University of Twente Student Theses


Macro-stability safety assessment for flood defenses with burried pipes

Groot, T. (2015) Macro-stability safety assessment for flood defenses with burried pipes.

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Abstract:A growing demand for multifunctional flood defenses causes the direct need for more knowledge and understanding of the effects of a structure placed inside a flood defense, called transition, to the physical condition of a flood defense (van Loon-Steensma & Vellinga, 2014). Constructors and policy makers do at the moment not have a complete understanding of the direct implications on safety caused by transitions (Morris et al., 2012). This leads to the situation that concepts of multifunctional flood defenses will not be approved (lack of norms) or executed (to high risk). Therefore the goal is to deliver geotechnical information which predicts the safety of a flood defenses under influence of a transition during an extreme loading situation. Focus of this research is mainly on making a model which predicts the effect of a buried pipe on the development of macro-stability failure mechanism since the structure element pipeline is in present-day one of the mostly applied structures within dikes (Finsbury, Steven L. Stockton, & Loudiere, 2013). The geotechnical conditions, which provide information about the effect of a pipeline on macro-stability are simulated with a 2D, stationary coupled model build in the finite element software COMSOL 4.4. This model applies a coupled combination of a Darcy model for modelling the flow of water in the porous media and a Drucker-Prager linear elastic perfectly plastic model for modelling the soil displacements in the soil skeleton. The model predicts the potential slip circle for the dike of the macro-stability failure mechanism, not the critical slip circle. Analysis of the effect of the pipe to the potential slip circle is based on the calculated effective plastic strains, a dimensionless monotonically increasing scalar value describing the unrecoverable portion of the true strain beyond the yield limit (Melnikova, 2014). Validation of the model is performed using the data of the Southdike, an IJkdijk real scale experiment. The Southdike has been brought to failure by means of macro-instability and it was coped with a comprehensive sensor system, therefore perfect for this research. The model predictions are validated on three aspects with this data; safety factor, hydraulic head and displacement. The safety of the Southdike is slightly overestimated due to a smaller slip surface. Hydraulic heads and displacements are modeled within an acceptable range. The model predicts the characteristic of the pipe such as location, size, material and the content of the pipe influence the development of the macro-stability failure mechanism. This can be a negative or positive influence, closely related to the principles of Bishop. The change in safety condition of the macro-stability failure mechanism cannot be addressed solely to a single characteristic of the pipe. A combination of characteristics of the pipe and the conditions of the dike determine the safety situation. The soil conditions and the loading conditions also have a significant influence to the effect of the pipe on the safety condition of the macro-stability failure mechanism. The general conclusion is that for certain conditions the characteristics of the pipe have a positive influence to the development of the macro-stability failure mechanism. Combinations of characteristics and conditions determine the gain in safety for macro-stability failure mechanism. Therefore the second conclusion is that for every individual situation with different conditions and characteristics of the pipe, a calculation has to be performed to know the safety situation to macro-stability of the flood defense.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:10 humanities in general
Programme:Civil Engineering and Management MSc (60026)
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