Key Rock Mechanics Problems and Countermeasures on Huge Diversion Tunnel of Baihetan Hydropower Station

ZHOU Chuiyi， CHEN Pingzhi， HE Shihai， CHEN Jianlin，LIU Ning， CHU Weijiang

(1. PowerChina Huadong Engineering Corporation Limited, Hangzhou 311122, Zhejiang, China;2. HydroChina-Itasca Research and Development Center, Hangzhou 310014, Zhejiang, China)

0 Introduction

Baihetan Hydropower Station is the largest hydropower station under construction in the world and also the second largest one after the Three Gorges Hydropower Station. It is ranked No.1 in all hydropower projects in terms of the complexity of engineering geological conditions, the scale of underground cavern groups and the span of caverns. The geological conditions of the huge diversion tunnel project are extremely complex, involving large size, complex engineering geological conditions, exposed interlayer fault zones in the tunnel section, weak fault, the development of columnar basalt with type Ⅰ and type Ⅱ hidden fissures and high-stress level. Many new challenges are raised in understanding and countermeasures of the problems of rock mechanics[1]are proposed.

The origin and engineering geological characteristics of hard brittle basalt are investigated widely, and the research mainly focused on its characteristics, composition, and mechanism of formation. Shi et al.[2]studied the fundamental mechanical properties of basalt and the deformation mechanism of rock mass under different loading conditions through engineering geological investigation and field tests. Müller[3]and Phillip et al.[4]systematically investigated basalt and performed the statistical analysis and empirical expressions on the column scale, morphology and texture of columnar jointed basalt. References[5-7] conducted numerical analysis by establishing a composite multi-weak plane constitutive model. In their study, the anisotropic characteristics, excavation unloading effect, distribution of fracture zones and size effect of columnar jointed rock mass were investigated. The cause, process and mechanism of forming the columnar Jointed basalt were studied by Kantha[8]and Xu.[9]The deformation mechanism of the interlayer staggered zone in the basalt rock mass was analysed by Hao et al.[10]Discrete element method was employed by Zhu, Di and Ning et al.[11-13]to investigate size effect, anisotropic property, in-situ deformation test characteristics and Cosserat anisotropic constitutive model of columnar joints. This study mainly focuses on the deformation mechanism of interlayer fault or the formation, classification, experimental test and numerical simulation of columnar Jointed basalt. However, the research is limited on the failure modes and mechanical response mechanisms under full impact conditions of medium-high stress, interlaminar motion zone and columnar joints in practical engineering, especially the in-depth analysis of response control measures for actual projects.

Based on the engineering practice of Baihetan Diversion Tunnel, we discussed the critical rock mechanics problems and control measures during the construction of the giant cavern under complicated geological conditions. This study can provide references for engineering constructions under similar geological conditions.

1 Project overview

Baihetan Hydropower Station is located at the lower reaches of Jinsha River in Ningnan County of Sichuan Province and Qiaojia County of Yunnan Province. It is the second cascade of four hydropower cascades at the lower reaches of Jinsha River, with the total installed capacity of 160 MW. The construction diversion in Baihetan Hydropower Project adopted the method of the year-round dewatering cofferdam and five diversion tunnels. The left bank has three diversion tunnels (#1, #2 and #3), and the right bank contains two diversion tunnels (#4 and #5). The cross-section of diversion tunnel is 17.5 m × 22.0 m with the city gate type, and the total tunnel length is 8 980.26 m. The diversion tunnel project has a large scale with the specific layout, as shown in Fig. 1.

Fig. 1 3D Layout of Baihetan Hydropower Station

The dam area of Baihetan Hydropower Station exhibits the landform of a medium-high mountain gorge, which is higher in the north and lower in the south and tilted to the east. Monoclinic mountains are distributed on both banks, and basalt-like structures form the layered rock slope on both banks. The geological structure in the engineering area is mainly characterised by primary structure and fracture structure (i.e., fault, interlayer and inner-layer dislocation and fissure). The development characteristics of the layered rock mass and growth belts between the layers in the project area are shown in Fig. 2. Fig. 3 shows the development characteristics of the columnar Jointed basalt.

