Physical and numerical pull-out modelling of ribbed piles

In situ cast concrete piles with helical ribs have been used in Shanghai, China for uplift piles. The piles are about 600 mm in diameter with half-cylindrical ribs of 75 to 100 mm in radius. Pull-out tests on model helical ribbed nylon piles of different rib spacing have been carried out in a large pull-out box under different confining pressures. Pull-out resistance and displacement of the piles revealed that the resistance varies with rib spacing and is at a maximum when the ratio of rib spacing to pile diameter is 1, and the resistance is twice that of a smooth pile with the same outer diameter. Three-dimensional finite-element analysis of the model piles revealed that the inclusion of ribs changed the soil and pile interaction mechanism. Shear wedges may form ahead of the ribs. The size of the wedges varies with rib spacing, and is the largest when the ratio of rib spacing to pile diameter is about 1. The wedges may connect to each other to form larger shear zones around the piles to increase the pile resistance. It is suggested that rib spacing is one of the most important factors that affect the pull-out resistance of the piles with the designed geometry.