Revision of the calibration equation for a river dike on macro stability

Abstract

Two types of calculations are currently used to assess dikes for macro stability. These are the semi-probabilistic calculation and the probabilistic calculation. In practice, the semi-probabilistic calculation is currently mostly used. However, the probabilistic calculation is the most accurate calculation of the two methods resulting in the most realistic results. Performing a probabilistic calculation is on the other hand less simple and a lot more time consuming. Therefore it would be more efficient to still regularly apply the semi-probabilistic calculation to save time and also find a way to make this calculation more accurate. Although the probabilistic calculation results directly in a reliability index, this is not the case with the semi-probabilistic calculation. To link the target reliability index to the factor of safety of the semi-probabilistic calculation, a calibration formula is used. This formula was made using a set of probabilistic calculations from the calibration study. The formula is designed in such a way that in reality 80% of dike profiles are calculated too conservatively with the semi-probabilistic calculation and 20% not strict enough. This means that this formula results in a lower reliability index for the majority of dike profiles than what the realistic value would be. The result of this conservative formula is that in some cases actual reliability of the dike profile can deviate reasonably from the probabilistic reliability index. Some profiles can therefore be made far too robust, while a few others may be approved unfairly. There could be a lot of profit to be made in both safety and in the amount of reinforcement required if the existing calibration formula can be further elaborated. This leads to the main question of this report: “How can the current calibration equation for the assessment of a dike on macro stability be elaborated by using the results of probabilistic calculations from the calibration study and SAFE?” As emerges in the research question, the probabilistic calculations of two projects are used for the research; the calibration study and the project SAFE. The calibration study is the project from Deltares on which the current calibration formula is based. The set of dikes in this project consists of a mix of local and regional dike profiles around the Netherlands. The set is arranged in such a way that almost every type of dike in the Netherlands appears in it so that it can be applied as widely as possible. In contrast to the dataset from the calibration study, the dataset from SAFE consists of dike profiles of only two dike sections at the Lek. The dike profiles belonging to SAFE have been compiled by the water authority and calculated probabilistically by Sweco and Arcadis. The data set from safe is composed in such a way that the critical dike locations of these 2 sections on the Lek are included. Several ways to see if the current calibration formula can be made more accurate are investigated. Initially, SAFE's location-specific set is compared to the calibration formula. It is checked to which extent SAFE corresponds to this and what the reasons can be for existing differences. It is also examined whether the current calibration formula can be improved on the basis of different dike characteristics. A distinction is made of several general , soil and geometric characteristics. The research shows that SAFE's dike profiles require a slightly different calibration line than the line from the calibration study. Instead of 80%, only 64% of the dike profiles lie below the calibration line of the calibration study from Deltares. Especially at the lower reliability indexes, the profiles from SAFE are not calculated safely enough. It is not demonstrable that the profiles from SAFE lead to different results due to the fairly similar type of dikes from the project or the specific location. The difference will therefore mainly be due to the different calculation methods of both projects. In addition, there were also two dike cases/characteristics that has an influence on the difference in reliability index; the design profiles and the berm. With Both, it seems useful to draw up a special calibration line that takes this into account. Especially a calibration line for only the design profiles produces a much more accurate result. This new calibration line results in 30% more accurate reliability indexes. The conclusion of the report is therefore that it would be effective to apply a separate calibration line for SAFE and for the design profiles. For SAFE, the difference with the current calibration line could also be solved with a better calibrated schematization factor, using frequently probabilistic calculations or by renewing the existing calibration study because the difference that occurs seems to be mainly due to the calculation method. Moreover, a separate calibration line for the design profiles thus produces a lot more accurate results. However, the consequence of these more accurate results is that fewer profiles will be on the positive side of the calibration line. The separate calibration line for the dike profiles without designs is 20% less accurate but resulted in safer calculations. When both formulas for designs and normal profiles are used, the complete data set gets a little more accurate with 8% and also even safer than the current calibration formula. So there is certainly something to be improved from the current calibration formula by distinguishing between dike profiles without design and with design.