Modelling and Dimensioning of a Natural Riprap Protection Dike Against Coastal Erosion on the Djiffère and Joal Spit

This work aims to study the design and modelling of a longitudinal riprap protection at the bottom of the beach in the areas most threatened by coastal erosion on the Djiffère and Joal spits. The choice of design and modelling techniques is based on the geotechnical and geological characteristics of the subgrade. The sizing allowed us to determine all the parameters necessary to quantify the riprap structure to be installed and the results show that it makes it possible to attenuate the energy of the waves but also to reduce their incident heights by 72%. The dike will help reduce the crossing and will act as a protection for the infrastructure located towards the mainland. The conditions of stability are assured, as well as an analysis of the impacts of the project on the sustainable development of the Joal and Djiffère coastline. Modelling with the Plaxis 3D software allowed us to model the dike with ease and to calculate the safety factor (determination of the balance, stability and displacements of the protective dike). It also allowed us to evaluate the sensitivity of the safety factor with respect to these parameters based on the finite element method using the Mohr Coulomb model. It appears that for a total stress of 1067 KPa, we observed a horizontal displacement of 10.19 cm for the rapid drawdown phase, which falls within the range of tolerable deformations for our dike, i.e. less than 20 cm. In this study, the rapid drawdown of the sea level in 5 days had no influence on the safety factor from ΣMsf = 1.861 to the value of the last phase ΣMsf = 1.972 satisfactory for the dike in terms of stability, i.e. less than 1.4. Thus, the rise in sea level has the effect of producing additional deformations. Keywords: Protective dike, dimensioning, drawdown, geotechnical modelling, 3D Plaxis.