Cavern Excavation Research Articles

Excavation of artificial caverns inside asteroids by leveraging rotational self-energy

Future space ventures will likely require exploitation of near-Earth asteroid resources. Moreover, it can be envisaged that asteroids may host habitats in their interiors. In fact, a cavern inside an asteroid would be a natural radiation shield against cosmic radiation and may also serve as a confined environment for storage of mined material such as water ice or other processed volatiles such as propellants. To this end, this paper proposes to leverage the asteroid rotational self-energy to remove material from the asteroid interiors and create a spherical cavern, by means of the orbital siphon concept. The siphon is a chain of tether-connected payload masses (the asteroid material), which exploits the rotation of the asteroid for the delivery of mass from the asteroid to escape. Under certain conditions the siphon can be initiated to ensure self-sustained flow of mass from the asteroid to escape. A net orbital siphon effect is generated by connecting new payloads at the bottom of the chain while releasing the upper payloads. Key parameters are discussed, such as the required siphon dimension and the maximum size of the internal cavity that can be excavated, as a function of the asteroid rotational period. Moreover, assuming elastic material behaviour, a closed-form expression for the stress tensor is found and a failure criterion is used to identify regions in the asteroid interiors subjected to the larger stresses. It is shown that the conditions for failure are relaxed as the radius of the internal void increases.

In Situ Monitoring and Numerical Experiments on Vertical Deformation Profiles of Large-Scale Underground Caverns in Giant Hydropower Stations

Spatial response is a feature of rock deformation in regions surrounding large-scale underground caverns and includes significant vertical components due to the construction method of layered excavation. This vertical response is different to the longitudinal response of excavation deformation surrounding shallow tunnels. The study of longitudinal deformation profiles (LDPs), which describe the spatial response of longitudinal tunnel excavation and surrounding rock deformation, is a mature field. However, there has been no independent discussion of the relationship between vertical excavation and the spatial response of deformation in large-scale underground caverns nor the incremental characteristics of layered excavation. In this paper, we define the attenuation function λ x of unloading strength based on theoretical analyses and numerical simulations. We also propose the concept and form of the vertical deformation profile (VDP) curve for the first time and apply it to the Baihetan and Lianghekou Hydropower Stations. After fitting the complete VDP curve with a Levenberg-Marquardt algorithm, we verify its validity by comparing predicted data with in situ monitoring data. The curve can be used to quantitatively analyze the relationship between layered excavation and incremental deformation of surrounding rock, providing a basis for the rapid evaluation of staged deformation during the excavation of large underground caverns. This study has practical significance for the control of deformation in rock surrounding excavations and decision-making during the construction progress.

Research on Excavation and Stability of Deep High Stress Chamber Group: A Case Study of Anju Coal Mine

As the underground transportation hub and power center, the stability of chamber group is one of the key points of coal mine construction. With the increasing of mining depth, it is more and more difficult to control the stability of roadway and chamber in coal mine, especially in the mining process. Because the excavation of chamber group is a dynamic and multi factor coupling process of, it is necessary to adopt dynamic construction mechanics to optimize the excavation scheme. Therefore, based on the engineering background of − 1155 m level at Anju coal mine in Shandong, the numerical model of deep high stress chamber group is established. Then the interaction of adjacent roadways in the process of excavation and the stress superposition of chamber intersection are studied. The excavation process of the chamber group is divided into three phases, and the construction optimization scheme is formulated. Through numerical simulation, the stress and plastic zone of each excavation scheme are analyzed, and the optimal construction sequence of each phase is determined. Finally, through the field implementation and engineering application, the optimal excavation scheme of tunnel group design is proved. The results provide a scientific basis for the excavation of deep high stress caverns and the improvement of surrounding rock support. At the same time, it also has a good reference significance for other similar underground engineering.

