2018 GHD Prize in Geomechanics

An experimental analysis was completed to determine the settlement behaviour of tailings from a coal mine in the Hunter Valley. Various properties were assessed to provide recommendations for use in industry.

The tailings by-products generated during coal extraction and processing consist of fine particles suspended in water-based slurries, often with solids concentrations as low as 25%. The industry guidelines recommend management of these materials by storage in large dams. The design of these facilities is complicated by the physio-chemical properties of the tailings. Throughout the design-life, settlement and consolidation under self-weight causes increase in material density. The rate of settlement is dependent on the initial solids concentration, drainage potential and presence of ions in the pore fluid.

Column tests were used to assess the tailings behaviour when reconstituted to low solids concentrations. Tests were completed at various effective stresses, in columns with varied flow capabilities and with a pore fluid ions representative of artificial sea water (ASW) and the standard process water (SPW) produced in the industry. Measurements were taken at regular intervals to determine the change in density and permeability over time. The consolidated material was analysed to evaluate the impact of the variables on the degree of settlement.
The slurries exhibited three primary stages of settlement: initial settlement, transitional settlement and prolonged consolidation. The material generally reached a final consolidated density of 30%. Increasing the initial solids content beyond this did not cause an increase in settlement. A positive correlation was found between the material permeability and void ratio. The introduction of pore fluid ions increased the rate of settlement but did not further increase the final density. The ASW was more effective than the SPW. The implementation of a permeable base layer to allow drainage was the only effective measure to increase both the rate of settlement and the final density. Centrifuge testing demonstrated that under applied forces higher degrees of consolidation could be achieved.

To ensure accurate sizing, the design of tailings storage facilities should involve consideration of the likely rate of settlement and final density. Additional measures should be implemented to increase these properties, with the most effective method being the inclusion of a permeable layer to allow drainage of pore fluid. Further investigation should be completed to determine the applicability of these findings to large-scale designs.

This study aimed to compare the efficiency and cost of contact and contact-less methods of gathering data about natural geological structures. Where contact sensors are physical devices such as a geological compass or smart phone application that can measure the dip angle and direction of a rock mass. Contact-less sensors are cameras used to develop an image set of the rock face to create a 3D model of the rock face. From this 3D model virtual measurements of the rock features can be taken for comparison with the measurements from the contact devices.

The project began with planning and research for the data acquisition stage. Once the data was acquired it was then processed using software such as Photoscan to develop point clouds from the photos and eventually a 3D model. CloudCompare was also used to virtually compare the accuracy of the models with each other and take measurements of the features. Once the virtual measurements were taken, they were plotted on scatter plots to visually compare how all the various methods of measurement weigh up against the geological compass measurements. The geological compass was used as a benchmark.

One important objective was to identify whether the smartphone applications were worth using or not, how reliable they are when compared to a geological compass. From preliminary results it seemed all the methods generally had a degree of reliability about them. Of the results from the smartphone applications, that of the iPhone 4 (an older generation) was the most unreliable. It provided inaccurate dip direction due to the device constantly requiring calibration. However as phones these days are becoming more and more accurate in terms of the GPS function, it means the potential of these devices will only grow.

The objective of this study was to produce and characterise a transparent soil surrogate that could be used in the physical modelling of soft clays. This was achieved through the use of the silica-based colloid, Laponite, with the ultimate goal of developing an improved understanding of the kinematics and failure mechanisms of real, soft clay-like soils.

Given the promising results of previous studies in the applicability of this material in the scaled modelling of soft clays, this study investigated the mechanical response of Laponite soil surrogates of varied configurations under vane shear loading; a common laboratory and in-situ test. This analysis of material behaviour focused on how the sensitivity of the peak and residual undrained shear strength of this material to: Laponite concentration, age time, rate of shearing, and pore water conductivity.

Utilising a highly sensitive laboratory vane shear apparatus, consistent and high accuracy results on how Laponite soil simulates respond to vane shearing were achieved. These results were then compared to laboratory tests, such as the fall cone test, performed outside of the scope of this project, in order to better establish a baseline behaviour of this material.

In addition to this, a key aspect of this project was to ascertain repeatable results, for future use of this material. The use of a standardised preparation and testing procedure allowed for highly consistent analyses to be undertaken. These results, as well as the continued refinement of the material preparation procedure will allow for the use of this material in future scaled modelling of complex geotechnical problems.