1. Project introduction
As the second-largest hydropower station in the world in terms of installed capacity, the Baihetan Hydropower Station plays a crucial role in China’s West-East electricity transmission project. Located in the lower reaches of the Jinsha River, the dam site straddles Ningnan County in Sichuan Province and Qiaojia County in Yunnan Province in China, controlling a basin area of 430 300 km2 and covering 91% of the Jinsha River’s catchment. The total reservoir capacity of the Baihetan Hydropower Station is 20.627 billion cubic meter, and its total installed capacity is 16 000 MW. While the main function of the project is hydroelectric power generation, the project also addresses flood control and navigation, as well as promoting local economic and social development.
The main structures of the hydropower project include the water-retaining structure, flood-discharge and energy-dissipation facilities, and underground hydroelectric power-generation system. The water-retaining structure is a concrete double-curvature arch dam standing 289 m tall with a crest elevation of 834 m. The flood-discharge and energy-dissipation facilities, which have a maximum discharge capacity of 42 350 m
3·s
−1, consist of six spillways and seven deep outlets within the dam, three discharge tunnels on the left bank, and a plunge pool with an auxiliary weir located downstream of the dam. The underground water-conveyance and power-generation system, embedded on both banks of the Jinsha River, houses eight hydroelectric generators on each side, with a unit capacity of up to 1000 MW—the largest in the world.
Fig. 1 presents a panoramic view of the Baihetan Hydropower Station, while
Fig. 2 illustrates the project layout.
Preliminary work for the Baihetan Hydropower Station commenced in the 1950s. Construction preparations began in October 2010, reservoir impoundment commenced on April 6, 2019, the first set of generators began operation on June 28, 2021, and all generators became fully operational by December 20, 2022.
The Baihetan Hydropower Station presented significant technical challenges, including the following issues [
1], [
2]:
(1) The site features complex geological conditions, being located in a basalt formation characterized by easily relaxed columnar jointed basalt, interlayer and intralayer fault zones, and intense unloading of the left bank’s rock mass. The area has a seismic basic intensity of VIII and a bedrock peak ground acceleration reaching 451 gal (1 gal = 1 cm·s−2), with a 2% exceedance probability within 100 years. These conditions impose high requirements for seismic precautions.
(2) The double-curvature arch dam, with a concrete volume of 8.03 million cubic meter, stands at a maximum height of 289 m and has a total hydraulic thrust of 16.5 million metric tons. Due to the asymmetrical topography of the left and right banks, the dam has a significant asymmetrical shape.
(3) The hydropower project’s maximum discharge capacity is 42 350 m3·s−1, with a flood-discharge drop reaching 220 m and a discharge power of up to 90 000 MW. The asymmetrical arch dam poses significant challenges in flood discharge and energy dissipation.
(4) A total of 364 underground caverns serving various functions are densely arranged in the water-conveyance and power-generation systems on both banks. The total excavation volume for the underground complex is as high as 25 million cubic meter, posing significant challenges for the stability control of the surrounding rock.
(5) Every two hydroelectric generators—where each generator has a capacity of 1000 MW—shares one tailrace tunnel. The hydraulic safety requirements for the operation of such high-capacity hydroelectric units are stringent.
(6) Multiple giant deformation bodies and extremely large debris flow gullies near the dam area have presented significant geological risks and safety challenges during both the construction and operation phases.
2. Engineering and technology achievements
2.1. The ultra-high arch dam and foundation treatment
2.1.1. The equilibrium design method for nonsymmetrical arch dams
The topography on the right bank of the dam site is steep and intact, whereas the left bank features a stepped terrain with alternating gentle and steep slopes. The length of the dam crest on the left bank is 405 m, compared with 304 m on the right bank, making the left bank 101 m longer. This significant asymmetry is further compounded by the left bank rock mass having a greater unloading depth and a larger exposure of easily relaxed columnar jointed basalt than the right bank. The topographical and geological conditions following the foundation excavation are markedly asymmetrical, making it the most nonsymmetrical ultra-high arch dam in both China and the world, as shown in
Fig. 3. The asymmetry in the shape and foundation bearing capacity of the arch dam heightened the construction difficulty of the ultra-high arch dam as a hyperstatic shell structure [
3].