An Insight into the Excavation-Induced Stress Paths on Mechanical Response of Weak Interlayer Zone in Underground Cavern Under High Geostress

A weak interlayer zone (WIZ) is a poor zonal geotechnical system with loose structure, weak mechanical properties, variable thickness, random distribution, and strong extension, that occurs between different rock strata (e.g., tuff and basalt), due to the intense tectonic movement, representing a potential threat to the overall stability of rock masses with WIZs in large underground cavern excavations. Focusing on the excavation-induced hazards in the weak interlayer zone (WIZ) occurring in underground cavern under high geostress, the mechanical response of WIZ under different loading and unloading stress paths has been well investigated and unearthed, by a series of automatic stress path controlled triaxial tests, as well as through scanning electron microscope (SEM) analysis. Results show that the mechanical characteristics of WIZ are closely related to the initial confining pressure and stress paths. Sudden increases in the circumferential strain and strong dilations of WIZ occur from the start of unloading, among which the unloading total strains are most strongly promoted by stress path II (i.e., axial pressure loading and confining pressure unloading), whereas the unloading stress path IV (i.e., axial pressure and confining pressure unloading) has the greatest promotion on enhancing the plastic volumetric dilatation. The deformation modulus and the Poisson’s ratio follow a deterioration law under the unloading process of confining pressure, and it is the unloading stress path II that most influences the damage of elastic parameters. The ultimate bearing strength, the internal friction angle, and the cohesion of WIZ appear extremely obvious degradation in course of unloading, among which the damage effect of stress path IV is the most remarkable. The macroscopic and mesoscopic analyses reveal that the failure mechanism of WIZ under complicated unloading stress paths lays in the accumulation and propagation of axial and circumferential cracks and fractures, as well as pervasive particle breakage, with the intergranular fractures, transcrystalline fractures, and the shear scratches on a meso level. The research could provide an effective basis and good lessons for the mechanical response and failure mechanism of WIZ under unloading stress paths in underground cavern under high geostress.

Optimized ANN model for predicting rock mass quality ahead of tunnel face using measure-while-drilling data

Rock mass quality assessment has a vital influence on the excavation of tunnels and caverns in rock mass. For this purpose, extensive field studies, including records of measure-while-drilling data and rock mass quality scores (RQS) from the observation reports of tunnel faces, have been conducted. In order to predict RQS, three optimized artificial neural network (ANN) models based on genetic algorithm (GA), particle swarm optimization (PSO), and imperialist competition algorithm (ICA) were developed. Six parameters of measure-while-drilling (MWD) data and their corresponding RQS constituted 1270 datasets, which were set as input and output of ANN, respectively. The traditional multiple linear regression (MLR), multiple nonlinear regression (MNR) statistical model, and ANN model were developed as comparative models. Comparison results reveal that PSO-ANN and ICA-ANN models are capable of predicting RQS with higher reliability than the MLR, MNR, ANN, and GA-ANN models. Results indicate that PSO-ANN and ICA-ANN models can be used to predict RQS; however, the PSO-ANN model has better performance.

Numerical analysis of the brittle–ductile transition of deeply buried marble using a discrete approach

In high geostress areas, rocks tend to experience plastic deformation processes and show ductility. Therefore, the study of the mechanical properties of rocks, especially the post-peak behavior, is crucial for underground geotechnical engineering. For this reason, the macroscopic mechanical property of marble is studied by using the PFC/PBM numerical method. An improved numerical marble sample with random properties is used by setting different contact strengths following a bimodal Weibull distribution. Thereafter, on the basis of an improved boundary servo mechanism, the brittle–ductile transition appearing in laboratory tests on marble is successfully realized by the discrete approach, and a servo velocity threshold function of axial loading speed and confining pressure is obtained. Furthermore, the marble model and servo method are applied in the excavation of typical underground caverns. The effects of constrained velocity and geostress level on the surrounding rock failure model of the cavern are described and analyzed. In addition, mechanism of the excavation failure under high geostress is discussed.

