Underground Quarrying for Marble: Stability Assessment through Modelling and Monitoring

This paper presents the approach for assessing the stability of underground quarries through the numerical modelling and the monitoring of the stress-strain state of the rock mass and of the support structures. On the basis of the practical experience and long term observation on different case histories, the key issues for a safe and profitable exploitation are given. The procedure is focused on the Candoglia cavern (named Cava Madre), a large mining void due to the exploitation of an high quality marble used for the Milan Cathedral (Duomo di Milano). The importance of the monitoring system for the back analysis of uncertain rock mass parameters and for the operation controls is discussed and emphasized, in order to provide practical design suggestion

Prediction of penetration per revolution in TBM tunneling as a function of intact rock and rock mass characteristics

A new empirical formulation is presented which can be used to estimate the penetration-per-revolution for TBM tunneling, derived from TBM monitoring data of alpine tunnels in the North-West of Italy. This formulation is easy to use and allows the contribution of both the intact rock and of the rock mass characteristics to be taken into account. The contribution of the intact rock is taken into consideration through the use of the uniaxial compression strength, while the influence of the rock mass is considered through the use of the GSI. A statistical interpretation procedure of numerous operative data from TBMs used for the excavation of tunnels in rock, and of the characterization of intact rock and of the rock mass, has been developed to determine the proposed formulation. In particular, the penetration-per-revolution (p) recorded during excavation, the forces applied to each disk (FN), the Geological Strength Index (GSI) and uniaxial compression strength of the intact rock (σc) along the stretch have been compared.The set-up formulation is simple to use and reliable for tunnels excavated in metamorphic rock, as it has successfully been compared with the TBM net advancement speed data of a well-known case history taken from the literature (the Maen tunnel in Italy). Comparisons with results obtained with the Norwegian School method (NTNU) and Barton[U+05F3]s calculation model have also led to positive results

A Numerical Approach for Evaluating the Convergence-Confinement Curve of a Rock Tunnel Considering Hoek-Brown Strength Criterion

The convergence-confinement method is frequently used for the analysis of the behavior of a tunnel and to define the support structures necessary to guarantee its stability. The reasons why it is used extensively, at least in the preliminary study phases of a tunnel are: The fundamental parameters of the problem (the diameter and depth of the tunnel, as well as the mechanical characteristics of the ground in which the tunnel is being excavated) can be considered in the calculation; the stresses and strains in the ground and the displacements of the tunnel wall can be obtained from the results of the calculation; the thickness of the plastic zone around the tunnel can be estimated by the method. The application of this method to rock masses, which are characterized by a non-linear strength criterion, has been limited to a restrictive number of works in the literature. In order to consider non-linear strength criteria, some simplifications are generally introduced in order to be able to obtain a simple expression of the stresses and strains in the rock mass and of the displacements of the tunnel wall. A numerical solution is presented in this study with which it is possible, in a simple way, to precisely describe the convergence-confinement curve of a tunnel excavated in rock masses, without the need of introducing any simplifying hypotheses for the description of the stress-strain relations in a plastic field

evaluation of the tunnel face stability through a ground stress analysis with a hemispherical geometry approximation

The evaluation of the stability of the excavation face is an important aspect in the design of a tunnel. When it is not possible to ensure excavation face stability in natural conditions, it is in fact necessary to intervene with remarkably costly reinforcement operations. The analysis of the stability conditions of an excavation face can be conducted, with a certain degree of detail, through numerical modelling. Simplified analytical models exist for shallow tunnels, but fewer are available for deep tunnels. One of the analytical methods most commonly used in the study of the stress conditions at the excavation face in deep tunnels is described in this study: The convergence-confinement method adapted to a spherical geometry. This method has here been extended to rock masses, which present a more complex rupture criterion (curved and not simply linear) than that of soils. The presented solution is of a finite difference numerical type. An extensive parametric analysis conducted on soils and rock masses has led to the estimation of the maximum lithostatic stress that still foresees the absence of a plastic zone around the hemisphere, which has been used to represent the excavation face. Therefore, this study makes it possible to obtain a preliminary estimation of the maximum depth of a tunnel in a certain type of soil or in a rock mass in which it is still possible to advance without the necessity of excavation face reinforcement operations. However, a more detailed and reliable analysis still requires more sophisticated instruments, such as numerical modelling

Productivity and working costs of modern trench-cutters for the construction of concrete diaphragms in an urban environment

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 effects of wear on the tools, it has been possible to make a preliminary estimation of the construction costs and the productive times of the concrete diaphragms for the different types of geolog

analysis of the interaction between the lining of a tbm tunnel and the ground using the convergence confinement method

