Underground Powerhouse Cavern Research Articles

Three-Dimensional Discontinuum Modeling and Rigid Block Stability Analysis of a Group of Three Caverns for Hydroproject in Northeast India

Abstract This paper presents various methods for calculating the factor of safety (FOS) and utilizes the Isoblock method to analyze the stability of underground powerhouse caverns located in the North-East state of India. A three-dimensional discontinuum analysis simulates the stability of the downstream sidewall, upstream sidewall, and crown of the caverns. The results indicate that the powerhouse cavern contains approximately 2.36% unstable blocks, while the Main Inlet Valve (MIV) and Transformer Caverns exhibit 0.14% and 0.84% unstable blocks, respectively. A comprehensive parametric analysis investigates the impact of joint stiffness, joint friction angle, in-situ stress, cover depth, and the orientation of the cavern's longer axis on stability. The findings reveal that the cavern's stability is significantly influenced by variations in lateral stress ratio and the direction of in-situ principal stress. Stability is optimized at stress ratios between 1.2 and 1.8, with maximum instability observed in the powerhouse cavern, correlating with its larger size. Notably, the stability of the crown is greater than that of the sidewalls, particularly when the cavern orientation is aligned between N68°E and N78°E. The study underscores the importance of these factors in the design and stability assessment of underground structures, providing valuable insights to enhance their safety and reliability. This analysis highlights the necessity for considering joint stiffness and in-situ stress orientations in future designs to mitigate risks associated with instability.

Performance Assessment and Optimization of Seepage Control System for Wunonglong Underground Powerhouse Cavern System

Performance Assessment and Optimization of Seepage Control System for Wunonglong Underground Powerhouse Cavern System

A case study on the stability of a big underground powerhouse cavern cut by an interlayer shear zone in the China Baihetan hydropower plant

AbstractThe big underground powerhouse cavern of the China Baihetan hydropower plant is 438 m long, 34 m wide, and 88.7 m high. It is cut by a weak interlayer shear zone and its high sidewall poses a huge stability problem. This paper reports our successful solution of this problem through numerical simulations and a replacement‐tunnel scheme in the detailed design stage and close site monitoring in the excavation stage. Particularly, in the detail design stage, mechanical parameters of the shear zone were carefully determined through laboratory experiments and site tests. Then, deformation of the surrounding rocks and the shear zone under high in situ stress conditions was predicted using 3 Dimensional Distinct Element Code (3DEC). Subsequently, a replacement‐tunnel scheme was proposed for the treatment on the shear zone to prevent severe unloading relaxation of surrounding rocks. In the construction period, excavation responses were closely monitored on deformations of surrounding rocks and the shear zone. The effect of local cracking in the replacement tunnels on sidewall stability was evaluated using the strength reduction method. These monitoring results were compared with the predicted numerical simulation in the detailed design stage. It is found that the shear zone greatly modified the deformation mode of the cavern surrounding rocks. Without any treatment, rock mass deformation on the downstream sidewall was larger than 125 mm and the shearing deformation of the shear zone was 60–70 mm. These preset replacement tunnels can reduce not only the unloading and relaxation of rock masses but also the maximum shearing deformation of the shear zone by 10–20 mm. The predictions by numerical simulation were in good agreement with the monitoring results. The proposed tunnel‐replacement scheme can not only restrain the shear zone deformation but also enhance the safety of surrounding rocks and concrete tunnels. This design procedure offers a good reference for interaction between a big underground cavern and a weak layer zone in the future.

Estimation of Modulus of Deformation Using Rock Mass Rating—A Review and Validation Using 3D Numerical Modelling

The Himalayan region has enormous potential for hydropower development. However, variations in geological and geotechnical conditions pose challenging tasks for the designers. If these variations are not tackled in a timely manner during underground excavations, especially for caverns, instabilities may occur, resulting in time and cost over-runs. For sustainable hydropower development, minimizing these over-runs is necessary. The modulus of deformation (Ed) of a rock mass is an essential input parameter required in the design of underground excavations. This study involves collecting the results of extensive in situ tested values for various hydroelectric projects in the Himalayan regions, along with the rock mass rating (RMR) values at 35 test sites. Ed is estimated empirically based on statistical analysis. Comparisons were made with the empirical equations already available in the literature, using RMR and the proposed equation for estimating Ed. Although different researchers have proposed many equations for estimating the value of Ed using RMR, a gap exists in validating such equations. In this regard, the proposed equation for Ed was verified by carrying out 3D numerical-modelling studies using FLAC3D, an explicit finite-difference software for an underground powerhouse cavern and comparing the displacement values with the field instrumentation data.

