Development of an FBG-Based Sensor for Multipoint Density Measurement in Settling Clay Suspensions

Monitoring density variation of soil suspensions is essential for understanding their settling process. However, current measurement methods, such as radiation attenuation and sampling, are limited by safety concerns, tedious operations, and temporal lag. This study developed a fiber Bragg grating (FBG)-based density profile sensor based on Archimedes’ principle. The buoyancy acting on independently suspended sensing shells was converted into FBG wavelength responses and the sensor adopted a modular design, allowing density to be measured at multiple elevations. Calibration in NaCl solutions and Hong Kong Marine Deposit (HKMD)-water mixtures showed a linear relationship between Bragg wavelength shift and density variation, with experimental calibration coefficients close to the theoretical value. The density resolution of the density sensing units was estimated to be 0.0029–0.0031 g/cm³. A prototype measuring density at eight elevations simultaneously was validated in settling column tests on remolded HKMD. Compared with physical sampling, the density profiles obtained from the proposed sensor showed maximum deviations of 0.82% and 2.33% for the two initial densities tested. Soil-water interface settlement and pore water pressure measured in a parallel test were consistent with the sensor-recorded density evolution, demonstrating the feasibility of the proposed sensor for multipoint density monitoring of high-water-content clay suspensions.