Model test of soil deformation response to draining-recharging conditions based on DFOS

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 chapged 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.