Large deformation

Large deformation was caused by excavation and unloading in the surrounding rocks of the underground powerhouse(Bizjak and Petkovˇsek, 2004). Based on research findings and extensive engineering experience, deformation greater than 50 mm can be defined as large deformation for the hard basalt at Baihetan (Li et al., 2017). The monitoring results show that the area with large roof arch deformation in the powerhouse on the left bank is located mainly in the sections affected by C2and LS3152,while the area with large roof arch deformation on the right bank is located in the sections affected by C3, C3-1, C4and RS411. Generally, the deformation of the powerhouse occurs mainly within a depth of 6.5 m,and the deformation at depths of 11 m and 17 m of the local tunnel section is also relatively large.Typical curves of deformation versus time with excavation steps of the roof arch on the left and right banks are depicted in Fig. 6. The monitoring results show that the deformation curves followed a stepwise rise with an increase in excavation steps, but this increase rate declined. However, the deeper surrounding rocks gradually deformed as excavation proceeded. Fig. 6a shows relative deformations at various depths of surrounding rocks in section L0-012 m of roof arch of the left bank powerhouse,which was affected by the intralayer shear band LS3152and the surrounding joints. The deformation is manifested mainly as the deformation at the orifice, which decreases gradually from the surface to the inside. Only a small deformation occurred at greater depth due to the redistribution of stress during excavation.The cumulative curve clearly shows that at the initial stage, the cumulative displacement at 1.5 m away from the orifice was only 2-3 mm,indicating that the rock mass of the roof arch had decent stability at that time and no large deformation occurred in the free face. However, when the upstream spandrel was excavated, the cumulative displacement at 1.5 m,3.5 m and 6.5 m away from the orifice increased significantly.

The relative deformations at various depths of surrounding rocks in sections R0-020 m and R0+076 m of the roof arch of the right bank powerhouses are shown in Fig.6b and c.These relative deformations were affected by the interlayer shear bands C3-1and C4and the intralayer shear band RS411, and all of them showed large deformation.However,there were specific differences in the deformation between them. In section R0-020 m, the overall deformation is more considerable. The surrounding rock deformation grows slowly with time at the early stage of excavation,and the initial deformation growth rate is less than 0.1 mm/d.Subsequently, the deformation shows an acceleration trend, and the deformation growth increases rapidly, with surface deformation reaching nearly 100 mm. Deep deformation occurs in section R0+076 m. According to field observations, continuous cracking occurred in September 2015 in the shotcrete on the upstream side of the spandrel of the right bank between sections R0+040 m and R0+140 m with an extensive damage range. Subsequent research found that this phenomenon is caused mainly by stress concentration in the excavation, which is also the reason why the deformation of Fig. 6c suddenly changed in September 2015. After the abrupt change, the deformation in section R0+133 m gradually slowed down. However, as of October 2019,the deformation at different depths increased gradually and did not seem to converge. Regarding deformation, section R0-20 m cave shows mainly surface deformation, while section R0+076 m cave still has relatively large deformation and continues to increase at the depth ranging from 11 m to 17 m.

Fig. 6. Typical monitoring results for the deformation of the surrounding rocks at the main powerhouse site: (a) Deformation process of the multipoint extensometer in section L0-012 m of the roof arch of the left bank powerhouse,(b)Deformation process of the multipoint extensometer in section R0-020 m of the roof arch of the right bank powerhouse, and (c) Deformation process of the multipoint extensometer in section R0+076 m of the roof arch of the right bank powerhouse.

In general, the monitoring data show that the deformation of surrounding rocks on the left bank is smaller and tends to converge compared with the deformation of surrounding rocks on the right bank. In contrast, many measuring points on the right bank show large displacement and continuous growth, mainly due to the different geological conditions of underground powerhouses on the left and right banks.The in situ stress on the right bank is higher than that on the left bank, and the weak structural planes are widely distributed.

