Real-time determination of soil suction in unsaturated clay based on POF sensor

Suction is a crucial property parameter of unsaturated soil, significantly influencing crop growth in agriculture and the strength, deformation, and seepage characteristics of unsaturated soil in geotechnical engineering. However, traditional methods for determining soil suction are challenging, time-consuming, and incapable of onsite monitoring of high suction region (>100 kPa). This paper proposes a novel method for determining soil suction by utilizing a Polymer Optical Fiber (POF) sensor coated with nanocellulosic film and graphene oxide film, based on the theory of thermodynamics (the relationship between suction and relative content of vapor water, characterizing the soil water retention curve) and optical fiber signal transmission. Compared with the filter paper method, a series of tests demonstrate that the proposed method can effectively measure the soil suction in unsaturated soil. The results indicate that it can measure soil suction within the range of 96 kPa to 911456 kPa, and within the temperature from 10 degrees C to 50 degrees C; when the soil moisture is balanced, the response time is 800 s. Additionally, the POF soil suction sensor features a compact size (5 mm), strong resistance to electromagnetic influence, high compressive strength, and suitability for large-scale civil engineering. This new method offers a valuable tool for addressing agricultural and engineering challenges related to unsaturated soil.