An equilibrium design method was developed to tackle the construction challenges of the asymmetrical arch dam. This method introduces an asymmetry index reflecting the degree of asymmetry in the arch dam’s shape and establishes an equilibrium coefficient for the arch dam based on the relationship between the arch thrust ratio, centerline translation, and deflection angle, thereby providing a basis for the adjustment of the arch dam’s stress and deformation. Using this method, the centerline of the arch dam was translated approximately 20 m toward the gentle slope bank from the centerline of the river channel and then further deflected counterclockwise by 5°. These adjustments reduced the arch dam’s asymmetry index from 1.76 to 1.42. Moreover, as the slightly weathered and fresh breccia covering part of the columnar jointed basalt in the riverbed was retained as the foundation of the arch dam, the exposed area of the easily relaxed columnar jointed basalt in the arch dam foundation was decreased from 55.5% to about 40.0%.
The shape design was based on two connected elliptical curves, and the curvature radius on the left side was increased to homogenize the stress distribution of the dam. A concrete pedestal was placed on the upper part of the left abutment, and a new type of expanded concrete foundation was installed on the middle and lower parts of the foundation, to further improve and adapt to the asymmetrical topographical and geological conditions. This strategy achieved a balanced coordination of thrust and deformation, enhancing the overall safety of the Baihetan arch dam.
2.1.2. Earthquake resistance of the ultra-high arch dam
The basic earthquake intensity at the project site is VIII. The arch dam was designed and verified according to seismic ground motion parameters, with 2% and 1% probability of exceedance over a 100-year reference period and horizontal ground motion peak accelerations of 451 and 534 gal, respectively, making these the highest peak accelerations in the world among 300 m-class high arch dams.
The Baihetan arch dam is the first ultra-high arch dam designed and evaluated for seismic safety according to the Seismic Ground Motion Parameters Zonation Map of China (GB18306-2015) and the code Seismic Design of the Hydraulic Structures of Hydropower Projects (NB35047-2015). In addition to the use of conventional methods to evaluate the seismic performance of the arch dam, comprehensive and in-depth studies were carried out using 3D nonlinear finite-element methods and seismic dynamic model testing methods. These studies examined various factors affecting the seismic performance of the arch dam, including the opening and closing of transverse joints in the dam body, foundation radiation damping, and interactions between the arch dam, rock foundation, concrete pedestal, and expanded concrete foundation. The impact of rock blocks formed by weak discontinuities in the foundation and the effects of seepage pressure were also considered.
In view of the importance of the project and its high seismic precaution standards, the structural design and foundation treatment measures were strengthened for parts of the dam with larger design seismic responses. These measures included emphasizing a smooth connection between the dam body and the expanded foundation and pedestal, using high-strength concrete, arranging seismic reinforcement on both the upstream and downstream dam surfaces, strengthening the anchoring of the arch abutments, enhancing the connection of the top beams of the spillways, reinforcing the transverse joint water stops, replacing weak structural planes in the rock masses, and consolidating the grouting of the dam foundation.
Thanks to the aforementioned measures, under the design seismic action, the stress in the dam body met the design requirements, and the residual seismic displacements of the arch dam and abutments were negligible after the simulated design earthquake. The cracking depth in the heel area of the dam was about 10% of the dam base width and did not extend to the grouting curtain. Moreover, the opening of transverse joints in the arch dam did not exceed 40 mm, ensuring that the waterstop structures in the transverse joints remained intact. Under the simulated verification earthquake, there was no sudden change in the displacement of the arch dam and its foundation, indicating the overall stability of the arch dam’s foundation system. The seismic overload capacity of the arch dam is controlled by the stability of the local blocks on both abutments, and the seismic overload safety factor of the dam’s foundation system is greater than 1.7. The stress and stability of the arch dam under the design seismic action met the design requirements, and the dam’s foundation system under the simulated verification earthquake complied with the seismic-precaution objective of preventing collapse.
