Baihetan Hydropower Station Research Articles

A new predictive method for the shear strength of interlayer shear weakness zone at field scales

The ISWZs (interlayer shear weakness zones) are the critical weak infilled discontinuity controlling rock mass stability in the underground caverns, dam foundations, and slopes. Given the complex occurrence characteristics of ISWZs and the limitations in the shear strength criterion, it is not easy to assess their strength variability in the field. This study presents a method to evaluate the shear strength parameters of the ISWZs in the Baihetan Hydropower Station. First, the ISWZs are classified based on the two parameters, i.e., the infilling ratio and clay content of ISWZs. Subsequently, the shear strength model for each class of ISWZs, involving the climbing/shearing off effects, is proposed according to the physical and mechanical characteristics. Finally, the strength characteristics of three typical ISWZs in the Baihetan Hydropower Station are studied systematically using the multi-index engineering geological classification method. Based on this study, a new predictive shear strength model was proposed. The obtained results in this study suggest that the strength parameters of ISWZs determined through field tests provide a more conservative result than the shear strength model presented in this study. This result may have engineering implications for designing and constructing underground caverns with large and weak ISWZs.

An Analytical Solution for Non-Darcian Flow on a Constant Head Packer Test in the Interlayer Staggered Zone

Groundwater flow in an aquifer has frequently been found to be non-Darcian by performing in situ tests. A novel analytic model is proposed in this study for describing the unsteady non-Darcian flow in a confined aquifer by taking advantage of the observed flow rate and injection pressure during the constant head packer test. A linearization approximation of the Izbash equation is used to approximate the nonlinear term in the governing equation. This analytic model is applied to describe the non-Darcian flow in the interlayer staggered zone at the Baihetan hydropower station, China. The test results inversed by the genetic algorithm show that non-Darcian flow happened during the test under the injection pressure 0.3 MPa with the power index n is 1.278, non-Darcian hydraulic conductivity k 1 is 1.613 × 10 − 5 cm/s and the specific storage S s is 9.757 × 10 − 5 m-1, respectively. The sensitivity analysis indicated that when the power index n or the specific storage S s is larger, and the hydraulic head will increase more slowly and needs longer to stabilize, but the non-Darcian hydraulic conductivity k 1 shows the opposite trend. Moreover, the hydraulic head is more sensitive to the power index n compared to other parameters at late times. The findings of this study reveal the non-Darcian flow during the constant head packer test and provide a simple and fast way to estimate parameters for more accurate seepage field simulation.

Study on Unloading Relaxation Characteristics of Columnar Jointed Rock Masses Based on Displacement Back Analysis

We aim to understand the relaxation of columnar joint rock masses during the excavation process of the diversion tunnel of the Baihetan hydropower station. This paper inverts the deformation parameters of the relaxed columnar joint rock based on the displacement monitoring data, and introduces a relaxation factor to describe the deterioration degree of anisotropic parameters of the relaxed columnar jointed rock. The equivalent strain is proposed as the criterion of unloading relaxation and the threshold is also given. Based on the software Flac3d, a program for calculating anisotropic elastoplastic model is developed. The distribution of the relaxation zone of the diversion tunnel after excavation is simulated, and compared with the results of the acoustic detection to verify the correctness and rationality of the program, which can provide a necessary reference for the design and construction of hydropower projects.

Analysis of Toe Formation in Stress-Concentration Zones on High Rock Slopes

Toe formation is a phenomenon resulting from unconnected cracks between blastholes,which occurred during blasting excavation of the abutment slot at Baihetan hydropower station. Analysis of crack development has tended not to focus on the high in-situ stress environment at the slope corner. Therefore, this study aimed to explore the toe formation process through theoretical analysis and numerical simulation. Firstly, a three-blasthole model was established to analyze the effects of blasting load and in-situ stress on crack extension. Then, a finite-element model of the abutment slot was built to simulate crack propagationunder different in-situ stress environment. Finally, optimization measures for avoiding toe were proposed. The results demonstrate that cracks grow preferentially in the direction of the maximum tangential tensile stress and are influenced by the blasting load and in-situ stress. On the lateral side of the abutment slot, the crack initiation and extension occur far from the centerline due to the deviation of the maximum tangential tensile stress. The deviation increased as the in-situ stress. Consequently, cracks between blastholes not being able to connect with each other results in toe formation. Increment of the linear density of the charge and optimization of the excavated profile are both effective in reducing the impact of the stress concentration caused by in-situ stress and preventing toe formation.

