## 3 Results and Discussion  
The data collection mainly focused on the period from September to October, which is the rainy season in South China and has complex weather patterns. Thus, the structural performance of the underground structures needs to be closely monitored during this period. The collected monitoring data from 17 sections was systematically analyzed, and the effects of vehicle induced vibration, temperature, tide and rainfall on time-dependent changes to crack width were investigated. The results show that the time-dependent changes to crack width were affected by the vehicle induced vibration on the time scale of seconds, temperature through the nature of thermal expansion of concrete on the time scale of hours, the tide on the time scale of days and rainfall on the time scale of months. In addition, it was found that rainfall could indirectly affect the influence of temperature on crack width through changes in the groundwater level.  
Influence of Variation in Tide Height and Temperature on the Change of Crack Width in the Box-Type Structure  
As the time-dependent changes in crack width in Sections 1, 3 and 4 due to the changes in temperature are similar, Section 1 was selected as a case study. Figure 11 shows the time-dependent variations in crack width, temperature and tides on September 1, 2 and 3 in 2009. In this study we investigated the effects of vehicle induced vibration, temperature, tide and rainfall on the time-dependent change in the width of an existing crack. A negative value of the change (%) represents a decrease in crack width, while a positive value of the change (%) represents an increase in crack width.  
Figure 11: Influence of variation in tide height and temperature on the change in crack width in the box-type structure (Section 1) on (a) 1, (b) 2 and (c) 3 September, 2009.  
During the tidal variation, the change of temperature had a great effect on the change of time-dependent crack width (Pearson correlation coefficient >0.6). For example, the variation of temperature can significantly affect the change in crack width (e.g., 120% increase in crack width) with Pearson correlation coefficients of 0.65, 0.84 and 0.86 for September 1, 2 and 3 in 2009, respectively. However, our results show that there is no strong correlation between the tidal variation and change in crack width. The change in crack width due to the change in temperature is a relatively slow process. Thus, the Pearson correlation analysis between temperature change and crack propagation were performed between the first and the third of September.  
To study the influence of temperature on the continuity of cracks, 15 days of uninterrupted monitoring were carried out on the cracks on the southern side of Section 1, as shown in Figure 12. By comparing the monitoring data for crack width with the temperature in the graph, the curve of width variation of the crack shows strong correlation with the temperature change curve. When the temperature increases, the width variation of cracks decreases, whereas when the temperature decreases, the width variation of cracks increases. Additionally, the temperature change effect on the width variation of cracks at high levels of the box-type structure is more obvious than that at low levels. Therefore, temperature has a greater impact on crack width, and is the main factor affecting crack changes. Simultaneously, the changes in cracks and temperature are not simultaneous, and there is a certain delay.