Ferreira, G, Garcia, M, Conceição, R, Lopes, I, Lopes, H & Rodrigues, V 2026, 'Partial drainage in tailings using the cone penetration test', in AB Fourie, M Horta, M Oliveira & S Wilson (eds), Paste 2026: Proceedings of the 28th International Conference on Paste, Thickened and Filtered Tailings, Australian Centre for Geomechanics, Perth, pp. 1-14, https://doi.org/10.36487/ACG_repo/2655_19
(https://papers.acg.uwa.edu.au/p/2655_19_Ferreira/)
Abstract: Due to the difficulty in obtaining good quality undisturbed samples, as well as the index properties and in situ state variability, the characterisation of tailings materials is often performed by in situ tests, such as the cone penetration test with pore pressure measurements (CPTu) or vane tests. CPTu is the most widely used in situ tests, for it allows for a quasi-continuous investigation reaching significant depths and encompasses a vast theoretical framework correlating with the most relevant soils’ physical and geomechanical parameters. However, while this framework was established and proven as suitable for natural soils, its complete applicability to tailings materials is often questioned. One of the common pitfalls is the use of the standard piezocone penetration rate (2 cm/s). This rate is based on the presupposition that permeable soils (sands) will respond in drained conditions and impermeable ones (clays) will present undrained behaviour. However, tailings materials often have high fine contents, mainly within the silt fraction, thus they have the potential for partially drained behaviour during penetration. In such cases, higher penetration rates are recommended. This paper presents the hydraulic characterisation of different tailings from different geographical origins, based on CPTu measurements at a constant penetration rate of 2.0 cm/s. The tailings are analysed in terms of their ore type (iron, aluminium, copper and zinc), index properties (grain size distribution, specific density, plasticity), in situ state (void ratio, water content) and deposition method (slurry, paste/thickened and filtered). The hydraulic properties are estimated based on the CPTu and dissipation tests, performed in different sites, and the results are discussed within the existing theoretical framework of partial drainage. As a result, the partially drained behaviour of a range of different tailings materials – from sandysilt iron ore to red mud – and the potential need to implement a different CPTu standard velocity for these tailings is assessed and discussed.

Keywords: tailings, permeability, penetration rate, CPTu, partial drainage