Study on Non-Darcian Flow in Interlayer Shear Weakness Zones in the Basalt by High-Pressure Packer Tests

The interlayer shear weakness zone (ISWZ) is a special structural plane with different widths and spacing in stratified rock masses, it has higher permeability compared with surrounding rocks which is a risk factor for the safety of the hydropower station project. The high-pressure packer test (HPPT) by step injection is always applied to characterize the permeability of ISWZ. However, the non-Darcian flow is easy to appear under high pressure, which makes the Darcy law model no longer applicable. In this study, two non-Darcian flow analytical methods for confined aquifer were proposed to investigate the non-Darcian flow permeability parameters. The equivalent permeability coefficients of different non-Darcian models were derived as well. The in situ tests were conducted on the ISWZs at the Baihetan hydropower station to verify the proposed methods. The results indicate that the flow is non-Darcian flow in the test section from integrity to destruction during the whole HPPT process. Izbash’s law has a better fit than Forchheimer’s law in this complicated test situation. The equivalent permeability coefficients after destruction are one or two orders of magnitude larger than those before. Meanwhile, it is necessary to pay attention to the increased difference of two expressions of the equivalent permeability coefficients under higher gradient (i) or velocity (v). In general, these methods can be used to evaluate the characteristic of ISWZ to analyze the impact on engineering stability.

Study on the mechanical behaviour of Columnar Jointed Rock Mass under uniaxial compression state

The columnar joint is a special type of primary joints which the cross sections are mostly quadrilateral, pentagonal and hexagonal. Intact rock mass is always cut by columnar joints into regular or irregular rock prisms, resulting in poor properties. With the expansion of large-scale engineering activities, a number of hydropower engineering such as Baihetan hydropower station, Xiluodu hydropower station and Ertan hydropower station, have encountered columnar jointed rock mass. The existence of columnar jointed rock mass poses a challenge to the design, construction and safe operation of these projects. To understand the mechanical behaviour of columnar jointed rock mass, uniaxial compression tests were carried out on artificial specimens made from rock-like materials. In the experiments, a self-designed pouring apparatus was used to prepare the columnar jointed rock-like specimens. Based on the experimental results of axial stress–axial strain curves, the strength and deformation of columnar jointed rock mass were investigated. The effect of columnar joint inclination angle on the mechanical behaviour of columnar jointed rock mass was analysed. It is found that the columnar jointed rock masses are characterized by significant anisotropy. The strength of columnar joint plays a significant role on the mechanical response of columnar jointed rock mass. The study Formatting the text

Analysis of large deformation mechanism of the underground powerhouse at the Baihetan hydropower station

Baihetan hydropower plant has the widest underground powerhouse and the largest scale of underground openings in the world. The surround rock mass of huge caverns is characterized by complex geological conditions and high in-situ stress of 30MPa. The monitoring deformation of the roof arch of the right bank powerhouse reached 165mm and the deformation of the side wall reaches 192mm, which had become the focus of the feedback analysis. In the paper, several field test methods, such as extensometer monitoring, grating fiber displacement monitoring, acoustic test and borehole image test, have been adopted to revealing the fracture expansion and time-dependent deformation of surrounding rock. Moreover, FLAC3D numerical simulation has been conducted integrated the monitoring results to analysis of large deformation mechanism. The results show that the combination of interlayer zone and high stress factors is the fundamental cause of large deformation caused by excavation. Finally, the reinforce measures and reinforcing range had been determined, and the stability of the powerhouse cavern has been evaluated according to the monitoring results for nearly 2 years after excavation. The studies, integrated the research approach of field geological survey, monitoring and numerical simulation, not only have shed light on the large deformation mechanism of excavation response of huge caverns, but also has provided a basis for dynamic optimization design and construction.

