Abutment stability assessment of the Hongrin arch dam using 3D distinct element method

The Hongrin north dam is a double curvature concrete arch dam located in western Swiss Prealps, which attains 125m high. The right bank abutment of the dam mainly consists of intensively jointed Neocomian limestone and exhibits zones of potential instability.At the time of construction (1965–1969), this slope was reinforced with rock anchors. Subsequent hydrogeological study and groundwater monitoring revealed the presence of water pressure due to a slight seepage flow through the rock joints in the dam foundation. This latter evidence raised an additional concern about the stability of the abutment. In a dedicated study, the stability of the right abutment the Hongrin north dam abutment has been assessed using continuumdiscontinuum numerical analysis. 3DEC (3-Dimensional Distinct Element Code) has been used to model the complicated slope geometry and to explore the role of rock discontinuity in the failure mechanisms. The rock mass is defined as deformable distinct blocks which interact along frictional discrete discontinuities representing the rock joint sets. The water pressure is introduced as fluid pressure boundary condition along the discontinuities, and the rock reinforcement is modeled as structural elements working across the discontinuities. The dam reaction forces, derived from a separate finite element analysis, are evaluated for their possible effects on the stability. The model examines the sensitivity of the abutment stability to the presence of joint water pressure and evaluates the improving effects of rock reinforcement. The results of the analysis allow achieving an enhanced understanding of potential failure mechanisms and helps in proposing further suitable measures to improve the stability of the abutment.

Numerical simulation of free surface flows

Vincent Maronnier, Marco Picasso, Jacques Rappaz

A numerical model is presented for the simulation of complex fluid flows with free surfaces. The unknowns are the velocity and pressure fields in the liquid region, together with a function defining the volume fraction of liquid. Although the mathematical formulation of the model is similar to the volume of fluid (VOF) method, the numerical schemes used to solve the problem are different. A splitting method is used for the time discretization. At each time step, two advection problems and a generalized Stokes problem are to be solved. Two different grids are used for the space discretization. The two advection problems are solved on a fixed, structured grid made out of small rectangular cells, using a forward characteristic method. The generalized Stokes problem is solved using a finite element method on a fixed, unstructured mesh. Numerical results are presented for several test cases: the filling of an S-shaped channel, the filling of a disk with core, the broken dam in a confined domain. (C) 1999 Academic Press.

1999

Numerical modelling of the hydrogeological and geomechanical behaviour of a large slope movement: The Triesenberg landslide (Liechtenstein)

Bertrand François, Lyesse Laloui, Laurent Tacher, Véronique Triguero

Using advanced hydrogeological and geomechanical finite element modelling, it has been possible to model the mechanical behaviour of a large slope movement, the Triesenberg landslide. This slope is located along the Rhine valley in the Principality of Liechtenstein, covering an area of around 5 km2 which includes two villages. Pore water pressure fields calculated by the hydrogeological model were used as input for the geomechanical model. The results obtained through numerical simulation agree fairly well with field measurements of peak velocity, spatial and temporal distribution of velocity and total displacements. Such results were obtained using a Modified Cam-Clay elasto-plastic constitutive model for which the required material parameters were obtained through careful geotechnical tests. These finite element models were carried out in 2 and 3 dimensions in order gradually to improve the understanding of the physical phenomena governing the hydrogeological conditions and the movements of the slope.

2007

Analysis and numerical simulation of free surface flows

Alexandre Caboussat

Mathematical and numerical aspects of free surface flows are investigated. On one hand, the mathematical analysis of some free surface flows is considered. A model problem in one space dimension is first investigated. The Burgers equation with diffusion has to be solved on a space interval with one free extremity. This extremity is unknown and moves in time. An ordinary differential equation for the position of the free extremity of the interval is added in order to close the mathematical problem. Local existence in time and uniqueness results are proved for the problem with given domain, then for the free surface problem. A priori and a posteriori error estimates are obtained for the semi-discretization in space. The stability and the convergence of an Eulerian time splitting scheme are investigated. The same methodology is then used to study free surface flows in two space dimensions. The incompressible unsteady Navier-Stokes equations with Neumann boundary conditions on the whole boundary are considered. The whole boundary is assumed to be the free surface. An additional equation is used to describe the moving domain. Local existence in time and uniqueness results are obtained. On the other hand, a model for free surface flows in two and three space dimensions is investigated. The liquid is assumed to be surrounded by a compressible gas. The incompressible unsteady Navier-Stokes equations are assumed to hold in the liquid region. A volume-of-fluid method is used to describe the motion of the liquid domain. The velocity in the gas is disregarded and the pressure is computed by the ideal gas law in each gas bubble trapped by the liquid. A numbering algorithm is presented to recognize the bubbles of gas. Gas pressure is applied as a normal force on the liquid-gas interface. Surface tension effects are also taken into account for the simulation of bubbles or droplets flows. A method for the computation of the curvature is presented. Convergence and accuracy of the approximation of the curvature are discussed. A time splitting scheme is used to decouple the various physical phenomena. Numerical simulations are made in the frame of mould filling to show that the influence of gas on the free surface cannot be neglected. Curvature-driven flows are also considered.

EPFL2004

Analysis and numerical simulation of free surface flows

Alexandre Caboussat

EPFL2004