3.4. Comparative analysis of the failure characteristics on the left and right banks

According to the on-site deformation and failure investigation of the underground powerhouse of Baihetan,the deformation and failure of the surrounding rocks during the excavation of the underground powerhouse on the left and right banks are different.The stress-controlled failure (spalling, fracture failure, slack collapse and shotcrete cracking) of the surrounding rocks represents the main stability issue for the underground powerhouse on both the left and right banks.In addition,the right bank also shows structural plane-controlled failure(unstable rock block and structural collapse) and large deformation. As shown in Table 3,the geological structure of the right bank is more complex,and the large structural planes are relatively more numerous and widely distributed.The left and right bank powerhouse areas have high in situ stress, but the stress distribution is different. The left bank has the unfavorable condition where the maximum principal stress intersects the axis of the powerhouse at a large angle.At the same time, the higher in situ stress environment where the intermediate principal stress of the left bank intersects the axis of the powerhouse at a large angle is also relatively unfavorable to the stability of the surrounding rocks on the right bank. These differences in the in situ stress and rock structure conditions are the internal determinant for the difference in deformation and failure of the left and right bank underground powerhouses.

Table 3 The differences in basic condition, deformation and failure of the underground powerhouse on the left and right banks.

Acoustic wave and borehole TV technology are essential means to reflect the difference of surrounding rock deformation and failure(Peyras et al.,2015).Field tests of the acoustic wave and borehole TV in Baihetan were carried out in the study,and the results are shown in Fig. 7.The wave velocity decreases with an increasing number of fractures and increases with increasing rock mass density. Joints in the surrounding rocks are generally believed to be the main reason for wave velocity reduction (Bao et al., 2020). The typical field test results of the acoustic wave curves of the sidewall downstream at section L0-012 m on the left bank and section R0+228 m on the right bank at an elevation of 601 m are shown in Fig.7a and b.Combining acoustic wave data and borehole TV images,a large number of cracks occur in the surficial rock mass under the influence of construction,resulting in a lower wave velocity (～3500 m/s). This feature is the same on the left and right banks, but the acoustic wave curve features in the deeper rock mass are quite different.In the deep region of the left bank, the acoustic waves have a higher velocity(～5000 m/s)with minor fluctuations,which indicates that the rock mass has fewer fractures and better properties.However,in the deep region of the right bank, the acoustic waves fluctuate considerably(reaching up to 5480 m/s at the maximum and only 3050 m/s at the minimum),indicating that the fractures in the right bank are widely distributed. This phenomenon is consistent with the above geological difference between the left and right banks.The relaxation depth(the thickness between the excavation face and the intact rock mass,where a sharp increase in acoustic wave velocity and little change in the deeper part can be defined as the relaxation depth of the surrounding rocks)of the surrounding rocks at an elevation of 608 m of the downstream rock anchor beam of the left and right banks is shown in Fig.7c and d as a function of the excavation progress.As the excavation proceeds,the relaxation depth increases gradually due to the disturbance of the surrounding rocks.However,from Fig.7c,the relaxation depth of the rock anchor beam on the left bank can be seen to rise slowly. When the layer VIII was excavated, the deformation rate gradually flattened out. The deformation and failure of the left bank are determined mainly by the in situ stress, and the relaxation depth is gradually adjusted concerning the stress redistribution in the excavation process.However,the relaxation depth on the right bank suddenly increases to a greater depth in a short time.It changes little in the subsequent excavation process,indicating that the higher in situ stress and the relatively widely distributed weak structural plane of the surrounding rocks play an essential role in the deformation and failure of the right bank.

Fig. 7. The field test results of acoustic wave and borehole TV techniques: The acoustic wave curves of the down sidewall at section L0-012 m on the left bank at an elevation of 601 m (a) and section R0+228 m on the right bank at an elevation of 601 m (b); The relaxation depth of the surrounding rock at an elevation of 608 m of the downstream rock anchor beam of the left bank (c) and right bank (d) with the development of excavation.

