Development of an FBG–MEMS-Based 3-D Principal Stress Monitoring Device in Soil

The stress state is the basis for evaluating soil deformation and strength. To study the 3-D stress state of the soil, a 3-D stress device based on fiber Bragg grating (FBG) and a microelectromechanical system (MEMS) was designed to measure the 3-D stress state, principal stress, and direction of principal stress. First, the FBG 1-D earth pressure cell is developed and calibrated. Based on the calculation relationship between the normal stress and the 3-D stress state, and considering the device rotation, the 3-D stress device was developed and verified in the large-scale triaxial test. The results show that the range of the 1-D earth pressure cell is 1 MPa, and the linearity is within 2.5% FS. The device can effectively measure soil's 3-D stress, principal stress, and direction. The measured results are consistent with the reference and theoretical results. The device provides a new method for effectively measuring 3-D stress in soil.