Evolution of shaft friction resistance in pile foundations of transmission towers under mining influence

A FLAC3D model of a single pile and its surrounding soil was established to investigate the evolution of shaft friction resistance and pile-soil deformation in transmission tower pile foundations subjected to mining-induced ground movement. Pile-head vertical loads of 0 kN,200 kN,400 kN,and 600 kN were considered. Changes in shaft shear stress,the neutral point,and the axial relative displacement between pile and soil were examined as the working face advanced,and the formation of negative skin friction was explained based on the interface shear transfer relationship. The results show that shaft friction generally evolves from being predominantly positive to a state of positive friction in the upper pile and negative skin friction in the lower pile. The negative skin friction zone extends upward; its boundary subsequently stabilizes while friction magnitudes continue to increase. Within the range of conditions investigated,lower pile-head loads lead to earlier onset,greater extent,and larger peak magnitudes of negative skin friction. Increasing the pile-head load from 0 kN to 600 kN increases the estimated critical advance distance for the onset of negative skin friction from approximately 184 m to 212 m. Mining-induced downward soil movement relative to the pile unloads the initial positive shaft friction and reverses it into negative skin friction. Upward migration of the neutral point corresponds to expansion of the negative skin friction zone in the lower pile. Under a 600 kN load,the neutral point remains at a depth of 14. 18 m-14. 46 m below the pile head for working-face advance distances of 350 m-500 m,while the magnitude of relative pile-soil displacement continues to increase. Stabilization of the zone boundary therefore does not imply that interface deformation has ceased to accumulate. These findings provide a reference for assessing pile foundation response and controlling deformation in mining-affected areas.