2.1.3. Relaxation control of the columnar jointed basalt dam foundation
The rock formation at the Baihetan dam site primarily consists of basalts from the upper Permian Emeishan Formation. The dam foundation exposes columnar jointed basalt with a thickness of approximately 76 m at medium to low elevations, which accounts for approximately 40% of the 80 000 m
2 foundation surface, as shown in
Fig. 4. The columns typically range from 2 to 3 m in length and 13 to 25 cm in diameter, with internal microcracks. Test results have shown that, after excavation and unloading, the rock mass is highly susceptible to relaxation, with a typical relaxation depth of 2-3 m and up to 4 m locally, as shown in
Fig. 5. Under relaxation, the deformation modulus of the relaxed rock mass decreases by about 50%, leading to a deterioration of the stress state in the dam body. Few studies have been carried out on the engineering characteristics of columnar jointed basalt, and its use as a foundation for a high dam was unprecedented. The relaxation control in the columnar jointed basalt foundation was one of the significant technical challenges of the Baihetan project.
Comprehensive research on the characteristics of columnar jointed basalt was conducted throughout the entire engineering design process. Extensive geological explorations, field tests, and analytical studies focused on the feasibility of using columnar jointed basalt as the foundation for an ultra-high arch dam, the adaptability of the arch dam structure, blasting excavation technology, anti-relaxation protection measures, and relaxation prediction methods. These studies elucidated the mechanisms of relaxation and the spatiotemporal evolution of the mechanical properties. A corresponding quality evaluation index system for basalt and standards for the utilization and control of relaxed rock masses have been established. In addition, a set of relaxation control technologies has been developed, including thick layer protection, grouting consolidation, deep anchoring, and precision-controlled blasting, thus pioneering the construction of ultra-high dams on easily relaxed rock mass foundations [
4].
After fully implementing this set of relaxation control technologies, the depth of rock mass relaxation caused by the extensive foundation excavation was effectively constrained to within 1.2-1.4 m. The wave velocity of the relaxed rock mass layer is not less than 4000 m·s−1, and the original columnar mosaic structure is maintained. The control effectiveness is comparable to that of the relaxed level of blocky basalt rock mass; thus, the issue of rock relaxation control has been completely resolved, and the quality of the rock mass at the arch dam foundation is ensured. Monitoring results following the reservoir impoundment indicated that the dam is operating safely and reliably.
2.2. Large-flow flood discharge and energy dissipation in a narrow river valley
2.2.1. Energy dissipation via asymmetric collision
The flood-discharge and energy-dissipation facilities at the Baihetan project include six spillways and seven deep outlets in the dam body, a counter-arched plunge pool and an auxiliary weir downstream of the dam, and three free-flow discharge tunnels on the left bank. During the dry season, the water surface width of the valley at the dam site is only 50-90 m, while the maximum discharge of the project in such a narrow valley will reach up to 42 350 m3·s−1, with a total discharge power exceeding 90 000 MW. More specifically, the maximum discharge capacity of the dam body is 30 100 m3·s−1, corresponding to a discharge power of 61 700 MW, and the specific energy-dissipation rate of the counter-arched plunge pool is 17.1 kW·m−3. The project is characterized by a narrow valley, high head, and substantial discharge, with hydraulic indicators such as the unit width discharge flow, dam body discharge power, and specific energy-dissipation rate being the highest among similar projects. Due to the asymmetric terrain and geological condition of the two banks, it was impractical to adopt symmetric discharge outlets in the dam body and conventional energy-dissipation techniques for safe and efficient energy dissipation.
