Daniel Dias

Probabilistic Analysis of Circular Tunnels in Homogeneous Soil Using Response Surface Methodology

A probabilistic analysis of a shallow circular tunnel driven by a pressurized shield in a frictional and/or cohesive soil is presented. Both the ultimate limit state (ULS) and serviceability limit state (SLS) are considered in the analysis. Two deterministic models based on numerical simulations are used. The first one computes the tunnel collapse pressure and the second one calculates the maximal settlement due to the applied face pressure. The response surface methodology is utilized for the assessment of the Hasofer-Lind reliability index for both limit states. Only the soil shear strength parameters are considered as random variables while studying the ULS. However, for the SLS, both the shear strength parameters and Youngs modulus of the soil are considered as random variables. For ULS, the assumption of uncorrelated variables was found conservative in comparison to the one of negatively correlated parameters. For both ULS and SLS, the assumption of nonnormal distribution for the random variables has almost no effect on the reliability index for the practical range of values of the applied pressure. Finally, it was found that the system reliability depends on both limit states. Notice however that the contribution of ULS to the system reliability was not significant. Thus, SLS can be used alone for the assessment of the tunnel reliability.

Extrusion movements of a tunnel head reinforced by finite length bolts—a closed‐form solution using homogenization approach

An analytical model on the general behaviour of a tunnel head, reinforced by finite length bolts is proposed. This model is based on the homogenization method and spherical symmetry assumption. Despite its simplicity, and in consequence its limits of validity, it does allow a quick estimation of the key design parameters: frontal displacement, extension of decompressed zone, ground stresses and bolt tension, and constitutes thereby a very useful and handy tool for design engineers. In particular, the influence of the reinforcement length, as well as other important design parameters, are studied by the proposed model. The charts resulting from the parametric studies are directly applicable. Otherwise, the comparison to a 3D numerical model is also presented in this paper. The first results provide the validation of the analytical solution, at least in terms of average extrusion movements. Copyright

Three-dimensional face stability analysis of circular tunnels by a kinematical approach

The upper-bound method of the limit analysis theory is used to calculate the active and passive pressures in front of a pressurized shield. Two translational kinematically admissible failure mechanisms composed of a sequence of truncated rigid cones are considered for the calculation schemes. The numerical results obtained are presented and compared to those given by other authors.

Three-dimensional face stability analysis of circular tunnels by numerical simulations

The face stability analysis of shallow circular tunnels driven by the pressurized shield is investigated by three-dimensional numerical simulations. Both the active and passive soil failures of the tunnel face are considered in the analysis. A comparison is performed between (i) the present numerical solutions of the ultimate pressure of the tunnel face, (ii) the results of the multiblock mechanism of the kinematical approach in limit analysis and (iii) the values of the ultimate tunnel pressure obtained from centrifuge model tests. The shortcomings of the multiblock failure mechanism are presented and discussed.

Extension of CSRSM for the Parametric Study of the Face Stability of Pressurized Tunnels

The Collocation-based Stochastic Response Surface Methodology (CSRSM) is a powerful probabilistic method. It aims at replacing a complex deterministic model by a simple analytical expression (called meta-model) to reduce the time cost of the classical probabilistic methods. The meta-model is based on a Polynomial Chaos Expansion (PCE). The coefficients of the PCE are computed in this paper by regression from the response of the deterministic model at a limited number of collocation points. The conventional formalism of CSRSM requires performing a new set of deterministic computations each time the probabilistic parameters of the input random variables are slightly modified. An extension of CSRSM is therefore proposed in this paper. It allows the realization of a parametric study at a limited time cost without loss of accuracy. This is demonstrated by comparing the results obtained from the original CSRSM and its extension when performing a parametric study concerning the stability analysis of a pressurized tunnel face.

Physical model and discrete element method analysis of a granular platform supported by piles in soft soil

Two-dimensional experiments were conducted on a small scale model simulating a granular platform over soft soil improved using vertical stiff piles, using the Schneebelis soil analog. The soft soil is simulated using foam elements and the stiff piles are modeled using steel block elements fixed to the rigid frame. The aim of this study is to investigate the load transfer mechanisms by arching in the platform and the settlement reduction and homogenization. The experimental parametric study has been used for comparison to numerical models. A discrete element method approach is implemented to simulate the granular platform. In this modeling, each rod is simulated by a rigid disk and the contact constitutive law between the rods is linear. The micromechanical parameters are calibrated on biaxial compression tests. The physical experiments are then simulated. The numerical results are compared to the experimental results and a good agreement is found in terms of load transfer onto the piles. The contact forces between particles are investigated.

Assessment of Soil Parameters Met during a Tunnel Excavation: Use of Inverse Analysis on In Situ Measurements — Case of Bois de Peu (France)

In this paper, an optimization method is tested on in-situ measurements carried out during the excavation of Bois de Peu tunnel in France. An important monitoring program was foreseen during this excavation because it was decided to apply the interactive design method in order to adapt support in real time. An optimization software, SiDoLo, is used. It is coupled with the finite element software CESAR-LCPC to evaluate soil parameters. Only the identification of the argillaceous unit parameters is presented in this paper. Experimental responses used in the inverse analysis approach and presented in this paper are strain measurements of the soil ahead the face by extrusometers. Numerical responses are obtained by 2D numerical models. This article shows the results obtained according to the number of used experimental responses. It shows difficulties which can be met during the identification of several parameters simultaneously.

Modélisation numérique du renforcement du front de taille d'un tunnel. Influence de la loi d'ancrage sol/boulon

ABSTRACT The design of reinforced structures requires a perfect knowledge of the behaviour of the soil, the bolts and of the soil/bolt interface. To determine the frictional law between the soil and the bolt, full-scale pullout tests were performed on radial anchor bolts, equipped with strain gauges. The analytical method developed by [BOUR 94] allows to derive the local frictional law parameters (assumed constant along the rod) from the global relationship between the total load and the head displacement. This frictional law is introduced in a 3D tunnel model with reinforced face. The 3D numerical model provides an accurate geometrical description of the tunnel structure, such as the lining behind the face, as well as the interface law between bolts and ground. To observe the influence of the soil/bolt interface, we study the influence of several frictional laws in terms of face displacements and axial forces in the bolts.

The Research and Application of Masonry Structure Reliability Rating Method Based on Extenics

Aiming at masonry structure housing, establish the structure reliability evaluation index system. Subsidiary factors index dimensionless processing, the traditional matter-element model is analyzed, and based on this, the article puts forward an improved correlation functions, establish correlation function of the correlation factors of the index layer. Consider the weight coefficients of members,gave the extension index,combining analysis of the structural member reliability and extension comprehensive evaluation of the integral structural reliability, establish the masonry structure housing reliability assessment method framework. And prove the feasibility of this method through engineering examples.

Probabilistic analysis of the face stability of circular tunnels

This paper presents a reliability-based approach for the threedimensional analysis of the face stability of a shallow circular tunnel driven by a pressurized shield. Only the collapse failure mode of the ultimate limit state is studied. The deterministic model is based on the upper-bound method of the limit analysis theory. The random variables used are the soil shear strength parameters. The Hasofer-Lind reliability index and the failure probability were determined. A sensitivity analysis was also performed. It was shown that (i) the assumption of negative correlation between the soil shear strength parameters gives a greater reliability of the tunnel face against collapse, with respect to the hypothesis of uncorrelated variables, (ii) the failure probability is much more influenced by the coefficient of variation of the angle of internal friction than that of the cohesion and (iii) when no correlation between shear strength parameters is considered, a more spread out CDF of the tunnel pressure was obtained in comparison to the case of correlated shear strength parameters.