Finite Element Stress-based Stability Analysis of Diaphragm Walls for Coastal Protection at Onne, Rivers State in the Niger Delta

Abstract

The aim of this research is to model the stability of diaphragm walls subjected to seepage and sediment transport using finite element method for different soil conditions in the Niger Delta. Most coastal structures are supported by deep foundations and diaphragm walls for coastal protections or serving as quay walls. Diaphragm walls serve the dual purpose of coastal protection and berthing of vessels. Transient seepage/sediment transport (flow boundary conditions) and stress-deformations (stress boundary conditions) are the major controlling factors influencing the stability of diaphragm walls in coastal protection works. In this study, a two-dimensional uncoupled transient seepage and stress- deformation analyses were carried out to determine the stability of the diaphragm wall in different soil stratigraphy in Onne, Niger Delta region using the finite element stress-based method. The finite element solutions are based on the Galerkin’s weighted residual method and the use of Lagrange isoparametric triangular or quadrilateral elements representative soil stratigraphy along the coastline of Onne in the Niger Delta region were subjected to different stress and flow conditions modeled using a finite element method-based product Geostudio 2018R2V9.1. Uniform surcharge loads of 40, 60, 80, 100 and 120KPa were applied to the different case studies at varying dredge depths of 10, 16 and 19m under transient seepage conditions. Stability factors for critical slip surfaces and local stability factor of each slice for both shallow and deep-seated failure modes of the diaphragm wall were determined. The results obtained showed that diaphragm walls embedded in Stiff to firm clay offered better resistance than those in sand or sand with clay intercalations for both shallow (10m) and deep-seated slip surface (19m) failure modes due to transient seepage effects. The results also showed that the stability of diaphragm wall in coastal protection is strongly dependent on the depth of embedment as the stability factors in the case studies for all time steps decrease with increase in the dredging depth. The results further showed that stability factors reduce with increase in surcharge loads application for the case studies. The solutions obtained were in agreement with literature. Therefore, it is recommended to incorporate uncoupled/coupled transient seepage and stress-deformation analyses (finite element stress -based method) in the stability analysis and design of diaphragm walls for coastal protection works in the Niger Delta region.