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The construction process for the enlargement of an underground railway station in Barcelona required an additional tunnel excavation below existing structures. The paper presents the case study, describing the details of the excavation method and the support systems used both to guarantee the stability of the tunnel excavation and to limit the effects of the underground constructions activities on the buildings above the alignment. The movements observed in the buildings, once the excavation was completed, are compared with some class A predictions. The construction works had a favourable outcome and no building damage was reported because of the excavation.

In an assault on a military fortress, the attacking side often attacks the fortress through the use of underground mining, so that the defensive side cannot be prepared. The existing monitoring methods make it difficult to monitor such underground excavation. One effective way to monitor for tunneling activity is to detect and identify seismic signals generated by underground excavation. However, the main problem facing the practical application of this technology is that many behaviors on the ground may generate seismic signals, and the monitoring system cannot identify whether a signal is generated by underground excavation or by someone walking on the ground, resulting in a high false alarm rate. To effectively identify underground excavation signals, we propose an approach for estimating speed based on a double point synchronization measurement. In our approach, we first formulate mathematical models of the velocities of underground and ground-level objects. Then, signals acquired by different seismic detectors are used to estimate the velocities of underground and ground-level objects. By analyzing the differences between velocities, signals due to human movement and underground excavation are effectively identified. Lastly, simulations and a field test are performed. It is found that the proposed approach can effectively distinguish between signals generated by a human moving at ground-level and underground excavation. Our approach can be helpful for reducing the false alarm rate of a monitoring system.

This work systematically quantifies groundwater’s effect on the roof stability of underground openings in shallow coal mines and indirectly assesses the potential for sinkhole formation. A specific stratigraphy was analyzed where the opening instability can lead to sinkhole formation, given the geological conditions, the overburden stratigraphy, and the depth of the openings. The groundwater recharge was investigated as an individual rainfall infiltration (short-term) and as groundwater dynamics (long-term), representing the accumulation of infiltrated rainwater with time. In the latter case, two approaches were employed for the porewater pressure calculation: (a) phreatic line with hydrostatic conditions and (b) steady-state flow based on constant groundwater head on the vertical boundaries at the model’s edges. The safety analysis was based on a simplified statics model relating the stability to the bending of the roof, and numerical analysis was employed for the stress analysis. The short-term safety remained unaffected as rainfall water accumulated on the aquitard to form perched water. The long-term safety deteriorated due to the increase of the tensile stresses in the roof of the underground openings with the rise of the groundwater table. The phreatic line approach is the most conservative, resulting in lower safety than steady-state flow.

Intersecting discontinuities are often encountered in rock engineering and sometimes associated with damaging geohazards. Our understanding of intersecting discontinuities instability remains vastly insufficient due to difficulties in comprehensively monitoring the failure process. Here we use microseismic (MS) monitoring to virtualize the MS events in the rock masses surrounding a powerhouse crown and investigate the effect of geological features on the occurrence of MS events. We subsequently build a three-dimensional numerical model and validate this model using the in-situ measurements by multipoint displacement meters. The numerical results demonstrate how the displacements of surrounding rock masses near the fault and the dyke increase and reveal possible causes, such as stress condition as well as geometry and orientation of rock discontinuities. We also discuss the correlation between the tempo-spatial distribution of MS events and the failure pattern of rock masses and confirm the weakened dyke as the main cause of the rock collapse. This study highlights that the stability of intersecting discontinuities can be controlled by both the geometrical and mechanical properties of individual discontinuities, and attentions should be paid to key properties favorable for rock instability. This work was supported by the Science Foundation for Distinguished Young Scholars of Sichuan Province (Grant No. 2020JDJQ0011) and the National Natural Science Foundation of China (Grant No. 51809221).

