1,721,082 research outputs found
Face stabilization of deep tunnels using longitudinal fibreglass dowels
No full textWhen the stability of an excavation face in a deep tunnel is not ensured by the strength characteristics of the rock mass, it is current practice to proceed with reinforcement of the face using longitudinal fibreglass dowels. An analysis of the effect of these reinforcements and a correct dimensioning can be obtained through three-dimensional numerical modelling, which is very burdensome from a computational point of view. The simplified methods that are at present available do not allow the maximum force in the dowels to be obtained and cannot therefore be used for their dimensioning.A finite difference procedure for deep tunnels is presented in this paper. This procedure is able to analyse the presence of dowels in the rock mass in a distinct manner, and it adopts the conceptual schematisation of a spherical geometry void for the approximation of the tunnel face zone. A calculation comparison of the simplified procedure and the three-dimensional numerical modelling has offered satisfactory results. The procedure has also been applied to a real case and the results agree with in situ measurement
The characteristics of the two-component grout on the stress-state in segmental lining
Full text linkIn this work, the effective influence of the mechanical characteristics of the filling material on the safety factors of the support system is analyzed. Through an extensive parametric analysis, developed by adopting proven analytical methods, on 243 different cases of tunnels excavated using a TBM in a soil mass, at different depths and with different excavation radii, it was possible to identify the conditions in which the safety factors can be effectively low. In all these cases, therefore, it is necessary to intervene on the mechanical characteristics of the filling material, requiring elastic modules and strengths such as to guarantee higher values of the safety factors, avoiding risks on the possible failure of the concrete that makes up the segmental lining and of the same filling material that connects the support system to the tunnel wall
A probabilistic approach for the evaluation of the stabilizing forces of fully grouted bolts
Full text linkThe essential task of the ground reinforcement techniques is to keep the rock as stable as possible. In particular passive rock bolt should resist the rock movements along its entire length and through the resulting reaction forces, to improve the load-bearing capacity of the rock. Among different calculation techniques, the calculations based on Block Reinforcement Procedure (BRP) was used in this paper, also adopting some simplified equations available in the scientific literature. However, parameters influencing the interaction are difficult to evaluate. Therefore, the problem of the reliable definition of the parameters that most influence the behavior of the bolts and the evaluation of the stabilizing forces of the potentially unstable block of rock remains. A new probabilistic approach is presented in this article, able to appropriately manage the uncertainties present on the fundamental parameters of the bolt-rock interaction and on the mechanical characteristics of the sliding surface of the block. Through the use of a Monte Carlo procedure, in fact, it was possible to obtain different samples of the safety factors of the rock block, one for each diameter of the steel bars used for its stabilization. Finally, the probabilistic management of the safety factor samples allowed the correct design of the steel bars, by evaluating the probability that the safety factor of the block with regard to potential slipping has a value lower than a pre-established limit. The probabilistic approach developed was applied to a real problem of stabilization of a potentially unstable rock block due to planar sliding, present on a municipal road in North Italy
Innovazione nella modellazione numerica di scavi sotterranei civili: il caso della galleria del Brennero
Full text linkTwo-dimensional numerical analysis on the rock/bolt interaction considering shear and normal relative displacements
Full text linkFully-grouted passive bolts are widely used in underground or surface rock excavations and in particular in stabilizing potentially unstable blocks of rock due to sliding on natural discontinuities. Their operating mechanism is complex, but it is possible to consider two stabilizing forces that each bolt applies to the block of rock. These forces depend on the mechanical parameters governing the bolt-rock interaction, which are difficult to evaluate. In this work, specific numerical analyses have been developed, able of evaluating the bolt-rock interaction (in shear and perpendicular to the interface that separates them) for numerous cases that were obtained by varying the main geometric parameters of the bolt, the mechanical properties of the binder material and rock. Thanks to this complex study, it was possible to describe the variability of the interaction parameters and to define, through graphs, the trend of the stabilization forces as the main geometric and mechanical parameters that can be encountered in practice change. The graphs obtained are a useful tool for the correct design of fully-grouted passive bolts and the stabilization of potentially unstable rock blocks on the walls of underground cavities or on the faces of surface excavations
A probabilistic approach to evaluate the risk due to a fire in unidirectional road tunnels ventilated by jet fans
Full text linkThe risk produced by a fire is one of the most important aspects to be analysed for the safety of a road tunnel. In one-way ventilated tunnels with jet-fans it may happen that during a fire the fumes move in the opposite direction to that of the fresh air entering the tunnel. This phenomenon (back-layering) can involve people fleeing towards the mouth of the mountain with dramatic consequences. Since many parameters that characterize the phenomenon are known only with a certain approximation, it is necessary to adopt a probabilistic approach. In this work this approach is illustrated, applying it to a road tunnel in Northern Italy. The probability that the fumes of the fire can reach people fleeing has been plotted as a function of the total number of jet-fans in the tunnel, so that the definition of the ventilation system can be carried out in the design phase in relation to safety during the emergency phase
