Analytical–Numerical Investigation of Secant Pile Retaining Walls for Slope Stability in Soft Clay Using PLAXIS 3D

Abstract

The construction of the Presidential Palace in Indonesia’s new capital city, Nusantara (IKN), is located in an area dominated by soft clay soil and sloping terrain, which increases the risk of slope instability and poses significant challenges for structural safety. This study aims to evaluate the effectiveness of secant pile retaining walls in improving slope stability and controlling soil deformation under such conditions. The research employs a combination of analytical and numerical approaches, including Rankine earth pressure theory and limit equilibrium methods for manual calculations, and finite element modeling using PLAXIS 3D for validation. Soil parameters were obtained from field investigations (CPT and SPT) and laboratory testing. The results indicate that the initial slope safety factor of 1.8, which is at the minimum acceptable threshold, significantly increased to 4.73 based on analytical calculations and further to 5.7 from numerical simulations after the installation of secant piles. In addition, the maximum vertical deformation was recorded at –3.54 mm, remaining well within allowable limits. Structural analysis also confirms that the designed secant pile and capping beam satisfy shear strength and stability requirements. These findings demonstrate that the secant pile retaining wall system is highly effective in enhancing slope stability and minimizing deformation, making it a reliable solution for infrastructure development in soft soil and sloping environments.