Underground excavation is a necessary process for constructing mines, tunnels and depots in granite rock mass. In this study, the numerical granite specimens were established by the discrete element method and confirmed by laboratory experiments in order to investigate the peak stress, cracking development and failure properties of pre-holed granite under coupled biaxial loading and unloading conditions. The results show that, for the specimens containing D-type and square holes, the peak biaxial unloading strengths first decrease, then increase and finally decrease as the inclination angles of the holes increase. For the specimens with elliptical holes, the peak biaxial unloading strengths first decrease and then increase with the increases in the inclination angles of the holes. The biaxial unloading strengths of specimens containing elliptical, circular, D-type and square holes decrease in that order. The cracks initially appear near the crossover points between the X-type shear fracturing plane and the pre-hole in the center and gradually expand along the X-type shear direction, which indicates that the failure of pre-holed granite is primarily shear failure. When the overall length of cracks expanding along the X-type shear direction extends to the size of the pre-hole in the center, the failure of the pre-holed specimen occurs. When the existing pre-hole in the center of the granite specimen extends to connect with the shear slip in the vicinity of the hole, this triggers overall failure.

International audience; The paper investigates the mode I fracture propagation in brittle rocks using numerical simulations based on the discrete element method. The rock material is represented as an assembly of bonded particles interacting according to an elastic-brittle contact law. The formulation of the method includes the ability for near neighbour interactions to occur so as to control the density of interparticle bonds in the medium, and thus ensures the non-dependence of the model's predictions with respect to its discretization. The adopted model was calibrated to reproduce the behaviour of a granitic rock with the aim to focus more specifically on brittle failure. Numerical samples with different discretizations were generated and subjected to different loading paths. Firstly, the response of laboratory scale pre-cracked specimens subjected to direct tension and three points bending tests was studied. Then, the fracture process zone was characterized by evaluating the volume of the damaged zone as well as the energy dissipated by cracks for different discretizations. It is shown that the proposed approach ensures the fracture process zone characteristics to be independent on the discretization of the model. In addition, large scale simulations of a well-documented underground excavation were performed in order to assess the model's capability to describe brittle failure mechanisms at the structure scale. The results confirm the independence of the model with respect to its discretization and highlight the pertinence of the proposed discrete approach for the study of large scale boundary value problems.

The numerical modelling of the excavation of an underground gallery in hard clay has been discussed in current article. A constitutive model is proposed to describe poromechanical behaviour of the hard clay. The main features of the hard clay observed in laboratory and in-situ experimental investigation have been taken into account in the proposed constitutive model, in particular the plastic deformation, the visco-plastic deformation, the damage, etc. The influence of the initial in-situ stress and the pore pressure has been taken into consideration. The numerical modelling of the underground excavation has been implemented by using a fully coupled hydro-mechanical finite element calculation code. The performance of the model is examined by comparing numerical simulations with in situ measurements. The proposed model and the calculation procedure for the modelling of the excavation of an underground gallery have the capacity to reproduce well the excavation damaged/distributed zone and other main features and phenomena observed during the excavation process. However, the in-situ observed convergence could not be reproduced correctly. More effort on the discontinuous problem should be made for the reproduce the observed convergence. Keywords: Full coupled hydro-mechanical processes, Elasto-plastic-damage model, Visco-plastic model, Underground excavation

The paper concerns shape optimization of a tunnel excavation cross-section. The study incorporates optimization procedure of the simulated annealing (SA). The form of a cost function derives from the energetic optimality condition, formulated in the authors’ previous papers. The utilized algorithm takes advantage of the optimization procedure already published by the authors. Unlike other approaches presented in literature, the one introduced in this paper takes into consideration a practical requirement of preserving fixed clearance gauge. Itasca Flac software is utilized in numerical examples. The optimal excavation shapes are determined for five different in situ stress ratios. This factor significantly affects the optimal topology of excavation. The resulting shapes are elongated in the direction of a principal stress greater value. Moreover, the obtained optimal shapes have smooth contours circumscribing the gauge.

