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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. [ABSTRACT FROM AUTHOR]

During underground excavations for civil or mining engineering purposes, the variations in rock mass quality are important, especially for the design of the most suitable support to be applied to ensure stability. The aim of this investigation is to model the expected behavior of the ground, and thus to predict the scenarios indicating potential variations in the quality of the rock mass during underground excavation. When considering the rock mass rating (RMR) values observed at the excavation face in six study cases, which together total more than 27 km in length of underground excavation by drilling and blasting (D&B), and based on the observed RMR values at the face, the most probable value (1-100) is estimate for the RMR index at the five subsequent front advances. It is concluded that, up to about 20 m ahead of the current face, the quality of the rock mass for the next advances is close to the quality observed at the present face, and that, with increasing distance, there is a greater deviation of RMR values with respect to the quality observed at the current face. [ABSTRACT FROM AUTHOR]

The present report highlights the methods used to monitor the rock mass behavior and performance of a cavern during excavation in order to verify and, if necessary, adjust the rock support to ensure safe construction at each stage of excavation. The present geotechnical monitoring practice for underground structures involves convergence monitoring with the help of optical targets and rock mass displacement with the help of borehole extensometers. More importantly, it involves detailed planning to finalize the position of each monitoring instrument, based on the location and orientation of geological features. It is equally important to ensure proper recording of monitoring data in order to analyze and take immediate action in case of any adverse situation. This requires dedicated high-end automated software, which can record, analyse and produce significant results from a large quantity of recorded data, to prevent what could otherwise be a catastrophic failure. Initially, geotechnical monitoring was carried out in the recently excavated zone of the cavern on a daily basis. Based on continuous monitoring data for at least 1 week, the frequency of subsequent monitoring can be decided. In most cases, the deformation of rock mass was considerably less than the alarming values which were calculated based on detailed design for different rock classes. [ABSTRACT FROM AUTHOR]

Design of underground mining operations, in their preliminary stages, often requires obtaining geomechanical data from re-logged rock core initially drilled for mineral exploration. The degree of alteration of this core due to handling and storage time increases uncertainty in the geotechnical parameters assessed and in the estimated required excavation support. Uncertainty is primarily related to changes in observed joint infill characteristics during storage. This paper presents a simple approach to assess this uncertainty quantitatively, illustrated with a case study in the Peruvian Cordillera. The Q-System was adopted for such assessment and to estimate the required excavation support. From results of the case study presented in this paper, uncertainty associated with the use of stored core was not considered significant when compared to the natural variability of the rock mass. Numerical simulations of excavation geometries for different rock mass qualities were performed to validate estimated support requirements through the Q-System. These simulations required estimations of strength characteristics of the rock mass and discontinuities. These were estimated from measurements in all core available (old and fresh) and considered the uncertainty in the adopted strength envelopes quantitatively. The range of model results and sensitivities to effects of modeling validated the recommended support requirements. During excavation, support requirements would be based on an assessment of Q as the excavation proceeds (an observational approach). [ABSTRACT FROM AUTHOR]

As an objective necessity of restriction the scope of the steel timbering used for the current metal support of the underground excavations, resulting from the not correlation of the symmetric construction supporting system with uneven maximum vector manifestation of pressure, as well as economic considerations, generated by the high costs and labor for the lamination of metal profiles, by applying to the supplier a additional treatments improve, it becomes increasingly necessary to introduce and generalization on how to consolidate rocks by anchored support. The paper analyzes the possibility of generalization the anchored support for underground excavations, as well as the specific features of this support system in its interaction with the surrounding rock, with the aim of making the option for choosing this method of consolidation the rocks. [ABSTRACT FROM AUTHOR]

This paper presents a case study of an underground crude oil storage project. This storage site is researched with detailed field and laboratory investigations. The layout, cross section, and elevation of the caverns are analyzed and designed based on geological and geotechnical information obtained during these investigations. In this study, two-dimensional (2D) elastoplastic analysis is performed using the finite-element code Phase 2D to study the rock mass behavior caused by staged excavation of the cavern (). The excavation of the cavern is simulated with different stages, i.e., excavation of a pilot tunnel and subsequent side slashing followed by excavation for a pilot bench and subsequent side slashing with a total of three benches. Displacement values obtained analytically are compared with the observed displacement as the excavation progresses. Because the 2D FEM code is based on the plain strain concept, it could not take into account the effect of excavation in other directions that is very obvious at the intersection with tunnels and shafts. Therefore, these intersections were analyzed using a linear three-dimensional (3D) boundary element code, Examine3D. The stability analysis of rock wedges formed from various joint patterns is also studied using the UNWEDGE software based on block theory for each section of the cavern. [ABSTRACT FROM AUTHOR]

Abstract: Probabilistic analysis of underground rock excavations is performed using response surface method and SORM, in which the quadratic polynomial with cross terms is used to approximate the implicit limit state surface at the design point. The response surface is found using an iterative algorithm and the probability of failure is evaluated using the first-order and the second-order reliability method (FORM/SORM). Independent standard normal variables in U-space are chosen as basic random variables and transformed into correlated non-normal variables in the original space of random variables for constructing the response surface. The proposed method is first illustrated for a circular tunnel with analytical solutions considering Mohr–Coulomb (M–C) and Hoek–Brown (H–B) yield criteria separately. The failure probability with respect to the plastic zone criterion and the tunnel convergence criterion are estimated from FORM/SORM and compared to those obtained from Monte Carlo Simulations. The results show that the support pressure has great influence on the failure probability of the two failure modes. For the M–C model, the hypothesis of uncorrelated friction angle and cohesion will generate higher non-performance probability in comparison to the case of negatively correlated shear strength parameters. Reliability analyses involving non-normal distributions are also investigated. Finally, an example of a horseshoe-shaped highway tunnel is presented to illustrate the feasibility and validity of the proposed method for practical applications where numerical procedures are needed to calculate the performance function values. [Copyright &y& Elsevier]

A closed-form solution is presented in this paper for the prediction of displacements around circular openings in a brittle rock mass subject to a hydrostatic stress field. The rock mass is assumed to be governed by Hoek–Brown yield criterion and a non-associated flow rule is used. For the elastic–brittle–plastic analysis of circular openings in an infinite Hoek–Brown medium, the existing analytical solutions were found to be incorrect. The present closed-form solution is based on a theoretically consistent method and the solution does not require the use of any numerical method.The present closed-form solution was validated by using the finite element method. In the finite element analysis, the infinite boundary was simulated “exactly” by using the newly developed elastic support method. Several cases were analyzed and the present closed-form solutions for stresses and displacements were found to be in an excellent agreement with those obtained by using the finite element method. [Copyright &y& Elsevier]

In this review the results are presented reached by the authors in the study of the strength characteristics of a group of rock massive of defferents undergroudn mines and tunnels of the the country's eastern region that are characterized by the affectation structural degree that possess. Initially judgment are given on the main methods that at the moment are employess for to value the mechanical characteristics in the joints. Ferther more to analize the defferents ways for the strength estimate in these rock massive. As result of the research expressions for the strength estimate are offered in these rock massive, that are of great importance, since they allow to face defferent project tasks and construction in the study areas with a bigger economy and releability, but olso it is possible to extrapolate to other rock massive. [ABSTRACT FROM AUTHOR]