Powerhouse Cavern Research Articles

Dynamic stability evaluation of underground cavern sidewalls against flexural toppling considering excavation-induced damage

Flexural toppling is a crucial engineering issue for high sidewalls of underground powerhouse caverns in low stress regions, when a small angle between cavern axis and bedding planes is formed. In this study, a novel dynamic method is proposed for stability evaluation of a high sidewall subjected to flexural-toppling, by integrating our derived criterion, continuum modelling and microseismic (MS) data. First, by virtue of cantilever slab theory, a geo-mechanical model is defined for analysing the toppling rock strata exposed on the high sidewalls of underground caverns. The Rayleigh-Ritz method is then adopted to deduce the equation for potential energies of toppling rock strata. A criterion for assessing the stability of high sidewalls considering a secondary stress field can thus be derived from catastrophe theory. Subsequently, stresses acting on the bedding planes are numerically obtained by continuum modelling coupled with MS data. Finally, a quantitative stability evaluation methodology is presented for flexural-toppling in view of progressive failure, considering the influence of fractures along bedding planes. A practical case of flexural toppling failure is analysed to verify our proposed approach. Good agreements indicate that our proposed approach is reliable for quantitatively evaluating the stability of larger underground powerhouse caverns against flexural toppling.

Erratum to: Mechanical response and stability analysis of rock mass in high geostress underground powerhouse caverns subjected to excavation

Erratum to: Mechanical response and stability analysis of rock mass in high geostress underground powerhouse caverns subjected to excavation

Norwegian oil and gas storage in rock caverns – Technology based on experience from hydropower development

Underground storage in rock caverns is widely used in Norway for many different petroleum products, such as crude oil, fuel, propane and butane. Basically, the caverns for such storages are unlined, i.e. containment is ensured without using any steel lining or membrane. The main basis for the storage technology originates from the extensive hydropower development in Norway. As part of this activity, about 4500 km of tunnels and shafts have been excavated, and around 200 large powerhouse caverns have been constructed. The hydropower tunnels are mainly unlined, with hydrostatic water pressure on unlined rock of up to 1000 m. Some of the projects also include air cushion chambers with volumes of up to 1 × 10 5 m 3 and air pressure up to 7.7 MPa. Many lessons which are valuable also for underground oil and gas storage have been learnt from these projects. For a storage project to become successful, systematic, well planned design and ground investigation procedures are crucial. The main steps of the design procedure are first to define the optimum location of the project, and then to optimize orientation, shape/geometry and dimensions of caverns and tunnels. As part of the procedure, ground investigations have to be carried out at several steps integrated with the progress of design. The investigation and design procedures, and the great significance of these for the project to become successful will be discussed. Case examples of oil and gas storage in unlined rock caverns are given, illustrating the relevancy of experience from high-pressure hydropower projects for planning and design of unlined caverns for oil and gas storage.

Studying the deformation and stability of rock mass surrounding the power station caverns using NA and GEP models

In this research, an optimum equation is proposed for displacement estimation at the key point and key zone of powerhouse cavern sidewall in elasto-plastic condition based on the numerical experiments. Moreover, a new predictive equation is suggested to predict occurred displacement at the key point using the gene expression programming (GEP). Based on the numerical analyses (NA), the location of key point on the reciprocal longitudinal walls of the powerhouse cavern is studied. It was specified that for cavern with 33x52 m cross section, key point might be placed on the rock sidewall (left wall) or on the pillar side (right wall) of cavern. On the other hand, for cavern with 18x30 m cross section in various geo-engineering conditions, the key point was located on the rock sidewall. Verification of the proposed equations (from NA and GEP) were conducted using some case studies measurements which proved a good agreement of the new equations with the real values and showed the higher accuracy compared to the similar available equation. Finally, using warning levels of tunnel stability loss proposed by Sakurai, a new a system classification scheme is proposed for key point surrounding the powerhouse cavern based on the critical displacement.