Deformation and Failure Analyses of the Surrounding Rock Mass with an Interlayer Shear Zone in the Baihetan Underground Powerhouse

In the process of underground cavern excavation, the existence of the interlayer shear zones or large faults often makes the surrounding rock tend to be unstable or even deformed. Under the influence of interlayer shear zone C2, different degrees of deformation and failure occurred in many parts during the excavation of the Baihetan left bank underground powerhouse. Based on field monitoring and numerical calculation, this paper studies the deformation and failure characteristics of the rock mass with C2 in the whole excavation process and the failure mechanisms are analyzed. The results show that C2 has poor mechanical properties. In the process of excavation, it mainly induces two failure modes: rock collapse and shear deformation, which specifically leads to rock collapses, large deformation and shotcrete cracking in the main powerhouse, and shear deformation in the omnibus bar caves. In addition, the similarities and differences between this study and other studies on the deformation and failure of surrounding rock of underground powerhouse in recent years are discussed, and the relevant treatment measures for C2 are given. The above research results can be a reference for other related studies.

Comparative Simulation Research on Stability of Layered and Homogeneous Surrounding Rocks

Layered surrounding rock mass is common in underground engineering, and it has obvious anisotropic mechanical properties. Therefore, the stability of the cavern excavation of layered surrounding rocks has always been a key issue in the engineering community. Based on an example of a hydraulic tunnel, the stability analysis of the excavation in cavern of the layered and homogeneous surrounding rocks are compared. The results show that the overall distribution of the stress field and displacement field of the layered and homogeneous surrounding rocks are basically the same. The stress field of the layered surrounding rocks has obvious discontinuous characteristics, and the deformation caused by excavation of the cavern is greater than the homogeneous surrounding rocks. The extent of the plastic zone along the joints of the layered surrounding rocks is also significantly larger than that of the homogeneous surrounding rocks.

Research on Surrounding Rock Deformation Prediction During Underground Excavation of Hydropower Station Based on Prototype Monitoring

This paper proposes an improved gray model of equal dimensional innovation based on time-varying parameters and the gray characteristics of small sample and poor information of the safety monitoring data during underground cavern construction, so as to predict and judge the deformation of surrounding rocks in underground caverns and to obtain relatively reliable results in the analysis of uncertain systems. Under certain conditions, the improved gray model of equal dimensional innovation based on time-varying parameters can be used to predict the deformation of surrounding rocks during the excavation of underground caverns, thereby providing mathematical analysis tools for safe construction.

Stress-induced brittle failure of surrounding rock mass and its engineering countermeasures in the Baihetan underground powerhouse

Stress-induced brittle failures, such as spalling, slabbing, and rockburst, are common failure modes of underground caverns in high-stress zones. In the Baihetan Hydropower Station, the brittle failures, embodied as spalling, rock fracturing, and time-dependent deformation, bring new challenges to the excavation and support system design of underground caverns. Based on the New Austrian Tunneling Method, the specific design principles for the underground caverns of Baihetan Hydropower Station are proposed, the keys of which are preconditioning surrounding rock mass, reducing the excavation damage, supporting quickly in the proper sequence, and controlling loosen zones. These principles are applied during construction, and a series of excavation and support methods are developed for the key parts of the powerhouse, such as the roof arch, the rock wall of rock-bolted crane girder, the high side wall, and the partition wall of the generator pit. Data obtained from visual appearance, displacement measurement, and acoustic testing show that the proposed design principles are appropriate and that the developed excavation and support methods can control the range and depth of stress-induced failure of the surrounding rock mass.

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 analysis of the surrounding rock mass stability in the underground caverns of Jinping I hydropower station in Southwest China

