Ground strain measuring system using optical fiber sensors

This paper presents a device to measure the dynamic horizontal shear strain of the ground during earthquakes. The proposed device consists of a bronze plate with fiber Bragg grating (FBG) sensors attached on it. The device is vertically installed in the ground, and horizontal shear strain of the ground is measured as deflection angle of the plate. Employment of optical fiber sensors makes the proposed device simple in mechanism and highly durable, which makes it easy to install our device in the ground. We conducted shaking table tests using ground model to verify applicability of the proposed device. The displacement measured by the proposed device is compared with displacement measured by the displacement meter. They gave identical results, which indicates that proposed device can measure the ground strain with sufficient accuracy. Displacement measured by the device also shows good agreement with displacement calculated from the ground acceleration. This result shows proposed device well represents the deformation of the ground. We also obtain the hysteresis curves of stress-strain relationship of the ground. It shows the temporal variation of the shear modulus during shaking. The experiment results show that dynamic horizontal shear strain of the ground can be measured accurately by the proposed device, and that the accuracy of the device is sufficient for practical purposes.