The geotextile filter design is particularly complex when the granular base soils are internally unstable. In these conditions, the design criteria available in literature are not always reliable. This paper deals with a new theoretical method developed to evaluate the internal stability of granular soils. In order to simulate, theoretically, the filtration process inside these soils a set of spherical particles and different soil relative densities have been considered. The soil has been represented by means of a sequence of parallel layers, containing constrictions and particles, placed one upon the other one at a distance, in the direction of hydraulic flow, function of the soil relative density. The movement of the fine particles through the different soil layers has been simulated by means of a mechanism that compares each particle contained in the i-layer with the constrictions contained in the next i+1- layer. The results of the numerical simulations allowed to evaluate the internal stability of the analyzed granular soil and the corresponding critical diameter of suffusion, Dc. Finally, the reliability of the proposed theoretical method has been evaluated by means of the results of experimental long term filtration tests performed using a rigid wall permeameter on different unstable granular soils.
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