Ponències/Comunicacions de congressos
http://hdl.handle.net/2117/3579
2015-08-01T05:53:58ZEPB tunnelling in mixed geological formations: a case study
http://hdl.handle.net/2117/27860
EPB tunnelling in mixed geological formations: a case study
Di Mariano, Alessandra; Gens Solé, Antonio
Tunnels for the new Barcelona Metro Line 9 are being constructed in a densely built urban area, close to sensitive structures. The paper presents a case history involving tunnelling in difficult mixed geological conditions belowan existing Metro Line (Line 4).Tunnelling was performed with a dual EPB boring machine and the closed mode of operation was prescribed beneath all sensitive structures. Despite this original prescription, an open mode excavation (with a partially filled pressure chamber) had to be adopted to avoid the interruption of the boring process in the area below the existing Metro Line 4. The paper describes the geological conditions of the site, presents some of the ground movement observations prior to the change of excavation mode and discusses the rationale that allowed the change in the prescribed tunnelling mode as well as the results obtained. Observations of displacements during the excavation in open mode confirmed that no soil inrush into the head chamber occurred and that in addition, with the new excavation procedure, ground movements were reduced significantly.
2014-01-01T00:00:00ZTHM analysis of pellet-based materials for engineered barriers for nuclear waste
http://hdl.handle.net/2117/27443
THM analysis of pellet-based materials for engineered barriers for nuclear waste
Gens Solé, Antonio; Barboza De Vasconcelos, Ramon; Villar Galicia, María Victoria
A promising alternative to form engineered barriers for high level nuclear waste repositories is the use of pellets-based material because high dry densities can be achieved with no or minimal compaction effort. It is also planned to subject these materials to temperatures significantly higher than 100ºC. The paper reports the results of coupled THM analyses of a heating-hydration test performed on a granular assembly of MX-80 bentonite pellets. The goal is to characterize the material under conditions akin to those of an engineered barrier. It is shown that the numerical analyses are able to reproduce satisfactorily the observations of the test. This enhances the confidence placed on the computational tool for predicting the behaviour of actual engineered barriers constructed with pellets-based
materials.
2014-01-01T00:00:00ZImportance of sample volume changes and material degradation on gas transport properties in Boom Clay
http://hdl.handle.net/2117/27439
Importance of sample volume changes and material degradation on gas transport properties in Boom Clay
González Blanco, Laura; Romero Morales, Enrique Edgar; Li, Xiang Ling; Sillen, X.
An experimental study on Boom Clay –one of thepotential deep clay formationsin Belgium for the geological disposal of long-living and heat-emitting radioactive waste–has been performed using controlled volume-rate gas injection tests on high-pressure oedometer cells. These experiments intend to analyse the influence of the gas injection rate, the stress state and deformation history, the anisotropic flow conditions (flows parallel and orthogonal to bedding) on the gas migration process. Results of selected tests on two sample orientations are presented, paying particular attention to sample volume changes on gas injection and dissipation stages. The gas pressurisation process at constant vertical stress acts as an unloading stage, which may induce expansion and degradation on the clay that have important consequences on gas migration properties.
2014-01-01T00:00:00ZA new efficient strategy to simulate particle breakage in DEM analyses
http://hdl.handle.net/2117/27434
A new efficient strategy to simulate particle breakage in DEM analyses
Ciantia, Matteo Oryem; Arroyo Alvarez de Toledo, Marcos; Gens Solé, Antonio; Calvetti, Francesco
The discrete element method (DEM) is increasingly gaining acceptance as a modelling tool for engineering problems of direct geotechnical relevance. One area for which the method seems naturally well adapted is that of crushable soils. Grain Crushing is generally modeled using the discrete element method (DEM) via two alternative methods: replacing the breaking grains with new, smaller fragments; or by using agglomerates. The latter, despite being very helpful for the understanding of the micromechanics occurring in a single grain, becomes an unpractical tool for the modeling of larger scale problems. In fact, when considering those alternatives there is always a need to balance computational expediency, accuracy of results and soundness of principle. This work focuses on the encounter of those two last requirements, as exemplified in a series of simulation of high pressure one-dimensional and isotropic compression of Fontainebleau sand. A recently developed model for crushable soils is briefly outlined. It is shown that the upscaling procedure adopted allows a considerable reduction of computational bargain without losing accuracy in terms of grain size distribution evolution and mechanical response.