To ensure safe flood discharge, extensive research has been conducted on the hydraulic coordination issues of flood discharge and energy dissipation. Challenges such as the spatial stratification and diversion of dense water inlets in front of the dam, the spatial distribution of energy-dissipation partitions, the allowable discharge volume in the river channel, and the control of scouring and accumulation have been addressed and solved.
A creative asymmetric collision energy-dissipation technology [
5] has been developed to directly adjust the trajectory of each water nappe. The layout and structural shape of the multilayer spillways and outlets have been adjusted. In the longitudinal direction of the discharge flow, a large differential grouping is used to spread out the water nappe landing points. In the transverse direction, plane diffusion structures are adopted to utilize the energy-dissipation space as much as possible. Additionally, in the vertical direction, the water nappes from the spillways and deep outlets overlap and collide to dissipate energy. This adjustment in the longitudinal and transverse flow distribution ensures sufficient collision among the water nappes. Under equivalent flood-discharge conditions, this asymmetric arrangement leads to a more uniform distribution of the dynamic water impact pressure in the plunge pool downstream of the dam, with the peak pressures being reduced by 18%.
Fig. 6 illustrates the layout for the flood discharge and energy dissipation at the asymmetric arch dam, while
Fig. 7 depicts the energy dissipation resulting from the collision of water nappes from the spillways and deep outlets.
2.2.2. The free-flow flood-discharge tunnel group
Three discharge tunnels are arranged in parallel on the left bank, featuring straight-line, dragon-tail, free-flow tunnels. These tunnels range in length from 2170 to 2317 m and have a maximum discharge capacity of 4100 m3·s−1, with the highest levels of single-tunnel discharge and operating water head in the world. In-depth research has been conducted on critical technical challenges, including the hydraulic safety of long, gentle slope sections at high velocities without air entrainment, effective air entrainment and ventilation in dragon-tail sections to reduce erosion and enhance air replenishment, safe energy dissipation at discharge tunnel outlets, and the complex structure of extra-wide inlet gates.
The maximum flow velocity in the 1908 m-long gentle slope section approaches 30 m·s−1, while the maximum flow velocity within the tunnel body reaches nearly 50 m·s−1, accompanied by a significant water depth and a low Froude number. To address the cavitation issues associated with the high-speed water flow, a hydraulic safety design method for free-flow discharge tunnels with a long gentle slope, high flow, and high water head has been established. For the dragon-tail sections, a straight-flow side air-entrainment structure and 3D non-isotropic air-entrainment technology have been developed to coordinate the bottom and side air entrainment.
An innovative air-entrainment bucket for large free-flow discharge tunnel groups incorporates isolated-top air-replenishment technology, resolving the challenge of insufficient effective air entrainment due to the limited roof space in the tunnels. In addition, surface-irregularity control standards for concrete lining construction and segmented anti-cavitation connection structures have been established, effectively addressing the challenge of surface cavitation at high flow velocities. Furthermore, a lateral three-arm support technology for eliminating uneven deformation and a three-point co-linear coaxial self-adapting technology have been developed to address structural safety concerns associated with the extra-wide inlet gates under immense water thrust [
6].
2.2.3. Structure and safety of the counter-arched plunge pool
With a discharge flow of 30 100 m
3·s
−1 and a flood-discharge power of 60 000 MW, the maximum pulsating uplift force on the bottom slab of the plunge pool exceeds the combined weight and anchoring force, making it challenging for conventional flat-bottom plunge pool structures to meet the safety requirements. To address this issue, the plunge pool employs a counter-arched structure composed of nearly 200 concrete slabs, each 4 m thick and measuring 15 m × 15 m, forming a discontinuous arched thin-shell structure. Hydraulic model tests indicated that the combined discharge from the spillways and deep outlets is the controlling condition for the dynamic water impact pressure and pulsating pressure in the plunge pool [
7]. The dynamic water loads borne by each plate are characterized by locality, randomness, and non-uniformity. When subjected to external loads, the discontinuous arched thin-shell structure exhibits local arch behavior, with the arch effect of different slabs gradually diminishing (from 80% to 20%) from the center of the ogee to the edge, resulting in a corresponding increase in the need for anchoring force.