4. Analyses of failure mechanisms

To explore the failure mechanisms,the influences of the in situ stress and rock mass structural planes are analyzed in this part.The maximum and intermediate principal stresses are the reason for the failure of stress control on the left and right banks,respectively.The widely distributed structural planes are the reason for the structural plane-controlled failure and substantial deformation on the right bank. The simulation results from 3DEC, measured deformation data from multipoint extensometers and stress statistics from anchor dynamometers support the failure mechanism.

4.1. Numerical simulation

To further explore the deformation and failure mechanisms of the surrounding rocks, a numerical analysis of the underground powerhouse on the left and right banks of Baihetan was carried out based on 3DEC software. During the calculation, the left bank and right bank underground powerhouse models considering the influence of structural planes(C2,LS3152,F719,F720,F721,T720and T721for the left bank, C3, C3-1, C4, C5, RS411, F20and T813for the right bank) were used to verify the failure mechanism (Fig. 8). Considering the complexity of the modeling process, the model was simplified in the numerical calculation, and the scope of some structural planes is expanded. Considering the small number and scale of the structures treated by this method,the influence of this method on the deformation and failure mechanism analysis of surrounding rocks is ignored in this paper.

According to the excavation process shown in Fig. 2, the excavation of seven floors of the powerhouse was considered. Considering the distribution of surrounding caverns, the calculation area sizes of both the left and right banks are 553 m× 234 m × 260 m(main powerhouse axis×water flow direction×vertical direction).The 460 m elevation was considered as the bottom boundary,198 blocks and 51,785 zones were generated for the left bank, and 195 blocks and 52,434 zones were generated for the right bank in 3DEC.The blocks were piled to form the overall structure, and then structural planes were created to build the excavation model.In the calculation process, all soft structural planes were simulated by structural elements in 3DEC,and the mechanical parameters of the structural planes were set according to the actual situation to reflect the influence of the structure plane on the stability of surround rocks. In order to simplify the calculation model, the surrounding rock is considered to be uniform in the calculation process. The mechanical parameters of surrounding rock III1with the largest proportion were used as the calculation parameters, as shown in Table 1. The parameters of the structural plane were selected according to Table 2. It is worth noting that all the values for the mechanical parameters were obtained through laboratory or in situ tests. In addition, the Mohr-Coulomb shear strength criterion was employed to model the yield and failure behaviors of the surrounding rocks during the numerical simulation.

Fig. 8. Calculation model of underground powerhouse excavation on the left and right banks of Baihetan in 3DEC.

In underground engineering construction,the initial stress field plays a vital role in the deformation and failure of surrounding rocks. In order to carry out realistic engineering numerical calculations, the reliability of the initial stress field must be guaranteed first.At Baihetan underground powerhouse site,the initial stress is mainly measured by the stress relief and hydrofracturing techniques.Based on the measured values in the field,we found that the initial stress distribution in the underground powerhouse area of the left and right banks had a close relationship with the buried depth, as shown in Table 4. The stress distribution functions in Table 4 were used for the simulation in this study. After the initial stress environment was set up, the excavation process of the underground powerhouse on the left and right banks was simulated and analyzed, and the displacements, stress and deformation distribution characteristics were obtained.

Table 4 Initial stress distribution in the underground powerhouse area on the left and right banks.

4.2. Mechanism of stress-controlled failure

The relationship between the in situ stress and the axis of the underground powerhouse determines the basic situation of surrounding rock failure(Fellner and Hobson,2008).According to the field investigation of the distribution of stress-controlled failure(such as spalling, fracture failure, slack collapse and shotcrete cracking)in the roof arch of the underground powerhouses on the left and right banks of Baihetan, stress-controlled failure occurred mainly at the position between the roof arch and spandrel upstream, which has an apparent spatial correlation with the in situ stress distribution in the powerhouse area. In general, stresscontrolled failure occurred on the left bank at a large angle or approximately perpendicular to the maximum principal stress direction on the cross-section of the powerhouse. The stresscontrolled failure occurred on the right bank at a large angle or approximately perpendicular to the intermediate principal stress direction on the cross-section of the powerhouse. This pattern of stress-controlled failure elucidated above is related mainly to the distribution of the in situ stress in the underground powerhouse area on the left and right banks,which is inclined slightly upstream.The tangential component of the in situ stress in the vertical excavation section is prominent after the excavation,which leads to stress concentration in the upstream spandrel, resulting in deformation and failure such as spalling, fracture failure and shotcrete cracking. Regarding the intensity of failure, the stress-controlled failure of the surrounding rocks of the underground powerhouse on the left bank was larger and more continuous. However, this failure also developed a specific scale on the right bank.

