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Piled foundations are commonly employed to reduce settlements of artificial earth embankments on soft soil strata and geosynthetic reinforcements are installed at the embankment base to increase pile spacing and reduce construction costs. Despite the well-documented effectiveness of this technique, the mechanical processes, developing during the construction in the different elements constituting the ‘geo-structure’, are not fully understood and the design approaches are based on very simplified assumptions. They disregard the deformability of the various elements constituting the system and cannot be employed to estimate settlements. With the aim of introducing a displacement-based design approach to optimise the use of reinforcements and piles, in this article, the mechanical response of the system during the embankment construction is studied by means of large displacement non-linear finite difference numerical analyses, in which the geosynthetic reinforcement is modelled as an elastic membrane. The arching effect developing within the embankment body is described and the evolution of the process zone, where shear strains localise, is discussed. The global system response is described in terms of (i) average, (ii) differential settlements at the top of the embankment and (iii) maximum tensile force within the geosynthetic reinforcement.

Cement-bentonite cut-off walls are commonly employed in geoenvironmental applications to limit ground water flow and pollutant transport. The wide diffusion of this artificial material in the current practice is not only due to its low permeability, but also to its simplicity of use. In this paper, experimental evidences about the role of curing on the hydro-mechanical behaviour of cement-bentonite mixtures are presented. Different curing times and curing conditions (representative for either water saturated or hydrocarbon polluted soils) have been considered, and their effects on both hydraulic conductivity and mechanical response in oedometer and triaxial conditions have been assessed. A unified hydro-mechanical framework, accounting for the changes of material fabric occurring with curing time and environment, is formulated. The hydraulic conductivity is very well predicted by a Kozeny-Carman like equation, whereas the mechanical behaviour is finely reproduced via an enhanced elastic–plastic constitutive model.

In the geotechnical community, the macroelement approach nowadays is largely considered to be a successful theoretical tool for solving soil–structure interaction problems. This approach is based on the definition of a generalised constitutive law putting in relation a small number of suitably defined generalised stress/strain variables. The macroelement formulations proposed in the literature take into consideration either drained or undrained cases, but disregard the hydro-mechanical coupling. In this paper, the authors intend to generalise the theory by introducing a new formulation for shallow foundations overpassing this limitation and capable of accounting for the influence of loading rate on the system response. To conceive and to calibrate the model, the authors numerically analysed the case of a shallow foundation positioned on a normally consolidated clayey soil stratum whose mechanical behaviour is reproduced by means of the modified Cam Clay model. Finally, the approach is discussed critically from the perspective of its use as a design tool according to approaches based on the ultimate limit state and displacement.

Tunnel faces excavated under particularly difficult ground conditions are commonly supported. In case of conventional tunnelling, this support is provided by either improving the mechanical propert...

Anchored wire meshes are commonly adopted to stabilise potentially unstable slopes in granular soils. In most of the cases, this reinforcement technique is employed either as an active or a passive anchoring system. In both cases, the definition of a characteristic curve putting in relation the stabilizing force with the displacement of the soil in the “far field” is crucial. To optimize the employment of these reinforcement techniques, the authors performed a series of 3D FEM numerical analyses aimed at highlighting the interaction mechanisms developing between soil and stabilizing system. To correctly reproduce the mechanical response of the wire mesh, this latter is modelled as a membrane and the numerical analyses are performed by employing a large displacement approach. The soil mechanical behaviour is modelled by means of an elastic perfectly plastic constitutive relationship: the failure condition is described by means of the Mohr Coulomb criterion and the flow rule is assumed to be non-associated.

The macroelement approach, commonly employed to study soil-structure interaction problems, stems from the intent of describing the mechanical response of a complex system by means of a single upscaled constitutive relationship, relating generalized stresses and strains.

Cement–bentonite mixtures are commonly used to build cutoff walls, which limit water flow and underground transport of pollutants. These artificial materials are employed due to their very ...

In the risk assessment of soil movement, the comprehension of unexpected collapses involving creeping landslides is crucial. The transition from creeping to fast landslides is very hard to justify theoretically and simulate numerically. In this paper, the authors intend to demonstrate that even the integration of a basic elastic strain softening constitutive relationship under simple shear conditions is sufficient to capture such a transition. With this aim, an infinite slope, a homogeneous cemented/bonded soil and an oscillating water table level are considered. The onset of instability is shown to be anticipated by a local increase in strain rate within the shear band, behaving like a precursory signal. The size of the shear band, evolving with time and not depending only on the material grain size distribution, is governed by both the amplitude of the imposed perturbation and the material constitutive parameters.

In the design of earth embankments on soft soil strata, piles are often employed as settlements reducers. Despite of their well-documented effectiveness, the complex interaction processes transferring loads to piles within both the embankment and the soft foundation soil, are not yet totally understood. As it is well known, the effectiveness of this intervention is strictly related to the construction rate. In fact, the consolidation process taking place in the foundation soil induces the accumulation of settlements with time. The settlements increment at the embankment base, induces stresses migration towards the pile.