The geological conditions of the underground caverns in the Jinping I hydropower station are particularly complicated, with high ground stress (the extreme value of the measuring point is 35.7 MPa), complicated deformation and failure mechanisms, and large unloading depth. The large deformation of the surrounding rock mass was beyond current engineering and academic understanding. During the construction period, the deformation and failure mechanisms were complex, and the unloading depth was large. The maximum deformation of the sidewall of the main powerhouse reached 246 mm. The monitoring results of multi-point extensometers basically converged two years after the completion of cavern excavation. Regarding the deformation and failure mechanism of the large underground caverns, the energy dissipation theory that can better describe the complex mechanical response analysis was proposed. The deformation failure mechanism, mechanical behaviour, supporting effect and stability evaluation of the large underground caverns were investigated from the perspectives of excavation unloading and energy dissipation and release. The excavation unloading-induced relaxation characteristics of the surrounding rock mass were analysed using a comprehensive approach incorporating the results of borehole television, acoustic wave testing, multipoint extensometer and numerical simulations. A criterion of the surrounding rock mass delineation incorporating the energy dissipation ratio, acoustic wave testing, wave velocity variation and elastic modulus variation was also proposed. Furthermore, the phenomena and law of deformation and failure of surrounding rock mass in the large underground caverns were revealed. The stability of surrounding rock mass in the underground caverns was evaluated. The related achievements can provide some references for similar deep underground rock engineering.

Back analysis of displacements for estimating spatial distribution of viscoelastic properties around an unlined rock cavern

A displacement back analysis algorithm is developed, considering the time-dependent effect of the rock mass. It can map spatially distributed the first elastic modulus (E1), viscidity coefficient (η1), and the second elastic modulus (E2) of the Kelvin-Voigt viscoelastic constitutive model (VCM) and the Poynting-Thomson VCM in a rock mass by fusion of the observed displacement data from the excavation of an unlined rock cavern. The algorithm is tested and validated using numerical experiments with a synthetic heterogeneous rock mass. The results of the experiments show that this approach yields unbiased estimates of E1, η1, and E2 fields and quantifies their uncertainty. Further, the estimated fields closely predict shear strain distribution and displacements field in the example.

Study on transient unloading loosening simulation test of excavation of jointed rock mass

Based on the abrupt displacement of a jointed surrounding rock mass during blasting excavation of deep underground caverns, a set of transient unloading loosening simulation tests was designed for multi-joint rock masses. Through the linear regression analysis of the unloading displacement of jointed rock masses, the opening displacement of a joint is determined to be proportional to the square of the initial unloading stress. The numerical simulation method is used to apply horizontal and vertical in situ stresses to the jointed rock mass model to study the opening displacement in the jointed rock mass at different lateral pressure coefficients of in situ stress. The results indicate that, with the unloading of the horizontal stress, a larger lateral pressure coefficient of the in situ stress leads to a larger opening displacement in the jointed rock mass. When the vertical stress is unloaded after the horizontal stress is totally unloaded, a lower lateral pressure coefficient of the in situ stress leads to a larger opening displacement. The influence of the stress lateral pressure coefficient on the open deformation of the jointed rock mass is obviously higher than that of the unloading rate on the rock mass.

Numerical analysis on rock blasting damage in Xiluodu underground powerhouse using an improved constitutive model

Damage effect on surrounding rock is unavoidable during blasting excavation of deep underground caverns. In order to accurately analyse the damage degree of surrounding rock caused by explosive load under high ground stress conditions, an improved damage constitutive model of rock mass is proposed based on dynamic finite element numerical simulation software, and the continuous change processes of the deterioration of rock mass material parameters and damage threshold under blasting dynamic disturbance are studied. Based on the monitoring data of blasting excavation vibration of underground powerhouse in Xiluodu Hydropower Station, the field measured data and numerical simulation results of several different constitutive models are compared, and the effects of different constitutive models and explosive loads on vibration propagation attenuation law of surrounding rock of this tunnel and adjacent tunnel are analysed. The results show that the calculated results of the improved damage constitutive model are closer to the field measured values. The damage depth of surrounding rock is mainly affected by the diameter of cartridge under the same charge quantity, and the change of the hole spacing will only affect the damage range of surrounding rock.

Influence of intermediate principal stress on the mechanical properties of rocks containing fractures

The excavation fracture zones develop during the staged excavation of caverns and tunnels, which still have bearing capacity. However, further excavation and support activities will change the stress state in these areas until these activities cease. The influence of intermediate principal stress, σ2, on the strength and deformation characteristics of the surrounding rocks is not clear. In this paper, multi-step true triaxial tests are carried out on rock specimens containing stress-induced fractures to investigate the influence of σ2 on the mechanical properties of these specimens. The experimental results show that Young's modulus decreases with decreasing σ2, while the ratios of deformation in the two lateral directions increase. However, due to the fractures induced by true triaxial stress parallel to σ2, this fractured rock strength is not influenced by σ2, so a strength criterion that does not consider σ2 effect should be adopted in this failure pattern. This study deepens the understanding of the deformation and the bearing capacity of surrounding rocks.

