Schematization uncertainties in the macrostability safety assessment

Abstract

The current macrostability safety assessment for primary river dike trajectories in the Netherlands is applied to approach the failure probability of a dike during high water events. However, in the current schematization process that is described in the Wettelijk Beoordelings Instrumentarium (WBI) to assess the macrostability, aleatory and epistemic uncertainties are approached ’sufficiently safe’ by applying design values based on expert judgement via a semi-probabilistic assessment. Several primary river dike sections in the Alblasserwaard do not suffice the current safety standard set for the failure mechanism macrostability. The region is composed of a highly complex subsurface with large spatial variation, resulting in large schematization uncertainties for the macrostability assessment of the primary dike trajectories of the Alblasserwaard. With the recent development of full-probabilistic analysis possibilities in software such as D-Stability, it becomes possible to consider uncertainties as a stochastic variable in the macrostability safety assessment. Including schematization uncertainties within the macrostability safety assessment will improve the approximation of the failure probability of the primary dike trajectory. The largest schematization uncertainties in the macrostability safety assessment are currently considered to be the schematization of the subsurface in a vertical soil profile and the uncertainties in the schematization process of the pore water pressures in the dike during high water events. These uncertainties will be included in the calculation process to investigate the influence on the expected reliability of the primary dikes in the Alblasserwaard region. The subsurface schematization uncertainties are investigated by using soil scenarios to investigate the influence of local subsurface schematization in the vertical soil profile. The simplification of the soil profile and position of the soil layers is considered. The pore water pressures are separated into three components: the hydraulic head in the aquifer, the intrusion length, and the phreatic line. Each component will be included as a stochastic variable in the stability analysis. Fragility curves can be applied to describe the distribution function for each pore water pressure component, where the combined fragility curve will provide the combined failure probability and reliability index that includes the schematization uncertainty of the pore water pressures considered.
The soil scenarios can be applied to include schematization uncertainties of the subsurface in the macrostability safety assessment. The analysis showed that the simplification of the subsurface schematization only has a minor influence on the reliability index and failure probability of the case study dike cross-section Kortenhoevendijk. The schematization uncertainties of the pore water pressures can be considered in the macrostability safety assessment by combining the fragility curves of each component describing the pore water pressures underneath the dike. Results of the pore water pressure analysis are that failure probability is improved significantly for case study Kortenhoevendijk by a factor 1000 and case study Bergstoep by a factor 10. The approach to consider schematization uncertainties in the macrostability safety assessment via a full-probabilistic analysis can be used for dike sections prone to the uplift mechanism. This approach provides insight into the influence of schematization uncertainties on the failure probability of a dike cross-section. Including the pore water pressure schematization uncertainties in the macrostability safety assessment can have a significant impact on the outcome of the assessment. Including these uncertainties can make the difference between deciding whether a dike trajectory needs reinforcement, or deciding that reinforcement is not necessary.