Execution of Safe Blasting Under Adverse Conditions of a Powerhouse Complex: A Revisit to Sardar Sarovar Project, India

When the excavation of the underground powerhouse of the Sardar Sarovar Project, India was nearly complete, cracks were observed on the upstream and downstream walls of the powerhouse, and the installed instrumentation readings sounded an alert for the instability of the powerhouse cavern that could possibly derail the project, further excavation in the powerhouse cavern was halted. After completing stabilisation measures, the remaining underground excavations by drill and blast method were to be completed. This paper revisits case studies of controlled blasting for the remaining excavations, namely a construction ramp, turbine pits, draft tube tunnels connecting the powerhouse, and the concrete plugs erected at the exit ends of the draft tube tunnels. To ensure overall stability around the excavations, blast vibration was controlled by planning the excavations in proper sequences. The damage outside the planned line of excavations was controlled by adopting modified line drilling/smooth blasting techniques. The details of the sequence of excavations, drilling and blasting parameters, compiled from previous publications, are presented in this paper. This paper also describes the reasons why concrete plugs were erected in the draft tube tunnels, the details of the concrete plugs, the optimised drilling and blasting procedure for safe removal of the plugs, and the method adopted to quantify the damage.

Dynamic Stability Evaluation of Underground Powerhouse Cavern Using Microseismic Monitoring

Construction of any excavation adds to the re-distribution of stresses around abutment and crown. This is stabilised by providing extra reinforcement. When a large underground cavern is excavated, stress re-distribution around abutment is obvious. Underground powerhouse cavern of Tala Hydropower Plant with adverse geological setting is one such cavern which encountered failure of a number of rock-bolts and series of other types of rockmass failure during its construction. It puts a question mark on the long-term stability of this powerhouse. To evaluate the dynamic stability of the rockmass (against induced stresses) around this powerhouse, microseismic monitoring network was installed. Microseismic data has been analysed in terms of static and dynamic parameters to assess the overall cavern stability. Spatio-temporal analysis of microseismic events could identify the potential risk zones which has significant cluster of events. To assess the overall stability due to these induced events, the ratio of transverse wave and longitudinal wave energy of these events was used as an index. It is found that cluster of events had shear failure. These events are basically of small non-damaging type and aid in releasing the built-up induced stresses. The slope of log–log plot of energy versus moment has a very high value. This indicates a stiffer rockmass which adds to its capability to absorb the induced stresses and aid to the overall stability of the region. Analysis reveals that no major seismic events are expected around this rockmass cavern and there is no dynamically unstable region against induced stresses.

Comprehensive Evaluation of Sensitivity of Surrounding Rock Parameters for Underground Powerhouse Experience Support

At present, the design and construction of underground powerhouse support are still in the experimental stage. Moreover, before the construction of the underground cavern group, the mechanical parameters of the surrounding rock mass cannot be accurately obtained. During the implementation process, these reasons may cause adjustments to the construction and design plans. To comprehensively evaluate the sensitivity of surrounding rock parameters, we used the multi-indicators, which have certain guiding significance for the accurate acquisition of surrounding rock parameters and the adjustment of empirical support schemes. This study combines the initial geological conditions of the underground powerhouse caverns of the hydropower station and the support scheme. We selected the key position deformation of the lining and the cable stress as the evaluation indicators. The elastic modulus, Poisson’s ratio, friction angle, and cohesion are the influencing factors for the comprehensive evaluation of sensitivity. The results show that the elastic modulus is a highly sensitive parameter for the empirical support scheme, the friction angle is a middling sensitive parameter, and the Poisson’s ratio is a low-sensitivity parameter. Moreover, it is reported that sensitivity is related to the selection of indicators.