Design of seepage control system around a large-scale underground powerhouse: A case study of Baihetan Hydropower Station

Located at the lower reach of Jinsha River, the Baihetan Hydropower Station is the world’s second largest hydropower station. It consists of a double-curvature arch dam with a maximum height of 289 m and two large-scale underground cavern systems on each side of the bank, which is the largest underground cavern system of hydropower project in the world. The construction site is characteristic of complex geological conditions, and multiple interlayer staggered zones are developed between different strata. Based on the results of on-site packer tests, the interlayer staggered zones generally have a higher hydraulic conductivity than the surrounding rocks, and therefore may act as the potential seepage flow path. Since the underground powerhouse is close to the upstream reservoir and these is a large difference of water level between the upstream and downstream reservoirs, the control of groundwater flow in the surrounding rocks of the underground caverns becomes one of the key technological issues. A large-scale seepage control system including grout curtains and drainage curtains was designed around the underground powerhouses. Besides, the hydraulic conductivity of the interlayer staggered zones was systematically characterized and several anti-seepage tunnels were designed and constructed specifically for the control of seepage flow through the interlayer staggered zones. On this basis, numerical simulation of seepage field was conducted, and the results show that the designed seepage control system is effective in lowering the groundwater level and reducing the amount of seepage around the powerhouse.

Study on the influence of transient unloading on surrounding rock deformation in expanding excavation blasting

The tensile stress generated by the transient unloading of the in-situ stress may cause deformation, damage or even destruction of the surrounding rock. Based on the equivalent load method and the deleted element method, the influence of blast loading and transient unloading on the surrounding rock deformation is studied in the present study. The field blast vibration tests in the BaiHeTan hydropower station were carried to verify the numerical results. The results shown that the deformation simulated by the LS-DYNA agrees well with that measured in the field tests only if the coupling effect of blast loading and transient unloading are considered. In expansive excavation blasting, the convergent deformation caused by transient unloading is about 4 to 6 times greater than that caused by blast loading, and the deformed rock mass area caused by transient unloading is about 2 to 3 times larger than that caused by blast loading. However, plastic zone induced by the blast loading is much wider (about 2.1 times) than that induced by transient unloading. The results show that blast loading tends to cause more damage of surrounding rock, while transient unloading tends to cause larger deformation.

Design of the dome shape of complicated huge cylindrical surge chamber in Baihetan hydropower station

Baihetan hydropower station located at lower reaches of Jinsha River in China, the hydropower station is characterized by its large scale and complicated layout. Four cylindrical throttled surge chambers sit in each bank with their heights about 100 m, and the diameters 43 to 48 m. The surrounding rock of the dome is affected by high in-situ stress, interlayer dislocation zone and columnar joints. The geological conditions are extremely complex and the stability of the surrounding rock is prominent. The numerical simulation method is used to build the calculation model of the dome with different shapes, and the excavation response characteristics of the domes are compared to determine the optimal shape. Research results: (1) The domes of the 1#∼3#, 5#, 6# surge chamber are mainly affected by the initial in-situ stress and surrounding rock lithology. The semi-circular shape is beneficial to improve the smooth transition of surrounding rock stress at the crown and spandrel. (2) The interlayer dislocation zone C4 and C5 jointly affect the dome of the 7# surge chamber. The semi-circular scheme can be used to control the relaxation deformation of the surrounding rock. (3) The streamlined scheme of the 8# surge chamber can reduce the exposure range of columnar joints, reduce the risk of the impact of columnar joints, and improve the conditions of the dome.

