Abstract

Pile foundations are widely used in civil engineering to transfer structural loads to deeper, more stable soil strata when surface soils lack sufficient bearing capacity. Many structures experience overturning moments such as retaining walls, high-rise buildings, offshore platforms, and transmission towers that produce axial compression on one side of the pile foundation and axial tension or uplift on the opposite side. The performance of piles depends largely on shaft resistance at the pile-soil interface, which controls settlement and load-bearing capacity under axial loads. Understanding shaft resistance behavior under compression, uplift, and loading sequences involving both is crucial for safe, economical design. Despite extensive research on shaft resistance of displacement piles in cohesionless soils, fundamental mechanisms remain inadequately understood, leading to inconsistencies in capacity predictions due to soil modeling complexity, limited field data, and often overlooked shaft resistance roles under certain loads. The concept of critical depth in cohesionless soils also remains debated without consensus.

This thesis presents a comprehensive literature review to identify gaps related to axial loading of vertical piles. An experimental investigation using scaled pile models, a steel tank, and precise loading systems was developed to measure shaft resistance distribution. A novel sand placement method ensures uniform relative density. Instrumented model piles, tested in overconsolidated cohesionless sands, vary parameters like pile diameter, embedment depth, relative density, angle of internal friction of sand, and slenderness ratio to assess their effects. Analytical models were developed to predict the earth pressure coefficients and shaft resistance as functions of embedment ratio for both compression and uplift scenarios. Moreover, an analytical equation was developed to calculate the overconsolidation ratio (OCR) as a function of relative density and depth.
The experimental program examines single piles under compressive-uplift sequences and pure uplift loading. Uplift tests with and without prior compression show that preloading enhances uplift capacity by increasing shaft resistance and earth pressure coefficients, although uplift capacity remains lower than compression due to soil compaction versus dilation behaviors. This study further analyzes earth pressure mobilization and investigates critical embedment depth effects. Through experimental and analytical efforts, a predictive shaft resistance model is developed and calibrated, with findings translated into practical design charts. These advances aim to improve shaft resistance predictions and enable safer, more cost-effective foundation designs under complex loadings. Finally, this study examines the uplift load-carrying capacity of single vertical piles embedded in granular soils, with a particular focus on developing and validating analytical models using the horizontal slice limit equilibrium approach. Experimental pile tests were employed to empirically determine the inclination angle of the failure surface, which was then incorporated into the model to enhance its accuracy.
The results address the critical depth for pile foundations in sand, piles subjected to loading and unloading, and piles to resist uploading.