A modified fiber Bragg grating (FBG) oedometer ring for measuring lateral strain and stress during 1D soil compression

Reliable assessment of stress state in soils during oedometer compression requires effective measurement of lateral stress. Traditional monitoring techniques, such as stress gauges and strain gauges, suffer from poor interference resistance, limited durability, and restricted spatial coverage, whereas fiber Bragg grating (FBG) sensors can effectively address these issues. This study develops an oedometer based on the FBG technique for continuously monitoring lateral stress evolution. The lateral stress was calculated based on a realistic elastic assumption. The FBG-based oedometer ring was calibrated using three different materials over a broad range of vertical stress (0 - 35 MPa). Moreover, oedometer tests were conducted on soils with varying water contents (16%, 18%, and 22%), with monitoring lateral stress. Results show that the developed FBG-based ring effectively captures later stress evolution, demonstrating feasibility in laboratory tests and potential for field applications. The evolution curve displays two distinct stages: an initial stage and a yield stage. During the initial stage, as the soil skeleton remains relatively stable under vertical compression, the soil structure gradually compacts, increasing lateral stress. Once vertical stress exceeds yield stress, the soil enters the yield stage, during which soil particles begin to slip and fill pores. The variation rate of lateral stress with vertical stress (δ σ h / δ σ V ) slows and eventually stabilizes, resulting in a linear relationship between lateral and vertical stresses. Comparing specimens with different water contents reveals that the soils with higher water content exhibit higher lateral stress and greater δ σ h /δσ V under a given vertical stress, suggesting that the increase rate of later stress depends on soil compressibility – the greater the compressibility, the faster the lateral stress increases. This work presents an effective method for monitoring lateral stress and offers insights into the underlying relationship between mechanical and microstructural behaviors of soil.