2014-01-01T00:00:00ZFluid effects in sand production simulations coupling DEM with CFD
http://hdl.handle.net/2117/27401
Fluid effects in sand production simulations coupling DEM with CFD
Climent Pera, Natalia; Arroyo Alvarez de Toledo, Marcos; Gens Solé, Antonio; O'Sullivan, Catherine
Sand production around a wellbore is here simulated using a three dimensional numerical model based on Discrete Element Method (DEM) coupled with Computational Fluid Dynamics (CFD). The model is calibrated against a well-known poro-elastoplastic analytical solution for the dry case, and later applied on hydrostatic conditions, at different levels of pore pressure and effective stress. The outcomes of the numerical model are examined and compared with the response predicted by the analytical solution. The microscopic asymmetry of the numerical model and the damping effect of the drag force are found to play a large role in the response.
2014-01-01T00:00:00ZParticle failure in DEM models of crushable soil response
http://hdl.handle.net/2117/27399
Particle failure in DEM models of crushable soil response
Ciantia, Matteo Oryem; Arroyo Alvarez de Toledo, Marcos; Gens Solé, Antonio; Calvetti, Francesco
The Discrete Element Method (DEM) is progressively gaining acceptance as a modelling tool for engineering problems of direct geotechnical relevance. One area for which the method seems naturally well adapted is that of crushable soils. To simulate crushing in soils using DEM a number of different alternatives are available. When considering those alternatives, as in other areas of applied numerical modelling, there is always a need to balance computational expediency, accuracy of results and soundness of principle. This communication focuses on the encounter of those two last requirements, as exemplified in a series of simulation of one-dimensional compression of a silica sand to high pressures (up to 100MPa).A recently developed model for crushable soils is briefly outlined and the role of several parameters is illuminated by a parametric analysis. It is shown that using the same model for single-grain platen crush tests results in a different choice of optimal
parameters than what will be inferred from simply matching the oedometric results. The apparent contradiction might be resolved by combining the particle crushing model with a more refined contact model.
2014-01-01T00:00:00ZExplicit finite deformation stress integration of the elasto-plastic constitutive equations
http://hdl.handle.net/2117/27395
Explicit finite deformation stress integration of the elasto-plastic constitutive equations
Monforte Vila, Lluís; Arroyo Alvarez de Toledo, Marcos; Gens Solé, Antonio; Carbonell Puigbó, Josep Maria
In this work, an explicit integration scheme for hyperelastic-based finite strains elasto-plasticity is presented. One step update equations are obtained from the large deformation multiplicative elasto-plasticity
theory, where an exponential variation of the plastic deformation gradient is assumed. In addition, the material tangent matrix is presented and has the same formal structure as the usual small strains elasto-plastic tangent matrix. In purely elastic regime, the proposal reduces to the usual large deformation elasticity equations. Routines to alleviate the main drawbacks of explicit methods are outlined, such as a yield surface drift correction scheme and an adaptive substepping method. Several examples of typical geotechnical tests using the Houlsby hyperelastic model along with the Modified Cam Clay plastic model are discussed. Results from a convergence test suggest that, using an adaptive substepping scheme, the error of the local problem is independent of the step size.
