Richard Kastner

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.

Etude de la stabilité d'ensemble des rideaux de soutènement ancrés

ABSTRACT Laboratory tests on a two-dimensional model of an anchored sheet pile structure are carried out to study the displacement field of a soil-structure system. The comparison of the experimental results with those obtained by the Kranz method shows significant differences. The overall stability analysis of these structures is then investigated by a theoretical model using the upper-bound method of the limit analysis theory. A translational failure mechanism is considered for the calculation scheme. This mechanism is composed of two rigid blocks in the active zone and the so-called log-sandwich mechanism in the passive zone. Numerical results show good agreement with results obtained from laboratory tests.

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.

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.

South Toulon tube: 3D numerical back-analysis on in situ measurements

Full face excavation with ground reinforcement has become a common technique to build large tunnels in soft rock or hard soil. Nevertheless, at the design phase, it remains difficult to assess the effect of the different construction and reinforcement elements on the control of the ground movements and settlements. In order to improve the understanding of ground response to this tunnelling method, a monitoring section has been installed during the construction of the south Toulon tunnel (France). An important database was obtained and subsequently used for numerical back-analysis. A 3D FE calculation, modeling the real reinforcement system and construction stages, was then performed. The satisfactory matching between the numerical results and the in situ measurements shows that the three-dimensional numerical model with an explicit modeling of the reinforcement elements is a reliable tool to simulate the complex phenomenon of interaction between the excavation process, the reinforcements and the ground reaction.

Quelques résultats de recherches acquis dans le cadre du projet national Microtunnels

ABSTRACT The aim of the paper is to present some results of the National Research Project Microtunnels dealing with trenchless technologies for pipes installation. After a short description of microtunneling techniques, one describes the objectives and the content of the “recommendations” published at the end of the project. Then the main results about the sol-pipe interaction mechanisms are commented, with special focus on the main parameters governing the behaviour: ground, overcutting, driving stop duration and lubrication. At the end, among the various other multidisciplinary aspects, are described the use of geophysical methods for investigation of buried obstacles and the evaluation of the “social cost”.

Étude de la butée sur un écran de largeur limitée en sol frottant

ABSTRACT This paper deals with the problem of the three dimensional passive earth pressures. First, the model tests carried out allows one to deduce the mode of failure of wall of limited breadth. Then, a theoretical study based on the kinematical approach of the limit analysis theory is investigated. Three translational kinematically admissible failure mechanisms are considered. The smallest upper-bound solutions of the three dimensional passive earth pressures are presented and discussed.