Automated High-Speed S-Wave Velocity Tomographic System for Laboratory Geotechnical Engineering Process Monitoring – A Study on Pile Installation

This paper reports the innovative development of a flexible, multiple-layer, economical but robust, and automated high-speed shear wave (Vs) tomographic control and data acquisition system for process monitoring in the laboratory and the associated validation for monitoring the dynamic process of model pile installation. This tailor-made tomographic system consists of four major parts: (i) the control unit built with a microcontroller board, (ii) a signal generating unit, (iii) three layers of bender element sensing arrays (including 14 source and receiver bender elements per layer), and (iv) a signal receiving unit. The system allows for sampling rates up to 1 GS/s (20 MS/s were used in this study); the required total measurement time is 2.94 seconds (i.e., for 196 ray path measurements) to allow for dynamic process monitoring. The obtained time-lapse Vs tomographic images during pile installation show that Vs first gradually increases when the pile toe is approaching the sensing layer and gradually decreases afterward as the pile toe penetrates through the layer. The validity of the Vs tomographic images is supported by similar responses in variations of the circumferential stress σc′ and radial stress σr′, measured by the tactile pressure sensors.