Optimization design of a large-scale seepage control system at a high arch dam site

The Baihetan Hydropower Station, located in the lower reaches of Jinsha River in southwest China, is the second largest hydropower station in the world. The site is characteristic of extremely complex geological and hydrogeological conditions, with fractures, faults, and shear zones that make water tables wedge-shaped. A large-scale seepage control system that contains grouting curtains covering an area of 780,000 m2 and 12,300 drainage holes, together with concrete plugs, were designed to control the seepage in the dam foundation and in the underground powerhouse areas as the reservoir water level will be raised by 225 m during operation. How to assess the performance and optimize the design of seepage control system becomes a key technological issue in the design and construction of this project. In this study, a three-dimensional finite element model that well represents the site conditions is established. The drainage holes are modelled with a substructure technique and the seepage field is solved with the variational inequality method of Signorini’s type. A procedure is proposed to optimize the layout of the seepage control system by taking the amount of leakage, the distribution of uplift pressure, the risk of seepage erosion and cost into account. Numerical results yield the appropriate parameters for the seepage control system, including the extension and depth of grouting curtains, and the spacing of drainage holes. With these optimized parameters, the seepage control system is effective in lowering the phreatic surface and limiting the amount of leakage at the dam site.

Stability control for surrounding rock of the underground powerhouse of Baihetan hydropower station in China

The geological conditions of Baihetan hydropower station are very complex, under which the stability control of surrounding rock encounters two major challenges: stress-induced large deformation and brittle failures, and dislocation deformation along interlayer shear zones. Brittle failure control is achieved through proper excavation sequence and fine excavation method to reduce the damage of excavation and avoid the superposition of the stress concentration area, and pre-treatment measures are applied for key parts of the powerhouse to control the excavation shape, with proper support sequence and parameters being used to strengthen support in stages and thus limit the development of fractures. Dislocation deformation control is achieved through the use of concrete replacement tunnels to promote shear resistance, proper excavation sequences to avoid rapid stress relaxation, and targeted supports on hanging wall to reduce rock mass relaxation to limit shear displacement and promote the stability of surrounding rock. The data obtained from visual appearance observation, displacement measurement, and acoustic testing show that such measures are effective.

Risk analysis of progressive cracking and failure of hard rock around deep underground caverns with high sidewall

The progressive cracking of hard rock is more significant under the strong unloading effect of excavation of deep buried large caverns, which often leads to the destruction of surrounding rock, the sharp increase of deformation, support damage and other engineering disasters. Relying on a domestic giant underground hydropower station that faces serious aging crack problems of surrounding rock during the excavation period, the risk analysis methods on the progressive cracking and failure of hard rock around deep underground caverns with high sidewall were presented. Firstly, the formation mechanism of cracking and failure risk of hard rock was revealed from the perspective of risk, based on the analysis of the influencing factors of the cracking behaviour. Secondly, the risk consequences and losses caused by progressive cracking of hard rock were summarized through typical historical observation cases. On this basis, the identification and estimation process of progressive cracking and failure risk of hard rock was put forward, according to the characteristics of complex excavation process of the large caverns and the development characteristics of progressive cracking of hard rock. Thirdly, based on the existing risk assessment guidelines for underground engineering, the risk classification principle of progressive cracking of hard rock for large underground cavern was preliminary proposed, including risk occurrence, risk loss and risk grading, and a set of methods for estimating the risk of progressive cracking and failure of hard rock in the underground powerhouse from Baihetan Hydropower Station project was applied to verify the applicability of the method proposed in this paper. This study has important guiding significance for risk assessment of similar disasters and support optimization design of large and deep underground engineering in hard rock.

RQD calculation method based on 3D rock discontinuity network simulation in the dam foundation of Baihetan hydropower station

The original rock quality designation (RQD) index has several limitations, such as one-dimensional (1D) direction and a single threshold. For these reasons, a novel RQD calculation method is proposed by using 3D discontinuity network simulation. In this method, a 3D discontinuity network model (3D-DNM) of the rock mass is generated by Monte–Carlo analogy procedure, in accordance with the optimal probability models of the rock discontinuity’s geometrical elements. Then, the RQD values at different azimuth on the three orthogonal planes of the 3D-DNM can be analyzed statistically. Based on the analysis result, the novel index RQD t is put forward. In this index, the threshold corresponding to the maximum standard deviation of the RQD t is defined as the optimal threshold. The RQD t with an optimal threshold has the greatest capacity to reflect the heterogeneity and anisotropy of rock mass. To illustrate the effectiveness of the proposed method, we apply it on the foundation rock mass of Baihetan hydropower station on Jinsha River. Finally, the optimal threshold is determined as 0.6 m, and the quality and anisotropy of the dam foundation rock mass are fully demonstrated by the generalized RQD 0.6.

