Rock Mass Excavation Research Articles

Modeling transient excavation-induced dynamic responses in rock mass using an elasto-plastic cellular automaton

The problem of transient excavation-induced dynamic response in rock mass is discretized on both spatial and temporal scales and is solved by using the cellular automaton (CA) technique and the Newmark scheme, respectively. Using this approach, a dynamic analysis version of 3D elasto-plastic cellular automaton (EPCA3D) is developed. The advantage of this method is it avoids the solution of large-scale linear equations since only the local CA rule is used for dynamic state updating. The Den Iseger algorithm is introduced to validate the numerical method. The evolution of stress and velocity for the transient excavation in rock mass under different conditions obtained by the EPCA3D and Den Iseger methods are in good agreement. The abilities of EPCA3D in the modeling of elasto-plastic dynamics of transient excavation in rock mass are well demonstrated. By considering different in situ stresses, excavation radiuses, unloading durations and unloading paths, the factors affecting nonlinear dynamic responses are investigated. It is found that the failure extent increases with the decrease of unloading time. When lateral pressure coefficient (σx/σy) equals to 1, the failure zone is evenly distributed around the tunnel. However, with the decrease of lateral pressure coefficient, the extent of failure localization increases. The fault around the tunnel makes the dynamic failure zone asymmetric distribution. The modeling helps to understand the major mechanism of rock mass damage for the transient excavation.

Analysis of Collapse Load and Open Hole Evaluation Based on Rock Mass Ratting (RMR) Method in Underground Mining

The excavation of a underground rock mass resulting a change in the balance of rock conditions such as the occurrence of rock collapse and changes in the dimensions of tunnel as a result of the displacement of the voltage distribution. It is desirable to have a good geotechnical analysis to provide an appropriate treatment of rocks excavated. This research was conducted to determine rock mass class, rock strength. Classification of rocks mass with RMR system And Q-System FK potential of wedge using unwedge software. From the measurement of several parameters of RMR method, RMR value of 48 ( Coal) and 60 (Sillstone) is obtained where as rock mass class III (fair rock), based on the Q-System obtained a value of 14 in class II for coal and 4 Class III for Sillstone. Based on the RMR-system classification, the value for span maximum is 3 m with a collapse time of 110 hours and the value of the load collapses is 1,56 tons / m2. Based on the Undwedge Software processing, there was an increase in FK values for both methods, Based on Support RMR Ground from 1,907 to 2,939 and Based on Ground Support Q-System from 1,907 to 2,078.

Numerical simulation method for vibration response of blasting excavation of deep-buried rock mass

In order to solve the safety problems caused by in-situ stress unloading and blasting impact in blasting excavation of deep-buried rock mass, the initial stress field and dynamic response of excavation unloading of deep-buried rock mass are calculated by ANSYS implicit and explicit solver. The results show that the stress wave induced by in-situ stress unloading diffracts at the foot of the wall and produces stress wave superposition in the surrounding rock above the wall. The vibration of surrounding rock caused by excavation of buried rock mass consists of blasting shock and ground stress unloading. With the increase of excavation depth, the vibration of surrounding rock caused by ground stress unloading exceeds blasting effect and gradually dominates the measured vibration.

Damage evolution mechanism in production blasting excavation under different stress fields

Deep rock mass stores large amounts of energy. Energy accumulation, dissipation, and release during the deformation and failure of rock mass occur when the rock mass is excavated via the drilling and blasting method. According to the energy effects, failure pattern, and effect of the strain rate on the rock strength, a Johnson–Holmquist rock (JHR) model was established by combining the energy-based tensile–compressive damage model and the Johnson–Holmquist ceramic model based on the coupling method for tensile–compressive damage in a rheological-dynamical analogy model. To apply the same method to the damage evolution mechanism of deep rock mass, the JHR model was incorporated into the LS-DYNA software. Subsequently, the self-developed numerical model was employed to investigate the damage evolution in the production blasting excavation of stopping holes under different stress fields. According to the failure law, the challenges in rock-mass excavation were analysed. This study provides a basis for the analysis and research regarding the blasting design in deep rock-mass excavation.

