Settlement monitoring using FBG sensors for soil under bridge approach slab

The settlement of soil under concrete slab road will cause road damage. When it occurs, a gap will form between the slab and the soil with air trapped in between. Normally, the thermal properties of the air and the soil are different. This paper tries to utilize this thermal disparity between them to address the issue of soil settlement under the concrete slab. FBG sensor is used in this paper for temperature monitoring. It has the attractive features such as excellent durability, immunity to electromagnetic interference(EMI), radio frequency interference(RFI), or electric current, and multiple sensors can be multiplexed along a single fiber. Its fabrication is described in this paper. To relate the temperature variation of the air and soil with the soil settlement, the temperature variation processes of air, and the materials of dry sand, saturated sand and water which represent the soil under the artificial temperature change situation and the atmosphere condition, are monitored and analyzed. Test results show that under the simulated temperature rising condition, the temperature variation of the air is far faster than the soil materials. Under the atmospheric condition, the temperature variation of the air is larger and more drastic than the soil material. These tests validate that, by identified the air position under the concrete slab, the occurrence and extent of the soil settlement can be indicated.