During devastating earthquakes, soil liquefaction has disastrous outcomes on bridge foundations, as mentioned in books and published research. To avoid foundation failure when the surrounding soil is fully liquefied, a bridge’s pile foundation design could be such that the bridge pier is directly resting on the top of a large-diameter monopile instead of the traditional multiple small-diameter piles.
This paper discusses the gap of insufficient studies on large-diameter monopiles to support railway bridges subjected to buckling instability and the lack of simplified tools to quickly assess structural reliability.
A framework could quickly assess the structural reliability by formulating a simplified reliability analysis.
This study focused on pure buckling with shear deformation and reliability assessment to calculate a monopile’s failure probability in fully liquefied soils. In reliability assessment, with the critical pile length (Lcrit) and the unsupported pile length (Luns), the limit state function g(x) = [Lcrit − Luns] thus forms the basis for assessing the safety and reliability of a structure, indicating the state of success or failure.
The Lcrit formulation is accomplished with a differential equation. Here, Luns assumes various depths of liquefied soil. The reliability index’s (β) formulation is achieved through the Hasofer–Lind concept and then double-checked through a normal or Gaussian distribution.
A case study was conducted using a high-speed railway bridge model from a published research to demonstrate the application of the proposed methodology. To validate the minimum pile diameter for buckling instability when a fully liquefied soil’s thickness reaches the condition that Lcrit = Luns, this study applies the published research of Bhattacharya and Tokimatsu. The validation results show good agreement for 0.85–0.90 m monopile diameters.
With a monopile diameter smaller than 0.85 m, the Lcrit = Luns limit was at lesser depths, while with a monopile diameter larger than 0.90 m, the Lcrit = Luns limit was at deeper depths.
A load increase notably affected the large-diameter monopiles because the Lcrit movement required a longer range. In fully liquefied soil, buckling will likely happen in piles with a diameter between 0.50 m and 1.60 m because the calculated probability of failure (Pf) value is nearly one.
Conversely, buckling instability will likely not happen in monopiles with a diameter of 1.80–2.20 m because the Pf value is zero.
Hence, the outcome of this case study suggests that the reliable monopile minimum diameter is 1.80 m for supporting a high-speed railway bridge.
Lastly, this paper analyzed the shear deformation effect on large-diameter monopiles, the result of which was 0.30% of Lcrit.
Shear deformation makes minimal contributions to large-diameter monopile buckling.