Application of rock mass classifications: a case study from Ituango hydroelectric diversion tunnel, Colombia

ABSTRACT The local site experience is a valuable component for the success of rock mass classification systems as tunnel design methods. The Ituango hydroelectric project is a very important source of information in order to evaluate the usefulness of the main rock mass classification systems. The objective of this research is to improve understanding of some important features of excavated rock mass, such as discontinuities, block size, shear strength and joint alteration, by analyzing some hundreds of data obtained during excavation cycle. The field study included a survey of exposures after drilling and blasting rounds. Rock mass classification and support measures for each type of terrain along 1400 m tunnel were performed. The rock mass classes could be better explained if shear strength, alteration or block size is calculated. The assessment of these data allows evaluating the block fall risk, improving support and liner. A local correlation between the RMR and Q system was also obtained.

Effect of Contributing Parameters on the Behaviour of a Bolted Rock Joint Subjected to Combined Pull-and-Shear Loading: A DEM Approach

The mechanical performance of fully grouted rock bolts is essential in the stability of underground excavations in jointed rock masses. This research implements a new cohesive contact model in distinct element codes (PFC2D) to investigate the fracturing response of rock-like and grout material, as well as the bolt–grout interface. The results are compared in detail with experimental observations. The proposed modelling approach, used in conjunction with the distinct element method (DEM), successfully predicted the behaviour of grout failure and the bolt–grout interface’s shear response. We then developed a novel numerical, stepwise pull-and-shear test (SPST) scheme to further analyse the mechanical behaviour of bolted rock joints subjected to simultaneous pull–shear loading. The cohesive DEM framework proposed in this paper was used to carry out the SPST scheme numerically. The mechanism involved in enhancing the shear strength of bolted rock joint was determined by monitoring the $$ \sigma_{\text{n}}^{\text{i}} $$ and its corresponding contact chain force network during the pull-out test. The influence of pretension stress, the rig angle of the bolt profile, and the constant normal stiffness (CNS) condition are assessed systematically. In particular, the pretension stress magnitude at which the synthetic rock bolting system exhibits the highest shear resistance is identified. The findings from this research highlight the sensitivity of bolted rock joints to the simultaneous pull-and-shear loading, boundary conditions, and bolt–grout interface configurations.

Brittle overbreak prediction in deep excavations for hydrothermally altered and heterogeneous rockmasses

In massive rockmasses under high in situ and deviatoric stresses, brittle spalling is the dominant damage process and failure mode and can result in significant depths of overbreak outside the excavation design dimensions during and after construction. Significant work has been done by researchers to develop empirical and mechanistic prediction tools for brittle overbreak in homogeneous rockmasses. Brittle overbreak in heterogeneous rockmasses that contain hydrothermal veins and breccia can behave quite differently than homogeneous rockmasses, with the result that the existing tools for homogeneous rocks are inadequate. In this study, overbreak profiles and excavation observations from part of the undercut level in the New Mine Level project at the El Teniente copper–porphyry mine in Chile and intact laboratory test data from the four local rockmass units are used to develop new functions based on brittle rock mechanics to improve the prediction of the depth of brittle overbreak for similar excavations in heterogeneous rockmasses. The dacite porphyry unit, which has few stockwork veins, most closely resembles the expected homogeneous brittle overbreak development. The stockwork mafic complex unit exhibits overbreak that exceeds the homogeneous prediction because the planar, pervasive quartz veins act as crack attractors and develop maximum crack propagation after initiation and immediate strength loss. The anhydrite breccia unit and a brecciated contact zone contain large clasts of contrasting mineralogy that act as crack arresters, resulting in minimal overbreak. The proposed functions to predict depths of brittle overbreak around tunnels in these heterogeneous rocks can ultimately be used to aid in excavation and ground support design.

A new classification of failure mechanisms at tunnels in stratified rock masses through physical and numerical modeling

Stability of tunnels in stratified rock masses under low stress conditions is governed by the rock mass structure (blocks) and excavation geometry and dimension. A physical model was built and the effects of bedding dip, discontinuity spacing, and tunnel dimensions on tunnel failure mechanisms were examined. Based on the image analysis, three zones were recognized around the excavation consisting: stationary, collapsed and buckling zones. The buckling zone was more affected by layer dip angle. The collapsed zone occurred in three modes of block falls and sliding, and toppling. Also, numerical simulation using distinct element method was performed and verified. After model calibration, numerical simulations were extended to a wide range of block size and discontinuity gradients. Finally, considering physical test results, image analysis, and numerical simulation results, failure mechanisms in stratified rocks were classified using “bedding dip” and so called “dimensional ratio”. Results lead us to a deep insight of ground behavior around the shallow underground excavations in stratified blocky rock masses under low stress condition. Additionally the deformation mechanisms of blocks such as buckling, shearing, sliding are discussed in detail.

