Shear wave velocity measurement for ultra-deep vibro stone column composite foundations using pile-hole CT method

The evaluation of ground improvement effects achieved by ultra-deep stone columns is critical for assessing the safety and performance of deep foundations. Conventional field testing methods, such as standard penetration tests, cone penetration tests, and surface wave methods, encounter difficulties such as low resolution and even impracticability in deep soil layers. To address these issues, this study proposes a new method for evaluating ground improvement, based on the principle of cross-hole CT scanning, to obtain the equivalent shear wave velocity of deep composite foundations. This approach utilizes the construction-induced vibrations from a deep vibrator as the wave source and synchronously collects the signals using a geophone array installed in deep boreholes. The equivalent shear wave velocity of the stone-column-improved ground is calculated using a partitioned-regularization-based wave-velocity inversion algorithm, thereby allowing for an assessment of the improvement effect from the perspective of shear wave velocity. This method was validated through a case study of an earth dam foundation in China improved with ultra-deep vibro stone columns. The results indicate that the spectral characteristics of the vibrations are distinctive, and the vibration amplitude is correlated with key construction parameters. The equivalent shear wave velocity obtained through inversion shows an average absolute difference of 3~5% compared with the results from the microtremor method. After improvement, the equivalent shear wave velocity of the composite foundation increased by an average of 9.5%. The findings provide an effective and reliable technique for shear wave velocity measurement in deep ground improvement projects.