Optimization Analysis of Suki Kinari Underground Powerhouse Caverns Based on an Efficient CATIA-Abaqus Model

Underground powerhouse caverns, which are large-scale, complex-type, and intersecting circuits, are considered as a complicated, constrained, and irreversible nonlinear system. They are divided into several layers for excavation. Various stress states occur whenever they are subjected to excavation; therefore, deformation and failure arise because of stress redistribution and concentration. Based on the determinate excavation scheme, the sectional shape of underground powerhouse caverns, especially the aspect ratio of the main powerhouse, is the key factor that affects the stability of the surrounding rock masses. Furthermore, during the excavation process, the integrity and stability of rock pillar, whose thickness is determined by the interval of caverns, are considered as crucial issues, because damage occurs in the thinner rock pillar. Consequently, the interval of caverns is the important parameter that needs to be investigated. In this study, we combine building information modeling with numerical analysis to investigate the effect of the aspect ratio of the main powerhouse and the interval between the main powerhouse and the main transform chamber. For the Suki Kinari (SK) underground powerhouse caverns, the 3D terrain and geological model was created in accordance with the detailed geological information. The parameterized underground powerhouse model and the supporting system were designed by CATIA, and the numerical model that involves the excavation scheme was generated by ABAQUS. The results show the following: (1) As the aspect ratio increases, the deformation of the main powerhouse exhibits various features, indicating that the arch crown settlement reduces while the deformation of the sidewalls increases. (2) As the aspect ratio increases, the overall deformation of the particular parts exhibits the characteristic of rapidly decreasing and then gradually forming a convergent trend. (3) Intense disturbance occurs on the rock pillar because of bidirectional unloading during the excavation process, and the rock pillar weakens rapidly and then gradually stabilizes as the thickness increases, suggesting the property of exponential function of the total deformation. Finally, the conclusions are applied on the SK underground powerhouse caverns to reveal the optimal aspect ratio and interval.

Mechanical response and stability analysis of rock mass in high geostress underground powerhouse caverns subjected to excavation

To investigate the stability of rock mass in high geostress underground powerhouse caverns subjected to excavation, a microseismic (MS) monitoring system was established and the discrete element method (DEM)-based numerical simulation was carried out. The tempo-spatial damage characteristics of rock mass were analyzed. The evolution laws of MS source parameters during the formation of a rock collapse controlled by high geostress and geological structure were investigated. Additionally, a three-dimensional DEM model of the underground powerhouse caverns was built to reveal the deformation characteristics of rock mass. The results indicated that the MS events induced by excavation of high geostress underground powerhouse caverns occurred frequently. The large-stake crown of the main powerhouse was the main damage area. Prior to the rock collapse, the MS event count and accumulated energy release increased rapidly, while the apparent stress sharply increased and then decreased. The amount and proportion of shear and mixed MS events remarkably increased. The maximum displacement was generally located near the spandrel areas. The MS monitoring data and numerical simulation were in good agreement, which can provide significant references for damage evaluation and disaster forecasting in high geostress underground powerhouse caverns.

Monitoring of Forces in the Active Anchorage System Installed in the Underground Structures According to Extensometer Data

The paper presents the method developed to evaluate acting forces in the active anchorage system based on the data provided by multipoint borehole extensometers installed in the chamber-type underground power house cavern of Rogun HPP (Republic of Tajikistan) and the results of its practical application. This method allows for quickly and reliably determination of the actual values of forces in uninstrumented anchors and evaluation of their remaining service life.

Treatment of Main Geological Defects during Excavation Construction of Wudongde Underground Powerhouse Caverns

Underground powerhouse is adopted by Wudongde hydropower plant on both left and right bank. The excavation scale of the main powerhouse is 333.0×32.5×89.8m (length×width×height ) that ranks top on the world. The underground powerhouse caverns are hosted in the fold basement of the Presinian system that belongs to the layered epimetamorphic rock. Geological condition is complicated and rock stratum is steep towards the downstream and characterizes in “thin, hard and crisp”. During excavation construction of the underground powerhouse, geological defects such as small angle between surfaces, B type breccia, carbon film attached on surface, blocks, large and long discontinuities with small angle are exposed. The targeted treatments for the geological defects are proposed and implemented based on numerical feedback analysis and references of similar finished projects. Excavation and supports of the main powerhouse are all constructed completely at present and monitoring displacements tend to be convergent. The proposed treatments are verified to be reasonable and effective and provide reference for subsequent similar projects.