Therefore, a stability analysis using the moment balance method for the ogee-shaped bottom slab was proposed to quickly solve the stability calculation of the bottom slab. Based on the distribution pattern of the dynamic water loads, while prioritizing the utilization of the arch effect of the slabs, a zoning anchoring principle has been developed. This approach has led to an innovative coupling technology for analyzing the interaction between the arch effect and the anchoring of the ogee-shaped bottom slab. To ensure the safety and easy maintenance of the plunge pool structure, several inventions have also been developed, including a joint structure adaptable to extrusion deformation, a flexible waterstop leakage-treatment technology integrated with dynamic monitoring, and a bidirectional self-flow drainage system.
The flood-discharge and energy-dissipation facilities have successfully undergone multiple operational tests over four flood seasons, operating safely for more than 260 h. Prototype observations and measurements confirm the smooth flow dynamics, stable monitoring indicators, and excellent performance of the flood-discharge and energy-dissipation structures.
2.3. The expansive cavern group of the underground powerhouse
2.3.1. Overall layout of the underground cavern group
The underground powerhouse of the Baihetan Hydropower Station is symmetrically arranged on both banks of the Jinsha River, with each bank housing eight water turbine generator units, each with a capacity of 1000 MW. The water-diversion and power-generation system comprises water-diversion tunnels, main and auxiliary powerhouse caverns, main transformer caverns, draft tube maintenance gate chambers, tailrace surge chambers, tailrace tunnels, and aboveground take-off yards. In addition, the layout must effectively coordinate the arrangements of five diversion tunnels and three flood-discharge tunnels. The confined terrain of the narrow valley presents significant challenges for the overall layout planning of the underground cavern group.
The inertia time constant
Tw is 55.96 s, indicating the need for a surge chamber with a cross-sectional diameter of up to 60 m for the tailrace tunnel. This presents considerable challenges due to the large span required in rock formations characterized by interlayer faults and columnar joints. Consequently, the initial arrangement of three units sharing one surge chamber was modified to two units sharing one surge chamber. Furthermore, the traditional parallel arrangement of three major caverns (i.e., the main and auxiliary powerhouse caverns, main transformer cavern, and tailrace surge chamber) was transformed into a layout featuring four major caverns by adding an independent maintenance gate chamber cavern for the draft tube, as shown in
Fig. 8. This adjustment reduced the excavation diameter of the surge chamber to 48 m.
In-depth research involving theoretical analyses and numerical simulations was carried out, using the failure degree of the rock mass as an indicator to mitigate the impact of the cavern group on the surrounding rock stability, thereby enhancing the load-bearing capacity of the deep surrounding rock. The separation between the main powerhouse cavern and the main transformer cavern was adjusted to 60.65 m, resulting in a rock-pillar-thickness to cavern-span ratio of 1.96, which exceeds the 1.55-1.70 range observed in similar projects.
Fig. 9 illustrates the spacing control values for the main caverns of the underground powerhouse. Combining the three tailrace tunnels on the left bank and the two on the right bank with the diversion tunnels not only reduced the excavation scope of the tailrace tunnel outlet slope but also permitted the optimal utilization of temporary structures during the construction period, resulting in significant cost savings.
2.3.2. The rock mass stability of the underground cavern group
The powerhouse area is subjected to high
in situ stress, with measured values exceeding 33 MPa. The rock-strength to stress ratio (
Rb/
σ1) varies between 2.85 and 5.89. The basalt in the area is characterized by developed micro-fractures, low crack initiation strength, and significant spalling issues [
8]. Interlayer and intralayer faults, which tend to soften upon exposure to water and exhibit low mechanical strength, diagonally intersect the main caverns on both banks. This geological condition can trigger deformation of the surrounding rock after excavation. Moreover, the columnar jointed basalt in the large dome of the tailrace surge chamber and the tailrace tunnel displays notable anisotropic characteristics, making it susceptible to unloading relaxation and potential structural disintegration during excavation.
