S. Maghous

Face stability of shallow circular tunnels driven under the water table: a numerical analysis

The stability analysis of a tunnel excavated in a water-saturated frictional soil is investigated in the light of a failure design approach. The soil strength properties being classically formulated in terms of effective stresses, it is first shown how the effect of seepage flow generated by the excavation process, may be accounted for in such an analysis by means of driving body forces derived from the gradient of an excess pore pressures distribution. The latter is obtained as the solution of a hydraulic boundary value problem, in which both water table evolution and soil deformability can be neglected. A variational formulation of this hydraulic problem in terms of filtration velocities is then presented, leading through appropriate numerical treatment, to a search for the minimum without constraints of a quadratic functional (hybrid formulation), which is formulated by a finite element method. Some numerical examples are given, which provide ample evidence of the crucial role played by seepage forces in the tunnel face stability, since the factor of stability may be divided by as much as three. The influence of such parameters as the tunnel relative depth or soil anisotropic permeability is finally discussed, thus offering a first illustration of the various capabilities of this numerical tool. Copyright

3D finite element model for mechanical and chemo-mechanical deformation in sedimentary basins

Sedimentary basins result from deposition of an appreciable amount of sediments which are transformed into rock through natural phenomena involving mechanical and chemo-mechanical deformation. Purely mechanical deformation prevails in the upper layers of basins, whereas chemo-mechanical deformation dominates at depth, where eifective stresses and temperature are higher. The aim of the present contribution is to provide a comprehensive 3D framework for modeling both mechanical and chemo-mechanical compaction in sedimentary basins. Extending concepts previously proposed for the modeling of purely mechanical compaction in finite poroplasticity, chemo-mechanical deformation induced by lntergranular Pressure-Solution (IPS) is addressed by means of additional viscoplastic terms in the state equations of the porous material. Special attention is given on the analysis of the eifects of large irreversible porosity changes on the poromechanical properties of the sediment material. The sedimentary basin is modeled as a fully saturated poro-elasto-visco-plastic material undergoing large strains. The numerical simulation is performed through the finite element method. An oceanic sedimentary basin is modelled as a horizontal infinite layer where sediments deposition occurs over tens of mil/ions of years. The evolution on time of the porous material constitutive model is presented and analyzed in conjunction with the overall behavior of the sedimentary basin.

Um modelo multifásico para estruturas em concreto armado considerando a fissuração do concreto

Determinar o comportamento global de materiais reforcados a partir das propriedades individuais de seus componentes vem sendo tema de um consideravel numero de trabalhos experimentais e teoricos nos ultimos anos. A modelagem multifasica e uma generalizacao alternativa do metodo de homogeneizacao cujo principio basico consiste em descrever o concreto armado como a superposicao geometrica da fase matriz (concreto) e da fase reforco (barras de aco). Esta tecnica foi empregada com sucesso em uma serie de trabalhos envolvendo estruturas geotecnicas. No caso da estruturas de concreto, Figueiredo et al (2009) estuda o comportamento mecânico de placas de concreto armado sob a acao de carregamento prescrito com o uso da modelagem multifasica em elastoplasticidade. No presente trabalho e acrescentado ao modelo previamente desenvolvido o algoritmo que leva em conta a fissuracao do concreto baseado no modelo de fissuras distribuidas apresentado por Hinton (1988). Sao realizadas comparacoes com simulacoes diretas classicas visando salientar as vantagens do modelo como a reducao significativa do custo computacional.