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Experimental study for evaluating the internal stability of gap-graded soils Khan, Abdul Sattar


Experience has shown that certain soils may be susceptible to internal erosion. These soils, for example gap-graded and broadly graded (concave-up) soils are vulnerable to migration of the fine fraction during permeation. Factors governing this phenomenon of internal stability may be categorized as geometric constraints (pore size constrictions, porosity and shape of gradation curve) and hydrodynamic actions (seepage force and direction of flow). The objective of this study was to evaluate the success of empirical rules developed for assessing the internal stability of granular media subjected to seepage flow. Specifically, it seeks to examine the influence of hydraulic gradient, vertical effective stress on the susceptibility of reconstituted gradation of glass beads to internal instability with reference to the boundary conditions of the rigid-wall permeameter. A total of twelve multi-stage permeability tests were executed on glass beads specimens of five different gradations. The specimens were reconstituted using a method of slurry preparation and discrete deposition that resulted in repeatable homogeneous specimens. Analysis of results reveals that the Kezdi (1979) proposed for a split gradation analysis could be used, with reasonable confidence, for assessing the internal stability of gap graded soils. In contrast the criterion of Kenny and Lau (1985, 1986), failed to adequately detect and properly differentiate between the gradations, characterised by a completely horizontal gap in the gradation. At a more fundamental level, it appears that potential for internal stability, and particle migration, diminishes with an increase in vertical effective stress. With regard to the apparatus, the influence of the boundary wall condition is very subtle. For specimens that are either prone to segregation or, on the other hand, stable (which are two extreme stability conditions), the boundary wall condition does not exert any influence. Whereas, for the intermediate gradations, it effects the material behaviour.

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