Investigation of land subsidence with the combination of distributed fiber optic sensing techniques and microstructure analysis of soils

Land subsidence is a global phenomenon caused by a variety of factors, and there are still many challenges in land subsidence monitoring. The results of an investigation of land subsidence using the combination of Distributed Fiber Optic Sensing (DFOS) techniques and microstructure analysis of soil are here presented. Two DFOS techniques: Brillouin optical time domain reflectometry (BOTDR) and fiber Bragg grating (FBG), were developed in two boreholes (namely GM03 and SZ01) in Su-Xi-Chang area, China, to monitor movement of strata crossed by the borehole and pore pressure in some specific strata. Scanning electron microscope (SEM) and Mercury intrusion porosimetry (MIP) were employed to quantify and characterize the microstructures of undisturbed soils from the boreholes. Based on the BOTDR monitoring results, the annual levels of land subsidence in GM03 and SZ01 are ~1.2 mm and ~2.9 mm, respectively and the movement history of strata were clearly tracked. For both boreholes, the main contributors to subsidence are two aquitards (Ad2 and Ad3) that exist adjacent to the second aquifer (Af2). The results obtained by FBG-based sensors indicate good agreement with BOTDR and it confirms that the deformation of aquifer is significantly controlled by pore pressure. SEM images were analyzed with Pore/Particle and Crack Analysis System (PCAS). Three statistical parameters including average form factor (ffavg), fractal dimension (Df) and area probability distribution index (b), were used to describe the geometric characteristics of pores. It is found that larger average form factor, larger fractal dimension and higher proportion of large pores in soil microstructures favors land subsidence. MIP results confirm that area probability distribution index can be used for the estimation of strata compression potential in land subsidence investigations.