Determination method for horizontal load-bearing capacity of pile group for large LNG tank

Objective The pile group foundation for large LNG tanks is subjected to relatively large horizontal forces under two seismic conditions: Operating Basis Earthquake (OBE) signifying a 10% probability of exceedance within a 50-year period and Safe Shutdown Earthquake (SSE) denoting a 2% probability of exceedance within the same timeframe. Hence, the checking calculation for the horizontal load-bearing capacity of piles is particularly critical. Nevertheless, the existing empirical formulas given in pertinent codes lack accounting for the superposition effect of pile-soil interactions at different positions within pile groups. As a result, these formulas yield conservative results in the checking calculations, potentially leading to inefficient investments in pile foundation construction.

Methods To address this shortcoming, this study conducted accuracy verification of soil layer parameters through geotechnical analysis software, PLAXIS 3D, by integrating existing data from field experiments. Numerical simulations were carried out to analyze the elevated pile cap foundation for 20×10⁴ m³ large LNG tanks under varying seismic loads, obtaining the horizontal force distribution characteristics of pile groups under different conditions.

Results The study revealed that the front-row piles in the force direction experienced greater horizontal forces compared to the center pile, with their ratio decreasing as horizontal loads increased. Traditional design methods outlined in relevant codes yield conservative results due to disregarding the superposition effect of pile-soil interactions at various positions within pile groups, potentially resulting in significant financial waste. To tackle this challenge, a method to determine the horizontal load-bearing capacity of pile groups for LNG tanks was introduced. This method, based on horizontal load-bearing capacity experiments on individual piles, was subsequently compared with traditional design methods.

Conclusion In addition to factors like the numbers of piles, pile spacings, pile diameters, and constraint modes at the pile tops, this method integrates real pile-soil interactions by adopting a reasonable soil constitutive model and soil layer parameters. On the premise of ensuring sufficient horizontal load-bearing capacity in pile groups, the proposed method significantly reduces design conservatism, serving as a reference in improving the cost efficiency of pile group foundation design with elevated pile caps for LNG tanks.