| Summary: | Metro tunnel sections in China can generally be divided into two types, those in granite formations (D = 6.0 m) and those in soft soils (D = 6.2 m), to which the same shield tunnel machine cannot be applied. The consequent low rate of machine utilization needs to be addressed. One solution is to modify shield machines which tend to tunnel granite strata with varying degrees of weathering (D = 6.0 m) into those applicable in soft soils (D = 6.2 m). Shield tunneling is a complex operation accompanied by potential risks, and accordingly it is demanded in risk evaluation and management. Hence, according to the construction features of modified shield machines in soft soil areas, this paper identifies relevant risks before establishing a specific model of risk evaluation by virtue of a fuzzy comprehensive evaluation method. This model weighs risk factors by triangular fuzzy numbers, and the membership function included is of L-R type that is frequently used in engineering. This risk evaluation model is applied to one section tunnel (Binhai New Town-Lianhua) of Metro Line 6 in Fuzhou City. Tunneling tests in the field uncover problems of the modified shield machine, including inappropriate tunneling parameters, segment dislocation, segment damage, and inadequate grouting. The result conforms to that produced by the risk evaluation model, which in turn proves the reliability of this model. Field data are also analyzed to address existing problems and to determine the appropriate tunneling parameters. The validity of these tunneling parameters is verified when surface settlement is measured.
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