Underground Cavern Excavation Research Articles

A physical model test for the stress and deformation analyses in underground caverns containing block during excavation

In order to investigate the influence of block on the stress and deformation of underground cavern with high sidewall during excavation and unloading, a physical model test of an underground cavern containing block was conducted based on similarity theory. The response pattern of surrounding rock stress and displacement during excavation is analyzed, and the influence and mechanism of block on surrounding rock stress and deformation are revealed. Results show that stress conduction in the model is closely related to the structural plane during loading. The stress barrier and stress concentration phenomena occur when stresses pass through the structural plane. After the model is loaded, stress loss takes place when stresses are transmitted from the boundary to the interior. During the underground cavern excavation, the tangential stress of the surrounding rock decreases within a certain range of the sidewall and increases at both the top and the shoulder of the arch. The change rate of stress before and after excavation is used as an index to determine the surrounding rock stress zone, and the influence range caused by excavation is 2.0–2.5 times the cave span. Compared with the sidewall without block, the sidewall with block has a smaller radial stress unloading zone and a larger tangential unloading zone.

Characterizing large-scale weak interlayer shear zones using conditional random field theory

The shear behavior of large-scale weak intercalation shear zones (WISZs) often governs the stability of foundations, rock slopes, and underground structures. However, due to their wide distribution, undulating morphology, complex fabrics, and varying degrees of contact states, characterizing the shear behavior of natural and complex large-scale WISZs precisely is challenging. This study proposes an analytical method to address this issue, based on geological fieldwork and relevant experimental results. The analytical method utilizes the random field theory and Kriging interpolation technique to simplify the spatial uncertainties of the structural and fabric features for WISZs into the spatial correlation and variability of their mechanical parameters. The Kriging conditional random field of the friction angle of WISZs is embedded in the discrete element software 3DEC, enabling activation analysis of WISZ C2 in the underground caverns of the Baihetan hydropower station. The results indicate that the activation scope of WISZ C2 induced by the excavation of underground caverns is approximately 0.5–1 times the main powerhouse span, showing local activation. Furthermore, the overall safety factor of WISZ C2 follows a normal distribution with an average value of 3.697.

Analysis of the Hydromechanical Properties of Compact Sandstone and Engineering Application

The paper proposes a method to simulate the mechanical behavior of compact rock considering hydromechanics by combining physical experiments and numerical analysis. The effectiveness of the constructed method is validated by the comparison between the numerical and physical results of triaxial shear experiments on sandstone in seepage conditions. Based on the validated method, the stability of underground water-sealed oil and gas storage caverns in surrounding compact sandstone during excavation is analyzed. The main findings are as follows: The intrinsic permeability of compact sandstone has a power function relationship with the porosity; the combination of the porous media elastic model and the modified Drucker–Prager plasticity model can preciously represent the mechanical properties of compact sandstone; the proposed method can accurately replicate the hydromechanical response of compact sandstone in seepage conditions; the effects of hydromechanical effects have significant impacts on the stability of surround compact sandstone during the excavation of underground water sealed oil and gas storage caverns, which causes the obvious increase in stress, deformation and plastic deformation zones of the surrounding compact sandstone and remarkable decrease in the stability safety factor.

Experimental Study on Mechanical Properties of Deeply Buried Granite During Layered Excavation of Large Underground Caverns

Experimental Study on Mechanical Properties of Deeply Buried Granite During Layered Excavation of Large Underground Caverns

Study of the Evolution Characteristics of Microseismic Events during the Excavation of Underground Caverns under High Geostress