In response to the complexities of the ultra-high, large-span underground cavern groups, a comprehensive strength-stress design technique that considers the post-peak strength of the surrounding rock has been developed [
9]. This method aims to fully leverage the load-bearing capacity of the surrounding rock. Utilizing the degree of rock mass damage as an indicator, a three-level design methodology for the stability of the surrounding rock has been established, encompassing macroscale overall layout control, mesoscale process control, and microscale focus reinforcement. Emphasizing timely closure, phased strengthening, maintenance of the confining pressure, and suppression of damage, a series of control technologies addressing the unloading relaxation of the low-fracture-strength brittle basalt along with corresponding support systems have been developed to ensure the stability of the surrounding rock in the giant underground cavern group under high-stress conditions.
For the interlayer fault zones that diagonally intersect the high sidewalls of the underground power house, a combined anti-shear technique involving advanced replacement to suppress movement and strong anchoring of the upper plate to control deformation was employed. This approach effectively managed the amount and extent of deformation in the fault zones, with the measured deformation of the upright high sidewall being limited to only 38 mm, despite a height of up to 89 m and length of 438 m. The technology used for the giant top-arch caverns included pre-anchoring of the crown, maintaining the surrounding pressure, and gradual arch formation. Pre-anchoring reinforcements were implemented through observation galleries situated in the cavern’s top arch, while a streamlined dome shape was adopted for the giant tailrace surge chamber. The excavation was conducted in finely segmented layers and blocks to reduce the unloading rate, with immediate support and rapid maintenance of the surrounding pressure, as illustrated in
Fig. 10. This approach was crucial for the stability of the large caverns with spans ranging from 34 to 48 m.
Deformation monitoring demonstrated that the excavation and support measures used for the ultra-high, large-span underground cavern group under complex geological conditions were effective. The deformations have stabilized, confirming the surrounding rock stability of the giant underground powerhouse cavern group.
2.4. Hydraulic safety of the million-kilowatt generating units
The Baihetan Hydropower Station’s water-conveyance system consists of several key components including intakes, headrace tunnels and shafts, draft tubes with maintenance gate chambers, tailrace surge chambers, and tailrace tunnels. The headrace system employs a single tunnel for each unit, while the tailrace system utilizes a dual-machine single-tunnel layout. The flow rate for the tailrace single tunnel is 1095 m3·s−1, and the hydraulic inertia of the tailrace system reaches 2.2 million t·m·s−1.
For longer tailrace systems, a one-tunnel multi-machine arrangement is typically employed to reduce project costs. However, given the million-kilowatt units at Baihetan, which have large individual capacities and high reference flows, hydraulic oscillations significantly impact the unit performance. To mitigate interference between units and ensure stable operation, it was crucial to minimize the number of units connected to each hydraulic system. Consequently, the tailwater hydraulic system employs a combination scheme in which only two machines share one surge chamber and one tailrace tunnel, with the dimensions of both the surge chamber and the tailrace tunnel being appropriately reduced.
The tailrace system at Baihetan exhibits substantial hydraulic inertia, necessitating heightened requirements for hydraulic safety control. To manage the minimum pressure at the tailrace pipe outlet, stabilize fluctuations in the surge chamber, and control the surge-wave water levels, the surge chamber was designed to be exceptionally large. Therefore, the draft tube maintenance gate chamber and the cylindrical surge chamber were configured separately. The design and hydraulic analysis were based on the combined action of a dual-chamber differential surge chamber, which enhances the cavern stability and effectively accelerates the decay rate of hydraulic oscillations.
