Study on the clay weakening characteristics in deep unconsolidated layer using the multi-point monitoring system of FBG sensor arrays

•Multi-point monitoring based on FBG sensor arrays has been proposed and implemented.•Evolution process of clay layer weakening has been detected and demonstrated.•Survival rate of FBG is illustrated and suggestions to ensure which is also provided. A large area of thick unconsolidated stratum layer overlying the coal seam of eastern China, which brought the regional disaster prevention issue under the circumstance of unconsolidated layer settlement triggered by mining activities. In this paper, the multi-point monitoring system based on fiber Bragg grating (FBG) sensors array was implemented into the deep unconsolidated layer to realize a long-term ground monitoring, which investigated the compression process of ground rock layers at different burial depths and revealed the mechanical weakening characteristics of clay. The deformation characteristics of the aquifers were analyzed, including a sand layer, gravel layer, and clay water barrier, as well as the weakening characteristics of the clay layer aquifer. The monitoring results of the FBG sensing system show that unconsolidated layer settlement usually occurs in thick and stable aquifers and adjacent clay layers, and the peripheral part of the deep clay layer in contact with the aquifer will become a weakened zone. The clay layer at such locations should also be a key monitoring layer with increasing depth. The survival rate of FBG sensors were discussed to propose the suggestions of system installation. These findings could provide important guidance for predicting the unconsolidated layer settlement process and for the timely implementation of governance measures.