Fig. 2 Stratified rock mass and big & long shear belt

Fig. 3 Columnar jointed basalt in engineering area

The diversion tunnel in Baihetan consists of entrance, entrance gate, cavern section, access gate for tailrace tunnel, diversion tunnel and tailrace tunnel junction and exit. Meanwhile, it adopts the concrete lining. For the diversion tunnel on the left and right banks, the excavation of the main tunnels started in February 2012 and April 2012, respectively, as shown in Fig. 4. All construction and overflow acceptance of intake and exit cofferdam were completed in April 2014. The construction period was in total 25 months, which created the fastest construction record in China.

2 Basic geological conditions

Monoclinic rock formations are distributed along the diversion tunnel in Baihetan, and Permian upper Emeishan Basalt is exposed. The overall attitude of rock formations is N40°- 45°E, SE∠15°-20°, and the exposed horizons are P2β2, P2β3, P2β4…P2β7, P2β8, P2β9layers from the downstream to the upstream. The geological structures are mainly composed of an interlayer

shear belt, inner-layer shear belt, fault and fracture. The attitude of the shear belt is consistent with the attitude of rock flow layer. The interlayers and inner-layers are weak structural planes. The interlayer shear belt is developed in the tuff and dominated by rock debris mixed up with mud. However, the inner-layer shear belt is mainly dominated by rock fragments and shows a fragmented structure. The typical geological section (taking the right bank as an example) is shown in Fig. 5. The unique geological condition of the diversion tunnel in Baihetan is beneficial for the development of relatively large-scale columnar Jointed basalt with type Ⅰ, type Ⅱ and type Ⅲ hidden fissures. The column surface is usually attached to smooth and soft chlorite film, and the column body is closely inlaid without the relaxation state. Since hidden fissures are developed in column body, the stability of surrounding rock is prominent in the cavern sections with the development of columnar jointed basalt.

Fig. 4 Diversion tunnel excavation site

Fig. 5 Typical geological profile of right bank

The diversion tunnel in Baihetan has many lithologies dominated by hard rock, and its lithologies mainly include phenocryst basalt, cryptocrystal basalt, amygdaloidal basalt, columnar Jointed basalt and breccia lava. There is a small amount of tuff on the top of each rock flow layer. The rock quality is weak and easy to soften after encountering water. The uniaxial compressive strength of surrounding rock is 90-110 MPa. For the diversion tunnel on the left and right bank, the proportions of tunnel length with a vertical burial length larger than 300 m account for 28% and 54%, respectively, and most of the tunnel sections are in the medium-high stress areas. The in-situ stress level on the right bank is generally higher than that on the left bank. The maximum principal stress on the left bank is 14.2-22.4 MPa, and the maximum principal stress of the deep burial section on the right bank is 18.4-28.0 MPa. In most tunnel sections, the ratios of uniaxial compressive strength to the stress of the tunnel surrounding rock are from 4 to 7, and the ratios are from 2 to 4 in some tunnel sections and less than 2 in local tunnel sections.

3 Critical rock mechanics problems

The geological conditions along the diversion tunnel of Baihetan are complex, and rock mass is characterised by hard brittleness of basalt. The initial stress meets several essential conditions for the development of medium-high stress level, structural surface and columnar joints. The typical surrounding rock response modes are revealed by the excavation process include: (1) controlled failure of weak structural plane, (2) relaxation of columnar joint rock mass, and (3) stress-induced failure of brittle basalt.

