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On the influence of effective stress and micro-structure on suffusion and suffosion Slangen, Paul Herman Henricus


The research presented within this thesis covers the development of a flexible wall permeameter, and a parametric laboratory investigation of the factors influencing seepage-induced internal instability in gap-graded granular materials. A flexible wall permeameter comprising a double-walled triaxial cell, a seepage control system, and instrumentation, with a novel measurement of volume change, has been designed and built. The apparatus was successfully commissioned and the test procedure demonstrated to yield repeatable results. Two commissioning tests, 23 tests on eight glass beads gradations and 16 tests on ten soil gradations were conducted. All gap-graded gradations were reconstituted using the modified slurry deposition method, isotropically consolidated to a cell pressure between 50 and 150 kPa, and subsequently subject to upward multi-stage seepage flow. Analysis of the test results identifies two distinct seepage-induced internal instability phenomena. First, migration of fine particles from a soil, termed suffusion, is characterised by a mass loss without change in volume, or with a small non-progressive change in volume, accompanied by an increase of hydraulic conductivity. Second, local or overall collapse of the soil structure, termed suffosion, is characterised by a seepage-induced mass loss, accompanied by a reduction in volume and a change in hydraulic conductivity. It is demonstrated that measurement of total volume change is necessary to avoid any mis-interpretation of the phenomenological response to seepage flow. It was found that the differential pore water pressure at the onset of suffosion increases with increasing mean effective stress. The micro-structure of the specimen was found to influence the susceptibility to seepage-induced internal instability: the portion of non-load bearing fine particles appears a useful parameter to quantify the potential for suffusion, whereas the proposed state parameters are predictors of the relative susceptibility to suffosion. Although particle shape does not affect the suffusive response in a transitional clast-supported micro-structure, sub-angular particles are found to yield a transitional micro-structure that is more resistant to suffosion than a similar micro-structure of spherical particles. A unified approach is presented to characterise suffosion.

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Attribution-NonCommercial-NoDerivs 2.5 Canada