The process of surrounding rock deformation and failure with the change in stress state in the excavation process of the main powerhouse under a high in situ stress environment is depicted in Fig. 9a. In the initial state, the rock mass is subjected to a combination of radial stress(σr),tangential stress(σt)and axial stress(σa)in a triaxial stress state.The radial stress(σr)is relatively large in the current in situ stress environment. During excavation, the radial stress(σr)is released so that the tangential stress is concentrated at the upstream spandrel and adjacent sidewalls. The rock mass is thus in an unfavorable state of biaxial compression (radial stress σr≈0).Sudden unloading will lead to the expansion of the original small cracks on the surface and in the shallow surrounding rocks.Under the action of tensile stress,the surrounding rock will deform towards the free face. Many parallel cracks will be formed in a specific range due to the extension of small cracks(Cai,2008).The surrounding rock will split along the deep joints under gravity or the construction disturbance,resulting in stress-controlled failure.

For the left bank powerhouse, the maximum principal stress intersects the powerhouse axis at a large angle (approximately 50°-70°), which is unfavorable to the stability of the surrounding rock.Taking the L0+152 m section from a numerical calculation as an example, the tangential component of the in situ stress in the vertical plane is significant after adjusting the in situ stress. Stress concentration occurred at the roof arch and the upstream spandrel,and the value of the compression reached 60 MPa, leading to rock mass crushing(Fig.9b).However,the angle between the maximum principal stress and the axis of the powerhouse on the right bank is only 10°-30°, which is relatively favorable for the stability of surrounding rocks. In addition, in situ stress on the right bank is generally higher than that on the left bank. As shown in Fig. 9c,there were also stress concentrations at the roof arch and the upstream spandrel,and the value of the compression reached 51 MPa,which is less than that on the left bank, resulting in the stresscontrolled failure in the right bank, which is affected mainly by the intermediate principal stress.

4.3. Mechanism of structural plane-controlled failure and large deformation

Overall, the exposure position and spatial distribution of the shear band,faults,cracks and other II and III structural planes have a close correlation with the structural plane-controlled failure of the surrounding rocks. The rock masses near weak geological structures are relatively broken and unstable rock blocks, and structural collapse can thus occur easily.The weak structure of the underground powerhouse on the right bank was more prominent,resulting in structural plane-controlled failure. The numerical calculation results(Fig.10)show that the soft structural planes have apparent control over the deformation of the surrounding rocks.Discontinuous deformation occurred when the structural planes cut through the sidewall,and the deformation difference between the two sides of the structural plane reached 60-80 mm.The roof arch cut by the structural plane is deformed, and there is a risk of collapse (Fig. 10b). The sidewall deforms considerably along the structural plane, and damage such as sliding will occur (Fig.10c).Before excavating the underground powerhouse, there are many interlaced structural planes in the surrounding rocks, making the rock masses relatively broken and unstable, but most of these interlaced structural planes are in a relatively stable state. The redistribution of stress in the surrounding rocks after excavation leads to deterioration of the stability of the surrounding rocks,which will eventually deform under gravity or other disturbances.Fig.11 shows the main mode of structural plane-controlled failure in the underground powerhouse of Baihetan hydropower station.There are two main types of structure-control failures in the underground powerhouse on the left bank.One type occurs at the roof arch when crossed obliquely by the weak structural plane(LS3152).Failure phenomena such as collapse and rock block falling could thus easily occur under the action of gravity or stress during the excavation process due to the control of the structural plane(Fig. 11a). This kind of failure is also common in the right bank around C4, RS411, etc. The other type occurs at the middle of the powerhouse, where the cavity is cut by the weak structural plane(C2). Under the influence of the structural plane, the upper and lower surrounding rocks of the structural plane may interact with each other, causing shear sliding and block collapse along the structural plane (Fig.11b). Similarly, this failure also occurs in the right bank around C3and C3-1. In addition to these two types of failure, there are other types of structural plane-controlled failure in the right bank underground powerhouse under the control of the weak structural planes. For example, under the combination of deterministic weak structural planes and faults, unstable blocks were formed at the entrance of the#10 tailwater diffusion section(Fig. 11c). Multiple groups of structural planes cut through the strata, and internal collapse will occur when the rock structure is squeezed (Fig. 11d), which is evident on the right bank between sections R0+182 m and R0+197 m.

