Key Technologies for Borehole Drilling by Rotary Drilling Machines in Heterogeneous Ground: Case Study on Bored Piles of Vertical Shaft of Xiangjiang River Crossing Tunnel

The bored piles of the vertical shaft of Xiangjiang river crossing tunnel on Yingpan road in Changsha are located in heterogeneous ground,i.e.,sandy cobble strata in the upper part and medium-weathered slate in the lower part.During the construction of the bored piles,serious borehole collapse,slurry leakage and other problems occur.Technologies,including strictly controlling the slurry indexes,using polymeric slurry,using wear-resistant drill bit with alloy bullet-shaped teeth and backfilling high quality soil,are adopted.In the end,the construction of the bored piles has been completed successfully and the pile quality is guaranteed.