2019 Young Geotechnical Professionals

Subsurface ground condition data is at the core of geotechnical engineering practice and research. High quality factual information can assist in innovative and efficient design and also minimise uncertainty in construction and post construction design life. In an urbanised modern society, we are increasingly having to innovate geotechnical investigation methods to limit interruption of existing operations, such as public transport, mining operations and other construction activity.

This limitation or requirement was particularly pertinent for a recent geotechnical investigation for a port expansion. Traditionally, jack-up barges are used for subsea investigations, however are limited by lack of manoeuvrability and quick mobilisation. However, in this setting, a key requirement was to not disrupt other vessel movements and to have the ability to demobilise in a very short period of time in case of emergency. The methodology of subsea drilling adopted involved placing a specially constructed drilling rig onto the sea floor operated by commercial divers. A support vessel accompanied the drilling rig which housed craning machinery, core logging facilities and diver control equipment.

The relatively untested methodology resulted in a number of technical issues during the drilling works. Additionally, drill rig performance whilst under water at depths of over 13m was also affected. Some examples of these issues include lack of control of water pressure at the bit face and a decrease in reaction force due to the effects of buoyancy on the submerged rig.
With refinement of the methodology and equipment together, subsea drilling can be a suitable methodology for constrained offshore geotechnical investigations in the future.

In recent years, liquefaction of tailings has become a topic of interest as it is the most common phenomenon behind many major tailings dam failures. It is essential to develop deeper understanding towards liquefaction of tailings to prevent failures, subsequently driving policy to improve tailings dam construction and maintenance.

This study set out to establish deeper insights into static liquefaction by performing a case study on Fundão Dam failure which is known to be the worst environmental disaster in Brazil’s history. The catastrophic failure leads to both environment tragedy and humanitarian crisis, as substantial amount of iron waste was released into the Doce River and hundreds of villagers were displaced.

In this study, static slope stability analysis was performed on left and right abutments of Fundão Dam. The analysis is undertaken using SLOPE/W software (GEO-SLOPE 2012). The Morgenstern-Price method (Morgenstern and Price 1965) was used to calculate the Factor of Safety (FOS) for the critical slip surfaces. Effective Stress Analyses (ESA) will be performed for tailings above the piezometric surface while those below will be modelled by Undrained Strength Analyses (USA). Key inputs of the stability analysis would be tailings stratigraphy, engineering properties of the tailings and pore pressures. All required data are taken from Fundão Tailings Dam reports prepared by members of Fundão Tailings Dam Review Panel.

The analysis results shown that in November 2015, Dike 1 right abutment had FOS smaller than 1 which indicated that failure could have occurred. However the failure did not initiate at right abutment but left abutment. This is largely because left abutment had higher pore pressure and most importantly significant amount of compressible slime lying beneath the embankment slope. The presence of slimes-enriched tailings inhibited drainage, enhanced saturation, and promoted undrained shearing. Together, these conditions contributed to static liquefaction of tailings.

Recognising the triggering mechanisms of static liquefaction in this particular failure would facilitate better informed decision making in tailings impoundment design in the future.

New development of infrastructure such as transport network, water and sewerage, transmission, power and communication systems are essential in growing cites. Yet, the modern cities are already congested, and acquisition of land for mass developments such as roads and railways is nearly impossible. Underground construction and elevated construction are other solutions for the land scarcity, and in general, the former is preferred as it is cost effective. However, in the case of underground construction, the impact of a tunnel excavation may extend to a far greater distance than of a deep foundation or a basement excavation. For example; the effect of a tunnel at 30 m depth may trigger deformations/settlements at the ground surface.

The underground of Sydney city is rapidly getting populated with tunnels, and it is inevitable to have them crossing each other with a relatively narrow separation. In this instance, the construction of a new tunnel may cause deformations and additional stresses on an existing tunnel and its liners. One such situation is the interaction of proposed road tunnels with a historical utility tunnel that is still in operation. This utility tunnel is lined with two concrete liners and a steel liner. The two road tunnels cross above the utility tunnel with only about 9 m clearance; thus imposing additional stresses and bending moments on the existing tunnel steel liner. These additional stresses are not intended on the steel pipe in the original design and understanding the impact of proposed road tunnel on the utility tunnel liners is crucial to ensure its functionality, during and after the proposed road tunnel construction.

A 3D finite element model was developed to investigate the steel liner response to the excavation of proposed road tunnels. ABAQUS finite element programme was used to develop the model as it facilitates the featuring of important details of the project such as: complex geometry of the utility tunnel including a ground water drain at the crown, joints in the steel pipe, concrete liner and steel liner interface interaction, evolution of stresses with new tunnel advancement, the fractured zone around the utility tunnel due to the drill and blast construction at early days and the large in-situ stress regime at great depths in Sydney bedrock. The bending moments, stresses and deformations of the steel pipe were obtained from the finite element model by modelling the steel pipe using shell elements. The interpretation of structural integrity of the utility tunnel in terms of sensible parameters such as circumferential bending of the steel liner, radial cracking of concrete liner, separation of steel liner joints, longitudinal bending of the pipe and deformation of the pipe cross section, using the general model outputs was a challenging aspect of the project. Based on the model results, it was assessed that the construction of the proposed road tunnel should not cause excessive deformations, damage the joints or induce local buckling of the steel liner.

Environmental restrictions as well as profit optimisation have created incentives for engineers to use waste materials as alternatives to natural aggregates in engineering projects. The study of the geotechnical behaviour of these materials has been considered in many studies in recent years. Coal wash, a byproduct of coal mining industries, was proved by past investigations to be adequate as a construction fill but the material had a high potential for breakage. In this study, rubber crumbs produced by shredding waste rubber tyres are mixed with coal wash to reduce the breakage potential of the latter and create an energy absorbing material for transportation infrastructure sublayers. Four coal wash-rubber crumbs (CWRC) mixtures were considered with 0%, 5%, 10% and 15% rubber content. First, the compaction characteristics of the mixtures were studied to evaluate the effect of rubber elasticity on the rearrangement of particles during compaction under different energy levels. It was found that a mixture with rubber can be compacted at a higher energy level to reach a target void ratio and still show a reduction in breakage. Static triaxial tests were also performed under three confining pressures (25, 50 and 75 kPa) to explore the effect of rubber on the strength and deformation properties of the CWRC mixtures considered in this study with special relevance to energy absorption potential. The peak friction angle decreased almost linearly with increasing RC content. However, even at 15% rubber content the friction angle was 44°, an acceptable value for transportation sublayers. The energy absorption potential of the mixture increased and the material became more ductile when rubber was added, thus preventing brittle and sudden failure when the loading stress exceeded the strength of the material.