On internal erosion in embankment dams : a literature survey of the phenomenon and the prospect to model it numerically

The main objective with this literature survey is to elucidate the state of the art of internal erosion in embankment dams in order to be able to formulate a research program for numerical modelling of internal erosion in a physically sound manner. Since these processes normally are localised to specific zones in a dam, the ordinary continuum approach frequently utilised in soil modelling will not, by itself, be successful. The plan of the research group is therefore to treat internal erosion numerically as a type of localisation and describe the constitutive behaviour with micromechanical models in localised zones. In the next step, the internal erosion model developed will be implemented in a mathematical consistent fashion in a continuum model, based on e.g. the finite element method. In such software, ordinary computations of stresses, strains, deformations and pore pressures in an embankment dam can be performed; results which possible lead to conclusions about the initiation of internal erosion processes. When internal erosion is initiated, the micromechanical models will describe these processes in localised zones. It was decided to restrict the literature survey to areas important for the research direction defined above. The chosen areas are thus: numerical modelling of embankment dams, internal erosion processes, embankment dam failures and accidents, filter design, micromechanical models of hydro dynamical loads on single particles and system of particles as well as micromechanical models for friction between single grains and system of grains. The most essential findings for each area, in the context of the research, will be described below. There exists much experience in the field numerical modelling of embankment dams which is exemplified by the large amount of published papers on the subject. However, the processes of internal erosion have not been modelled in a general manner, as outlined in the research idea above, in any of the papers cited in this literature survey. There are three main internal erosion processes that can initiate piping: backward erosion, concentrated leak and suffusion. Piping can occur in the embankment, through the foundation and from the embankment into the foundation. Embankment dams are normally constructed in zones of different materials. Different compressibilities of the various zones might lead to internal redistribution of stresses and uneven settlements, which might cause cracks or "soft zones" where internal erosion can be initiated. Cracks can also occur later on by hydraulic fracturing. All of that described above are examples of mechanical processes that are theoretically possible to model numerically. To receive information about potential internal erosion problems in a dam at an early stage, geophysical methods are a promising alternative or complement to numerical methods. However, much research and development remain before the results from geophysical methods utilised for permanent monitoring and surveillance of dam structures are completely trustful. It is often very difficult to determine the exact reason for a dam accident or a failure since the processes involved have a tendency to destroy evidence which might have existed. Statistical data, however, shows that failure by overtopping and piping are the most common modes of failure while failure by slides is less common. Many piping failures occur very fast, leaving a short time for proper actions. The safety of large embankment dams is strongly dependent on the reliability of the performance of their critical filters. Existing filter design criteria are in most cases empirically derived, implying that a lot of knowledge can be gained by taking a mechanical approach to the problem. More research would also be desirable in the field of ageing effects of dams and uncertainties of core/filter appearance during and after accidents, incidents etc. For this, numerical modelling also seems to be a promising approach. Continuum formulations of flow through porous media often results in equations consisting of a few unknown parameters such as the permeability. The physical background of such parameters can often be traced to the detailed flow in the pores and it is therefore in place to study the flow on this level, as well. The forces on individual particles have been exploited for certain geometries and for a number of flow conditions. We however need to investigate further higher Reynolds number flows, more complex geometries and instationary conditions. The forces from the micromechanical models must be balanced by gravity and forces that emanates from particle interactions for the dam to be stable. As a first, and most simple, criteria the size distribution of the particles being subjected to hydrodynamic pressure is compared to the pore size distribution of the porous medium. Hence the particles will move if they are small enough independent on magnitude of the force on it. In reality, however, the hydraulic forces must exceed particle interaction forces and or gravitational forces keeping the particles in place at normal conditions. The forces are strongly dependent on the size of the particles and therefore dependent on different phenomena such as cohesion, adhesion and static friction. In the final part of the literature survey a concept for numerical modelling of internal erosion is presented based on ideas that have emerged from this work. The concept involves as well mathematical developments in order to formulate a micromechanical model for internal erosion as laboratory tests. The theoretical work and the practical laboratory work should be performed simultaneously and in an interactive manner. A software containing a model that simulates internal erosion could be useful for: an increase of the knowledge about internal erosion processes, evaluating the risk for dam incidents caused by internal erosion, estimating the time for progression of internal erosion and piping, studying self-healing of leaks, changes in filter behaviour subsequent to particle accumulation and ageing effects in dams, analysing the amount of instrumentation needed in a dam and the proper location for monitoring and surveillance as well as designing dams to mention a few examples. It is therefore apparent that the route suggested has a high potential to become a tool for future improvements of dam safety.Godkänd; 2008; 20081017 (ysko)