2014-01-01T00:00:00ZHybrid minimization algorithm applied to tunnel back analysis
http://hdl.handle.net/2117/27394
Hybrid minimization algorithm applied to tunnel back analysis
Santos Rodríguez, Cristian de; Ledesma Villalba, Alberto; Gens Solé, Antonio
The paper describes a procedure to perform backanalysis in an automatic manner in the context of a tunnel excavation. The measured displacements are compared with the calculated ones obtained from a
Finite Element Analysis, forming the objective function. The parameters that best represent the measured data are those that minimize the objective function and a suitable minimization algorithm is required. In this paper two different minimization algorithms were combined in order to define a hybrid method that makes the most of both. One of the algorithms is a Genetic Algorithm (GA) inspired by Darwin’s theory of evolution and formally proposed by Holland (1975) and initially introduced in the field of geotechnics by Levasseur et al (2008). The
other algorithm is based on the gradient method of Gauss-Newton type, described in Ledesma et al (1996). The Finite Element code Plaxis was used as a tool for the direct analysis. The “Hardening soil model” defined by Schanz et al (1999) and subsequently implemented in Plaxis has been the constitutive model selected to simulate the soil behavior. The parameters to identify are the coefficient of lateral earth pressure (K0) and the reference Young’s modulus for unloading and reloading (E ref ur ), to the reference pressure (p ref ). The geometry considered is a synthetic case involving the excavation of a circular tunnel. The paper shows in detail the structure and the different aspects of combining in serial form genetic algorithms with gradient based methods. In particular, a strategy is defined in which the genetic algorithm is used as a first stage to define a smaller search space, located near the minimum, and the gradient method is used as a second stage to finally find the minimum in an efficient manner. The method proposed strives to combine efficiently the advantages of the two types of algorithms and to avoid performing unfruitful gradient searches around local minima.
2014-01-01T00:00:00ZSmall-strain shear stiffness of compacted clays: initial state and microstructural features
http://hdl.handle.net/2117/27370
Small-strain shear stiffness of compacted clays: initial state and microstructural features
Suriol Castellví, Josep; Romero Morales, Enrique Edgar; Lloret Morancho, Antonio; Vaunat, Jean
The small-strain shear stiffness of compacted Barcelona silty clay is studied focusing on the effects of the initial state and the microstructure set on compaction. The Proctor plane is used to map these states, which cover a wide range of degrees of saturation. Stiffness data at different confining stresses are determined using a resonant column apparatus. The microstructural characterization of the material is carried out by mercury intrusion and extrusion porosimetry tests and water retention curves. Results indicate an increase in the small-strain shear modulus with the reduction of degree of saturation for samples prepared at degrees of saturation greater than 0.3. The results are interpreted considering the evolution of the microstructure, and a constitutive stress incorporating degree of saturation and microstructural features. Based on these aspects, a microstructural framework is proposed to interpret small-strain stiffness. The characterization of the microstructural void ratio is critical in order to obtain appropriate results.
2014-01-01T00:00:00ZFreezing-thawing tests on natural pyroclastic samples
http://hdl.handle.net/2117/27369
Freezing-thawing tests on natural pyroclastic samples
Pelaez, R.R; Casini, Francesca; Romero Morales, Enrique Edgar; Gens Solé, Antonio; Viggiani, Giulia M.B.
Artificial Ground Freezing is one of the construction techniques that were extensively adopted during construction of Line 1 of Napoli Underground, to ensure stability and waterproofing of the platform tunnels and inclined passageways during excavation below the ground water table through loose granular soils of pyroclastic origin (Pozzolana) and a fractured soft rock (Neapolitan Yellow Tuff). The aim of the paper is to study the behavior of the Pozzolana and Neapolitan Yellow Tuff subject to freezing and thawing cycle at constant vertical stress under oedometric condition. As the temperature decreases the ice content increases and a liquid ice-surface tension, s-li, develops at the interface of the two phases as the temperature decreases. This tension must be balanced by the suction s defined as the difference in frozen and liquid water pressure Pi and Pl respectively. Two oedometers working under suction and temperature controlled conditions have been modified to work with temperature below zero degree. The stress path followed by samples reproduce the in situ vertical stress interested by freezing and thawing
2014-01-01T00:00:00Z