University of Twente Student Theses
Finding safe boundary conditions in D-Geo Flow to assess the failure mechanism of piping : insights from two case studies
Gestel, D. (2024) Finding safe boundary conditions in D-Geo Flow to assess the failure mechanism of piping : insights from two case studies.
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| Abstract: | One of the important failure mechanisms within the safety assessment of dikes is piping or backward erosion. Piping is the failure mechanism where due to a high head difference between the river and the polder, groundwater starts to stream through the under the dike laying aquifer towards the hinterland behind the polder, where because of high effective stresses from the groundwater the covering layer bursts, resulting in heave which is the sub-mechanism of piping where liquefied sand flows up onto the surface in a sand boil through the bursting channel. With a strong enough seepage flow the pipe will continue growing from the heave towards the river, to potentially grow progressive and reach the river causing short-circuiting. When the pipe reaches the river the stream velocity in the pipe will grow exponentially, resulting in more transportation of water and sand from the aquifer to the sand boil, which in turn results in subsidence of the crest and possibly the dike losing its flood retaining function. In the Netherlands, dikes are assessed on their safety against piping periodically. Primary flood defence safety in the Netherlands has undergone changes in assessment over the past decade, and in 2024 the current assessment method is the BOI. Under the BOI, regional water authorities are given the task to assess their primary flood defences based on nationwide assessment rules, but also to incorporate regional characteristics. In case of piping, the presence of a poorly permeable foreland is important to consider in the assessment of the safety of the dike, because they might be of great significance to the sensitivity of the dike to piping. One of the assessment tools for piping in the BOI is D-Geo Flow. D-Geo Flow is a software developed by Deltares to calculate the pipe length for a given river head. In the BOI, D-Geo Flow is used to calculate whether a pipe grows longer than the dike base, in which case a further pipe analysis for the dike should be performed. Another function of D-Geo Flow is to calculate the critical pipe length and head for different schematizations, however this is not yet incorporated in the BOI. One of the reasons for this is that it is not yet known how a dike’s foreland and hinterland should be schematized in D-Geo Flow, and what boundary conditions are important to follow in setting up these schematizations. In this thesis the main geohydrological boundary conditions are determined to be the schematization of the heave boundary, the hinterland and the foreland, all three for an open / closed hinterland. In D-Geo Flow a closed hinterland assumes that all the groundwater leaves the system via the heave boundary, which is very conservative, because in reality a part of the groundwater will stream land inward. The results shows that the heave boundary should be schematized as two times the covering layer thickness; the hinterland should be set open and should be schematized land inward for a certain distance based on the specific schematization; and the foreland, if poorly permeable, should be schematized until the entry point of the pipe, but without the rest of the foreland or riverbed. When these three main geohydrological boundaries are kept in mind, it is possible to simulate more realistic results on the critical head and pipe. In further research it is recommended to investigate the influence of the aquifer thickness on the results to check if the conclusions still hold. Furthermore, it is recommended to perform calculations on the pipe length based on a given river head to see what effect the schematizations have on the calculated pipe length for a consistent head level. This could provide interesting insight in how D-Geo Flow simulates the influence of the initiation or progression dominated critical head, due to change in schematization. Results from these future projects could help in providing even more insight into the workings of D-Geo Flow. |
| Item Type: | Essay (Bachelor) |
| Faculty: | ET: Engineering Technology |
| Programme: | Civil Engineering BSc (56952) |
| Link to this item: | https://purl.utwente.nl/essays/104485 |
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