3.1 Controlled failure of weak structural surface

Many gently inclined interlayer shear belts are developed along the diversion tunnel, such as C2, C3, C3-1, C4, and their attitudes are similar with that of rock formations. The diversion tunnel also develops the inner-layer shear belts RS411, RS334, RS311and RS321as well as faults F16, F20and F17. The interlayer shear belt has a weak mechanical property and is largely located in the middle upper part of the tuff. An obvious mud phenomenon is found in the internal-splitting physicochemical tectonite. The fracture influencing zone is relatively large, which has a significant influence on the stability of surrounding rock in the diversion tunnel. The controlled destruction of project site on the weak structural surface is reflected by: (1) the collapse and destruction of the roof of the interlayer (inner-layer) shear belt with a low-angle dip, (2) the substantial shearing deformation failure of side wall in the interlayer (inner-layer) shear belt with a large and long low-angle dip, and (3) the combination of the gently inclined interlayer (inner-layer) zone with the steep structure easily leading to the instability failure of local blocks.

When the interlayer (inner-layer) shear belt with a low-angle dip is exposed on the roof of diversion tunnel and the roof area of cutting tunnel is close to the tunnel roof, rock mass at the lower part of the shear belt suffers impacts of relaxation, self-gravity, explosion and cutting off other structural surfaces. The local rock mass at the heading side of shear belt readily induces instability and collapse (Fig. 6). When the interlayer (inner-layer) shear belt with a large and long low-angle dip is exposed on the side wall of the diversion tunnel, the shearing deformation of rock mass at the heading side of the belt becomes more substantial and even causes the shearing deformation failure, as shown in Fig. 7. When the interlayer (inner-layer) shear belt and steep-inclined structural plane combine to form local blocks on the tunnel roof, it easily causes the instability failure of blocks (Fig. 8).

Fig. 6 Collapse of roof caused by interlayer shear belt

Fig. 7 Shear failure on sidewall caused by interlayer shear belt

Fig. 8 Local block damage caused by combination of interlayer shear belt and steep structure

The diversion tunnel in Baihetan is subjected to localized stress damage under the influence of large, long and weak interlayer belt. Fig. 9 shows that the collapse area in the site that is consistent with the stress anomaly at the heading side of the localized stress C2under the influence of the interlayer shear belt during the excavation of diversion tunnel. The influence range of the localized stress field is related to the properties of large and long structural surface and the direction of regional tectonic stress, which increases from several meters to tens of meters. The localized stress field nearby the structural surface can cause the stress-induced failure of surrounding rock, and the structural surface itself participates in the destruction to form the chippings or collapse boundary.

(a) Geological sketch

(b) Numerical calculalion sketch

3.2 Relaxation failure of columnar joint rock mass

Each diversion tunnel in Baihetan has a columnar joint section with a length of 400-500 m, and columnar Jointed basalt has an apparent unloading-relaxation failure. The combination of columnar joint, interlayer belt and fault makes the rock mechanics problems much more complicated and prominent[14-15]. The failure characteristics revealed on the diversion tunnel project site are mainly the relaxation of unloading deformation of the columnar joint and stress-induced disintegration relaxation of the columnar joint.

Hidden joints are developed in the columnar jointed basalt, which quickly results in unloading, deformation and relaxation of the columnar jointed basalt after excavation. When rock mass is undisturbed, the column body is tightly inlaid, and the rock bearing capacity is high. After excavation, its unloading and relaxation become apparent, and the depth of relaxation ring can reach a depth of 3-6 m. Mechanical properties of rock mass are significantly weakened after relaxation[16], which influences the stability of surrounding rock of the high sidewall. With certain confining pressure, the columnar joint rock mass shows shear failure along the structural surface. When the confining pressure near the tunnel wall is very low or absent, the column body is pressed and pulled, and the columnar jointed rock mass shows obvious tensile failure (Fig. 10).

Fig. 10 Relaxation damage of columnar jointed basalt

When the inner-layer shear belt is developed in the columnar jointed basalt, the inlaid structure and the integrity of the column body are destructed due to the cut of shear belt. When the inner-layer shear belt combines with the structural surfaces such as the fault, the unloading deformation and relaxation phenomena become more severe, and the self-stability of surrounding rock is reduced. The local rock mass at the lower part of the shear belt easily leads to instability and collapse under the effects of relaxation and self-gravity of rock mass, as shown in Fig. 11.