Statistical Interpretation of Jet Grouting Field Data Regarding Its Strength and Stiffness
Full text linkJet-grouting technique is one of the most widespread methods for soil reinforcement, used to make soil suitable to withstand forces transmitted by structures or the redistribution of stresses following excavations. The result of the treatment however is influenced by several factors, including type of treatment, type of soil, original stress state of the soil, machine operating parameters, and water-to-cement ratio. In particular, the strength of the treated material and its stiffness are very useful for designing the intervention for a specific case. Due to the uncertainty of the final strength and stiffness values, a test field is almost always required. But the design of the test field itself and the subsequent design of the intervention require some preliminary information on the expected values of the strength and the elastic modulus. An in-depth analysis of the scientific and technical literature was conducted, allowing for the selection of a significant number of cases. For each case, the type of treatment, the type of soil, uniaxial compressive strength (UCS), and elastic modulus (E) values were recorded. It was possible to determine the trend of the average UCS values and the E/UCS ratio for each type of analyzed soil and for the two studied jet-grouting techniques. In addition to the average values, the variability intervals, centered on the mean, capable of enclosing 90% of the data in the sample were identified. The obtained graphs represent a useful tool for obtaining a preliminary estimate of the strength and stiffness of the treated soil using the jet-grouting technique
Probabilistic estimation of the advancement rate of the Tunnel Boring Machines on the basis of rock mass characteristics
No full textAbstract: The prediction of the advancement rate of a Tunnel Boring Machine (TBM) is one of the most important aspects of the design phase of a rock tunnel. Many authors have deepened the argument, coming to estimate this value as a function of some characteristics of the rock and of the excavation machine. Among the many developed systems, one of the most complete and effective is the one proposed by Norwegian University of Science and Technology (NTNU). Unfortunately, the complexity of the phenomenon studied requires evaluating many characteristic parameters of the rock mass in some cases not known or known only with poor precision. In this work a new probabilistic procedure is developed, based on the study of the NTNU, able to estimate the variability interval of the advancement of the TBM per revolution of the TBM head i, with the associated reliability, referring to the most common characteristic parameters of the rock mass: number of discontinuity sets, type of discontinuities, average spacing between discontinuities, uniaxial compressive strength of the intact rock (UCS). Some abacuses have been pre-arranged in order to facilitate the evaluation for all the practical cases that can be encountered in the excavation of rock tunnels. An application to the well-known case of the Tunnel of Varzo (North-West of Italy) made it possible to compare the results of the probabilistic study with the net rate actually measured on the TBM. Article highlights: The NTNU system is one of the most important prediction model for advancement rate for TBM.A new probabilistic procedure is developed, based on the NTNU model.The variability interval of i is estimated considering some characteristic parameters of the rock mass
A simplified mathematical approach for the evaluation of the stabilizing forces applied by a passive cemented bolt to a sliding rock block
Full text linkPassive bolting is used to stabilise unstable rock blocks in surface and underground structures due to the various advantages it offers. Despite its use, the design phase still presents aspects of considerable complexity because the fact that the load of the bolt and therefore, its static action depends on its interaction with the block and the stable rock. In the present work, a mathematical model was developed which is capable of directly calculating the stabilisation forces as a function of the characteristic parameters of the bolt and of its interaction with the rock. This discussion is based on a simplified hypothesis of bolt behaviour, which provides negligible errors, and on the observation that the critical point is positioned at the intersection of the bolt with one of the lateral surfaces that separate it from the portion of stable rock. The formulation of the stabilisation forces obtained made it possible to evaluate the static contribution of each single bolt to the stability of the rock block, by varying the diameter of the steel bar and then designing the bolting operation to achieve acceptable stability conditions for the rock block. The application of stabilising equations to a real case, for which the results of load tests on bolt tests were available, allowed us to outline steps to be taken in the bolt design process
Statistical and mathematical preliminary interpretation of mechanical test results on sands grouted with colloidal silica
No full textColloidal silica is a relatively new grouting low-carbon material used in soil improvement projects. A series of mechanical tests on a sand with different solid contents of the binder, i.e. 40, 30 and 20% solid content have been performed. Unconfined compressive strength at 7, 28 and 56 days, shear tests and permeability tests at 7 days were assessed. A statistical interpretation of these data has been performed and values were interpreted. A mathematical prediction model was suggested to estimate the mechanical strength of the grouted sand as the dilution grade varies. Furthermore, some considerations useful for geotechnical design on the evolution over time of the mechanical characteristics of the grouted sands have been carried out. Results show, from a statistical and mathematical view, that colloidal silica is a promising binder for ground improvement applications
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