Estimating a pile shaft’s frictional capacity is challenging and has been a controversial subject among researchers. In this study, the shaft friction resistance of non-displacement (pre-installed) model piles under axial load was investigated. Four different model piles were used, including steel, timber, and two composite piles (FRP and PVC filled with concrete). The angle of interface friction (δ) between test sand, and pile materials was determined using an interface shear test (IST) at four relative densities. Axial pile load experiments were implemented in a soil tank and piles were embedded into loose to very dense sand. Model pile load tests were performed in such a way that there was no end (point) bearing capacity (only friction was generated), and lateral friction resistance between the pile material and the soil along the pile shaft formed the complete bearing capacity of the model pile. According to experimental results, it was observed that, with increasing sand relative density and surface roughness of the pile material, the shaft friction resistance of the model pile increases. A back-calculation analysis was also performed to find the values of lateral earth pressure coefficient (K) using Burland’s (1973) equation with the help of measured shaft friction capacity of the model pile load test. By performing multivariate regression analysis, an equation was obtained between the back-calculated lateral earth pressure coefficient (K) and other parameters. The obtained equation was used to calculate the K values given in other studies in the literature. It was determined that the obtained equation was in good agreement with the data in other studies. This equation can be beneficial in practice and can be advantageous for further study in the future.

In this study, the presence of a foundation near the sheet pile wall built on sand soil was simulated in the laboratory, similar to the field conditions. For this purpose, laboratory tests of sheet pile wall (SPW) (defo) and bearing capacity (q_ult) of the foundation were carried out by applying vertical loads to the steel model foundation. Besides, laboratory tests were represented by the Mohr‐Coulomb material model under 3D plane deformation conditions, and numerical analyses were performed. These studies were performed by varying the distance among the SPW, the foundation (L), and the penetration depth of SPW (Hp). The results demonstrated that L had a greater effect on q_ult and δmax than the H_p. When L was kept constant and H_p was increased, it was seen that q_ult increased and δmax decreased. Moreover, the raise in L has resulted in an increment in the qult and δmax decreased. In addition, a good agreement between numerical analysis and experimental test was observed.

In this study, the load distribution of the piles in the pile group was investigated in the case of the pile raft foundations failure under the effect of vertical loads in cohesive soils. For this purpose, loading tests were carried out on model pile groups in the laboratory. In addition, the effects of these conditions on the load distribution between the piles were examined by changing the pile spacing and soil properties. As a result of the experiments, it was determined that the piles in the corner region carried more load than the ones in the center region in small settlement amounts as in the literature. However, it has been determined that as the amount of settlement in the pile foundation increases, the soil between piles behaves like a block and approaches failure, and the piles in the center region of the pile group carry more load. As a result of excessive settlement, a fracture surface formed around the block and therefore the corner piles of the block began to carry less load. It has been determined that failure analyses should be performed considering the load carried by the corner piles after failure due to excessive settlement.