Numerical investigation on the stability of deforming fractured rocks using discrete fracture networks: a case study of underground excavation

The stability of a fractured rock mass around a subsurface opening is critical to tunnel excavation. The traditional underground excavation analysis is based on a continuum description of randomly distributed discrete pre-existing fractures. In contrast to this, we have developed an improved hybrid finite element method (FEM) to investigate the stability of fractured rocks around an excavation by incorporating the outputs into the FEM codes. The proposed model consists of a discrete fracture network (DFN) model and cohesive zone model (CZM). The DFN model automatically generates a fracture network with a given fracture opening distribution and provides grid generation strategy to the FEM. The CZM captures material failure and intersection and surface contacts through the fracture element with different constitutive laws. As a case study, a DFN model was formulated for the underground excavation at Jinping Hydropower Station, in light of the input requirements of our model, and its deformation analysis was performed. The comparison analysis of rock deformation was made for excavation in both intact rock mass and fractured rock mass under the same boundary conditions. The numerical results show that two different modes of rock failure exist in these two rock masses and that intense deformation at the fractures intersected by the tunnel is responsible for fractured rock mass instability. The proposed approach was verified using data from field investigations. A larger displacement can be produced if the rock mass is weakened by a key block. A sensitivity analysis was carried out to investigate the effects of different model parameters on deformation variations. This study provides an insightful understanding of the deformation of fractured rock mass during tunnel excavation.

Simulation of cross-correlated non-Gaussian random fields for layered rock mass mechanical parameters

In order to represent the spatial variability of layered rock mass mechanical parameters in Finite Difference Method (FDM) grids, an approach based on the numerical simulation of cross-correlated non-Gaussian random fields was proposed in this paper. First, the discretization of a Gaussian random field was performed using the Karhunen-Loève (K-L) expansion. Then, the Gaussian random field for layered rock mass parameter was built by geostatistics and the random field size calculation. Next, the cross-correlated non-Gaussian random fields were simulated using Nataf transformation and isoprobabilistic transformation methods. Furthermore, a grid model coupled to the random fields was finally built using regroupment and element traversal methods. The engineering application showed that the spatial variability, autocorrelation, cross-correlation and non-Gaussianity described by the random fields agreed closely with the statistical characteristics. The correspondence between the random fields and grid model verified that the random parameters in the grid model can represent the anisotropy of layered rock mass parameters. This study provides a computational basis for quantitative risk assessment during tunnel excavation in layered rock mass.

Correlational fractal characterisation of stress and acoustic emission during coal and rock failure under multilevel dynamic loading

The fractal characterisation of stress and acoustic emission (AE) of coal and rock failure may provide quantitative guidance for analysing the stability of rock mass during excavation engineering. The correlation between stress and AE data was calculated using fractal theory methods and Grassberger-Procaccia (G-P) algorithms to evaluate the damage and degree of flaw in coal and rock materials under multilevel dynamic loading (MDL). First, the mechanical properties of the testing device were developed to obtain synchronous data regarding the stress and AEs of coal and rock specimens under MDL. Second, strength deterioration behaviour and the Felicity effect during coal and rock failure are compared under different levels of dynamic loading. The results show that cumulative dynamic loading are less than the uniaxial compression strength of coal and rock. The Felicity effect is notable, and Felicity ratios decrease to 1 with increased loading velocity. Finally, stresses and AE signals are considered as the data mining specimens of correlation dimensions based on the G-P algorithm. Fractal characteristics on stress and AE coupling properties become more notable as the correlated dimensions become larger. However, the Felicity effect is less prominent, reflecting a lesser degree of damage and flaws in coal and rock materials. Therefore, this study suggests a data mining method for the correlation dimension to be applied in an in-situ monitoring system for rock mass excavation engineering.

Stress Field Distribution and Deformation Law of Large Deformation Tunnel Excavation in Soft Rock Mass

The problem of large deformation is very prominent in deep-buried tunnel excavation in soft rock, which brings serious potential safety hazards and economic losses to projects. Knowledge of the stress field distribution and deformation law is the key to ensuring rational design and safe construction in large deformation tunnels of soft rock. As described in this paper, theoretical analysis, numerical simulation and field monitoring were employed to investigate the surrounding rock stress and displacement state in the Dongsong hydropower station in Sichuan Province, China. The results show that the short-bench construction method can effectively control the deformation of surrounding rock and range of the plastic zone. In order to reserve enough working space, the optimum bench length in the actual construction was 10 to 14 m. The peripheral displacement and plastic radius decreased with the increase of tunnel support strength and the advance of supporting time. The displacement can be effectively controlled by applying the second lining in time at a position about twice the diameter of the hole (16 m) from the working face. A reasonable reserved deformation should be adopted to avoid secondary expanding excavation. The values of different positions in the tunnel laterally and longitudinally may be different, and adjustments are needed according to the actual situation.