Lithospheric Equilibrium, Environmental Changes, and Potential Induced-Earthquake Risk around the Newly Impounded Baihetan Reservoir, China

The Baihetan hydropower station is the second largest hydropower station worldwide. It began to store water in April 2021. We conducted a dense hybrid gravity and GNSS survey at 223 stations, obtained the free-air and Bouguer gravity anomalies, inversed the lithospheric density structure, and calculated the isostatic additional force (IAF) borne by lithosphere in the reservoir area. Moreover, we studied the gravity change and Coulomb stress change around the Baihetan reservoir due to impoundment. The main findings are the following. (1) Hybrid gravity and GNSS observations significantly improved the spatial resolution of the gravity field, and the maximum improvement reached up to 150 mGal. (2) A new method for risk assessment of reservoir-induced earthquakes is proposed from the perspective of lithospheric equilibrium. It was found that there is an IAF of −30 MPa at approximately 20 km upstream of the Baihetan dam, and the risk of a reservoir-induced earthquake in this area warrants attention. (3) It was found that the Coulomb stress variation on the Xiaojiang fault near Qiaojia at a depth of 10 km exceeded the threshold for inducing an earthquake (0.1 bar).

Technological innovations in construction of underground caverns in basaltic rocks at Baihetan Hydropower Station on Jinsha River

The two underground powerhouses on the left and right banks of Baihetan Hydropower Station (BHT-HS) are symmetrically arranged inside basaltic rocks in the alpine gorge reach of the lower Jinsha River. Each of them is equipped with 8 sets of 1,000 MW hydraulic turbine generators, with the largest unit capacity, total installed capacity, span of main powerhouse (i.e. 34 m), diameter of dome-shaped tail-regulating chamber (max diameter of 48 m) and scale of underground caverns around the world. Due to complex layout, interaction between caverns and high hollowing rate, the construction process of the underground cavern group is faced many challenges. For example, the cavern excavation is located in the high ground stress area dominated by horizontal tectonic stress; the underlying basalt fractures are well developed, and the rock is hard and brittle, with low crack initiation strength; columnar jointed basalts develop locally; a number of large and weak gently-sloped shear zones develop diagonally cutting the main powerhouse. Following the construction ideology of “understanding, utilization, protection, monitoring, and feedback analysis on the surrounding rock masses during the construction period”, and with the engineering construction procedure of “the excavation, analysis, prediction and inspection of each rock layer”, this paper proposes a set of innovation construction technologies including “advanced pre-control, thin vertical layer, small planar partition, short excavation progress, fine blasting, strong and quick support, short interval monitoring, quick feedback, precise dynamic optimization” to control the internal cracking of the hard and brittle basaltic rocks, discontinuous deformation of staggered zones and spatio-temporal deformation of underground caverns in high geostress. It has promoted the safe and orderly construction of BHT-HS, the technical progress of the construction of hydropower station projects in China, and may also be applicable to the design and construction of similar projects worldwide.

