A New Method for Piping Risk Evaluation on Unconfined Aquifers under Dewatering of Deep Foundation Pits

This paper presents a new method to evaluate the piping risk of unconfined sandy cobble aquifers during the dewatering of deep foundation pits. Firstly, the possibility of piping is judged according to the particle size distribution and pore diameter of the sandy cobble stratum. Secondly, the critical hydraulic gradients of particles with different sizes are calculated according to the pipe flow model. Thirdly, the hydraulic gradient around the foundation pit during the dewatering process is calculated by numerical method. Then, combined with the distribution of hydraulic gradient around the foundation pit and the critical hydraulic gradients of particles with different sizes, the range of moveable particles with different sizes in the surrounding stratum is evaluated. Finally, according to the sandy cobble aquifers conditions and the influence of particle movement on the stratum, the level of piping risk is determined. This method was successfully applied to evaluate the piping risk of Tawan Station in Luoyang Metro Line No.1, and according to the risk evaluation the pensile curtain was recommended for the retaining structure in this case. From the satisfied applications, this method is a reference for the evaluation of piping risk and selection of retaining structure when considering the impact of the dewatering in deep foundation pit with unconfined aquifers on the surrounding stratum.