The stability of deep excavations in cohesionless soils depends strongly on groundwater conditions and excavation geometry. Two-dimensional analyses, although widely applied, often fail to capture three-dimensional effects, leading to conservative designs. This study investigates an optimum hybrid support system; secant pile walls combined with prestressed tie-back anchors, through a three-stage methodology: A limit equilibrium analysis, followed by finite element simulations in PLAXIS 2D and PLAXIS 3D. A total of 81 scenarios were analyzed, considering soil density (loose, medium, stiff sands), surcharge loading (30 to 90 kN/m²), excavation depths (6 to 18 m), and groundwater levels. The results show that groundwater level is the dominant factor governing anchor demand, with normalized anchor consumption (β) increasing significantly as the water table rises. The hybrid system ensured stability across soil types by mobilizing pile resistance at depth and anchor efficiency against lateral displacement. Comparisons between 2D and 3D models revealed substantial differences: wall deformations predicted by 3D analyses were up to 62% smaller for 20 m wide excavations, 36% for 30 m, and 9% smaller for 50 m. Bending moments also decreased by as much as 62% in narrow excavations. The findings demonstrate that 3D modeling provides a more realistic representation of excavation performance. Moreover, the proposed β ratio further assists in evaluating anchor efficiency, helping prevent over-design while ensuring safety. Keywords: 2D-3D Finite element analysis, Limit equilibrium method, Anchorage, Scant piles, Optimum design.
Mohammadkeya Khosravi (Mon,) studied this question.