Abstract

Robertson (1986) soil classification on Sensitive Fine Grained Soils using CPTu is of interest since the variation of these soils include those of Consolidating Soils. However, the classification does not include into detail such as the influence of the existence of excess pore pressure prior to penetration testing or what is the contribution of the excess pore pressure to the total penetration resistance. Rahardjo et al (2008) studied this phenomena by extrapolation of the dissipation test data to determine the residual excess pore pressure which play an important role in CPTu testing. Further, Rahardjo et al (2016) continued investigation of the CPTu in Consolidating Soils using Bq, Bq* and Effective Stress Concept (Rahardjo et al. 2017). These findings have been very useful when using CPTu in very soft soils and ultrasoft soils where the undrained shear strength of the soils is very low. This paper is the results of several studies of CPTu testing in marine clays, lacustrine, ultrasoft soils or even peats. The author found that there is significant value of Bq and Bq*, hence are parameters of importance when dealing with soft soils and discussed in more detail. Recent applications are in very soft soils and peats in Sumatera and East Java mud eruption which are are also included.

Abstract

Historically, the analyses of tailings storage facilities (TSFs) have primarily focused on understanding the characteristics of tailings, while often overlooking a comprehensive evaluation of the foundation, as seen at Mount Polley in 2014. The Global Industry Standard on Tailings Management (GISTM) Requirement 5.4 stresses the importance of comprehensively addressing all potential failure modes. Numerous platinum TSFs in Southern Africa are underlain by residual clay, specifically a residual mafic rock from the Bushveld Complex. Similar soils, known as "tropical black clay soils", are found in other regions of the world. Surprisingly, there's limited public information on testing and modelling the behaviour of this clay foundation. This paper presents a case study that employs a novel approach to assess the impact of loading from tailings deposition on the underlying clay foundation of an upstream TSF. The analysis investigates how shear behaviour and consolidation characteristics of the clay foundation change with varying TSF heights and construction rates. The approach includes quantifying excess pore water pressures and their influence on the clay foundation's effective stress. The analyses primarily employ traditional limit equilibrium methods to assess TSF stability and foundation behaviour, with the potential to expand to numerical modelling. The study concludes that excess pore water pressures will significantly affect the Factor of Safety (FoS) of a TSF, particularly with adverse consolidation characteristics and increasing TSF height. This is primarily due to the low permeability and changing consolidation coefficient (cv) of the clay layer as the surcharge load increases. Furthermore, the research reveals that, depending on the rate of rise, tailings deposition may induce excess pore water pressures, potentially reducing the FoS. The ability to quantify excess pore water pressures using this novel approach enables a more accurate estimation of the FoS for facilities underlain by low-permeability materials, either residual or transported.