PhD Oral Exam - Yasir Mohammed Alharthi, Civil Engineering
When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
Piles are structural members that transfer the applied load of superstructures to deep supportive layers of soil or bedrock. Besides controlling the settlement of structures, piles provide sufficient capacity that other foundations cannot provide or provide only at a high cost. Despite ample research on the shaft resistance of displacement piles in cohesionless soils, the mechanism of such resistance remains unclear. Consequently, theories on shaft resistance have generated several discrepancies in predicting the capacity of displacement piles in cohesionless soils, not only due to the complexity of modeling cohesionless materials and collecting field data but also because the role of overconsolidation in such soils is often neglected. Whether occurring naturally or artificially, overconsolidation in cohesionless soils directly affects the lateral earth pressure that acts upon the pile shafts and thus upon pile capacity. Overconsolidation can occur when the ground surface is subjected to erosion, excavation, or unloading, often due to glacial melting, the demolition of structures, raised water tables, compaction, or vibration. Although the critical depth of pile foundation in cohesionless soils has long been debated, definite conclusions have yet to be drawn.
This thesis presents an experimental investigation into the capacity of driven piles in overconsolidated cohesionless soils. Tests, with an emphasis on the shaft resistance and the critical depth, were conducted on long piles in a setup that permitted measuring the overconsolidation ratio in the test tank as well as the total and local shaft resistance on the pile shafts. Shear stress distribution along the pile’s shaft showed some dependency on embedment depth ratio (L / D). As a result, an empirical design procedure was developed for driven piles in overconsolidated cohesionless soils to estimate the shear stress along the pile shafts and describe its distribution. Also, critical depth was observed for shaft resistance only when mean shaft resistance was analyzed, and was in line with Meyerhof’s (1976) results.
An analytical model was also developed based on limit equilibrium analysis using the horizontal slice method to predict the shaft resistance of a pile driven into normally consolidated cohesionless soils. The model assumes an inclined failure surface around the pile that accounts for the shear and normal stresses upon it. As a result, design charts and a procedure were formulated to predict the shaft resistance of driven piles in normally consolidated cohesionless soils. Notably, critical depth was not only observed but also increased linearly as the angle of shearing resistance increased.
Ultimately, a three-dimensional numerical model was developed and validated experimentally to perform 200 pile load tests in soils with various densities and at a range of embedment depths. Numerical as well as experimental and previous analytical results were analyzed to propose design charts for estimating the value of β, which combines the mobilized lateral earth pressure coefficient and the pile–soil friction coefficient (Ks tan δ), at different embedment depth and overconsolidation ratios. Among other findings, the value of β decreased as the angle of shearing resistance increased at the same overconsolidation ratio and embedment depth.