Probability distributions of shotcrete parameters for reliability-based analyses of rock tunnel support

A common support measure for underground excavations in jointed rock masses to support loose blocks is to apply a thin shotcrete layer to the periphery of the excavation and systematically install rockbolts into the surrounding rock mass. In this support system, large blocks are carried by the rockbolts and small blocks are carried by the thin shotcrete layer. To verify the shotcrete layer’s load-bearing capacity and to stringently account for the large uncertainties incorporated in the variables involved in determining its capacity, analytical calculations in combination with reliability-based methods can be used. However, a lack of knowledge exists regarding the magnitude and uncertainty of shotcrete characteristics (thickness, adhesion, flexural tensile strength, residual flexural tensile strength, and compressive strength), making it difficult to apply reliability-based methods. A statistical quantification of these characteristics is therefore important to facilitate reliability-based methods in design and verification of shotcrete support. In this paper, we illustrate how shotcrete support against small loose blocks can be viewed as a correlated conditional structural system and how this system can be analyzed using reliability-based methods. In addition, we present a unique amount of data for the aforementioned variables, which are all incorporated in the design and verification of a shotcrete layer’s ability to sustain loads from small loose blocks. Based on the presented data, we statistically quantify and propose suitable probability distributions for each variable. Lastly, we illustrate how the proposed probability distributions can be used in the design process to calculate the probability of exceeding the shotcrete’s load-bearing capacity. Both the probabilistic quantification and the defined correlated conditional structural system along with the illustrative calculation example are followed by a discussion of their implications.

Stress redistribution and formation of the pressure arch above underground excavation in rock mass

Pressure arch utilises the self-stabilizing capacity of surrounding rock and is benefit to the roof stability of underground excavations. The analysis of it provides a new stability assessment tool for underground engineering. In this paper, a series of numerical simulations were carried out to investigate the formation and properties of pressure arch and the self-stabilisation mechanism of surrounding rock. It is found that stress redistribution results in the formation of the pressure arch. The inner and outer boundary of arching area are determined as the principal stress trajectories through the inflection points of the principle stress-depth curves on the centre line of caverns. Furthermore, parametric analysis illustrates that in situ stress ratio and excavation size have more significant effect on the inner and outer boundaries and the arch thickness than roof shape and overburden depth. Finally, it can be observed that the various trends of the inner boundary height with different influence factors are same with the displacement of the crown, and it could be used as an indicator for stability assessment. Based on the observed phenomena, a design scheme is proposed to determine the maximum size of the underground openings cross-section without support in agreement with the geomechanical conditions.

Study on the Time-lag Failure of Sandstone With Different Degrees of Unloading Damage

The unloading effect of rock mass excavation is an inevitable practice, and it’s often characterized by a relatively large-scale engineering hazard with a noticeable time lag.A set of unloading triaxial tests were conducted on a sandstone rock to establish the deformation law and the threshold time. Based on the renormalization group theory, the unloading sandstone model was developed by considering the interaction between particles. Similarly, a logistic model was used to predict the unloading damage of sandstone. The unloading time lag damage of sandstone rock was predicted by using the damage threshold. The research shows that: (1) The higher the degree of unloading, the shorter the time-lag failure. (2) The damage range of critical values was optimized. (3) The error between the predicted value and the experimental value of the time threshold was almost less than 5 %, the prediction result was found to be good, and the employed logistic evolution model was reasonable. The findings of this research provide a prediction method and precise information about the mechanism of unloading time lag deformation. Therefore, it can be used as a reference for excavation-support design of underground structures.

Experimental Investigation of Energy Evolution in Sandstone Failure during Triaxial Unloading Confining Pressure Tests

The deformation and failure of sandstone samples are closely related to energy changes in the material. To explore the energy evolution during the process of sandstone sample damage, loading and unloading tests with different test paths were conducted. The results show that more energy is stored and consumed before the stress reaches its peak, while after the peak stress, more energy is released and consumed. Energy dissipation increases internal cracking, leads to sample damage and lithologic deterioration, and reduces the bearing capacity of the sample. During triaxial unloading of the confining pressure, the higher the initial unloading confining pressure, the more the elastic energy stored, and the more the energy released when the sandstone sample fails, resulting in more severe damage. Therefore, during the excavation of high‐stress rock masses, large amounts of elastic energy stored in sandstone can be rapidly released, leading to rock burst disasters. Additionally, during triaxial unloading confining pressure tests, the damage in sandstone when the sample is close to failure increases more rapidly than that during conventional triaxial compression tests because of the unloading effect of the confining pressure. This phenomenon also illustrates that the failure of sandstone induced by unloading is more sudden than that induced by loading.

