1,720,994 research outputs found
Geosynthetic-reinforced and pile-supported embankments: theoretical discussion of finite difference numerical analyses results
Full text linkPiled 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.</p
A New Strategy for Mitigating Pipeline Uplift in Liquefied Soils
No full textSeismic-induced liquefaction is one of the main hazards for pipelines buried in saturated granular materials. When soil is partially or completely fluidized, a lifeline, although installed in superficial trenches in which the coarse backfill soil usually is compacted, may experience a sudden uplift and damages. To reduce pipeline uplift and thus limit the associated risks, the authors propose a sustainable and original mitigation strategy, suitable for both existing and new lifelines, based on both the use of a geomembrane and the compaction of the soil surrounding the pipeline. According to the design method proposed, the intervention geometry is selected on the basis of the pipeline maximum admissible displacement, whereas the minimum required relative density can be designed, based on the site-specific seismic demand, to avoid cyclically induced local accumulation in excess pore-water pressure. To prove the effectiveness of this strategy, a series of 1-g small-scale laboratory tests was performed on a pipe buried in a fluidized sand layer. A simplified displacement-based design approach, which was validated against the experimental data, is proposed
Numerical analysis of the mechanical interaction among adjacent foundations: towards a simplified design approach
No full textThe increasing demand of sustainability of the construction of new structures and the necessity of verifying/retrofitting the existing ones require reliable design tools. To this aim, fundamental is gathering new insights in the mechanical response of foundations. As far as shallow foundations are concerned, one aspect commonly disregarded in the current engineering practice is the influence of the mechanical interaction among adjacent foundations. The authors approach this problem by performing a parametric finite element non-linear numerical study on a squared shallow foundation, loaded by a vertical centred load under fully drained conditions. The numerical results put in evidence that the interaction, for any spacing value, causes an initial increase in settlements, whereas the dependence of bearing capacity on spacing seems to be, according to the case, either beneficial or detrimental. For particular geometries and mechanical properties, the reduction in bearing capacity may reach about the 30%. These dependences are justified in the light of the findings at the local scale, i.e. discussing the spatial distributions of irreversible strain, displacement and stress fields as the applied load increases. Finally, the authors have proposed a simplified design approach to derive not only the bearing capacity, but also a preliminary prediction in terms of displacements accounting for the reciprocal foundation interaction
Effects of curing on the hydro-mechanical behaviour of cement-bentonite mixtures for cut-off walls
Full text linkCement-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
Deep tunnel fronts in cohesive soils under undrained conditions: a displacement-based approach for the design of fibreglass reinforcements
No full textThe 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
A simplified approach to estimate settlements of earth embankments on piled foundations: the role of pile shaft roughness
No full textThe use of piles as settlement reducers in the design of artificial embankments on soft soil strata is nowadays very common. The design methods employed in the current engineering practice are based on simplified approaches not allowing the assessment of average and differential settlements at the top of the embankment. In this paper, a model to estimate both differential and average displacements at the top of the embankment is introduced. This, based on the choice of substructuring the spatial domain and employing a suitably conceived upscaling procedure, is an extension to the case of rough pile shafts of a model originally conceived by the authors for smooth piles. To conceive and calibrate the model, the authors performed a series of numerical simulations mainly aimed at highlighting the mechanical processes taking place at the pile shaft. From a practical point of view, this model can fruitfully be employed in displacement based design approaches and to optimize pile diameter and spacing
Basal reinforced earth embankments on piled foundations: The role of embankment construction process
Full text linkThe current design methods for Geosynthetic-Reinforced and Pile-Supported embankments disregard on one side the effect of the embankment construction and on the other one the stiffness of embankment, foundation soil, column and geosynthetics. What is missing nowadays is a simplified design method capable of taking all these aspects into account. To this aim, in this paper the authors present the results of a series of numerical analyses simulating the embankment construction. In particular, the evolution during construction of embankment displacements is discussed and the maximum tensile force in the geosynthetic reinforcements is compared with the one suggested by the most popular standards. To clearly highlight the mechanical processes taking place in the embankment, an ideal problem is considered: the pile shaft is assumed to be smooth, the piles to be founded on a rigid bedrock and the embankment construction to take place under drained conditions
Influence of the excavation rate on the mechanical response of deep tunnel fronts in cohesive soils
No full textIn conventional tunnelling, the mechanical response of the tunnel front is a main concern and depends on both the geometry (tunnel diameter and cover) and soil mechanical properties. Moreover, in case of excavations in soils characterized by a low value of permeability, even the time factor plays an important role: in case of particularly problematic soils, the displacements of the front are expected to progressively increase with time and tunnel fronts stable under short term conditions potentially either develop unacceptable displacements or become unstable under long term conditions. In this paper, the mechanical response of deep tunnel fronts excavated in a homogeneous cohesive soil stratum are analysed by both performing experimental 1g small scale model tests and non-linear 3D FEM analyses. The numerical results are obtained by assuming the material to be isotropic, homogeneous and characterized by an elastic-perfectly plastic constitutive relationship. The results are presented in terms of the tunnel front characteristic curve, defined in analogy with the well-known characteristic curve for the tunnel cavity and by employing a suitable non-dimensional variable depending on the excavation time, the soil hydraulic/mechanical properties and the tunnel geometry. Finally, the authors introduce a rapid procedure allowing the front displacements estimation without performing ad hoc numerical analyses
Influence of the fibreglass reinforcement stiffness on the mechanical response of deep tunnel fronts in cohesive soils under undrained conditions
No full textThe fronts of tunnels excavated under particularly difficult ground conditions are commonly reinforced by inserting fibreglass pipes. This technique is particularly popular since it is very simple to adapt the reinforcement number/length according to the nature of soils encountered. In this paper, the authors illustrate the results of a 3D FEM numerical campaign aimed at analysing the influence of the reinforcements on the system response, con-veniently summarized by employing a suitably normalized front characteristic curve. The numerical analyses were performed by assuming the material to be isotropic, homogeneous and characterized by an elastic-perfectly plastic constitutive relationship. Only undrained conditions are taken into account: the failure locus is defined according to the Tresca criterion and the flow rule is assumed to be associated. The numerical results show that the effectiveness of the inclusions is not related to the absolute value of the inclusion stiffness but to the value of a suitably defined non-dimensional variable. By employing this non-dimensional variable the authors show that it is possible to tailor the reinforcement stiffness according to the nature of the soil encountered
- …
