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Reflection: Macro stability – better parameters or models or do we need to reinforce the dikes?

Posted at 14/06/2021 by Bas Jonkman

Macro instability of the inner slope is an important failure mechanism that has a large influence on the costs of dike reinforcements and their impacts on the landscape. There are various uncertainties considering the strength parameters and models. There seem to be various options when a dike does not meet the standards: better parameter estimation, better models, or realize a conservative and expansive reinforcement. Which options could we explore to deal with macro instability in an efficient manner?

Contact details Bas Jonkman

Delft University of Technology

This reflection emerged from the discussion between the webinar team and 40 participants to the All-Risk webinar organised in June 14, 2021.

Would you like to view the presentations of the webinar team? You can view them in the above video.

Contributions, discussion and reflection

During the introduction presentation, Martin Schepers discussed the challenges in the field. He signalled that there are many uncertainties in the knowledge and information, and that expertise is not equally distributed amongst organisations. He also emphasised that there is a gap between theory and practice. Arny Lengkeek summarised his research as part of All-Risk, which also focussed on the Eemdijk full-scale field test. He presented new insights in soil classification and parameter estimation, particularly for weak deltaic soils that are present in the Netherlands. The new methods can contribute to a better estimation of the safety of a dike and more efficient reinforcements.

A single slide, OR Subsequent slides that can lead to flooding.

Guido Remmerswaal presented his research in the field of MPM (Material Point Method). The method – which is still under development – enables the user to analyse multiple subsequent slides and therefore gives insight into residual strength. The residual strength is much dependent on the heterogeneity of the soil strength. Follow up slides are possible due to correlations between weak soil layers.

What is the most important challenge for macro-stability?

The discussion focused on multiple topics

Participants seemed to agree that the main challenge is the lack of knowledge. This concerns multiple types of knowledge, and their combination, e.g. project experience, and expertise in various fields such as geotechnics, models and probability. The participants acknowledged that there is a need to share and transfer knowledge. There is no one recipe for success, but multiple key success factors. This need is about sharing knowledge (also between engineering firms) and establishing an open environment to share experiences concerning topics that can be sensitive, such as slides, deformations, and incidents during projects. Webinars seem a suitable format. Knowledge from experts in other fields, such as archaeology or soil experts, should also not be forgotten. Knowledge from other domains such as archaeology and physical geography should also be involved.

How do we get the required knowledge at a the next level?

Also other factors and initiatives were considered important: More soil investigations and better soil classification methods. Not all participants agreed that including residual strength was a good idea. This also became clear in the case study that was discussed. While some participants would accept large slides and deformations as long as flooding did not occur, others did not consider slides acceptable. It was mentioned that for dike managers, other functions (e.g. road function) and perception of safety would make it undesired to experience regular instabilities.

The participants emphasise that the positive effects of residual strength can only be included in a design if all negative effects are also included (e.g. residual loads and/or ignored causes of failure). When modelling residual strength, it is therefore important to calculate the entire failure path. In addition, side functions (e.g. road function) must also be taken into account, for which again different acceptable probabilities apply. The discussion also focussed on approaches that could contribute to a better soil characterisation. The majority seemed to support an approach in which expert knowledge would be combined with smart methods for automatic parameter estimation. Also, novel techniques such as artificial intelligence (AI) could offer opportunities and could be useful to utilize large data collections. When applying new techniques, experts should still be involved to avoid black boxes. Automated approaches may shift the balance toward automation and quantity of calculations, at the expense of quality.

All-Risk recommendations:

  • Share experience and expertise through webinars and expert communities. This could concern academic knowledge, but also practical experiences, also concerning slide incidents and experiences with reinforcements.
  • Utilise newly developed classification methods for soft soils.
  • Reinforcing dikes with sheet piles can contribute to more robust failure behaviour (more residual strength and fewer deformations and dike breaches), and it reduces the sensitivity for the presence of local weak soil layers.
  • Develop MPM toward broader applicability (stable calculations for multiple soil types and dike geometries).
  • Knowledge concerning parameters and soil behaviour during large deformations is essential in this respect.
  • Explore possibilities to combine existing methods for instability assessment with MPM to gain more insight into the possibility and likelihood of follow up slides and failure processes.
  • Define criteria for acceptable deformation of dikes (also linked to the Eurocode) considering the extent and acceptable frequency of deformations.

Related updates

Joint HWBP and All-Risk effort to make maximum use of the results of the sheet pile test in Eemdijk

31/01/2021 by Matthijs Kok

The paths of Matthijs Kok (program leader of All-Risk), Arny Lengkeek (advisor geotechnical engineering with 20 years of experience and PhD student in All-Risk...

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Last modified: 01/11/2021