Study on negative skin friction of piles induced by mining subsidence using transparent soil model tests

Due to surface subsidence caused by coal mining, the negative skin friction (NSF) has seriously threatened the safety of piled building foundations. In order to study the mechanism of the NSF of piles during the mining subsidence process, a set of pile test model using transparent soil was developed. Based on the law of the ground movements and deformations, and according to the test result, the mechanism of pile NSF in mining subsidence is obviously different from that in traditional nature foundation. Test results show: The NSF of the pile foundation was first generated at the pile tip. With the increase of surface settlement, consequently, the pile-soil displacement increased; furthermore, the neutral position of pile NSF gradually extended upward along the pile, and moved up as high as 0.85 L of the pile; in consequent, as the pile settlement increased rapidly, and the relative pile-soil displacement decreased gradually, therefore the neutral position gradually decreased. Comparison analysis between the bearing capacity of the friction pile and that of friction end-bearing pile induced by mining subsidence, friction end bearing piles are not suitable in mining subsidence area.