Fig.9. Schematic diagram of the stress-controlled failure mechanism of the underground powerhouse of Baihetan:(a)A typical change in the stress state and the failure process at the near-surface surrounding rocks during the excavation, (b) The maximum compression stress of the L0+152 m section in the left bank calculated by 3DEC and the stresscontrolled failure mechanism,and(c)The maximum compression stress of the R0+215 m section in the right bank calculated by 3DEC and the stress-controlled failure mechanism.

At the same time,when the interlayer zone is located above the roof arch,the deep deformation of the rock mass at the bottom of the interlayer zone will increase significantly. The numerical calculation and monitoring results for roof arch deformation of the underground powerhouses on the left and right banks of Baihetan are shown in Fig.12.According to the deformation data,the roof arch of the underground powerhouse on the left bank affected by LS3152and the roof arch of the underground powerhouse on the right bank affected by C4show more significant deformation. The deformation depth is also more considerable than the deformation depth of the section free from the influence of interlayer shear bands. However, in terms of the magnitude of deformation, the large deformation of the surrounding rocks caused by the influence of the structural plane is more evident in the right bank. The maximum deformation of the right bank reaches 127.76 mm at section R0+028 m (Fig. 12b), while the maximum deformation value of the left bank at section L0+076 m is only 41.69 mm (Fig.12a).

Fig.12. The numerical results calculated by 3DEC and the monitoring results for roof arch deformation on the (a) left and (b) right bank Baihetan underground powerhouse.

After excavation,the deformation of the left bank powerhouse was detected in eight testing sections,and the deformations of the right bank powerhouse were detected in nine testing sections.Deformations of the surrounding rocks were monitored with multipoint extensometers, and stresses were monitored with anchor dynamometers during excavation in each section. Fig.13a shows the statistical results for the multipoint accumulative displacement of the extensometers at a depth of 1.5 m from the orifice in the area of the arch, rock anchor beam and sidewall on January 4, 2020. The large deformation of the underground powerhouse on the right bank is more evident than that on the left bank. The displacements over 50 mm make up a comparatively large share,accounting for 42.5%on the right bank and 25%on the left bank. Fig. 13b shows the statistical results for the accumulative stress statistics of the anchor dynamometers in the area of arch, rock anchor beam and sidewall on January 4, 2020.The magnitude of anchor dynamometers measured on the right bank is also much higher than that measured on the left bank.The proportion on the right bank of the anchor dynamometer with the monitoring value reaching 2000 kN is approximately 50%, while the proportion of the left bank is only 44.2%.The main reason for these phenomena is that the overall in situ stress on the right bank is higher than that on the left bank, the geological conditions on the right bank are more complicated than that on the left bank,and the distribution of faults, shear bands and fissures is more extensive.

Fig.10. Discontinuous deformation of the surrounding rocks caused by the cutting action of the soft structural planes calculated by 3DEC:(a)Deformation of the upstream sidewall,(b) Deformation of the sidewall and the roof arch caused by C3 and RS411, and (c) Deformation distribution along the R0+185 m section.

