Comparative Numerical Analysis of Surcharge Effects on a Cantilever Sheet Pile Wall in Stiff Clay

Cantilever sheet pile walls rely on passive resistance below the excavation level and are therefore sensitive to nearby surface loads. This study evaluates the combined effects of surcharge magnitude and distance on a 4 m deep excavation supported by a 9 m long steel sheet pile wall in homogeneous stiff clay. A two-dimensional plane-strain model was developed in PLAXIS 2D. The clay was represented by a drained Mohr-Coulomb model, while the wall was modelled as an elastic plate with interface elements on both sides. Uniform 4 m wide surcharges of 50 and 100 kPa were applied at normalised distances x/H of 0, 0.5, 1.0 and 2.0. A no-surcharge model was also analysed. The excavation was modelled as a staged construction process with four successive 1 m excavation stages, and stability was evaluated using the strength reduction method. When the 100 kPa surcharge started immediately behind the wall, the maximum wall displacement, maximum bending moment and excavation-induced surface settlement reached 31.0 mm, 107.3 kN m/m and 11.2 mm, respectively, while the factor of safety fell to 1.40. Moving the same load to x/H = 2.0 reduced the displacement and bending moment by 54% and 78%, and increased the factor of safety to 2.10. For the 50 kPa series, the wall displacement approached the no-surcharge condition once x/H reached 0.5, whereas the bending moment approached the reference value more gradually and became comparable at x/H = 2.0. The results show that increasing the surcharge distance substantially reduces wall deformation. However, the factor of safety remains lower than the no-surcharge value, even when the deformation response becomes comparable to the reference case.