Distributed acoustic sensing in ground improvement by dry deep mixing

This paper presents a full-scale field trial where distributed acoustic sensing (DAS) was explored as a novel technology to assess the quality of dry deep mixed columns, both for downhole testing and for surface wave testing. Optical fibres were installed vertically in columns and horizontally over the improved ground with geophones placed parallel to the optical fibre for validation. The test results show that DAS could reliably be used for downhole testing and to quantify the shear wave velocity (Vs) in the improved columns. Interpreted Vs ranged from around 190 to 400 m/s depending on column configurations, with a difference within ≈12% between the geophone and DAS data. Laboratory testing and cone penetration tests were also used to predict Vs in the columns, which gave results similar to those obtained from geophone and DAS data. The surface wave testing showed limitations in resolving the deep mixing columns using either geophone or DAS data, possibly due to a short array length, relatively low area replacement factor, and columns stiffness. Important practical considerations of DAS applicability in deep mixing are discussed, particularly gauge length versus wavelengths effects, coupling, and DAS directional sensitivity.