Brittleness of rock is one of the most critical features for design of underground excavation project. Therefore, proper assessing of rock brittleness can be very useful for designers and evaluators of geotechnical applications. In this study, feasibility of genetic programming (GP) model and non-linear multiple regression (NLMR) in predicting brittleness of intact rocks is examined. For this purpose, a dataset developed by conducting various rock tests including uniaxial compressive strength, Brazilian tensile strength, unit weight and brittleness via punch penetration on rock samples gathered from 48 tunnels projects around the world is utilized herein. Considering multiple inputs, several GP models were constructed to estimate brittleness index of the rock and finally, the best GP model was selected. Note that, GP can make an equation for predicting output of the system using model inputs. To show applicability of the developed GP model, non-linear multiple regression (NLMR) was also applied and developed. Considering some model performance indices, performance prediction of the GP and NLMR models were evaluated and it was found that the GP model is superior to NLMR one. Based on coefficient of determination (R2) of testing datasets, by proposing GP model, it can be improved from 0.882 (obtained by NLMR model) to 0.904. It is worth mentioning that the proposed predictive models in this study should be planned and used for the similar types of rock and the established inputs ranges. © 2016, Springer-Verlag London.

As our understanding and modeling capabilities evolve, an ever-increasing complexity of models representing the behaviour of geologic medium are analyzed. One way to evaluate these substantial problems is to optimize the underlying discretization of the governing differential equations by concentrating finite elements where solution accuracy counts the most. This paper develops and evaluates the performance of a priori local p-refinement method for finite element mesh improvement for stress analysis of underground excavations. This type of refinement entails a mesh with higher-order elements near the region of interest and lower-order elements elsewhere. The focus of the paper is the automated insertion of transitional elements at the interface of the two regions. The method relies on transitional finite elements in order to connect a mesh of quadratic interpolation order elements with a mesh of linear interpolation order elements. Four types of transitional elements were considered (4-node and 5-node triangles, 5-node and 7-node quadrilaterals). These were incorporated into a finite element code, and their performance was tested using representative problems such as a pressurized cavity or tunnelling through rock. For these problems the global stiffness matrix size was reduced on average by 85% and by 81% for the models using triangles and quadrilaterals, respectively, as a result, the calculation times were considerably shortened as well. While the average percentage of error with respect to the models without improvement, measured at critical points, was 0.04% and 0.02% in the case of triangular and quadrilateral elements, respectively.

In the excavation of shallow underground works in an urban environment using the ‘cut and cover’ method, the choice is often made to use concrete diaphragms in order to support the side walls, before proceeding with the excavation of the ground. When these diaphragms exceed a depth of about 20 m, trench-cutters are generally used to excavate the panels, using a supply of bentonite mud. A remarkable development of trench-cutters has taken place over the last 30 years and these machines today allow panels to be excavated in any type of ground whatsoever, even when it is highly cemented. The experience that has been gained in Turin (Italy) in recent years can be considered interesting, because of the huge number of diaphragms that have been completed and the varying characteristics of the ground in the urban area, which ranges from loose sand and gravel to highly cemented ones. On the basis of detailed analysis of the in situ behaviour of trench-cutters in Turin and of laboratory investigations on the effect...

The most common excavation methods for the construction of tunnels in urban areas are the ‘cut and cover’ method and the traditional method of excavation at the tunnel face. The first method is usually used for works at shallow depths, even though it frequently involves burdensome deviations and shifting of the existing underground plants and networks. It has been used to realise the Underground Railway Link in Turin (Italy), where excavation was made using trench-cutters and clam excavators. The results of a technical–operational comparison between the two techniques are presented in this work, from which the advantages concerning the use of trench-cutters compared to the use of clamshell bucket excavators have been determined.

A methodology to model and simulate joint zone for stability analysis of the underground excavation engineering is proposed. Firstly, the jointed rockmass is regarded as a layered anisotropic rockmass zone and a mechanical model of the layered anisotropic rockmass is put forward based on the finite element method (FEM). Secondly, a set of indexes are introduced as the stability assessment indicators to analyze the stability of the surrounding rock using FEM calculation results. Finally, a case study is further illustrated with excavation of an underground pumping house using the proposed methodology to analyze the effect of the steep joints on the surrounding rock stability. Compared to the isotropic condition, when considering the layered anisotropic property with different horizontal and normal elastic modulus, surrounding rock experiences more stress disturbance that resulting in increased plastic zones, dissipation energy and deformation around opening, which is unfavorable for the stability of surrounding rock of the underground caverns. The mechanical model of layered anisotropic rockmass is proved rational and suitable to simulate the joint zone for practical engineering application and of reference.