Deep foundations are commonly employed as settlement reducers for earth embankments on soft soil layers. Owing to the presence of piles, both stresses transmitted to the foundation soil and average settlements reduce. Since the piles may be interpreted as a vertical heterogeneity for the system, during the embankment construction and the soil consolidation, differential settlements accumulate at both the base and the top of the embankment. This complex interaction mechanism is severely influenced by the embankment height and by the relative stiffness of the various elements constituting this peculiar geostructure. Nevertheless, the approaches commonly adopted in the current engineering practice and the ones suggested by design codes do not explicitly consider the material deformability and do not allow the embankment settlement estimation.

In recent years, the employment of deep foundations as settlement reducers has become increasingly popular in the design of earth embankments over soft soil strata. To further improve the system response, geosynthetic layers are often positioned at the embankment base. Owing to the presence of both piles and geosynthetics, complex interaction mechanisms, transferring stresses towards the piles and reducing those on the soft foundation soil, take place. Although these mechanisms are governed by the relative stiffness of the various elements constituting this system (piles, foundation soil, embankment and georeinforcements), the approaches commonly adopted to design these “geostructures” do not explicitly take into consideration the stiffness of the single element as design parameters. Moreover, the effect of the stepwise embankment construction process is often disregarded. As a consequence, the settlements at the top of the embankment cannot quantitatively be estimated.

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.

Anchored wire meshes, commonly adopted to stabilise potentially unstable slopes in granular soils, are composed of wire meshes, geosynthetics, anchorage bars/ties and spike steel plates. In many cases, this reinforcement system is employed as a “passive” reinforcement, i.e. it transfers loads to the unstable domain only when this develops irreversible displacements. During this displacement accumulation, complex interaction mechanisms between the deformable reinforcement system and the underlying material take place. To study these interaction mechanisms, a series of large displacement 3D FEM numerical analyses is performed. The mechanical behaviour of the material constituting the wire mesh and the soil is reproduced by means of elastic-perfectly plastic constitutive relationships. In particular, for the mesh a von Mises failure condition and an associated flow rule are employed, whereas for the soil a Mohr-Coulomb criterion and a non-associated flow rule. The constitutive parameters are calibrated on a series of experimental punching test results. The numerical results are reported by means of the characteristic curve, relating the penetration of the steel plate to the system reaction force. In order to stress the role of (i) both soil and wire mechanical behaviour and (ii) the spike plate dimensions in influencing the mechanical response of the whole system, the results of a parametric study are finally discussed.

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.

The fronts of tunnels excavated under particularly difficult ground conditions by employing conventional tunnelling methods are commonly supported: the stabilization is usually achieved either by improving the mechanical properties of the soil (injections, jet grouting, soil freezing, etc.) or by introducing linear inclusions. This last technique, consisting in the introduction of pipes (usually made of fibreglass reinforced polymers) in the front, is particularly popular since it is very simple to adapt the reinforcement geometry, length and number to the different conditions encountered during the excavation. The design of this reinforcement technique is nowadays based on very simplified approaches: on either empirical formula or the limit equilibrium method. In a previous paper, the authors numerically studied the mechanical response of unreinforced fronts in cohesive soils and defined a non-dimensional front characteristic curve. In this paper, the authors intend to take into consideration the role of reinforcements by following the same approach. A procedure allowing the definition of the reinforced non-dimensional front characteristic curve, once the reinforcement pattern is assigned, is introduced. The practical use of this curve is described.

In conventional tunnelling, under difficult ground conditions, the fronts are commonly stabilized by employing expensive and time-consuming soil reinforcement techniques. The design of these soil reinforcement techniques is commonly based on the analysis of unreinforced front mechanical response.

Macroelement plasticity models are being increasingly applied to study non-linear soil–foundation interaction (SFI) problems. Macroelement models are particularly appealing from a computational standpoint, as they can capture the essence of SFI by means of a few degrees of freedom. However, all the macroelement formulations available in the literature suffer from the same limitation, that is the incapability of accounting for changes in both geometry and loading/boundary conditions. Accordingly, macroelement models are usually calibrated to analyse a given boundary value problem, with no chance of handling situations with significant variations in embedment, lateral surcharge and/or phreatic level. The present work shows how standard soil modelling concepts can be exploited to reproduce relevant ‘configurational features’ of non-linear SFI. A macroelement framework is here proposed to simulate the drained load–settlement response of shallow footings on sand in the presence of varying surface/body forces. As a first step, the ideal case of a weightless soil layer is exclusively considered. The macroelement constitutive equations are conceived/calibrated on a minimal set of finite-element results; the satisfactory predictive capabilities of the macroelement model are finally demonstrated by retrospectively simulating selected finite-element tests.

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.