The support used when a tunnel is excavated by a shielded TBM is complex because it consists of two different materials: A concrete segmental lining and a filler annulus between the lining and the tunnel wall. A detailed analysis of the behavior of such a support system requires a three-dimensional numerical modeling and should also consider the presence of the TBM machine. This article presents an analytical calculation procedure that allows to assess the interaction between the support system and the tunnel using the convergence-confinement method and Vlachopoulos-Diederichs method. Furthermore, it is defined the overall stiffness of the support system starting from the detailed analysis of the stresses and strains developed in the two constituent materials. From the analysis of the evolution of the radial displacements in the longitudinal section, it was then possible to evaluate another fundamental parameter: The radial displacement of the tunnel wall at the point of installation of the support system. The computational procedure was then applied to a specific case, showing the influence of the stiffness of the filling material on the final loads acting on the segmental lining

A review of the benefits of electronic detonators

Computerized drilling and the electronic timing of detonations are two technological breakthroughs which have had an important role in updating drilling and blasting excavation methods, although the electronic timing of detonators is still a comparatively infrequent technical solution to precision blasting problems. On the basis of an extensive collection of published cases, this paper reviews the successes achieved and the main expected advantages from the electronic ignition devices. After describing the primary characteristics of these detonators, some elements will be considered, in order to better understand their applications in different conditions, both in open pit and underground sites: extension of the time delay number, freedom in the choice of time intervals between detonations, timing accuracy, reduction of vibrations, control of back-break and fragmentation. The results are compared to those obtained by pyrotechnical timing devices, and summarized in the concluding remarks

The influence of the two-component grout on the behaviour of a segmental lining in tunnelling

Filling material is present around the segment lining when a shielded Tunnel Boring Machine is used to excavate a tunnel. The two-component grout is becoming lately one of the most used filling materials. Its mechanical properties evolve over time. Unfortunately, there are not many studies in the literature on the specific mechanical characteristics of these materials. This work presents the results obtained from an extensive laboratory test campaign that allowed to fully characterize the two-component filling material during the setting period. In particular, the values of the stiffness and resistance parameters were obtained over time, where uniaxial compression tests and oedometer tests were carried out. A detailed study of the effect of the presence of the filling material on the behavior of the support system (segmental lining + filling material) was developed for two of the most widespread analytical methods for the analysis of the behavior of tunnels and structures of support: the convergence-confinement method and the Einstein and Schwartz method. Subsequent parametric analyses made it possible to consider the variability of the influencing parameters within the typical variability ranges obtained from the laboratory test campaign or known from the available scientific literature. From the study carried out, it was possible to note that it is necessary to consider the presence of the filling material in the evaluation of the stiffness of the support system, when using the convergence-confinement method to estimate the loads acting on segmental lining. In this regard, it is necessary to have a reliable estimate of the elastic modulus of the filling material in the period of loading of the segmental lining. On the other hand, the presence of the ring of filling material is negligible when evaluating the state of stress of the segmental lining with specific methods capable of considering the rock-support interaction. In particular, adopting the Einstein and Schwartz method, it is possible to define the bending moments and normal forces acting in the support structure, referring to the stiffness parameters of the segmental lining alone

Analysis of predictor equations for determining the blast-induced vibration in rock blasting

The paper proposes a new empirical correlation designed to complement the "site laws" currently used to evaluate the attenuation in the rock masses of vibrations induced by rock blasting. The formula contains a deformed exponential known as the K-exponential, which seems to well represent a large number of both natural and artificial phenomena ranging from astrophysics to quantum mechanics, with some extension in the field of economics and finance. Experimental validation of the formula was performed via the analysis of vibration data covering a number of case studies, which differed in terms of both operation and rock type. A total of 12 experimental cases were analysed and the proposed formulation exhibited a good performance in 11 of them. In particular, the proposed law, which was built using blast test data, produced very good approximations of the points representing the vibration measurements and would thus be useful in organising production blasts. However, the developed formula was found to work less well when a correlation obtained for a given site was applied to another presenting similar types of rocks and operations, and thus should not be employed in the absence of measurements from test data. Keywords: Rock blasting, Vibrations, Predictor equation, Site law, K-statistics, K-exponentia

Problems concerning cutting tool performance during TBM work: modelisation and testing of the rock under the action of the tool

In rock excavation engineering, cutting performance prediction plays an important role in machine and cutting tool design. In the past, experimental and theoretical studies on the interaction between rock and TBM tools were conducted for the explanation of the intimate mechanism of rock failure and to obtain knowledge that was able to improve the performances of the TBMs. Nevertheless,, despite several decades of academic rock mechanics and practical rock engineering, there is still a lack of some fundamental knowledge on rock-tool interaction. The objective of this paper is to evaluate a model that predicts the distributed displacement and stresses under a TBM disk, taking into account, in the failure mode, the constitutive law of the rock, the presence of the tunnel head, the amount of the natural stress due to the high overburden, and the forces caused by the TBM disk on the tunnel head. Some laboratory tests were also planned to validate the theoretical results