Distributed Fiber-Optic Strain Sensing in Deep Mixed Columns

This paper presents a novel case study where distributed fiber-optic sensing (DFOS) was successfully employed in wet deep mixed columns for distributed strain sensing (DSS). Sensors were installed in cement-improved columns and interrogated with optical frequency domain reflectometry using Rayleigh backscattering to obtain a spatial resolution of 2–3 cm. The improved ground was gradually loaded with an up to 5.5-m-thick fill during frequent interrogation of the sensors. The sensors were shown to be highly responsive to loading, and measured strains ranged from around 150 to around 3,900 με, indicating a considerable vertical column variability resulting in large strain gradients, i.e., change of strain per unit length of sensor. Special attention was given to strain coupling between sensors and improved soil, and stress equilibrium considerations indicated that the interface shear strength was sufficient to maintain the measured strain gradients. Current design methodologies of strain magnitude and strain rate also agreed well with the measured strains, excluding the effect of a few weakness zones exhibiting large strains. The paper is completed with a discussion around practical issues concerning installation of DFOS sensors in deep mixing applications. In all, it is shown that DFOS is also a highly promising technology in the field of ground improvement by deep mixing and can reveal the performance of an improved ground with unprecedented spatial resolution.