Sampling disturbance during thin-walled tube sampling in gold tailings

Abstract

Tube sampling is a widely used method for obtaining soil specimens in geotechnical investigations. However, the process inherently disturbs the soil, altering its in-situ state and potentially affecting measured properties used for design. Quantifying this disturbance is challenging because the undisturbed soil state cannot be directly measured, making it difficult to establish a baseline for comparison. While extensive research has examined sampling disturbance in conventional soils such as clays and sands, the specific response of soft tailings materials to tube sampling remains poorly understood. In particular, pore pressure generation during sampling and the resulting soil deformation patterns have not been quantified for tailings materials. Understanding the mechanisms of disturbance alludes to an improvement of sample quality assessment and the development of effective sampling procedures for tailings characterisation. The purpose of this study was to investigate the driving factors contributing to sample disturbance during tube sampling of soft gold tailings material under controlled laboratory conditions. Two complementary experimental approaches were employed: First, pore water pressure responses were monitored using tensiometers (capable of measuring both positive and negative pore pressures) positioned inside the sampling tube and at fixed radial distances in the surrounding soil during all phases of the sampling process, i.e. tube insertion, pore pressure dissipation, vacuum application, tube extraction, and post-sampling handling. Second, Particle Image Velocimetry (PIV) was used with modified half-tube samplers inserted against an observation window to track and quantify soil particle trajectories and strain development during tube penetration. Three thin-walled tube sampler diameters (50mm, 75mm, and 100mm) with identical cutting edge geometry were tested to assess the influence of sampler size on disturbance mechanisms. The study reached several important conclusions. Pore pressure monitoring demonstrated that tube sampling generates significant pore pressure responses in soft tailings, with the magnitude of response directly related to sampler diameter. Distinct pore pressure responses were recorded for each phase of the sampling cycle, providing evidence of mechanical response of the material during penetration. These measurements confirm that there are effects on the in-situ stress state occurring even under controlled conditions in homogeneous, fully saturated materials. The study also indicated that the largest diameter sampler (100 mm) had the least amount of disturbance than compared to the two smaller counterparts (50 mm and 75 mm). The PIV study showed that sampler diameter influences both the extent and magnitude of soil deformation. All three tube diameters produced measurable soil disturbance through the development of shear and volumetric strains. While larger diameter samplers showed somewhat different strain magnitudes compared to smaller samplers, the differences were not substantial, all configurations disturbed the soil significantly. The calculated centreline strain paths did not show agreement with existing theoretical models. Clear relationships were established between strain magnitudes and sampler geometry parameters, including area ratio, inside clearance ratio, and outer cutting edge taper angle. This research provides an understanding of disturbance mechanisms in soft tailings during tube sampling by linking pore pressure generation and physical strain development. The findings demonstrate that both mechanical insertion and stress redistribution contribute significantly to overall sample disturbance, with sampler diameter playing a key role in the magnitude of these effects. These results have practical implications for site characterisation investigations and contribute to informed sampling disturbance mechanisms for soft, saturated materials.