Abstract: The structural abnormal response of underwater shield tunnel under operation is the prerequisite of structural abnormality early warning. Based on the monitoring segment strains of typical sections of the Wuhan Yangtze River tunnel during normal operation period (2013–2020), the distributions of strain increments, the relations of strain increment with temperature increment and water level increment, and relations of strain increments between adjacent segments (i.e. spatial effects) are thoroughly studied. An early warning method of structural abnormal response based on strain increment is proposed and the corresponding early warning threshold value is determined. Study results show that: 1) The distribution of strain increments does not obey the normal distribution but has a fat-tail. The exponential distribution can be used to describe the distribution of the absolute value of the strain increment. 2) The temporal strain increment is difficult to predict, but its variation range can be well determined through statistical analysis. Thus, an early warning method for structural abnormality is proposed based on the monitoring strain increment of a single measuring point, and its threshold value is determined. 3) There is a deformation mode of "one expand, the other contract" between adjacent segments during normal operation, which means the strain increases of one segment but decreases of the adjacent segment. This spatial effect constraints the average strain increments of adjacent segments in a relatively small variation range. Thus, another early warning method can be developed based on the monitoring strain increments of several adjacent measuring points, and its threshold value is determined.

Key words: shield tunnel, structural health monitoring, segment strain, statistic method, early warning, threshold value