Fig. 13. Statistics of the surrounding rock deformation distribution (a) and monitoring values of the anchor cable dynamometer (b) of the left and right bank underground powerhouses.

5. Discussion

The above analysis concluded that the deformation and cracking of the surrounding rocks of the underground powerhouse at Baihetan hydropower station are closely related to the in situ stress and rock mass structural plane. The types of surrounding rock failure revealed by the excavation of underground powerhouses are divided into two categories:stress-controlled failure and structural plane-controlled failure.The large deformation of the surrounding rocks was also prominent. Stress-controlled failure accounts for a large proportion of failure on both the left and right banks of the underground powerhouse. In addition, the right bank also shows structural plane-controlled failure and large deformation. Studies of other underground powerhouses have also found that these types of failures are common.

Because of the high in situ stress environment,stress-controlled failure (e.g. spalling, splitting, shotcrete cracking, rockburst)occurred in the underground powerhouse of Jinping I hydropower station at the section of haunch,arch seat and sidewalls(Qian et al.,2018). Stress-controlled failure (e.g. splitting, slabbing, bolt head caving in) also occurred in the underground powerhouse of Houziyan hydropower station (Xu et al., 2015). Although the deformation and failure of the surrounding rocks of the underground powerhouse for these three hydropower stations are essentially caused by the high in situ stress, the in situ stress characteristics and mechanisms of the three hydropower stations are quite different. The maximum principal stress intersects the axis of the powerhouse at a large angle, which leads to the stress concentration in the upstream spandrel,and this is the reason for the stresscontrolled failure for the underground powerhouse on the left bank of Baihetan hydropower station. The maximum principal stress of the powerhouse of the Houziyan hydropower station intersects the axis of the powerhouse at a small angle. The deformation and failure of surrounding rocks are controlled mainly by the intermediate principal stress,which is nearly perpendicular to the axis,similar to the failure of the underground powerhouse on the right bank of Baihetan hydropower station. The maximum principal stress of the powerhouse of Jinping I hydropower station also intersects the axis of the powerhouse at a small angle, but the relatively low rock strength under a high in situ stress environment leads to the stress-controlled failure of the surrounding rocks. In general, stress-controlled failure under a high in situ stress environment has become a common problem in the excavation process of large underground powerhouses.

Fig.11. The mechanism of structural plane-controlled failure of the underground powerhouse of Baihetan:The weak structural plane cutting the powerhouse obliquely in the area of the roof arch(a)and the middle of sidewalls(b)leads to collapse and slide,and deterministic weak structural planes(c)and multiple groups of structural planes cut rock strata(d) leads to the instability of the surrounding rock of the top arch.

Under the control of a complex and disintegrated rock mass structure, the large-scale collapse occurred in the section affected by diabase dikes of the underground powerhouse of Dagangshan hydropower station(Shen et al.,2017).Numerous potential sliding blocks occurred in the underground powerhouse of Xiluodu Hydropower Station under the control of structural plane combinations(Wang et al.,2020).These failures are not only dominated by rock properties but also dominated by in situ stress (Martin et al.,1999). Each project has unique topographic and geological conditions, and its failure mode and intensity are also different. Local collapse and unstable rock blocks caused by structural plane cutting are the main types of structural plane-controlled failure in the excavation process of large underground powerhouses. The structural plane-controlled failure caused by C3,C3-1,C4and RS411in the underground powerhouse on the right bank of Baihetan represents a unique phenomenon.