A microseismic (MS) monitoring system was established, and numerical modeling was performed using Fast Lagrangian Analysis of Continua in 3 Dimensions (FLAC3D) to examine the evolution characteristics of MS events during the excavation of underground caverns under high geostress. Specifically, the spatial and temporal damage characteristics of the rock mass, the dynamic relationship between the evolution of MS events, the site construction conditions, and the geological structures under high geostress were also analyzed. In addition, a three-dimensional numerical model of underground caverns was built to demonstrate the deformation characteristics of the rock mass. As a result, the characteristics of a large number of MS events, multiple concentration zones of MS events, and long periods of stress adjustment were discovered in underground caverns under high geostress. It was found that the primary cause of MS events was site blasting construction. In particular, the distribution of the MS events was strongly correlated with the location of the site blasting construction, and the frequency of the MS events was positively correlated with it. The structural plane is a discontinuous plane with very low or no tensile strength, and its presence also increased the number of MS events and raised the possibility of the surrounding rock becoming unstable. Moreover, the MS monitoring data agreed with the numerical modeling results, which can be useful in estimating damage to underground caverns under high geostress and predicting disasters.

Numerical Simulation Analysis Method of the Surrounding Rock and Support Bearing Capacity in Underground Cavern

After the excavation of an underground cavern, how the surrounding rock and the support work together to bear the excavation load is an important prerequisite to correctly analyze the joint force characteristics; effectively play the role of support; and ensure the safety, efficiency, and economy of underground cavern construction. Starting from the elastic-plastic load release characteristics of surrounding rock, this paper proposes a calculation method of the elastic load coefficient of surrounding rock and a graded release method of plastic load, which ensures the actual effect of the synergistic action of the first support and surrounding rock. Based on the elastic-plastic damage evolution characteristics of surrounding rock, a weighted iterative calculation method of elastic-plastic damage is proposed, and an evaluation method of load release ultimate bearing capacity of surrounding rock is determined. By monitoring the change law of rock acoustic wave velocity with surrounding rock damage, the relationship between the wave velocity and the damage coefficient of the surrounding rock in the excavation process is deduced, and it is proposed to determine the latest support time for first support by using the measured rock damage wave velocity. Through the numerical simulation analysis of a diversion tunnel excavation and support, the damage evolution law of the surrounding rock with the release of the excavation load is studied. The ultimate bearing capacity of various surrounding rocks and supporting opportunity is determined. The results demonstrate the validity and practicality of the analysis and calculation methods in this paper, which provide a new idea and analysis method for quantifying the bearing capacity of surrounding rock and determining the support timing in the excavation and support design of underground 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.

Microseismic Monitoring of Rock Mass Fracture Response to Blasting Excavation of Large Underground Caverns Under High Geostress

Microseismic Monitoring of Rock Mass Fracture Response to Blasting Excavation of Large Underground Caverns Under High Geostress

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.

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.

Study on timing sequence control fracture blasting excavation of deep rock masses with filled joints

During the blasting excavation of deep underground caverns, the effects of the structural surface on crack propagation are usually considered in addition to the clamping effects of high in situ stress. Based on the notched borehole and timing sequence control (TSC) fracture blasting method, this paper studies the effects of different borehole shapes on the degree of damage of the surrounding rock and profile flatness of the rock anchor beams and the effects of different filled joint characteristics on the blasting crack propagation rules. The results show that the damage depth of the surrounding rocks by round hole smooth blasting is approximately twice that by notched hole smooth blasting, by which the profile formed is flatter. The notched primary borehole (PBH) remains a strong guidance for crack propagation in a rock mass with filled joints, while the stress concentration effects of the round target borehole (TBH) cannot fully guide the cracks until they fall within a certain distance between the PBH and TBH. It is favourable for cracks to propagate along the lines between boreholes with larger filled joint strengths and larger angles between boreholes.

Mechanical properties and stability analysis of surrounding rock of underground cavern under various stress loading paths

Under the influence of geological tectonic movement and human engineering activities, the geological environment of rock mass is often very complex, and its stress state is also complex and constantly changing. During the excavation of underground caverns, the stress of surrounding rock will be redistributed, and its radial stress and tangential stress are constantly changing and adjusting. From this point of view, the excavation of underground chamber causes strong stress differentiation in surrounding rock, and the experimental study under simple stress path cannot fully reflect the actual state of rock engineering. In order to comprehensively understand the damage mechanism of rock under complex stress paths, this paper takes the granite of an underground powerhouse of a power plant as the research object, designs and carries out a variety of laboratory tests such as conventional triaxial test, unloading test and creep test, and obtains the deformation and failure characteristics of granite under different stress loading paths. Based on the laboratory results, it puts forward specific surrounding rock support recommendations. This study can provide more targeted supporting measures for surrounding rock under different stress states, which is of practical significance to maintain stability of surrounding rock in the factory area.

