Search

In spite of the increasing diffusion of tunnel boring machines, conventional tunnelling is still preferred for economic reasons in case of short tunnels, unconventional cross sections or irregular tunnel trajectories. In conventional tunnelling, the mechanical response of the tunnel front is a main concern and, when tunnels are excavated in cohesive soils, this is dominated by the time factor, related to geometry, to the mean excavation rate and to the hydro-mechanical properties of the materials involved. This is particularly evident during excavation standstill: front displacements progressively increase with time and, in many cases, the system response under long-term conditions becomes unstable. In conventional tunnelling, a common technique employed to improve the system response (under both short- and long-term conditions) is the installation of fibreglass tubes within the advance core. In this paper, the mechanical response of both unreinforced and reinforced deep tunnel fronts in cohesive soils is experimentally analysed. In particular, the results of a series of 1g small-scale tests, taking into account both the influence of the excavation rate (the unloading time) on the system response and the evolution with time of the tunnel face displacements, induced by a rapid reduction in the horizontal stress applied on the tunnel face, are reported.

In spite of the increasing diffusion of tunnel boring machines, conventional tunnelling is still largely employed in the excavation of both deep and shallow tunnels characterized by a particularly irregular tracing. Under difficult ground conditions, in conventional tunnelling, the front is frequently reinforced by using fibreglass tubes partially removed during the excavation. This technique is expensive, time-consuming and its design is based on either empirical or very simplified theoretical formulas. Thus, the ultimate objective of the research developed by the authors is to introduce a more sophisticated design approach for this front reinforcement technique. A first step in this direction is this numerical study, in which the mechanical response of deep tunnel faces under undrained conditions is analysed by employing the front characteristic curve: a useful tool largely employed in the literature in analogy with what done for the cavity. The main result of this paper is the “Front Mother Characteristic” curve, obtained by introducing appropriate non-dimensional variables, allowing the designer, once both the system geometry and the soil mechanical properties are assigned, to assess the displacements of tunnel fronts without performing any numerical analysis.

Piled foundations are largely employed as settlement reducers in the design of artificial embankments on soft soil strata. The commonly employed design methods are, however, based on simplified approaches not allowing the assessment of average and differential settlements at the top of the embankment. With this objective, the authors introduce a generalised constitutive relationship capable of accounting for irreversibilities stemming from both geometrical evolution and soil yielding. The model derives from the interpretation of the results of a campaign of three-dimensional elastic–plastic finite-difference numerical analyses describing the embankment construction process. Numerous simplifying assumptions (for instance smooth pile shaft, drained conditions) have been employed. Nevertheless, this does not compromise the theoretical value of the proposed approach. From a practical point of view, this model is a useful tool for geotechnical engineers to employ with a displacement-based design perspective.

Soft and highly porous rocks such as calcarenites are very common in all Mediterranean region. Due to their porous calcareous structure these rocks are prone to water induced weathering mechanisms. Natural onshore and inland underground cavities are evidence of such phenomena. The collapse of cliffs and underground cavities is usually the long-term result of a complex hydro-chemo mechanical process taking place at the micro-scale. Experimental results mainly give evidence of: (a) a marked and instantaneous reduction in strength and stiffness for these porous rocks when macro-pores are filled with water, (b) a slow successive reduction in strength and stiffness occurring in the long-term due to dissolution processes; (c) a more pronounced weakening of the rock material as a consequence of wetting and drying cycles. In the present work a methodological path to cope with deterministic assessment of the stability of natural and anthropic caves will be presented. The following steps will be adopted: (i) experimental study: execution of an experimental campaign to identify the physics of the processes taking place at both the micro-scale and the macro-scale; (ii) theoretical study: extend the concept of strain hardening-non mechanical softening to the time evolution of c-fi reduction; (iii) numerical study: present the 3D numerical results of a real case-study showing the capability of the proposed methodology to cope with risk assessment in complex geomechanical situations concerning weathering, as for underground cavities.

To model the temporal evolution of complex landslides, a 1D pseudo-dynamic visco-plastic approach, based on Perzyna’s theory, has been modified. In the original version of the model, the viscous nucleus has been assumed to be bi-linear: where irreversible deformations develop uniquely for positive yield function values whereas, in a more general case, it can also develop for negative values. In this work the model has been enriched by considering: (1) an exponential viscous nucleus, (2) a strain-rate softening to reduce friction angle as the sliding velocity increases and (3) block interaction forces to cope with complex 3D geometries for the sliding mass.

To model the evolution with time of the Bindo-Cortenova translational landslide (Italian Prealps, Lombardy, Italy), a 1D pseudo-dynamic visco-plastic approach, based on Perzyna's theory, has been conceived. The viscous nucleus has been assumed to be bilinear: in one case, irreversible deformations develop uniquely for positive yield function values; in a more general case, visco-plastic deformations develop even for negative values. The model has been calibrated and subsequently validated on a long temporal series of monitoring data and it seems quite reliable in simulating the in situ data. © Springer-Verlag Berlin Heidelberg 2013.

After the abandonment occurred in the '70s, the mining system (rooms and pillars) located in S. Lazzaro di Savena (BO, Italy), grown on three levels with the method rooms and pillars, has been progressively more and more affected by degradation processes due to water infiltration. The mine is located underneath a residential area causing significant concern to the local municipality. On the basis of in situ surveys, laboratory and in situ geomechanical tests, some critical scenarios were adopted in the analyses to simulate the progressive collapse of pillars and of roofs in the most critical sectors of the mine. A first set of numerical analyses using 3D geotechnical FEM codes were performed to predict the extension of the subsidence area and its interaction with buildings. Secondly 3D CFD analyses were used to evaluated the amount of water that could be eventually ejected outside the mine and eventually flooding the downstream village. The predicted extension of the subsidence area together with the predicted amount of the ejected water have been used to design possible remedial measurements.