Flowchart of DEM Modeling Stability Analysis of Large Underground Powerhouse Caverns

The power of the Distinct Element Method (DEM) in solving the stability of underground powerhouse caverns (UPC) is discovered, but when does the DEM need to be adopted? What problems can be solved? The flowchart is provided and applied to analyze the stability of UPC in this paper. With the guide of the flowchart, the damage index (Di) is used as a failure type (gravity‐controlled or stress‐induced) judgment indicator. Through the calculation of three typical engineering, the problems of random blocks stability, dynamic calculation, and support system evaluation are studied, respectively, with the help of the DEM code 3DEC. The method and results of this paper can give reference to engineering projects of its category.

Influence of amygdale on crack evolution and failure behavior of basalt

Amygdaloidal basalt is one of the two major types of surrounding rocks that are widely exposed in the super large-scale underground powerhouse caverns of Baihetan Hydropower Station, China. Excavation of the caverns induced a series of mechanical responses, including cracking and fracture relaxation. This study comprehensively explored the deformation and failure characteristics and the fracture evolution process of the amygdaloidal basalt using conventional MTS triaxial laboratory testing equipment and continuous–discontinuous modeling (FDEM) with the aim of clarifying its failure mechanism. The research results indicate that: (1) amygdales significantly changed the deformation and failure characteristics of basalt. Whereas the post-peak mechanical behavior of cryptocrystalline basalt was brittle, that of amygdaloidal basalt was brittle under low confining stresses and ductile under high confining stresses. Meanwhile, whereas the failure mode of cryptocrystalline basalt was fragmentation failure, that of amygdaloidal basalt was splitting failure at low confining stresses and shear failure at high confining stresses. (2) An FDEM model considering the heterogeneity of the rock mesostructure was established based on the statistical laws of the amygdales in real samples, and this confirmed the stress–strain curve characteristics, failure modes, and characteristic strengths measured in laboratory tests. (3) A complicated inhomogeneous stress field formed readily around amygdales and had a significant effect on the fracture path. (4) With an increase in confining stresses, the degree of tensile stress concentration at the upper and lower tips of amygdales decreased and the lateral displacement became restricted. Consequently, the total number of micro-cracks and the ratio of tensile cracks to shear cracks decreased, the crack initiation mode changed from tensional cracking to shear cracking at the tips of the amygdale, and the distribution of crack orientations transitioned from an approximately vertical direction to preferential concentration along fracture planes. The research results form a solid foundation for the accurate understanding of the mechanical response and fracture mechanism of the rock surrounding underground powerhouse caverns at Baihetan hydropower station, and also provide a reference for the prediction of the mechanical properties of rock with flaws and the stability control of engineering rockmasses.

Assessment of plastic zones surrounding the power station cavern using numerical, fuzzy and statistical models

Plastic zones evaluation around the powerhouse caverns is a very crucial issue in designing and constructing these structures and accurate determination of their related optimum support systems. Due to inherent difficulties during the field measurement of plastic zones around the powerhouse caverns and shortcomings of the available methods in this field, applying new predictive models is an attractive and helpful topic. Accordingly, plastic zones around the powerhouse caverns have been investigated in this research using numerical analysis (NA), fuzzy inference system (FIS) and multivariate regression (MVR) model. Based on the numerical simulations, a new predictive equation has been developed to determine the plastic zone at middle point of sidewall and induced key point around a cavern. The basic parameters including rock geomechanical properties and geometrical characteristics of cavern structures have been considered as input variables in plastic zones modeling at middle points of roof, floor, left sidewall and right sidewall as well as at key point. For FIS and MVR models construction, sufficient datasets were introduced based on the numerical simulations. Performance of established models has been assessed applying testing dataset and utilizing powerful statistical indices. Accordingly, it is proved that the derived results from FIS and NA models are more precise than MVR model and they are more satisfactory in plastic zone estimation. Finally, parametric study results revealed that lateral stress coefficient, depth of overburden and rock mass rating are the most effectual parameters and tensile strength is the least influencing parameter on the plastic zone around a cavern.