Fig. 11 Relaxation and collapse of columnar joints

Due to the development of internal primary joint, blind joint, micro-fissure and inner-layer shear belt, the columnar jointed rock mass is favourable to the stress release compared with the energy of intact rock. When the burial depth is large and the stress level reaches a certain degree, the columnar joint rock mass also undergo the stress-induced failure, similar to the typical intact rock.

The columnar joint induces the stress-type column disintegration that is further subjected to relaxation failure under the influence of stress adjustment, as shown in Fig. 12.

Fig. 12 Column stress type disintegration

3.3 Stress-induced failure of brittle basalt

The rock mass in Baihetan Diversion Tunnel belongs to hard brittle lithology, and the deep buried tunnel section is medium-high stress field. The stress-induced failure is primarily distributed in the stress concentration areas of the hard brittle rock tunnel sections such as cryptocrystal basalt, phenocryst basalt and amygdaloidal basalt. The high-stress failure phenomena mainly involve the spalling peeling-off, fracture and slight rockburst. In the project site, the development of some hard structural surfaces is observed in the high-stress failure areas of the tunnel sections influenced by the interlayer zone. The multi-factor combination leads to a significant failure resulting from the relatively severe local in-situ stress concentration.

Micro-fissures are usually developed in the stress concentration location of basalt mass in the diversion tunnel, which causes the reduction of rock mass strength[14-15]. Under the high-stress condition, the expansion of micro-fissures further leads to the stress-induced collapse. When the surrounding rock is influenced by the large and long interlayer belt, the local stress concentration happens in a specific range of the hanging and heading sides, which quickly aggravates the occurrence of spalling and stress-induced collapse. Fig. 13 shows the stress spalling of cryptocrystal basalt and the fracture propagation failure. The failure over a period tends to stop and eventually realise self-balance when it finally forms the V-shaped destruction pit, matching with the in-situ stress field conditions. Meanwhile, during the excavation of columnar jointed tunnel sections in the project site, the stress concentration area of the roof frequently occurs apparent high-stress peeling destruction under the influence of stress adjustment, as shown in Fig. 14.

Fig. 13 Stress spalling of cryptocrystalline basalt

Fig. 14 Stress peeling off

4 Main engineering countermeasures

4.1 Supporting measures

The diversion tunnel cavern in Baihetan has a vast scale and complex geological conditions. In addition to the systematic shotcrete support, differentiated supporting measures are adopted against the typical problems of surrounding rock.

4.1.1 Tunnel sections influenced by large, long and weak structural surfaces

The huge diversion tunnel in Baihetan develops large, long and weak structural surfaces, due to the influences of the interlayer shear belt, inner-layer belt and fault. The structural surfaces result in controlled destruction at the project site. Therefore, the following designed supporting measures are taken against this kind of destruction:

(1) The conventional suspended net shotcrete support is applied for tunnel sections influenced by large, soft and weak interlayer (inner-layer) belt. In addition, a 9 m pre-stressed anchor and a local 15-20 m anchor rope are also provided to reinforce supports in the range of 8-15 m of the belt influenced by the hanging and heading sides of the interlayer belt.

(2) For the effect of the interlayer belt cutting on the roof, the pre-stressed anchor and local anchor rope are employed for supporting the tunnel sections with significant deformation of surrounding rock at the heading side of the interlayer belt.

(3) Due to the influence of interlayer belt on the side wall, the discontinuous deformation of the local rock mass is significant at the exit (heading side of interlayer belt). The surface support is reinforced after removal of the relaxed rock mass. Besides, locking support is provided in the exit area of the interlayer zone, and anchor bundles are increased in the local area to form the efficient supporting system.

4.1.2 Tunnel sections with the development of columnar joints

The following designed supporting measures are taken for the tunnel sections influenced by the columnar joints of the diversion tunnel:

(1) The confining pressure of tunnel can be effectively improved, and the stability of surrounding rock can be increased after tunnel excavation through timely and decisive supporting measures, including a quick system supporting with pre-stressed anchor, secondary shotcrete with the suspended net and the group anchoring effect by the joint effect of anchor and shotcrete layer.

