Abstract: The effect of drilling velocity ratio, the ratio between the rotation and the penetration velocity, on the boring mechanism of auger piling was investigated with a series of model tests on both loose and dense Fujian standard sands. The variation of reaction force and rotational moment of the auger with the penetration depth, and the induced vertical and radial stresses within the soils during auger penetration were analyzed at the macro level. The variation of sand relative density and the evolution of soil fabric due to auger penetration were microscopically examined using the epoxy resin solidifying method and the digital image analyzing technique. It is found that the reaction force, the rotational moment and the induced maximum radial stress decrease with the increment of drilling velocity ratio, independent on soil initial density. In loose sand, the soils at the auger tip and around the borehole exhibits contraction behavior, whilst the dilation behavior is captured in dense sand. The extent of contact normal anisotropy on the circumferential cutting face of the solidified sand block reduces with the increment of drilling velocity ratio, yet the distribution of contact normal on the radial cutting face is not significantly affected by the drilling velocity ratio. This regularity is independent of the soil’s initial density.

Key words: soil displacement screw pile, piling auger, drilling velocity ratio, sand, indoor model test