Deformation characteristics of sidewall and anchorage mechanisms of prestressed cables in layered rock strata dipping steeply into the inner space of underground powerhouse cavern

Local stability of underground powerhouse cavern subject to low to moderate geo-stresses, is frequently threatened by the unpredictable displacements of high sidewall in layered rock masses dipping into cavern space. Further understanding these displacements and the anchoring behaviours helps provide effective measures for ensuring the safe, rapid construction of major projects. This study investigates the deformation features of sidewall and anchoring mechanisms of long prestressed cables in layered strata steeply inclined toward the inner space of underground powerhouse, using field tests and numerical tools. First, the development characteristics of sidewall in layered strata steeply dipping into cavern space are analysed based on the deformation monitoring data. Time-dependent relationships between those displacements and depths from the excavation surface distinguished from that in other typical underground caverns in layered rock masses, are critically discussed. Second, damage characteristics of these strata caused by excavating activities are presented based on the microseismic (MS) monitoring data. Effects of damage zone on the evolution of deep deformations distinctively involved, are described in detail. Then, the calculation platform coupling MS data and continuum modelling, suitable for simulating progressive damage of layered rock masses, is introduced to reveal new insights into the mechanisms behind those distinctive deformations. Finally, the modified models properly capturing the mechanical responses of prestressed anchor cables, are integrated into such a calculation platform. Anchoring mechanisms of long prestressed cables associated with those deformations of layered strata dipping into the cavern are thus further investigated. This can explain why the prestressed anchor cables provide less effective control effects on deep deformations mainly induced by shear dislocations on bedding planes.

Performance evaluation of hybrid YYPO-RF, BWOA-RF and SMA-RF models to predict plastic zones around underground powerhouse caverns

Performance evaluation of hybrid YYPO-RF, BWOA-RF and SMA-RF models to predict plastic zones around underground powerhouse caverns

Effects of discontinuities on stress redistribution and rock failure: A case of underground caverns

Excavation of the left bank underground powerhouse caverns of the Baihetan hydropower station has been undertaken in a deep rock mass with discontinuities under high in situ stresses. Rock failures such as spalling were encountered during the excavation of the large caverns. A discrete element model containing discontinuities was established and used to investigate the failure mechanism of the rock masses using 3DEC to reproduce the evolution of the displacement and stress fields. Based on the modeling, a risk boundary, significantly affected by discontinuities, is proposed and applied to evaluate roof stability. Owing to the discontinuities, the excavation of the top arch induced spalling at the crown arch and upstream sidewall and opening of the rock mass at the downstream sidewall and floor. Excavation of the subsequent sections increased stress at the crown arch, revealing the arching effect; simultaneously, the stress release and deformation of sidewalls were facilitated by the discontinuities. The simulation results were verified by on-site monitored data. Based on the study it was concluded that discontinuities influence stress redistribution, inducing the arching and sidewall effect. The result of this study is expected to aid in the design of excavation and support systems for underground engineering projects.

Digital design and stability simulation for large underground powerhouse caverns with parametric model based on BIM-based framework

Large infrastructures involving underground engineering like the underground powerhouse caverns constitute multi-medium, complicated and interaction systems, which require geotechnical data management, parametric model construction, optimal design alternatives, and real-time safety assessment. However, even though the data needed for numerical analysis is recorded in the Building Information Modeling (BIM) parametric models, the transformation of geometric characteristics between BIM and numerical models is cumbersome. Therefore, an automatic numerical simulation analysis framework was proposed, which can not only convert 3D parametric structural geometry model by indirect method, but also realize automatic pre-treatment and analysis, such as mechanical properties definition, boundary conditions assignment and mesh generation. In this study, the digital model of underground powerhouse caverns accompanying with cavern support system could be constructed automatically via secondary development on CATIA software. Reorganized borehole logs at geological profiles accompanying with topographic-geologic map are also used to define the 3D subsoil model. Python codes are programmed for pre-treatment of numerical model to enhance the calculation efficiency and alleviate the manual interventions by linking differ-format data files. Furthermore, a case study of Suki Kinari underground powerhouse caverns was exemplified the advantages of this framework in terms of modeling and pre-treatment efficiency, and error-free geometric information translation. The proposed framework provides a fundamental process for automatic design-to-analysis, hence mitigates user intervention and error-free structural calculation for design-making.

