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Piping under transient conditions : investigation of time-dependent erosion under dikes

Kramer, Renier (2014) Piping under transient conditions : investigation of time-dependent erosion under dikes.

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Abstract:Flood defenses have several failure mechanisms. One of those failure mechanisms is piping. Piping is the process of pipe formation in a sandy aquifer under river dikes. During high water periods, this process manifests itself by the formation of sand boils - formed by sand carried out of the aquifer by seepage – landwards of the dike. Piping will not lead to dike failure until the pipe length below the dike has reached the critical pipe length. From then, the piping process is irreversible and the dike will collapse. The current piping safety assessment of Sellmeijer (1988) in the Netherlands assumes that dike failure will occur when the difference between the water levels on both sides of the dike, the hydraulic head difference, exceeds the critical hydraulic head difference. This is a constant value that can be calculated for each dike. The critical head difference is related to a critical pipe length approximately half the dike width. In reality, however, it takes time before the critical pipe length has been reached. It is likely that the critical pipe length will not be reached in case the hydraulic head difference equals or even exceeds the critical head difference for a brief moment and then quickly drops. The influence of a variable head difference on piping has been investigated by, amongst others, Ozkan (2003) and Shamy et al. (2004). However, these studies do not involve the pipe length but focus on a process (exit gradient) that occurs before the start of piping, which is everywhere in the world, except for the Netherlands, used to indicate the chance of dike failure. Bonelli et al. (2007) researched the time-dependency of pipe widening (the last process of piping before dike failure) for a constant head difference. Recently, Van Esch et al. (2012) developed a piping model that is able to resolve the transient behavior of the groundwater pressure field of piping. Their model is meant to be part of the Dutch piping assessment and design from 2017. Until then, the model is not available and thus cannot be used for this study. Wang et al. (2014) derived formulae that can be used to determine the erosion velocity (pipe length increase per time step) under a constant head difference. It is, however, not clear what the effect of a variable head difference is on the erosion velocity. This Master’s thesis project investigates the effect of piping under transient conditions by taking into account the time dependency of the piping process. The objective of this study is to develop a piping model using existing theories of Sellmeijer (1988) and Wang et al. (2014) to investigate when progressive erosion occurs under realistic transient conditions. A transient piping model (TPM) has been developed in which the existing theory of Sellmeijer (1988) is extended with an erosion velocity formula of Wang et al. (2014) to account for time and to simulate piping under a variable head difference. The most important limitation is that the TPM is only valid for piping under dikes and only for the idealized dike geometry of Sellmeijer. This dike consists of an impermeable clay dike is situated on top of a homogenous sandy aquifer with uniform thickness, the pipe entry and exit locations are predefined, and the slope of the pipe is zero. It is possible to simulate dikes that differ from the idealized Sellmeijer dike, but then this dike has to be simulated in the Sellmeijer model first and subsequently in the TPM.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
Link to this item:http://purl.utwente.nl/essays/66159
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