Seepage Flow Properties of a Columnar Jointed Rock Mass in a True Triaxial Experiment

A columnar jointed rock mass is a type of rock mass with strong geometric anisotropy and high interface permeability. Its seepage characteristics pose new challenges to the construction and maintenance of the Baihetan Hydropower Station on the Jinsha River. The research object in this study is the columnar jointed rock mass (basalt) in the dam area of Baihetan Hydropower Station. Similar-material model samples of the columnar jointed rock mass with different column dip angles ( α = 0 ° ~90°) were prepared following a similar principle. A true triaxial seepage–stress coupling test was conducted to evaluate the seepage characteristics of similar-material samples with different dip angles under intermediate principal stress and minimum principal stress. The experimental results showed that the columnar jointed rock mass exhibited apparent seepage anisotropy. The relationship curve between the volume flow rate Q and the pressure gradient − d P / d L of the samples with different dip angles showed evident nonlinear seepage under intermediate principal stress, which could be well expressed using the Forchheimer equation. It shows the characteristics of a typical linear Darcy flow under minimum principal stress. The law of variations in the permeability of the samples with different dip angles under intermediate principal stress can be well expressed using the one-dimensional quadratic function equation k = a + b σ 2 + c σ 2 2 , and the law of variations in the permeability of the samples with different dip angles under minimum principal stress can be well expressed using the logarithmic function k = a + b ln σ 3 . The permeabilities of the columnar jointed rock mass with dip angles of 0°, 15°, 30°, and 60° were most sensitive to changes in stress, and the seepage characteristics increased in complexity after changes in stress.

Arch dam temperature control curve adjustment and its impact

With the improvement of intelligent temperature control technology, the “three phases and nine stages” temperature control curve used in the casting of extra-high arch dams also needs to be adjusted according to project requirements. In Baihetan Hydropower Station, the design is 120 days to reach the arch sealing temperature, but in the actual construction sometimes need to shorten or extend the length of time through the water, resulting in the arch dam concrete stress changes, so that the arch dam temperature control anti-cracking increased the difficulty. In this paper, we designed 90 days and 150 days to reach the arch sealing temperature through simulation calculation, and compared it with the standard working conditions. The results found that: shortening or extending the arch sealing temperature by 30 days brings less stress changes to the arch dam concrete, and the arch dam as a whole is in a safe range, and the temperature control curve can be reasonably adjusted according to the actual needs of the project.

Numerical simulation of hydraulic transients in hydropower station with complicated water conveyance system

With the construction of Jinping II hydropower station with extra-large diversion tunnel, Baihetan hydropower station with complicated tailwater system, Yangfanggou hydropower station with complicated surge chamber, High requirements are put forward for the calculation and analysis of hydraulic transient process. In the water transmission and power generation system design of these projects, the following technical problems was solved by Power China Huadong Engineering Corporation Limited: simulation of variable impedance coefficient of differential surge chamber, interface tracking analysis of free-surface-pressure flow system, narrow upper chamber open channel effect and simulation of trigeminy surge chamber. The solutions to those technical problems and were summarized, and the hydraulic transients simulation and calculation software of complex waterway system - Hysim was developed. Based on the field test of Jinping II Hydropower Station, Baihetan hydropower station and Yangfanggou hydropower station, the simulation calculations were carried out. The accuracy of the hydraulic transients simulation software is verified by comparing the inversion results with the test values.

Study on creep models and parameter inversion of columnar jointed basalt rock masses

Taking the Baihetan hydropower station in Southwest China as an example, creep models and parameter inversion of the columnar jointed basalt rock mass were investigated in this study. Based on the large in situ flexible double pillow bearing plate compression creep tests, the applicability of empirical creep models, such as the power function model, the logarithmic model and the exponential model, for the creep characteristics of columnar jointed basalt rock masses in the dam site area was analyzed first. Then, the compression creep formula of the measuring points in the middle of the flexible double pillow pressure plate was derived based on the generalized Kelvin model. Finally, three optimization algorithms, including the traditional least squares method (LSM), particle swarm optimization algorithm with a compression factor (CFPSO), and particle swarm optimization algorithm with a compression factor based on the simulated annealing algorithm (SA-CFPSO), were used to identify the creep parameters of the generalized Kelvin model. The results confirmed that the generalized Kelvin model based on the flexible double pillow pressure plate outperforms the traditional empirical models, and the performance of the proposed SA-CFPSO algorithm is superior to that of the LSM and CFPSO algorithms in identifying the creep parameters of the generalized Kelvin model.