Abstract:

Some diseases, i.e. uneven settlement, excessive structural deformation, seepage and local structural deterioration, would occur to metro shield tunnels crossing deep soft soil under longterm cyclic loading, which would affect the whole service life of tunnel structures. Hence, by taking Foshan Metro Line No. 3 for example, the reinforcement methods for soft soil commonly used in domestic metro project are analyzed from the perspective of construction risk, environmental impact, investment and structural safety in operation period; and the grilletype triaxial mixing pile reinforcement technology is proposed as a pretreatment measure. Two kinds of reinforcement schemes of deep cement mixing piles with three axes are compared and selected by numerical calculation analysis; the difference between internal force and deformation of segment is compared, the recommended scheme is determined, and it is verified by field monitoring of engineering practice. The results of numerical calculation and field monitoring show that: (1) When shield tunnel passes through deep silt layer, the reinforcement width should not be less than 3 m outside the arch waist, and the depth should be about 0.5 m below the better bearing layer. (2) Compared with the unconsolidated stratum, the deformation of the tunnel after foundation reinforcement can be reduced by 30% to 40%, and there is no obvious stress concentration in the structure.

Key words: shield tunnel, soft soil ground, ground treatment, numerical simulation, site monitoring