Hydraulic failure by heave and piping

Garai, J. (2016) Hydraulic failure by heave and piping. In: ICSE 2016 (8th International Conference on Scour and Erosion), 12-15 September 2016, Oxford, UK.

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Abstract

It is generally agreed the same criterion for hydraulic heave and for piping can be used. In vertical upward flow the theory is well established. The weight of the soil must be greater than the seepage force of the water. Despite this well established theory piping and sand boiling occurs at much lower hydraulic gradient than the theoretical value. It is concluded that this discrepancy resulting from a conceptual error by not making distinction between the local and the global equilibrium. In the case of hydraulic heave the equilibrium of the entire soil prism (global equilibrium), while for the initiation of piping on the surface of the soil the equilibrium of one grain (local equilibrium) should be investigated. Investigating the local equilibrium of a single grain physical model for the initiation of piping is proposed. Criterion for piping on the top of a horizontal surface in granular soil subject to an upward flow deduced from first principles. It is concluded that hydraulic gradient alone does not sufficiently describe the hydraulic stability of the soil. The particle size and its distribution also have effect on the hydraulic stability of the soil. Critical grain diameter corresponding to the critical hydraulic gradient is introduced. If the diameters of the grains in a soil matrix are bigger than the critical diameter then the Terzaghi criterion is valid. Grains with smaller diameters lose their stability at lower hydraulic gradient than the critical one. The removals of grains from the soil matrix increase both the hydraulic gradient and the permeability of the soil. Thus the initial grain removal from the surface of the soil can progressively lead to sand boiling; therefore, the stronger criterion of piping should also be required for hydraulic heave.

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