1,720,986 research outputs found
Mechanised deep tunnel excavation in saturated clayey soils: a pre-design hydro-mechanically coupled method for the assessment of both spoil and face volume loss
No full textMechanized tunnel excavation in soils causes over-excavations, potentially leading to large amounts of spoil and settlements at ground level. An accurate estimation of over-excavations is crucial in the pre-design phase for assessing costs, determining the appropriate excavation method and choosing the muck management strategy. Currently, the estimation is based on experience and data from similar projects, but it becomes difficult when project conditions are heterogeneous. Alternatively, finite element analyses are time-consuming and not suitable for early design stages and, therefore, simplified tools are needed. In this paper, the authors present a simplified approach putting in relation face extrusion with estimated spoil mass and face volume loss. This approach, conceived for deep tunnels, is the extension to the case of mechanized tunnelling of a hydro-mechanical coupled meta-model derived from finite element numerical analyses for tunnels in clayey soils excavated by using conventional techniques (i.e. without any use of tunnel boring machines). The model has been validated against field data relative to a case study. The approach can be used in the early design process to identify tunnel boring machine characteristics and provide preliminary cost estimates. Additionally, during the construction phase, the method can be employed to interpret monitoring data and pre-design mitigation measures for unforeseen soil profile variations
Mechanical Behavior and Constitutive Modeling of Cement–Bentonite Mixtures for Cutoff Walls
Full text linkCement–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 low permeability and adequate shear strength and ductility. In this paper, experimental results about the microstructure and mechanical behavior of three different cement–bentonite mixtures are presented. Specimens of these mixtures were subjected to oedometer and consolidated-undrained triaxial tests. These results were then used as a basis for the definition of a suitable constitutive framework. A quite good reproduction of the experimental results up to the peak strength was obtained
using the classical Modified Cam Clay model, which could then be used satisfactorily when conventional analyses aimed at assessing the stability of cutoff walls are required. The reproduction of the strength degradation and the strains occurring in the postpeak stage requires, however, a more advanced constitutive model. To this extent, the Modified Cam Clay framework was enhanced by introducing some features commonly employed to reproduce the mechanical behavior of granular materials. This model may be useful for the real-scale analysis of more critical cases when local failure mechanisms are likely to occur and may influence the functionality of the wall
Simulazione numerica del processo di consolidazione durante lo scavo di gallerie profonde in materiali coesivi e influenza del tempo di scavo sulla curva caratteristica del fronte
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Mathematical modelling of the mechanical response of earth embankments on piled foundations
No full textPiled 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
An innovative design approach for anchored wire meshes
Full text linkAnchored wire meshes are commonly adopted to stabilize potentially unstable soil slopes. This reinforcement technique, employed either as an active or a passive anchoring system, is commonly designed according to ultimate limit state approaches. In this paper, an interaction model, useful for the design of anchored wire meshes, is proposed. The model is based on the results of a series of 3D large displacement finite element numerical analyses, in which the wire mesh mechanical behaviour is modelled as either an elastic or an elastic–plastic membrane. The model is inspired to standard load–displacement curves for shallow foundations, and the wire mesh presence is taken into account by suitably modifying the bearing capacity formula. The proposed model predictions are compared with experimental punching test results. The use of the model, only requiring the definition of geometry and soil–wire mesh mechanical properties, allows the pre-design of the reinforcement system without performing ad hoc finite element numerical analyses.Geo-engineerin
Mathematical modelling of the mechanical response of geosynthetic-reinforced and pile-supported embankments
Full text linkPiled foundations are commonly employed to reduce settlements in artificial earth embankments founded on soft soil strata. To limit the number of piles and, consequently, construction costs, popular is the use of geosynthetic reinforcements laid at the embankment base. Nowadays, the complex interaction between geosynthetics, piles and soil is not yet fully understood and, in the scientific literature, simplified displacement-based approaches to choose reinforcements, pile diameter and spacing are missing. In this paper, the authors, starting from the critical analysis and theoretical interpretation of finite difference numerical results, introduce a new mathematical model to rapidly assess both (i) differential/average settlements at the top of the embankment and (ii) maximum tensile forces in the basal reinforcement. The model, conceived to reproduce the response of a pile belonging to the central part of the embankment, is the result of an upscaling procedure based on a suitable sub-structuring of the spatial domain (an axisymmetric unit cell) and on the concept of plane of equal settlements. For the foundation soil, drained conditions are considered, the pile skin roughness is disregarded, and piles are assumed to get the rigid bedrock. As generalised kinematic variables average and differential settlements are employed, whereas as generalized static ones the embankment height and the geosynthetic axial force. The model is validated against field measurements (where layered foundation soil and pile caps are included) and an application example of the model, used as a preliminary design tool in a displacement-based perspective, is finally provided
Mathematical model for piled embankments on saturated soft clay
Full text linkPiled embankments are traditionally designed by using either guidelines based on simplified limit-equilibrium theories or advanced finite-element (FE) numerical analyses. Both methods have limitations: the former do not allow the assessment of settlements at the top of the embankment, whereas the latter easily become overly complex, hence limiting practical applications. This paper introduces a new mathematical model capable of reproducing, with minimal computational effort, the mechanical response of piled embankments modelled by means of FEs. The model is based on a set of fundamental principles, assumptions and phenomenological equations obtained from a deep understanding of the mechanics behind the FE problem. The model, evaluating average and differential settlements at the top of the embankment during the consolidation of the soft soil, is validated against full-scale test data and benchmarked against independent numerical results. The results are compared with existing formulas to evaluate the critical height of the embankment, demonstrating the great potential of the new model for engineering practice (giving nearly instantaneous displacement-based solutions for the design of piled embankments in a preliminary stage)
A macroelement framework for shallow foundations including changes in configuration
Full text linkMacroelement 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.Geo-engineerin
A numerical exercise for the definition under undrained conditions of the deep tunnel front characteristic curve
No full textIn 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 âFront Mother Characteristicâ 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
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