Characteristics of microseismic b-value associated with rock mass large deformation in underground powerhouse caverns at different stress levels

Characteristics of microseismic b-value associated with rock mass large deformation in underground powerhouse caverns at different stress levels

Deformation mechanisms of sidewall in layered rock strata dipping steeply against the inner space of large underground powerhouse cavern

In low to moderate stress regions, deformations of the high sidewall dominated by bedding planes dipping against the cavern space, often pose an unpredictable threat to local stability of underground powerhouse. In this study, detailed field surveys, in-situ tests, beam theory, and the discrete element method (DEM) are used to critically investigate the deformation mechanisms of high sidewall in layered rock strata steeply inclined against the inner space of the underground powerhouse cavern. First, an approach integrating in-situ surveys, deformation monitoring and acoustic wave tests, is adopted to analyse the evolutionary features of displacements of a high sidewall in layered strata steeply dipping against cavern space. Differences of deformation behaviours and their relationships with excavating activities in low to moderate and high geo-stress regions, were distinguished. Second, two zones with different deformation mechanisms in layered strata involved are presented, which could be evidenced by the presence of various displacement modes near the top area of a high sidewall. Then, based on the conceptual beam models, mechanisms of displacements controlled by bedding planes in two zones are investigated. Synergistic effects arising from two zones promoting the overall deformation profile are presented. Third, combining with DEM simulations, the mechanical influences of bedding thickness on the excessive deformation modes near the top of sidewall, are revealed. Displacements of a sidewall partly affected by an intersecting chamber are described in detail. Finally, suggestions for applicable supports of the high sidewall with bedding planes dipping steeply toward the internal area of surrounding rocks during cavern excavation are presented. The aforementioned investigations can provide valuable guidelines for the construction of similar underground powerhouse caverns.

Determination of representative elementary volume (REV) for jointed rock masses exhibiting scale-dependent behavior: a numerical investigation

Representative elementary volume (REV) is defined as the usual size of a rock mass structure beyond which its mechanical properties are homogenous and isotropic, and its behavior can be modeled using the equivalent continuum approach. Determination of REV is a complex problem in rock engineering due to its definition ambiguity and application area. This study is one of the first attempts to define a REV for jointed rock masses using the equivalent continuum approach. It is aimed to numerically search a ratio between the characteristic size of an engineering structure and pre-existing joint spacing, which are the two most important contributing elements in assessing REV. For this purpose, four hypothetical engineering cases were investigated using the RS2 (Phase2 v. 9.0) finite element (FE) analysis program. An underground circular opening with a constant diameter, an open-pit mine with varying bench heights, a single bench with a constant height, and an underground powerhouse cavern with a known dimension were executed for possible changes in the safety factor and total displacement measurements under several joint spacing values. Different cut-off REVs were calculated for FE models depending on the type of excavation and measurement method. An average REV size of 19.0, ranging between a minimum of 2 for tunnels and a maximum of 48 for slopes, was found in numerical analysis. The calculated sizes of REV were significantly larger than the range of values (5 to 10) commonly reported in the relevant geotechnical literature.

Rockburst Inducement Mechanism and Its Prediction Based on Microseismic Monitoring

The rockburst disaster in the hard rock caused by excavation and unloading of deep underground caverns threatens the safety of engineering construction. In recent years, the microseismic monitoring technology, which can dynamically monitor the whole process of progressive failure of rock mass in real time, has been widely used in rockburst monitoring and early warning of underground engineering. In view of the slight rockburst in local surrounding rock during the excavation of underground powerhouse of Huanggou Pumped Storage Power Station, a rockburst microseismic monitoring system is constructed. And through the analysis of the temporal and spatial activity of microseisms during the monitoring period, the potential risk areas of rockbursts are identified and delineated. The monitoring results show that the microseismic system can effectively capture the blasting and microseismic signals during construction. The microseismic activity is closely related to the intensity of field blasting disturbance. The potential risk areas of rockburst are the upstream side arch shoulder and the intersection between lower drainage corridor and workshop installation room. The research results can provide technical support for later excavation and support of underground powerhouse caverns of Huanggou Hydropower Station.

3D Numerical Analysis for Rock Support Design of an Underground Powerhouse Cavern in Complex Geological Conditions

A new underground powerhouse that combines a run-of river scheme with a pumped storage scheme is planned in course of the Renewal of Forbach HPP in Germany. The powerhouse cavern is designed to have a maximum span and height of 18.9 and 42 m, and a total length of 123 m. The rock mass conditions were found favourable in general. However, comprehensive investigations revealed weakness planes crossing the cavern. The support system was determined by joint evaluation of empirical methods, wedge analysis and the 2-D finite element analyses. Due to the complicated geometry of the cavern and the orientation of the weakness planes, the effect of weakness zones on the stability and deformations could not be evaluated by only 2-D analyses. Implementing an additional 3-D analysis and comparison of the results with the common 2-D approach allowed a more realistic assessment of the rock behaviour, especially under the existence of the weakness zones through the cavern. A very detailed 3-D finite element model for the whole cavern is still not practical, so a relatively larger mesh was adopted. Nevertheless, a reduced model for a limited part of the cavern was also analysed by using a finer mesh. Obtained results were similar indicating the suitability of the adopted numerical models for the rock support assessment. The deformations and the size of the plastic zones were identified as significantly larger for weakness zones passing through the cavern. For these intersection areas additional rock support measures including pre-stressed anchors were therefore planned.

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

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.

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.

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.

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.