Direct shield cutting of large-diameter reinforced concrete group piles: Case study on Shenyang Metro construction

Abstract

Increasing demand for subways or metros has resulted in the need to build new tunnels in close proximity of pile foundations of existing underground structures. This study presents the details of cutting of existing large diameters reinforced concrete group piles which lie in the excavation path of the new tunnel directly by shield. Investigation is made based on the on-site case study of the construction of the right tunnel of Shenyang Metro Line 4, which crosses the Shenyang North Station of Metro Line 2. During the process, 17 reinforced concrete piles with diameters ranging from 800 to 1000 mm were cut. The details of the ground improvement, the replacement of the cutter, the cutting of the reinforced concrete piles, and the wear of the cutter are presented in this paper. From the monitoring data, it is found that the advancing speed is lower during the cutting of reinforced concrete piles, and the values of thrust and torque fluctuate. The field measurements show that the settlements and horizontal displacements in the existing tunnel and metro station are within the allowable limits when the reinforced concrete piles are cut directly with the shield. During the cutting of the reinforced concrete piles, the steel rebars usually break under tensile stress, and the tension fracture accounts for 66.2 % of the total fracture of steel rebars. The cutters installed at different heights have a cumulative effect on the cutting of piles, and the cutters installed at higher positions have a protective effect on the cutters installed at lower heights. The average wear amount of rippers at 165, 175, and 210 heights are 6 mm, 15.2 mm, and 24.8 mm respectively. The construction method used in this project can serve as a reference and experience for similar future projects.