Interface behaviour of fibre optic strain sensors in cement-improved soil

Abstract This paper investigates distributed fibre optic strain sensing (DFOSS) in cement-improved soil, with focus on sensor–soil interface behaviour. Pull-out tests were conducted to evaluate interface bond strength and shear stiffness of four commercially available fibre optic sensors. Subsequently, laboratory-scale column compression tests, both uniform and layered, were conducted using either four identical sensor cables or four different types of sensor cables. The experimental results were interpreted using analytical methods and a finite element model incorporating a disturbed state concept-based interface model. The findings show that the sensor type has a significant influence on the interface behaviour with varying bond strengths and interface stiffnesses. In layered columns, interface shear stresses were mobilised at the interlayer boundary due to relative displacements between the sensor and the soil, indicative of localised strain decoupling. Between these decoupled zones, fully coupled zones were observed, characterised by uniform strain profiles. The coupled zones exhibited compressive stiffnesses in an intermediate strain range considering nonlinear soil characteristics, which was evaluated using a stiffness degradation model. Although the study focuses on cement-improved soils, the findings are applicable to DFOSS in layered and heterogeneous ground conditions in general.