Development and Engineering Application of a Vertically Installed FBG Sensor for Subgrade Strain Monitoring

Subgrade deformation under load is accumulated from the local compressive strain of each layer. Solely relying on the overall settlement monitoring at the top surface makes it difficult to reveal the internal strain distribution and the evolutionary characteristics of early damage. Therefore, acquiring the local compressive strain field along the depth direction is of great engineering significance for subgrade condition assessment and safety early warning. In this paper, a vertically installed FBG sensor (VI-FBG) with GFRP packaging suitable for layered strain measurement of subgrade is developed. The effectiveness of the sensor is verified by direct compression tests and compressive strain tests with the sensor embedded in concrete. Furthermore, the sensor was buried in the subgrade of a high-speed railway to carry out strain monitoring. The test results show that the sensor has favorable sensing performance, with a strain sensitivity of 1.2 pm/με and a temperature sensitivity of 16.84 pm/°C. The measured compressive strain of concrete is in good agreement with the theoretical value, and the relative error is less than 8%. The field monitoring data can truly reflect the vertical compressive strain during subgrade filling. The measured time-history curve exhibits good consistency with the displacement time-history curve obtained by the adjacent FBG settlement sensor. This study verifies that the sensor is applicable to the monitoring of local vertical compressive deformation of subgrade, and its GFRP encapsulation endows it with good engineering applicability.