Assessment of internal stability of filters under static and cyclic loading: An experimental and theoretical treatment

The occurrence of internal instability may significantly affect geo-mechanical characteristics of granular filters such as permeability and particle size distribution, consequently rendering them ineffective in retaining the protected base soils and thereby endangering the structural stability. This paper presents the results of 65 hydraulic tests performed on ten different granular soils compacted at varying relative densities between 0 and 100% and subjected to an upward hydraulic flow under both static and cyclic conditions. It was observed that the internal stability is a function of particle gradation and relative density in tandem, i.e. constriction size distribution, under static conditions. However, the agitation and pore pressure development under cyclic loading triggered excessively premature internal erosion in filters. Based on the analysis, new constriction-based criteria proposed for both static and cyclic conditions that showed remarkable accuracy in correctly assessing the potential of instability of filters compared to many existing criteria. Moreover, a new hydromechanical model is presented that could accurately capture the correct potential of instability of filters, thereby contributing toward increased confidence level for practical design of filters. Two practical design examples presented to demonstrate the implications of this research study in practice to conclude this paper.