Field test and numerical analysis of a helical steel pile under vertical compression loads considering installation effects

An in-situ vertical compression load test of a helical steel pile was conducted in silty sand and sand in order to study the bearing characteristics of helical piles in sandy soils under vertical compression loads. The difference of the existing methods for determining the ultimate bearing capacity of the helical pile tested was discussed. A finite element model of the helical pile under a vertical load considering installing effects was established based on soil layer parameters obtained from field cone penetration tests, and comparison of the pile load-displacement curves between the calculation and measuring results was performed. Based on the numerical simulation, the distributions of the axial force and shaft friction force of the pile along the depth under different load levels were explored, and the magnitudes of the shaft and end resistances and their ratio were investigated. The results show that the ultimate bearing capacity calculated by lgP-s method is larger than those obtained by Livneh&E1 Naggar and modified Davisson approaches, which indicates the later two methods over-conservative and that the simulation P-s curve considering installation effects gives better agreement with field measurement than that without considering installation effects. Under vertical compressive loading, the shaft resistance of the pile and the end resistance of middle-upper plates of the pile take effect almost simultaneously, and with increasing the vertical load, the end resistance provided by lower plates comes into play and increases gradually up to a peak value about 71% of the ultimate load with a displacement of the pile head exceeding about 4%of the diameter of plates. © 2019, Science Press. All right reserved.