The aim of this article is to create a new methodology that is simpler, faster and cheaper than those that exist today to estimate the wear of the cutters of a TBM operating on hard rock, using a statistical development that has allowed to establish an empirical formula. They have been identified and assigned values to the different rock properties that mainly influence the wear of the discs, obtaining the CLH index, which estimates the wear of the cutting head cutters of a TBM, thus being able to predict the duration thereof. A thorough statistical study of the results has been performed to demonstrate the reliability of the method, as well as a quantification of possible errors that can be committed and levels of confidence., El objetivo de este artículo es crear una nueva metodología más sencilla, rápida y barata de las que existen en la actualidad para estimar el desgaste de los cortadores de una tuneladora operando en roca dura, utilizando para ello un desarrollo estadístico que ha permitido establecer una fórmula empírica. Se han identificado y asignando valores a las diferentes propiedades de las rocas que influyen principalmente en el desgaste de los discos, obteniendo el índice CLH, que estima el desgaste de los cortadores de la cabeza de corte de una tuneladora TBM, pudiendo así prever la duración de los mismos. Se ha realizado un profundo estudio estadístico de los resultados para demostrar la fiabilidad del método, así como una cuantificación de los posibles errores que se puedan cometer y niveles de confianza.

This paper analyses the stability of the mining blocks by the Monte Carlo method in Estonian oil shale mines, where the room-and-pillar mining system is used. The pillars are arranged in a singular grid. The oil shale bed is embedded at the depth of 40-75 m. The processes in overburden rocks and pillars have caused the subsidence of the ground surface. Visual Basic for Application was used for the modeling. Through Monte Carlo simulation, room-and-pillar stable parameters can be calculated. Model allows determination of the probability of spontaneous collapse of the pillars and surface subsidence by the parameters of registered collapsed mining blocks. Proposed method suits as an express-method for stability analysis and failure prognosis. It is applicable in different geological conditions, where the room-and-pillar mining system is used.

A probabilistic framework to perform inverse analysis of geotechnical problems is presented. The formulation allows the incorporation of existing prior information on the parameters in a consistent way. The method is based on the maximum likelihood approach that allows a straightforward introduction of the error structure of field measurements and prior information. The difficulty of ascribing definite values to the uncertainties associated with the various types of observations is overcome by including the corresponding variances in the set of parameters to be identified. The inverse analysis results in a minimization problem that is solved by coupling the optimization technique to the finite element method. Two examples are presented to illustrate the performance of the method. The first one corresponds to a synthetic case simulating the excavation of a tunnel. Young’s modulus, KO value and measurements variances are identified. The second case concerns the excavation of a large underground cavern in which again Young’s modulus and KO are identified. It is shown that introduction of prior information permits the estimation of parameters more consistent with all available informations that include not only monitored displacements but also results from in situ tests carried out during the site investigation stage.

The run-of-river hydro power plant of Lavey, situated in the Western part of Switzerland, is an important source of renewable energy for the city of Lausanne. With its annual production of roughly 400 GWh it covers approximately a third of its consumptions per year. Even given the relatively short distance of 4 km between its water intake and power plant, the total head of the hydroelectric development is still roughly 40 meters and with the design flow given at 200m3/s, the installed power is considerable. To increase the energy production of the power plant, an extension project has been proposed. It includes the construction of a second head race tunnel and the extension of the existing underground cavern for the installation of an additional turbine group. This would allow for a bigger volume of water to be used for electricity production, as well as optimize the operation of the plant by reducing energy losses and giving it better flexibility and a wider range of operation. The excavations of underground structures such as the large diameter head race tunnel, the vertical pressure shaft and the extension of the existing cavern, represent big challenges from an engineering point of view and the planning and design of such underground works require a pluri-disciplinal approach. This study focuses on the geotechnical aspects which need to be considered during the excavation of these underground cavities. Moreover, it outlines the challenges and constraints originating from the fact that the construction of the new power plant is to be done during normal operation of the existing scheme. These analyses allow to stress the importance of a sound planning process including the determination of the excavation methods to be used and the excavation sequences in a project of such magnitude and complexity.