An Enhanced Ubiquitous-Joint Model for a Rock Mass With Conjugate Joints and Its Application on Excavation Simulation of Large Underground Caverns

A conjugate jointed rock mass (CJRM) is a rock mass with two sets of intersecting joints formed from intact rock under shear. Its mechanical properties and excavation-induced hazards of large underground caverns are different from those of common rock masses because of the unique geological origin thereof. To demonstrate numerically the excavation responses of CJRM, the ubiquitous-joint model is enhanced by consideration of the specific mechanical behaviors of the rock mass. In the enhanced model, CJRM is considered as the composite of columns of rock and two sets of weak planes of joints. The local coordinates, failure modes, and failure sequences of the rock columns and joints are redefined based on the composite characteristics of CJRM, and the failure criteria and plastic potential functions are accordingly modified. The enhanced model is verified numerically by triaxial compression tests and then employed to simulate the excavation of large underground caverns of a pumped storage power station in China. Results show that the modification of the local coordinate system, failure modes, and failure sequences made in the enhanced model is suited to the simulation of the mechanical behaviors of CJRM. Compared with the original ubiquitous-joint model, the enhanced model allows better predictions of the distribution of plastic zones and magnitudes of deformations in simulating underground excavations in CJRM and helps to assess the excavation-triggered hazards more accurately.

Understanding the failure mechanism of a large underground cavern in steeply dipping layered rock mass using an enhanced ubiquitous-joint model

To understand the mechanical behavior and failure mechanism of the surrounding layered rock masses during the excavation of underground caverns for a hydropower station, an enhanced equivalent continuum model based on the ubiquitous-joint concept is developed and compiled as a plugin DLL file in FLAC3D. This model is then applied to analyze two engineering geological issues arisen during the excavations of a large underground powerhouse. Both cases are the typical responses controlled primarily by the internal structure of layered rock mass. The first case is mainly concerned with the continuous increase in the displacement of the upstream sidewall after removal of an auxiliary tunnel crown. Numerical simulation reveals the mixed shear-tensile fractures developed along bedding planes. The preserved crown thickness can affect the maximum displacement of sidewall. Failure region will progressively deepen into the interior if no effective reinforcements are adopted. The second case exhibits as the gradual cracking or slabbing of the shotcrete at downstream roof. Stress concentration is the main cause of shotcrete cracking as shown by numerical simulation. The degree of stress concentration can be influenced largely by the angle between rock strata and cavern axis. This degree also varies with the initial stress level and with excavation process, which is confirmed by field monitoring data. The validity and capability of the newly developed model are thus verified by the actual engineering issues.

Deformation characteristics observed during multi-step excavation of underground oil storage caverns based on field monitoring and numerical simulation

The sequential excavation method (SEM) is extremely useful for excavating caverns with high sidewalls and large spans. A new SEM for excavating large-scale underground water-sealing oil storage caverns and protecting groundwater level is proposed. The method is applied to practice successfully and the excavation characteristics were studied using monitoring datasets. Through finite element simulations, the differences between the proposed method and three-bench three-step method were compared. The influence of the section shape, bench division, and face-advancing sequences on the deformation characteristics observed in multi-step excavation was analyzed. The results showed that the maximum deformation of the excavation face was approximately 4–5 mm, and the maximum deformation rate of surrounding rock was approximately 0.8 mm/day. At 20 days after excavation, although the deformation characteristics of surrounding rock were stable, internal displacement would continue for a long time. The standard circular arch and one-step arching were useful for settlement control, and the middle drift facilitated geological forecast and water exploration. However, the more complex the bench division of the arch, the more noticeable is the stress concentration.

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.

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.

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.