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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.

In this study, a model test was carried out to investigate the hydro-mechanical properties of calcareous sand, and distributed fiber optic sensing technologies were utilized to monitor the entire process. The test procedure had three stages: one-way water supply, one-way drainage, and two-way drainage. The actively heated fiber optic (AHFO) method was used to monitor the soil moisture profiles, and soil deformation was captured using the high-accuracy optical frequency domain reflectometry technique. The Pearson correlation function was used to describe the relationship between soil compression and moisture content. The results indicated that the AHFO method allowed soil moisture measurement in saturated and unsaturated zones with a bias of 0.027 m3/m3. Water infiltration in calcareous sand can lead to uneven settlement, toppling, and horizontal deformation, with most settlements occurring during the watering stage. The ground settlement was mainly caused by particle sliding and rotation during the watering and drainage processes. The results show that settlement occurred earlier when the soil was closer to the water supply chamber. Moreover, tension cracks can easily appear between the calcareous sand and fine-grained soil interlayers. The Pearson correlation coefficients were larger than 0.8, indicating that the soil layer was compressed owing to particle sliding and rotation, and the moisture contents had a linear correlation with compressive strains.

Intense groundwater exploitation and seasonal fluctuations of water level in phreatic aquifer cause land subsidence disasters. To better understand the land subsidence mechanism caused by groundwater withdrawal, a small-scale sand-clay interbedded model box was built to carry out consolidation and rebound tests during two drainage-recharge cycles. Distributed fiber optical sensing (DFOS) technologies were introduced for coupled monitoring of soil strain and water content to analyze response characteristics of each layer to water level changes. The results indicate that soil layers were compressed during drainage and rebound during recharge, with clay layer deformation more obvious than sand layer deformation. Deformation of the sand layer was synchronous with water content changes, while that of the clay layer lagged behind water content changes due to its lower permeability coefficient. The segmented compression clay layer curve shows that the water content rapidly decreased when it was lower than the liquid limit, while the clay layer compression rate obviously accelerated during drainage. When the water content was higher than that of the liquid limit during recharge, the rebound rate significantly increased. Under the condition of repeated drainage and recharge, the soil microstructure changed with decreasing porosity and permeability coefficient, leading to a longer consolidation time. With increasing cycling times, the soil gradually tended toward elastic deformation. The observations propose a novel technology, DFOS technology, for monitoring land subsidence caused by seasonal water level fluctuations and dewatering projects. The test results are of great significance to investigate land subsidence mechanisms and evaluate soil layer compression potential.