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UBC Theses and Dissertations

An improved hydromechanical understanding of seepage-induced instability phenomena in soil Crawford-Flett, Kaley A.


Internal instability describes phenomena that occur when a soil cannot prevent the loss of its own small particles in the presence of forces induced by seepage flow. A significant proportion of incidents and failures in water-retaining structures and their foundations are attributed to the consequences of internal instability. There is a concern that existing dam and canal infrastructure may be vulnerable to internal instability as a consequence of deficiencies attributed to the state-of-practice at the time of design. In order to manage infrastructure risk, there is a need for an improved science-based understanding of the state-of-art 'hydromechanical framework' that describes the interacting material, stress and hydraulic factors understood to govern internal instability. A series of seepage tests were undertaken on five gradations in a large permeameter. The objectives were to observe critical seepage-induced phenomena at the 'upper bound', and to verify the presence of an extreme 'lower bound' to the hydromechanical space. A modified slurry deposition technique was used to reconstitute saturated and homogeneous specimens, and a multi-stage seepage regime was found satisfactory to identify the critical hydromechanical condition. Phenomena of fluidization and hydraulic uplift were found to characterize internally stable behaviour at the 'upper bound' of hydromechanical instability. A stress-independent 'lower bound' was experimentally defined by tests on two very unstable suffusive soils. Existing geometric methods were evaluated and found to inadequately characterize material behaviour in widely-graded till materials: rather, the presence of plasticity was found to inhibit internal instability. The present study quantified three necessary conditions for internal instability in gap-gradations: (1) a theoretical porosity-based microstructure framework was adapted to identify the 'α ≈ 0' 'particle detachment' condition, (2) the critical seepage condition at the hydromechanical 'lower bound' was verified in terms of a critical seepage velocity, and (3) a novel constriction size criterion was proposed to assess the 'transportation potential' for a particle in a porous medium. It is concluded that the hydromechanical space is not fully characterized by the stress-reduction factor 'α' alone: the present study characterizes two distinct and necessary components of material susceptibility for gap-gradations: (1) particle detachment, and (2) transportation potential.

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