The knowledge of the internal stability of granular soils is a key factor for the design of granular and filter for the geotechnical infrastructures such as dykes, barrages, weirs and roads embankment. To evaluate the internal instability of granular soils different criteria are generally used in the practice. However, the results of these criteria on the same soil may lead to different evaluations of the internal instability. In this paper the common criteria used for the internal instability have been presented and compared as far as possible. It was found that the most internal instability criteria define a limit value for the secant slope of the grain size distribution curve of the granular soils. Based on this finding an own criterion for the evaluation of the internal instability of granular soil has been developed and compared to the common criteria. A very good agreement between some criteria was found. Furthermore, a site specific assessment for the evaluation of the internal instability of granular soil has been proposed in order to get more confidence in this evaluation.
In unstable soils, a special erosion process termed suffusion can occur under the effect of relatively low hydraulic gradient. The critical hydraulic gradient of an unstable soil is smaller than in stable soils, which is described by a reduction factor α. According to a theory of Skempton and Brogan (1994) [1], this reduction factor is related to the stress conditions in the soil. In an unstable soil, the average stresses acting in the fine portion are believed to be smaller than the average stresses in the coarse portion. It is assumed that the stress ratio and the reduction factor for the hydraulic gradient are almost equal. In order to prove this theory, laboratory tests and discrete element modelings are carried out. Models of stable and unstable soils are established, and the stresses inside the sample are analysed. It is found that indeed in unstable soils the coarse grains are subject to larger stresses. The stress ratios in stable soils are almost unity, whereas in unstable soils smaller stress ratios, which are dependent on the soil composition and on the relative density of the soil, are obtained. A comparison between the results of erosion tests and numerical modeling shows that the stress ratios and the reduction factors are strongly related, as assumed by Skempton and Brogan (1994) [1].
Piping is an erosion mechanism that plays a significant role in river barrages, such as dikes and dams, that are founded on poorly graded cohesionless soils like, for instance, fine or medium sands. Likewise, in gap-graded or widely graded cohesionless soil, finer grains can pass through the pore matrix of coarse soil because of seepage. This phenomenon, which is called internal instability or suffusion, can occur in granular filter in or under dams, dikes, barrages, or any other water-retaining structure. For economic reasons, unstable soils are generally used in practice if the hydraulic force or the hydrodynamic energy is smaller than the critical hydraulic force or the critical hydrodynamic energy. Therefore, an approach of combined geometric-hydraulic criterion is expected to yield better results. Based on this premise, laboratory tests of piping and internal erosion are carried out using five cohesionless soils. An analysis of the results considering the instability index shows that the critical hydraulic gradient required to initiate the piping and internal erosion depends on the curve of the grain size distribution, the initial relative density of the soil, the seepage direction, and the initial stress condition in the soil. Based on these laboratory results and the results reported by several other authors, a combined geometric-hydraulic criterion with respect to piping and internal erosion in cohesionless soils is developed and proposed.
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