Based on a critical water-level analysis for clear open-channel and full flow conditions, a three-stage bottom slope arrangement is employed for the tailrace tunnel to ensure structural safety under these conditions. The layout of the water-conveyance and power-generation system is illustrated in
Fig. 11. This arrangement significantly improves the hydraulic characteristics for open-channel and full flow, meeting the operational stability requirements of the million-kilowatt units [
10].
To control the hydraulic vibrations, a theory of hydraulic vibration analysis for multi-flow state hydraulic systems and a technology for hydraulic resonance assessment have been established. A comprehensive analysis of the impact of the open-channel and full flow on the vibration characteristics has been conducted. Additionally, an analytical method for assessing the influence of the unit dynamic characteristics on the system’s characteristic frequencies has been developed. This has led to the creation of a hydraulic resonance safety-assessment technology for both free and forced vibrations, effectively addressing the safety-assessment challenges in the water-conveyance and power-generation systems of the million-kilowatt units [
11]. Furthermore, a hydraulic transient simulation analysis system has been developed, enabling the full-element, high-precision simulation analysis of complex water-conveyance and power-generation systems at the million-kilowatt level.
2.5. Temperature control and crack prevention of dam concrete
The Baihetan ultra-high arch dam features six spillways, seven deep outlets, and five bottom diversion outlets, resulting in a complex dam structure. C18040 concrete is utilized in areas with strong constraints, with a maximum casting block length of almost 100 m (without longitudinal joints) and a width of 23 m, significantly exceeding those of similar projects. Located in the dry and hot river valley of the Jinsha River, the site experiences prolonged high temperatures, sharp temperature drops in autumn and winter, substantial diurnal temperature variations, and frequent strong winds—all of which complicate temperature control and crack prevention in the dam’s concrete.
The dam’s concrete volume is 8.03 million cubic meter. After extensive research and assessment, low-heat Portland cement concrete was selected for the entire structure. Compared with medium-heat cement, low-heat cement exhibits slower heat release in the early stages and generates less hydration heat, reducing the adiabatic temperature rise by 4-6 °C. This effectively lowers the maximum temperature of the concrete, enhancing its resistance to cracking. However, low-heat cement concrete develops strength more slowly in the early stages, resulting in significant differences in temperature-control standards, measures, effects, and construction techniques compared with medium-heat cement concrete.
Temperature-control index systems and measures specifically tailored to the characteristics of low-heat cement concrete have been developed for Baihetan. A temperature-control strategy that focuses on small temperature differences, slow cooling, early protection, and long curing has been adopted; this strategy differs from previous cooling measures by implementing a full-cycle continuous cooling method, as illustrated in
Fig. 12. In line with the slow early heat release of low-heat cement concrete, a method for controlling the maximum temperature range has been introduced, achieving dual control of the upper and lower limits of the concrete’s highest temperature for the first time and maintaining it between 21 and 27 °C. After the peak temperature was reached, a comparatively high temperature was moderately maintained to promote the hydration reactions and early strength development of the low-heat cement. The middle cooling stage was optimized, ensuring uniform and continuous slow cooling throughout the process, which minimized temperature gradients. At the end of the middle cooling stage, temperature control was strengthened to coordinate the synchronized two-stage cooling of the same arch ring, ensuring the proper opening of the transverse joint and significantly reducing the temperature drop gradients to manage the risk of concrete cracking. Following the optimization of temperature-control indicators and measures, the assurance rate for various temperature-control metrics greatly improved, and the crack-resistance safety factor of the arch dam increased from 1.8 to 2.0 [
12].
An exclusive standardization process for casting low-heat cement concrete has been established, incorporating techniques such as delaying mold removal in winter, ensuring warmth and moisture, using a parallel demolding process that prevents damage to the transverse joint surfaces, and ensuring optimal timing and pressure for scouring low-heat cement concrete. A complete set of intelligent temperature-control equipment was utilized to ensure precise control of the water cooling and concrete temperature throughout the entire process. The construction process of the dam was simulated, with real-time monitoring of crack risks in the concrete due to temperature variations across different seasons and periods.
