Finite-Element Approach to the Erosion and Transport of Fine Particles in Granular Soils

The increase of the water table level presently occurring in the city of Milan led to some leakage of groundwater in underground facilities and subway tunnels. These structures, in fact, were completed about three decades ago when the water table was deeply located. To locally lower the ground water, and to eliminate or limit its flow towards the submerged openings, a series of pumping wells was placed in their vicinity. This provision, however, could lead to possible erosion of the fine fraction of the granular soil and to consequent settlements of nearby buildings. To investigate this phenomenon a finite-element approach has been developed for the analysis of the erosion and transport of the fine particles of granular soils subjected to a seepage flow. First, the continuity equation governing the problem and its finite-element formulation are discussed. Then, on the basis of the results of erosion tests presented in the literature, a law is derived that accounts for the nonlinear relationship between the total amount of eroded material, for time tending to infinity, and the velocity of seepage. Finally, this law is applied to the solution of one- and two-dimensional test examples, and some conclusions are drawn on the limits of the developed numerical model and on its possible improvement.