Numerical Simulation Research of Smooth Wall Blasting Using the Timing Sequence Control Method under Different Primary Blast Hole Shapes

To make sure the integrity and stability of surrounding rock structure during blasting excavation of important structural planes in deep underground caverns, two kinds of fine blasting methods, timing sequence control fracture blasting network and notch blast hole, are innovatively combined and the formation of cracks between smooth blasting holes with different delay initiation and different shapes of primary blast holes (PBHs) are compared and analyzed. The results show that when the delay initiation time between the successive explosion holes is greater than or equal to the transverse wave of the PBH propagates to the target blast hole (TBH), the concentrated stress along the connection direction of the hole on the wall of the TBH is larger than the other directions of the hole wall. After the TBH is detonated, cracks will preferentially expand along the connection direction of the blast holes. If the PBH is the notch blast hole, more explosive energy will be directed to the wall of the TBH so that the hole wall along the connection direction of the blast holes will be subjected to greater tension stress before the initiation of the TBH. In this way, the interval between successive holes can be increased and the efficiency of blasting excavation of rock mass can be improved accordingly.

Analytical solution of circular tunnel in elastic-viscoplastic rock mass

Abstract This paper deals with a deep circular tunnel excavated in infinite homogeneous and isotropic elasto-viscoplastic rock mass subjected to a hydrostatic initial stresses. The tunnel is divided into the initial plastic, viscoplastic, and elastic zones. By combining the generalized Bingham model with Mohr-Coulomb yield criteria, analytical solutions of the circular tunnel are derived with the consideration of non-associated flow rule and elasto-viscoplasticity. The initial plastic zone is defined as the instantaneous change of rock mass excavation. Based on the initial plastic zone, the stresses in viscoplastic zone are transferred to the deep surrounding rock with time due to the initial earth stresses. The results are compared with the elastic-brittle solution at static conditions, and the solutions of this paper are validated. Moreover, the presented results shows that the stress and displacement of the surrounding rock varies with time, and the solutions can be obtained at different instants of time.

Energy-Based Forming Mechanism and Criterion for Zonal Disintegration

Abstract The law of energy dissipation and release during rock damage is studied, and the energy-based criterion for rock damage is obtained. Based on these findings, the mechanism and criterion for zonal disintegration are studied. An equivalent excavation model is proposed to obtain the releasable energy during the rock mass excavation. This releasable energy distribution in rock mass shows the elastoplastic boundary fractured with the release of the conserved energy. The zonal disintegration is thus revealed to be the circulation of this fracture along with the constant formation of the new elastoplastic boundary. Meanwhile, a sliding fracture model in polar coordinates is proposed, and the dissipated energy is calculated for the circle fracture formation. Consequently, the criterion for Zonal Disintegration Phenomenon (ZDP) is constructed according to the energy conservation law. Additionally, a formula is inferred through the criterion to determine the numbers of disintegration zones. This energy-based theory is verified as effective by applying it to an actual deep cavern where ZDP occurs.

Influence of deviatoric stress on rockburst occurrence: An experimental study

As mining delves deeper into the crust, it is necessary to investigate the complex rock responses associated with higher stress gradients. Therefore, it is essential to better understand the mechanisms associated with the rockburst phenomenon. However, due to the large-scale and difficult monitoring of real mining excavations, laboratory scale tests must be utilised to determine the conditions conducive to burst. To this end, this research focuses on the implementation of a new rockburst testing apparatus to replicate the stress conditions of a rock mass excavation at the time of bursting. This apparatus allows the determination of rockburst stresses and a relationship between deviatoric stress and in-situ pressure/depth. Using this relationship it is then possible to estimate the standardised stress levels for a certain rock type which might lead to rockburst occurrence. Furthermore, it is demonstrated that with increasing in-situ pressure, the likelihood (measured as a lower differential stress) and the extent (indicated by the increasing range of deviatoric stress) of rockburst increases. These findings provide valuable information about the conditions necessary for bursting in deep mining.