(2) There is a time effect in the unloading and relaxation of the columnar jointed rock mass, the unloading relaxation develops quickly in the initial period of rock mass[11-13], and a certain period is required for the system supporting after tunnel excavation. Initial concrete shotcrete support and closure should be first implemented to efficiently control the quick development and destruction of relaxation deformation of the columnar joint rock mass after excavation. Moreover, the shotcrete thickness should be guaranteed to prevent the relaxation and chipping of the surface. The confining pressure should be provided in time to inhibit the further expansion of unloading relaxation. To improve intensity and tenacity of the initial shotcrete, nano-fibre or steel fibre reinforced concrete can be used for the initial shotcrete.

(3) For large-scale excavated tunnel caverns, both sides at the bottom of the previous layer should be reinforced by the pre-stressed locking support before the excavation of the next layer. The purpose is to facilitate the effective control of the unloading relaxation at the side wall location during the blasting excavation disturbance of the next layer and formation of the higher sidewall.

(4) Consolidation grouting treatment in the full cross-section is adopted, according to the relaxation characteristic of the columnar jointed tunnel sections.

4.1.3 Areas influenced by the stress-induced failure

Local reinforcement measures should be carried for the stress-induced failure widely distributed in the diversion tunnel of Baihetan. For obviously influenced areas by stress-induced failure at roof and spandrel locations, the reinforced and pre-stressed 6 m and 9 m anchor supports are usually adopted as primary supporting method, supplemented by the systematically suspended net shotcrete and large baseplate sprayed shotcrete.

4.2 Supporting time

There is a correlation between time and space in the excavation and supporting of tunnel cavern, and the layered excavation is usually adopted for the large-scale tunnel cavern. Before the blasting excavation of the next layer, all systematic supporting measures for the previous layer must be implemented to reduce the influence of blasting disturbance on the unloading relaxation of tunnel caverns. The diversion tunnel in Baihetan generally requires to implement initial shotcrete and closure immediately after the blasting and slagging, and thus the pre-stressed system bolts are equipped within 5-10 m behind the tunnel face as soon as possible. The tunnel scale and stress environment are slightly different, which should be determined according to the actual monitoring and inspection results. In case of the cutting of discontinuous structural surface, the system supporting should be guaranteed to strictly follow the tunnel face, to realize one support for one blasting.

4.3 Construction method

Due to the apparent time effect and spatial effect of the unloading relaxation of the columnar jointed basalt, the excavation scheme and the blasting control are equally important to its stability.

(1) Excavation scheme: Observations or experimental results of deformation, stress and ultrasonic wave velocity show that the amplitude of stress adjustment is obviously different by using different excavation layering methods. There are significant impacts of the single-layer thickness, single-layer footage and single-layer cross-section area of the excavation layers on the unloading relaxation of the columnar joints. During the overlaying of the sidewall, larger unloading relaxation depth is caused by larger single-layer excavation thickness, excavation footage and cross-section area. The excavation of lower layer has a substantial impact on the unloading relaxation of the upper layer of the side wall.

The specific measures are taken after fully considering these characteristics include: multiple layering (such as control of single-layer excavation height less than 4 m), short footage (such as control of excavation cycle footage less than 3 m) and multi-framing excavation (such as the expanded excavation in the middle drift firstly and left or right half-frame excavation or excavation in two half-frames).

(2) Blasting control: Due to the characteristics of the development of internal joint, micro-fissure and hidden joint of the columnar jointed basalt, the effect of the excavation blasting on the unloading relaxation is also apparent. Hence, blasting control is an essential aspect of controlling the unloading relaxation of the columnar jointed rock mass. The blasting control measures for the diversion tunnel include weak blasting, pre-splitting blasting or smooth blasting and reserved protective layers.

