The Master’s thesis by researcher Nabaa Kareem Kadhim—supervised by Dr. Osama Salem Abdul-Karim—was defended at the Department of Civil Engineering, College of Engineering, University of Basrah. The thesis is titled: Numerical study of Weak Subgrade Soil Reinforced With Geogrid
Weak subgrade soils, characterized by low bearing capacity and high deformability, represent a major challenge in pavement engineering because they can cause premature deterioration of pavement layers and increase maintenance costs. This study aims to evaluate the effect of using geogrid reinforcement on the performance of flexible pavements constructed over weak subgrade soil, with particular emphasis on the geogrid location and axial stiffness. A three-dimensional numerical analysis was conducted using PLAXIS 3D 2024, and the numerical results were compared with data obtained from previously published field tests.
A parametric study was conducted using subgrade CBR values of 2%, 5%, 7.5%, and 10%, considering two geogrid locations: at the interface between the base and subbase layers, and at the interface between the subbase and subgrade layers. Three geogrid axial stiffness values of 800, 1000, and 1200 kN/m were also investigated to assess their effects on settlement reduction and pavement response. The results showed that geogrid reinforcement reduced vertical deformation and improved the distribution of loads and stresses. The most effective geogrid location was found to be at the interface between the base and subbase layers.
The results also indicated that increasing geogrid axial stiffness generally reduced pavement settlement, with the effect being more pronounced for the weakest subgrade condition (CBR = 2%), where settlement reduction exceeded 40% when the higher-stiffness geogrid was used compared with the unreinforced condition. The results further showed that increasing subgrade strength does not necessarily lead to a proportional increase in reinforcement efficiency. The study confirms the importance of geogrid reinforcement in improving the functional and structural performance of flexible pavements constructed over weak subgrades and highlights the significance of modeling soil–geogrid interaction when selecting the appropriate reinforcement location and stiffness in pavement design