An automatic classification method for microseismic events and blasts during rock excavation of underground caverns

Accurately acquiring microseismic (MS) signals is the cornerstone of MS monitoring during underground rock excavation. This study developed a classification method to automatically recognize MS signals under the interference of blasting signals during construction period. Our proposed method consists of the improved complete ensemble empirical mode decomposition with adaptive noise (I-CEEMDAN), singular value decomposition (SVD) and k-nearest neighbors algorithm (k-NN). I-CEEMDAN was taken to decompose original multi-frequency to a few mono-frequency signal subcomponents and SVD was adopted to extract singular values from matrices formed by the decomposed results. These obtained singular values, regarded as input features, were imported into k-NN to establish an automatic classification model to identify MS signals. The 500 signals collected from Wudongde hydropower station in Southwest China were analyzed using our method. Our numerical experiments indicated that I-CEEMDAN and SVD can extract key characteristics of the signals, and k-NN has higher identification accuracy and computational efficiency compared with other machine learning algorithms. Our proposed method can be applied in MS monitoring techniques to offer more accurate MS signals for subsequent source analysis to achieve disasters warning.

Dynamic Unloading Instability Mechanism of Underground Cavern Based on Seepage-Damage Coupling

The seepage-damage coupling effect will aggravate the instability of the surrounding rock during the unloading process of underground cavern excavation. Considering this coupling effect and excavation disturbance, the theoretical solution of the stress state of surrounding rock is derived by using the elastic-brittle damage model. The dynamic criterion of the instability and water inrush is presented. Based on the theoretical derivation, the calculation program for the seepage-damage analysis of the surrounding rock under dynamic unloading is programmed, and the seepage flow and the radius of the damage zone of the surrounding rock are calculated. By analysing the variation of radius of the damaged zone with pore water pressure and excavation radius under different calculation conditions, the influence of dynamic unloading disturbance on the damaged zone of the surrounding rock is discussed. The radius of the damaged zone increases with the pore water pressure and excavation radius. Considering the effect of dynamic unloading, the calculation result of the damaged zone radius and seepage discharge of underground cavern are much larger than the theoretical calculation and coupling calculation of seepage-damage without dynamic unloading. Research methods and results can provide guidance and reference for similar engineering research.

Microseismic monitoring and stability analysis for the large-scale underground caverns at the Wudongde hydropower station

To analyse the stability of rock mass subjected to excavation of the underground caverns at the Wudongde hydropower station, a high-resolution microseismic (MS) monitoring system was applied. Based on the temporal evolution characteristics of MS events, the relationship between construction state and MS activity was established. Combining the spatial evolution characteristics of MS events with traditional monitoring, geological survey and field observation, several main damage regions were identified and the key factors causing rock mass damage were revealed. Furthermore, the evolution laws of MS source multi-parameters during the large deformation of rock mass were systemically analysed. Prior to the large deformation of rock mass, the MS source multi-parameters behaved differently, including an obvious increment of the number of MS events, a sharp increment of the energy index with a progressive increment of the apparent volume and a decreasing trend of the centre frequency. The present study reveals that the integrated analytical method incorporating MS monitoring, traditional monitoring, geological survey and field observation leads to a better understanding of the behaviour of surrounding rock mass subjected to excavation, which is of great significance for the risk management of rock mass deformation and failures in the large-scale underground caverns.

Displacement Back-Analysis of Rock Mass Parameters for Underground Caverns Using a Novel Intelligent Optimization Method

Abstract During the excavation of large-scale underground caverns, in which dynamic feedback analysis is required, the efficiency and accuracy in determining mechanical parameters of surrounding ro...