4.4 Monitoring and feedback method

For each monitored cross-section in the diversion tunnel, multipoint displacement gauges and anchor stress gauges are arranged on the roof and side walls to monitor the deformation of surrounding rock and the stress of the anchor bolts. Reinforcement meters, strain gauges and stress-free strainmeters are buried in the concrete lining to monitor the stress. The jointed meter is embedded at the joint located between the concrete lining and surrounding rock to monitor joint deformation. The osmometer is arranged on the contact surface between the surrounding rock and concrete lining as well as at the periphery of grouting treatment, to monitor the external water pressure of surrounding rock. Full-section relaxation depth test (acoustic emission test) and full-section convergence monitoring are performed for columnar jointed tunnel sections. During the project construction period, comprehensive measures are adopted to realise the real-time dynamic monitoring and feedback on the deformation of surrounding rock, the stress of supporting system and relaxation depth of surrounding rock. Thus, after the systemic analysis, reasonable supporting measures and suitable supporting timing are suggested to guarantee the safety construction and smooth completion of the enormous diversion tunnel project in Baihetan.

5 Conclusions and discussion

5.1 Conclusions

Since the huge diversion tunnel in Baihetan has complex geological conditions and prominent problems of rock mechanics, the project construction is directly influenced by the interlayer shear belt, columnar jointed basalt and high in-situ stress. In accordance with the experience accumulated in the construction of diversion tunnels, this paper systematically analysed the in-depth mechanism of the failures mentioned above and proposed the targeted engineering countermeasures.

(1) The basalt in diversion tunnel is characterised by strong features of hard brittleness, high in-situ stress and developments of structural surface and columnar joint. During the excavation process, various kinds of failures are revealed, including the controlled failure of soft and weak structural surface, relaxation of the columnar joint rock mass and stress-induced failure of brittle basalt.

(2) For the surrounding rock within the influence of the interlayer shear belt, rock mass at the hanging and heading sides undergoes significant shearing deformation under the unloading effect of excavation. Local instability and collapse of rock occur under the effects of relaxation, localised stress and self-gravity. The combination of pre-stressed anchors and deep support of anchorage cables is a useful supporting measure for the influence zone of surrounding rock, side walls and top arch in the interlayers.

(3) The hard brittle columnar jointed basalt easily leads to the expansion of joint surface among the column body of columnar jointed rock mass under the influence of high and steep dip angle and low confining pressure of the joint surface. Obvious characteristics of tension failure and unloading relaxation mechanism are analysed. Reasonable and effective means are provided for the excavation and support of the columnar jointed rock mass, including short footage, weak blasting, multiple layering, multiple framing, pre-splitting control and timely support. These measures can efficiently control its relaxation depth and inhibit the gradual expansion of unloading relaxation to achieve the stability control of surrounding rock.

(4) The deep burial tunnel sections in the diversion tunnel of Baihetan is the medium-high in-situ stress field. The stress-induced failure is mainly distributed in the stress concentrated areas of hard brittle tunnel sections, such as the cryptocrystal basalt,phenocryst basalt and amygdaloidal basalt. The high-stress failure phenomena are studied, such as the spalling chipping, fracture and slight rock burst. The timely support provided by confining pressure can effectively control the expansion of brittle stress failure.

5.2 Discussion

The practice has proved that the engineering countermeasures adopted in the construction process of Baihetan diversion tunnel are reasonable and valid. However, there are still some deficiencies in the preliminary work, which need to be further discussed and solved in the subsequent engineering construction.

(1) Basalt has the characteristics of hard brittleness and easily disintegrated columnar joint. There are difficulties in non-destructive sampling and large discreteness of mechanical results in the study of mechanical properties. It is necessary to explore new technologies and methods to solve the problems of rock mechanics.

(2) The time and space effects of unloading and relaxation of columnar jointed basalt show apparent nonlinear characteristics. Further study should be conducted on accurately and quantitatively predicting the relationship between the development of unloading relaxation depth and supporting time, which is helpful to determine engineering countermeasures accurately.

(3) The damage depth of surrounding rock is affected by various factors, including the development of internal joints, micro-fissures, hidden joints, excavation and unloading, stress adjustment and blasting vibration. Moreover, the effects of various factors on the supporting time also need to be further explored.

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