The effective stresses in saturated soils are crucial for geotechnical engineering, particularly in the ocean environment, but no current transducers can directly measure both vertical and lateral effective stresses. Thus, a novel effective stress transducer based on fiber Bragg grating (FBG) technology is developed to directly measure three-dimensional (3D) effective stress in saturated soils. The design of the transducer ensures that pore water pressures inside and outside the transducer are balanced, allowing the strain to solely reflect the effective stress sustained by the soil skeleton. Two FBG sensing elements of the 3D effective stress transducer are designed to measure the vertical and lateral effective stresses by sensing the strain in the thin plate and the sensing cylindrical shell through the porous disk, respectively. Experimental results indicate that the transducer accurately captures the evolution of effective stress under complex static loads and precisely tracks cyclic stress variations under cyclic loadings. Compared to traditional transducers, the lateral earth pressure coefficient derived from the measurement data of the new effective stress transducer shows advanced accuracy and stability. Moreover, the FBG-based transducer effectively monitors effective stress changes during the excavation, capturing soil stress variations and enabling precise excavation stability assessments. The novel 3D FBG-based effective stress transducer offers a vital method for directly measuring the vertical and lateral effective stresses of saturated soils.
