Monitoring and evaluation of land subsidence based on BOTDR in Xuwei near Lianyungang

Located in northern of Jiangsu Province near the coastal area, Lianyungang is undergoing large-scale subsidence, and the rate of land subsidence is more than 20 mm/a in a lot places, such as Xuwei. Aiming at observing land subsidence intensively, the authors adopted the Brillouin optical time domain reflectometry (BOTDR) technique to monitoring a borehole with a depth of 127 m in Xuwei for more than 2 years. The results show that there are four confined aquifer groups in Xuwei, the accumulated settlement of soil in aquitard I-1 and I-2 accounts for 70.29% and 24.59%, respectively, and the maximum settlement of aquifer accounts for 1.38 %. The soil of 44 m in thickness in aquitard I-1 and I-2 includs muddy clay (L2), mild clay (L3), silt and sand layer (L4), and the main cause for subsidence is water from aquitard flowing to adjacent aquifer. Furthermore, additional load from engineering play a significant role in development of subsidence. The current situation of land subsidence in Xuwei is that the settlement is on the rise while its rate is decreasing. Therefore, BOTDR technique can monitoring the deformation information of all layers, which can help to monitor and evaluate the land subsidence in the study area.