In the Baihetan underground powerhouse, shotcrete, mortar anchor bars and prestressed anchor bars are the main means to control the deformation and failure of the surrounding rocks. In similar projects, more targeted measures need to be adopted to strengthen the control effect. For stress-controlled failure, a reasonable arrangement of the excavation sequence and the use of thin-layer excavation should be undertaken to reduce the amount of stress release and surrounding rock stress concentration after excavation.When the magnitude of the in situ stress is high,short bolt supports should be carried out first,and systematic anchor bar supports and anchor cables (Fig.14a and b) need to be carried out after the in situ stress is released, which is the most effective method to ensure the stability of the surrounding rocks in a high in situ stress environment. For structural plane-controlled failure,providing confining pressure and strengthening the rock mass quickly by shotcrete-bolt support to improve the bearing capacity of the rock mass is an important measure to control the deformation of the rock mass and reduce unloading relaxation (Fig. 14c).When the rock mass shows continuous deformation without convergence, low-pressure consolidation grouting, systematic anchor bar supports or thru-anchor cables, and other measures are required to reinforce the surrounding rocks when necessary.

Fig.14. Treatment measures for surrounding rock deformation and failure:(a)System anchor bar supports and anchor cables for spalling and splitting at the roof arch and spandrel and (b) for fracture failure at the sidewall, and (c) Shotcrete-bolt support for the unstable rock blocks.

At the same time,the research of other scholars also shows that the construction disturbance is also an essential factor for the deformation and failure of surrounding rocks. The deformation of surrounding rocks presents a stepwise increase with the excavation process,and the deformation gradually decreases from the surface to the inside (Li et al., 2017). It can be seen from the deformation curve of surrounding rocks in Fig. 6 that the deformation of the Baihetan underground powerhouse also shows similar characteristics. Meanwhile, construction disturbance can also lead to the local loosening of surrounding rocks, an essential inducement for the collapse and other damage. In future research of the surrounding rock deformation of Baihetan, the influence of construction disturbance on surrounding rock deformation and failure is worthy of studying.In addition,the deformation and failure of the surrounding rocks are generally controlled by the comprehensive effect of different factors. Therefore, it is necessary to study the synergistic mechanism of rock deformation in underground powerhouses under the control of multiple factors in the future and propose corresponding countermeasures.

6. Conclusions

In this study, the surrounding rock deformation and failure characteristics in the Baihetan underground powerhouse were summarized. The difference in deformation and failure of underground powerhouses on the left and right banks was discussed.The stress-controlled and structural plane-controlled failures of the surrounding rocks were analyzed. Through the analysis of the deformation and failure of the surrounding rocks, the following conclusions can be drawn:

(1) The types of instability of the surrounding rocks in the underground powerhouse can be summarized as stresscontrolled failure (spalling, fracture cracking, slack collapse and concrete cracking), structural plane-controlled failure(unstable rock block and structural collapse) and large deformation.The deformation and failure of the surrounding rocks of the underground powerhouses on the left and right banks of Baihetan are mainly stress-controlled failure. In addition, the right bank also shows structural planecontrolled failure and large deformation.

(2) The stress redistribution caused by excavation is the direct inducement to produce the stress-controlled failure under the condition of high in situ stress, and the relationship between the in situ stress and the axis of the powerhouse also has an essential influence on the stress-controlled failure.

(3) The weakening effect of the soft structural plane on the mechanical properties of the local surrounding rocks is the main reason for the structural plane-controlled failure, and the exposure position and spatial distribution of the soft structural plane have a close correlation with the structural plane-controlled failure of surrounding rocks.

(4) For Baihetan, the unfavorable condition that the maximum principal stress of the left bank poses a considerable angle to the powerhouse axis is the main reason for the excessive development of stress-controlled failure. The higher intermediate principal stress,which is perpendicular to the axis of the powerhouse,is the main reason for the stress-controlled failure on the right bank. The more complex geological conditions and the extensive distribution of faults and fractures of the surrounding rocks are the main reasons for the structural plane-controlled failure on the right bank. This also explains why the large deformation of the surrounding rocks on the right bank is more prominent than that on the left bank.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors gratefully thank the support of the National Natural Science Foundation of China(Grant No.11902210)and the Graduate Student’s Research Innovation Foundation of Sichuan University(Grant No.2018YJSY076).