Abstract

Tailings storage facilities pose an increasingly significant global challenge for mining companies. Given that tailings, the by-products of mine processing plants, demand meticulous management due to the potential hazards associated with storage facility failures, continuous attention is imperative. The repercussions of such incidents can lead to profound and enduring environmental damage in downstream regions. Therefore, consistently monitoring the structural stability of these facilities is paramount to ensure the safety and sustainability of both active mining sites and legacy mines. This paper introduces an innovative approach using muon tomography to analyze the internal density structures of tailings dams. The research, conducted at the BHP Prominent Hill Mine site in South Australia, presents a pioneering method and technology for examining the internal density distribution of tailings storage facilities using muons. Muons, being a natural source of radiation capable of generating X-ray-like scans of large structures, offer a distinctive opportunity to gain insights into the inner structure of tailings storage facilities. Moreover, this technology holds the potential to serve as a foundational platform for future research in the development of early warning systems for detecting structural changes.

Abstract

Secondary compression can be an important source of settlement in artificial fills, even when these fills are well constructed. In some cases, especially when the fill thickness is greater than about 15 m, the resulting long-term settlements can adversely impact the performance of structures and infrastructure, and thus may necessitate special preventive design provisions. Yet, this source of settlement is often mistakenly overlooked. Secondary compression can be even more problematic when the fill is poorly constructed. Backfills of former open-pit mines are examples of practical projects where assessments of long-term secondary compression settlement are necessary, especially when these backfills are deep and/or not properly engineered. Laboratory assessments of secondary compression in these materials are inherently problematic and become impossible when the fill contains large particles or has other complicating characteristics. However, this problem is an excellent opportunity to apply the observational method where the coefficient of secondary compression, Cαε, is assessed in-situ using settlement monument data. This Cαε value is then used to forecast future settlements, which typically continue for decades, and thus provides essential information for the site-specific design of structures and infrastructure. However, the experimental and analytical processes for conducting these evaluations are more difficult than might be expected, and missteps can lead to significant errors in the computed future settlements. Some of these difficulties are due to limitations in our knowledge of the underlying physical processes and in the analytical models used to describe them. Methods of collecting the required field data and conducting these settlement evaluations are discussed based on experience with deep fills in California as well as published data from elsewhere.

Abstract

International standards for monitoring under ISO have been under development since 2010 and the intentions were presented during the 8th International Symposium on Field Monitoring in Geomechanics (FMGM 2011) in Berlin. Progress on this project was reported at FMGM 2015 in Sydney and ISFMG 2022 in London. The base standard on general rules was published in 2015, part 2, the standard on extensometers, in 2016 and in 2017 the third part on inclinometers. Part 4 on piezometers was published in 2020, Part 5 on pressure cells in 2019 and Part 8 on load cells in 2023. These standards have been published as ISO Standards in English and French worldwide. In Europe the standards have been published under EN ISO 18674 member countries of CEN have to publish these standards in their country. The standards on settlement measurements by hydraulic systems (part 6), geodetic measurements (part 9) and strain measurement (part 7) are under development and will be submitted to enquiry in the next few months.

Abstract

The weathering profile in mudstone outcrops can range from weathered clay to unweathered mudstone. The strength and small-strain stiffness of these materials, and of stiff clays and weak rocks in general, is critical to the design of geotechnical structures. Monitoring data and ground investigation data were gathered during the construction of a trial embankment founded on weathered, Jurassic-aged mudstone, for the HS2 high-speed railway project (UK). These data included outputs from in-situ downhole geophysical tests and extensometers at the embankment location. These were compared to ground investigation data from the wider mudstone outcrop over an 18.2 km stretch of the route. The installation of extensometers prior to loading by the embankment construction enabled the measurement of in-situ strains for a range of small and medium strains. It was therefore possible to characterise the stress-strain behaviour of the individual layers within the ground profile. The results showed that stiff clays derived from weathered mudstone at shallow depth (<10 mbgl) are characterised by low undrained shear strength (<300 kPa) and by maximum stiffness values that are comparable to laboratory measurements from fine-grained soils. However, the unweathered mudstone at greater depth (>15 mbgl) showed higher strength (>300 kPa) and higher maximum stiffness than the overlying clay. Both the strength and stiffness profiles showed a transition zone between the weathered clay and the unweathered mudstone. This zone included a partially weathered mudstone that had the visual appearance and index properties of a mudstone, but the strength of a stiff clay (i.e. τu < 300 kPa).

Abstract

Piezometers are instruments that can produce high-quality information if suitable and effective installation and monitoring procedures are followed. In order to minimise errors and optimise quality of the obtained information, we must pay attention when prescribing the type of piezometer and its installation, as highlighted in the ISO/EN18674 standards. The use of piezometers as a geotechnical instrument is a commodity. This paper aims to exemplify how the use of piezometers can be optimised and how the most common errors can be avoided.

Abstract

Drilling boreholes for geotechnical purposes such as sampling and in situ tests is usually performed with the aid of drilling machines. In France, typically, the machine’s drilling parameters such as applied torque and thrust are monitored in realtime and registered so that information about the subsoil’s structure can be acquired. As these machines tend to use hydraulic systems, the oil pressures fed into each motor and actuator are the parameters commonly monitored. Recently, a new type of sensor and its application in geotechnical investigations through the measuring of drilling parameters was presented. This sensor is directly mounted on top of the drill string of a drilling machine and records the actual thrust, torque, and rotation speed transmitted to the drill string and bit. This bypasses the energy losses present in the hydraulic circuit between the machine’s pressure sensors and the hydraulic actuators. This mounting position allows a more accurate measurement of the forces and, furthermore, the effective rotation speed applied to the drill bit. The data is transmitted through a wireless Bluetooth connection enabling real time monitoring. Nevertheless, finding the right balance between a soil’s resistance, the drill rig’s power and the sensor’s optimal measuring range remains a topic for further development. This analysis of several worksites with different soils throughout France contributes to a better understanding of the last generation’s sensor’s precision and application range.

Abstract

Recent advances in distributed fibre optic sensing enable new opportunities in geotechnical monitoring and characterization. Distributed acoustic sensing (DAS) is a distributed fibre optic sensing technology relying on Rayleigh backscattering of light to detect and locate disturbances in near real-time along tens of kilometres of fibre optic cable. The dynamic strain sensing capabilities of DAS have prompted numerous research initiatives from the seismology community over the past decade. Although research on DAS for seismic applications is well established, studies on DAS for geotechnical monitoring applications are less common. Here, we present a summary of two recent case studies involving DAS for geotechnical monitoring. The first study considers a slow-moving landslide where DAS data were acquired over a three-day period of rainfall. The DAS aseismic strain and strain-rate data support the interpretation of the triggering and retrogression failure mechanism of the landslide. The second study considers an active mine site in northern Canada. Data were acquired from a cable installed ~1m below a tailings dam crest. Passive seismic interferometry was applied to DAS data to infer changes in seismic velocities in the uppermost several meters of the subsurface. These findings represent a first step towards advancing continuous monitoring techniques with fiber optic sensing technologies. However, further research is needed to improve our understanding of DAS performance for geotechnical monitoring applications over longer-term periods.