A new explanation on causes of fractures in the ancient City Wall, Nanjing: observation, measurement, and modeling

The ancient City Wall of Nanjing (CWN) has suffered various damages since it was built more than 600 years ago. Numerous fractures and dislocations can be found on the wall. Furthermore, deterioration or destruction on several segments of the wall often occurred soon after restoration work was done, and brings some threats again and again on the people who either walk or live near the wall. The characteristics of these damages and their causes are unclear. In this paper the major fractures and dislocations on CWN were investigated through a field survey. It was observed that the apertures and dislocations of the CWN are usually large near the ground surface and decrease upwards and the most active fractures are located in the segments of the wall along a fault in NWW direction (290°). Twelve joint meters were installed across those fractures and the changes of the fractures' aperture were measured monthly from April, 2010 to December, 2011. The measured data show that the fractures' apertures are changing with time significantly, and the maximum cumulative change of apertures is 2.3 mm during 20 months. 80 bases were installed for the GPS measurement on top of the wall, and each site was measured monthly for the same time period as the joint meter measurements, the position of the wall moves significantly with time and the range of the displacement is ± 15.00 mm with the largest value of 20.00 mm. Based on the field survey and measurement, it is hypothesized that the changes of the fractures' aperture of the wall were caused by the non-uniform crustal movement. A three-dimensional mechanical modeling was built to test the hypothesis and the results confirmed the field observation. The findings from this study are significant and important to protect ancient walls and other human-made structures since the crustal movement may have never been considered before.