An experimental study on applying spatial TDR to determine bentonite suspension penetration

• Introducing a pioneer study on measuring suspension penetration via spatial TDR. • Analysing TDR-waveforms to determine suspension penetration depth. • Identifying suspensions with varying bentonite and sand concentrations. • Applying mixture equations on suspensions with varying bentonite and sand contents. • Predicting suspension penetration from measured permittivity. In slurry shield tunnelling, the penetration of the supporting bentonite suspension must be reduced to a critical value to ensure safety and cost-efficiency during construction. Aiming to measure bentonite suspension penetration, this study adopted the spatial time domain reflectometry (spatial TDR) technique. Although traditional TDR can detect point-wise changes in bentonite suspension concentration of pore fluid, this technique has rarely been extended to spatial profile detection. Spatioscale tests with a flat ribbon cable TDR sensor demonstrated the potential of TDR waveform analysis for determining penetration depth. Relationships between penetration depth and waveform characteristics were established. The travel time specified by the dual tangents method decreased with increasing slurry penetration, and the determined travel time agrees well with that calculated by a newly proposed mixture equation. This novel approach enables the determination of penetration depth without visual observation, providing a powerful measuring solution for laboratory studies and slurry shield tunnelling.