A fibre Bragg grating stress cell for geotechnical engineering applications

Abstract

Existing technology used to measure stress in granular materials is susceptible to water ingress and resulting damage to the electrical components, which limits this technology's use in long-term monitoring of soil structures. The connections of these instruments to readout or recording devices are also fragile and easily damaged. In order to explore the efficacy of fibre optic technology not just as a replacement component, but rather as a material of robust elastic properties, a series of experiments was devised to test the possibility of applying a transverse lateral stress to the fibre and measuring its transformed longitudinal strain. The fibre was inscribed with a 5 mm long Bragg grating and encapsulated in a softer material of high Poisson's ratio in an attempt to enhance the longitudinal strain developed in the fibre. This encapsulated device was then subjected to one-dimensional stress in a standard sand, and the strain in the fibre measured. The experiment showed that satisfactory correlation exists between the measured strain output when converted to an applied stress and the mathematically (and numerically) derived stresses. The fibre/silicone bonding was also modelled and results showed that slippage on that interface could be considered insignificant. In both theoretical and practical applications, this experiment can be regarded as successfully validating the principle of deriving stress from a longitudinal elastic strain measured normal to the applied stress. Hence, development can move towards both miniaturization (for research) and more robust construction (to withstand field conditions). Further research will encompass investigating the response of the cell to water and saturated soil conditions, particularly the device sensitivity to transient stress conditions. In addition, development of the sensor to read the complete three-dimensional state of stress in a soil remains the ultimate goal.

FBG-Based Monitoring of Geohazards: Current Status and Trends

“…On the basis of indirect sensing using epoxy resin-embedded FBG sensors, Correia et al developed a pressure sensor with a resolution of 0.66 kPa over a measurement range of 300 kPa, which was suitable for measuring pore water pressures in soil [54]. A similar sensor, but instead encapsulated in a softer material of high Poisson’s ratio, was devised by Legge et al with the advantage of enhancing the longitudinal strain induced in the fiber [4]. Huang et al measured the pore pressure profile in boreholes using FBG sensors and evaluated the stability of rainfall-induced deep landslides [55] (Figure 5).…”

“…For instance, the prices of an interrogator and a FBG sensor are about £20,000 and £50–300, respectively. But it is predictable that once the FBG system becomes common practice, the cost will decrease significantly [4]. …”

“…The readings taken in the field are often sensitive to electromagnetic interference (EMI) [3]. The quality of waterproof measures also significantly affects the durability (long-term performance) of these sensors [4]. In order to capture dynamic deformations and evaluate stability conditions of geological structures in real time, it is of great importance to develop an improved smart monitoring system which can overcome the above shortcomings.…”

Structural Health Monitoring of Civil Infrastructure Using Optical Fiber Sensing Technology: A Comprehensive Review

“…Kim et al [ 144 ] developed a specially designed FBG-embedded tendon for the monitoring of the prestress force and load transfer of the ground anchor and the feasibility of the device was verified through laboratory and field tests. Legge et al [ 145 ] developed an FBG-based stress cell for determination of the full state of three-dimensional stress at any accessible or predetermined point in a soil mass or structure.…”

Patents and Techniques of Contact Pressure Measurement in Geotechnical Engineering

“…An interesting example that demonstrates the application of FBG in contact pressure measurement is the work of Legge et al [32]. The fibre Bragg grating stress cell was made by encapsulating the FBG sensor in a silicon casing.…”