Bearing Characteristics and Negative Skin Friction Preventive Measures for Highway Bridge Pile Foundations in Collapsible Loess Areas Under Water Immersion

In collapsible loess sites, large-scale collapsible settlement may occur after water immersion, which will reduce the bearing capacity of existing highway bridge pile foundations and pose serious potential safety hazards. Given this, a large-scale field pile foundation immersion–loading test was conducted in a collapsible loess site. The settlement law of collapsible loess during the immersion was obtained, the bearing characteristics of pile foundations under the loading and immersion–loading conditions were compared and analyzed, and the formation mechanism of negative skin friction was discussed. The results show that the degree of collapsible deformation is related to the duration of immersion, external load, boundary conditions, and soil layer depth. Whether the collapsible loess site is immersed or not can only change the value and transfer rate of the axial force of the pile foundation but cannot change its transfer law. The collapsible deformation will increase the utilization rate of the pile tip resistance. During the collapsible settlement process, part of the gravity of the soil around the pile will be transferred to the pile, generating negative skin friction on the pile shaft. On this basis, eight preventive measures for reducing the negative skin friction of pile foundations in collapsible loess sites were proposed. The research findings can serve as a valuable reference for the design and construction of highway bridge pile foundations in collapsible loess areas.