Optimization study on the reconstruction and expansion of an underground rail transit center in Shanghai soft ground

To meet the demand of rapid growing passenger flow, Shanghai-Century-Avenue-Transit-Center (SCATC) was reconstructed to integrate two new metro stations with the existing ones. The entire reconstruction process of SCATC involved a cluster of complicated construction cases and procedures including dismantling and restructuring of old station structures as well as deep excavations. In order to control the disturbance of the reconstruction to a minimum, a series of numerical simulations coupling with system control conceptions were conducted for optimizing the reconstruction process. All the involved major cases and their included procedures had been identified and thoroughly investigated to ensure that the incremental effect of each step and overall impact on the old or remaining stations were all within the control criterion for metro operation. The primary findings obtained were: (1) An approach of coupling system control conceptions with numerical simulations incorporated updating procedures was conducted to determine the optimal sequence for the reconstruction, and proved to be efficient and pragmatic; (2) A robust anti-upheaval system was applied and proved very effective in reducing the upheaval effects induced by dismantling structures. With the help of the anti-upheaval system, the displacements induced by subsequent actions had also been reduced much more than expected; (3) In the cutting-hole procedure on walls, by setting the hole size of per-cutting within 1 m and the hole-spacing of no less than 4 m with compensating measures for concentrated forces, the potential damages to the station structures had been prevented effectively; (4) By dividing the large pit into small independent pits with compartment walls, excavating each individual pit and forming its inside structures separately before the adjacent pit started, the rail displacement induced by the large pit excavation had been controlled within 10 mm in Shanghai soft ground; (5) The optimal overall sequence for the SCATC reconstruction was determined by investigating and comparing the effects of all construction paths defined by permutation of the major cases. The optimal sequence was executed and proved successful by the field measurements and observations.