An analytical model for strength of jointed rock masses

An analytical model of rock mass with non-persistent joints is utilized to develop linear and nonlinear failure criteria based on intact rock strength, joint shear strength and joint persistence. The analytical model matches the solutions for the strength of fracture-free and fully jointed rocks and provides a reasonable transition of strength for rocks with non-persistent joints. Consistent with empirical evidence, the model predicts that increasing joint intensity and decreasing joint shear strength cause a reduction in the cohesive and frictional components of rock mass strength. Joint persistence in the analytical model has not only a physical definition as the normalized jointed area, but also a mechanical interpretation as the normalized reduction in strength due to the presence of joints. The relationship between the analytical model and the empirical Hoek-Brown failure criterion with Geological Strength Index (GSI) is explored. The analytical model of rock mass is utilized to develop and calibrate a quantitative GSI chart. The analytical model is validated using the results of comprehensive tests on undisturbed samples of naturally jointed Panguna andesite and thermally granulated Carrara marble. Practical application of the analytical model is illustrated by numerical back analysis of displacements around the Nathpa Jhakri powerhouse cavern.

Stability analysis of the underground infrastructure for pumped storage hydropower plants in closed coal mines

Electricity storage systems are necessary to increase the efficiency of variable renewable energies. Mine water in closed underground coal mines can be used for underground pumped-storage hydropower plants. Subsurface energy storage systems require the excavation of a powerhouse cavern and a network of tunnels as lower water reservoir. To prevent build-up of high air pressures in the network of tunnels during the water filling process it is necessary to excavate ventilation shafts. In this paper, fluid dynamics and geomechanical behavior are combined in the lower reservoir in order to know the feasibility of the underground infrastructure. Depending on the diameter of the ventilation shafts and water flow rate, air pressures up to 420 kPa can be reached. The stability of the powerhouse cavern and the effect of air pressure on the tunnels and shafts during the operation phase of the turbine are analyzed. Coal mines in Northern Spain have been selected as a case study to investigate the behavior of the underground infrastructure of a hydropower plant. Three-dimensional numerical models were developed and analyzed for different rock mass scenarios. The results obtained show that the effect of air pressure on the tunnels and shafts is moderate. The direction of air flow switches during operation time and long-term fatigue damage could be produced to the contour of excavations. The excavation of the powerhouse cavern is technically feasible in case of the application of a proper designed support system.

Dynamic analysis of rock mass deformation in large underground caverns considering microseismic data

The underground powerhouse of a hydropower project is one of the most significant structures that will be permanently in use. Large scale excavation induced rock mass damage may pose a threat to the stability of the underground powerhouse. In the present study, a microseismic (MS) monitoring system was established to capture microcracks induced by excavation of the underground powerhouse caverns at the Houziyan hydropower station. Based on tempo-spatial characteristics of the MS events, the relation between MS activities and field construction was analyzed and the main damage regions were revealed. A damage-driven model considering the effect of MS source sizes is adopted for dynamic analysis of rock mass deformation of the underground caverns. The rock mass damage is simulated by a micro-mechanics-based constitutive model described by the fracture density determined by the MS source radius. A numerical model of a typical profile of the underground powerhouse was established. The MS data monitored in each bench excavation period of the underground powerhouse caverns was imported to the numerical model for dynamic analysis of deformation. The results showed that the effect of rock damage played a significant role in the rock mass deformation of the underground powerhouse during each bench excavation. The displacements of rock mass considering MS data obtained from the numerical analysis can provide better predictions for real conditions on site.