A comprehensive inspection of the entire dam revealed no common temperature-induced cracks, and the world’s longest concrete core sample—measuring 36.74 m in length—was successfully retrieved.
2.6. Comprehensive management technology for large debris flow gullies
The Baihetan Hydropower Station is situated in a mountainous gorge characterized by strong weathering and unloading of rock. The gullies in this area are deeply incised with significant longitudinal gradients, making many of them susceptible to debris flow. Five debris flow gullies near the dam—namely, the Dazhai, Aizi, Haizi, Yanji, and Baihetan Gullies—posed challenges to the project’s construction and operation. In particular, the Dazhai, Aizi, and Haizi Gullies have previously experienced significant debris flow events.
Dazhai Gully, which is located on the right bank adjacent to the upstream side of the power station’s water inlet, presents a direct threat to the operational safety of the water inlet. In conjunction with the treatment of the creeping deformation mass at Hongyan on the right bank near Dazhai Gully’s outlet, a comprehensive management strategy has been established for addressing the debris flow in Dazhai Gully and the giant creeping deformation mass at Hongyan. This strategy integrates a diverse range of engineering measures, including barriers, drainage systems, spoil pile slope reinforcement, and anti-slide piles. It also considers the needs of surrounding town traffic, vegetation restoration, and landscape aesthetics.
On the left bank, a giant spoil dump with a total capacity of approximately 40 million cubic meter is located at the mouth of Aizi Gully, 6.1 km upstream of the dam site. The prevention and control measures for debris flow in Aizi Gully primarily focus on retention with supplementary drainage. Spoil was used to form a blocking dam in advance, creating a sedimentation reservoir several times larger than the total volume of a single debris flow event. After the separation of water and detritus, the clean water was drained through a channel set on one side of the spoil pile, while the detritus was retained in the sedimentation reservoir. This method effectively mitigates the adverse impacts of large debris flows on the giant spoil dump.
Yanji Gully, which is situated on the left bank above the water inlet and the dam, consists of multiple debris flow gullies. To address the dispersed, multi-branch characteristics of the slope debris flow in the basin of Yanji Gully, a high-position debris flow discharge technology comprising zoned multilevel horizontal drainage plus vertical drainage has been developed, solving the challenge of discharging debris flows from high positions with a 300 m drop on the slope.
Leveraging modern information technology, a rainfall-monitoring system has been established in the source area of the basin, along with a debris flow early-warning system. A rapid response mechanism for emergencies has been implemented, and a comprehensive risk management and control system for debris flow disaster prevention and mitigation of the hydropower project has been established.
3. Conclusions and outlook
Since the reservoir impoundment, the Baihetan Hydropower Station infrastructure has undergone three flood seasons and has demonstrated exceptional performance. The monitoring data for various structures—including the dam, flood-discharge, and energy-dissipation facilities and the water-conveyance and power-generation systems on both banks—have consistently outperformed the established technical control indicators, thereby fulfilling all technical expectations for the project.
The design and construction of the Baihetan Hydropower Station have effectively addressed 16 world-class engineering challenges, resulting in the development of 127 key technologies. This remarkable achievement has significantly enhanced China’s capabilities in the design and manufacturing of ultra-high arch dams, expansive underground structures, and advanced equipment, positioning the nation at the forefront of global hydropower engineering technology. Notably, the design, construction, manufacturing, and installation of the dam’s electromechanical systems were all executed with full autonomy, thereby setting a new benchmark in the international hydropower engineering sector.
Looking ahead, hydropower engineering construction in China will face challenges characterized by extreme macroscales, extreme microscales, extreme conditions, and extreme interdisciplinary integration. The potential for innovation is boundless; therefore, it is essential to rely on independent scientific and technological advancements to resolve critical engineering challenges and achieve the high-quality development of hydropower engineering in China.
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