Microseismic monitoring and deformation early warning of the underground caverns of Lianghekou hydropower station, Southwest China

The underground powerhouse caverns of the Lianghekou hydropower station are characterized by their depth, high geo-stress, and complicated surrounding geological conditions. To identify the excavation-induced damage zones and realize the deformation early warning of surrounding rock masses of the underground powerhouse, a high-resolution microseismic (MS) monitoring system was adopted to conduct real-time monitoring of the inner microfracture activity of the surrounding rock mass. The spatiotemporal evolution of the MS events recorded in the aggregation areas prior to the deformation of the surrounding rock mass was analyzed. The analysis results show that two typical MS event clustering zones were closely related to the blasting excavation disturbances of the omnibus bar caves and the working surfaces. The MS activity shows an obvious spatiotemporal migration evolution pattern controlled by excavation, which can be analyzed to provide some reference for future support measures. The Es/Ep value of approximately 75% of the MS events in one typical clustering zone was below 10, which indicates that the rock microfractures were dominated by non-shear failures. Furthermore, prior to the deformation relaxation of the surrounding rock mass, the MS events in the selected clustering zone increased and became rapidly concentrated, the apparent stress of the corresponding MS events suddenly decreased, and the cumulative apparent volume increased dramatically. This phenomenon of MS parameter variations can be regarded as an evaluation index of the deformation early warning of surrounding rock masses. The analysis results prove that MS monitoring is essential for the assessment and mitigation of rib spalling and collapse risks in deep underground caverns, and can be used to minimize catastrophic events involving equipment and personnel. The results of the current study may be valuable for future excavation management and safety evaluations for similar underground caverns with high in situ stresses.

Microseismic monitoring application for primary stability evaluation of the powerhouse of the Tapovan Vishnugad Hydropower Project

The study of micro-cracks, shear zone and redistribution of stress after excavation in the rockmass is required for a stability analysis of the underground structure. The powerhouse of the Tapovan Vishnugad Hydropower Project (TVHPP) of NTPC Ltd. is located in the seismic zone V of India and is positioned at the junction of the middle and higher Himalayan region under challenging geological conditions. The powerhouse cavern constructed by drill and blast method encountered a number of rockbursts of various intensities during and after construction. So, a real-time remote microseismic monitoring network was used to evaluate the stability of the powerhouse cavern rockmass. Strata condition, rockburst occurrence during construction and drilling feasibility for sensor installation were studied for an array of designs of the microseismic monitoring network. The primary monitoring period demarcated the micro-crack locations and probable failure zones in the powerhouse rockmass by processing and analysis of various temporal and spatial variations of microseismic parameters to evaluate the underground powerhouse structural stability. Although a large number of microseismic events occurred in the powerhouse which resulted in high displacement in a few areas, due to the low amount of energy released from the events occurring inside the powerhouse, the structure appears to be stable.

Engineering Geological and Geotechnical Approaches for the Construction of Powerhouse Cavern of Tehri Pumped Storage Plant (1000 MW) - A Case Study

Construction of underground Cavern in the Himalayan region is full of challenges and uncertainties. Experience has shown that construction in Himalayan regions requires good understanding of geology, adequate site investigations, proper design and selection of suitable construction methodology and technology. The most commonly encountered geological problems during excavation of underground structure in Hydroelectric Projects are, Fault/Thrust/Shear Zones squeezing and swelling, wedge block failure etc. Tehri Pumped Storage Plant (PSP) is located at the left bank of river Bhagirathi in the state of Uttarakhand in Northern India. This case study indicates about the geological challenges faced and their remedial measures during the construction of Tehri PSP Powerhouse Cavern having dimension of 203m x 24m x 58m.3D-geological mapping with 1:100 scales was carried out in excavated central drift of powerhouse to evaluate the rock composition, behavior of rock mass, structural features and further investigation to finalize the layout and orientation. During the investigation Sheared Phyllite with bands of thinly Phyllite Quartzite rock were encountered in the end portion of central drift of powerhouse which had posed a mammoth challenge in designing the powerhouse cavern. Keeping in view the recommendations of geotechnical experts and the design consultants, decision were made to shift the cavern further by 50 m to avoid Sheared Phyllite bands. The shifting of cavern led to the reorientation of structures like control room, service bay and location of units etc. This paper briefly describes the Engineering Geological and Geotechnical set up of powerhouse with proper investigation approaches and excavation sequences highlighting the importance of orientation and Sheared Phyllite Zone.