As Sydney expands west, excavations in Western Sydney get larger and deeper. Based on the Sydney Metro Western Sydney Airport Station Boxes and Tunnelling Works Project (SM-WSA-SBT), this paper discusses the compositional variability of the local rock Bringelly Shale. Extensive geological mapping along the project alignment indicate that the Bringelly Shale unit is compositionally non-uniform, complex in lithology and structural character. This paper provides commentary on the lithological composition of Bringelly Shale from an engineering geology perspective and discusses how inter- and intra-bed variations and alterations persist through the unit. Structural characteristics such as bedding partings, joints, faults and regional folding also contributes to this variability. This geological variability leads to challenges in characterising the rock mass, engineering parameter interpretation and designing to allow for the behaviour of the rock mass and expected construction performance, particularly in excavation, support installation, and movement prediction. Some of these challenges for retention structure design are discussed. This paper offers some practical insights for excavation, design, and construction in Bringelly Shale. To manage the inherent geotechnical risks, a three-stage approach is recommended, a robust design geotechnical model, designing in flexibility and redundancies, and a comprehensive geological mapping and monitoring regime at construction.
This paper describes various evaluations carried out using an unorthodox Markovian process on single tailings storage facilities dams (TSFs) and a small portfolio of those structures. The Markovian model was described in the first part of this discussion (Markov Processes For Tailings Dams. Part 1: The Model, Oboni, 2026 in this Journal) and the application described in this paper uses a proven quantitative risk assessment (QRA) procedure (Oboni, Oboni, 2025) for the prior estimates of the probabilities of failure (PoFs), but any other validated, equally proven QRA method delivering the initial estimates of the various PoFs needed for the Markovian model could be substituted. Thus, the Markovian model proposed in this paper is agnostic to the QRA procedure, under the condition that it can deliver the required data. As described in the companion paper, the driving idea is to offer users a way to optimize costly QRA updates frequency and to guide the selection of adequate TARP types along the life of the considered dam, while showing the model capabilities and offer insight on possible practical uses. The five real-life dams of the considered portfolio have initial probabilities of failure covering almost three orders of magnitude, from the low bound of the world-wide TSF benchmark up to what is considered pre-catastrophic conditions. The comparison of the Markovian results with the a priori QRA is very encouraging and in the rare case of apparent discrepancies, intuitive convincing explanations delivered. The Markovian approach shows that better dams require longer steps sequences to attain the so-called Markovian steady state, i.e. a state where the probabilities of each individual failure mode and causalities remain constant over time. The global PoF of the dams varies during the path to steady state, as the probabilities of the hazardous conditions vary at each step. As none of the considered dams exhibits an absorbing state, i.e. a certainty of a single failure mode (FM)/causality within the studied path to steady state, several probable failure modes (PFM) remain jointly possible through the “life” of the dam including at steady state. However, the fundamental difference between dams is that better-quality ones sport a larger number of dominant PFMs at steady state whereas lesser quality dams concentrate toward one or two PFMs. That means that better dams need a combination of multiple driving conditions to collapse as their resilience is higher whereas “weaker” dams just require a small trigger to collapse following one of the PFMs. A second phase of the development led to analyse the decay of a dam through a progressive relaxation of the standard of care. Seven levels of reduction were simulated varying the causalities, but not the initial QRA. This is a very theoretical case, as it is obvious that past a certain level of decay the QRA should be updated. It is inferred that the Markovian model would guide users to optimize updating frequency. On an initially good quality dam some cracks, leaks etc. are not necessarily fatal conditions as shown by numerous real-life cases of damaged dams surviving for years, sometimes even without repairs (not a recommended practice, of course). However, as expected, the number of steps to steady state reduces significantly as the standard of care decays, which is an indicator of the need to re-evaluate specific TARPs. Also, the initial post QRA observations significantly increase the probability of failure, shift the PoFs to a higher level, almost half order of magnitude. As the decay continues the PoF evolves during the considered life (up to the steady state).
Modulus of subgrade reaction, also known as spring stiffness, is commonly referenced in engineering textbooks and routinely adopted by structural engineers in their slabs on ground design for industrial facilities (e.g., warehouse slabs). From a geotechnical perspective, the spring stiffness is not an intrinsic material property and has limitations when adopted in the design. On the other hand, Young’s Modulus (E) is a material property and should be used in the design as the basic tool. While various literature has shown the shortcomings of spring stiffness, the limitations of adopting a springs model as compared to the use of Young’s Modulus model, especially in the design of industrial slabs (i.e., large slabs on grade) have not been investigated. In this paper, the effect of loaded area to derive the equivalent spring value is first computed followed by an analysis case demonstrating that the results from analyses using uncoupled spring are unrealistic. Then, the resulting deflection and bending moment profiles of slabs obtained from a spring model and a Young’s Modulus model are investigated and compared. Uniformly and pattern loaded slabs are also investigated. The results from the studies show that the results from the two analyses (models) are very different depending on the springs configuration and could lead to inaccurate designs.
Over nearly three decades, I have navigated a geotechnical engineering career spanning three continents - Europe (UK), the Middle East (UAE), and Australia. From substructure design in London Clay to major building and infrastructure projects across arid soils in the Middle East and expansive ground in Australia, each phase has shaped my approach to engineering, problem-solving, and professional growth. My early years in the UK were defined by procedural design and established professional standards. The transition to the Middle East in the mid-2000s was transformational, demanding greater flexibility, speed, and an openness to innovation. It was a region where risk perception and project ambition differed significantly, and where I had to adapt rapidly. In the late 2010s, I relocated to Australia, a continent whose geological diversity demands region-specific knowledge and an adaptation to local design and construction practices. Here, I found myself in Australia’s construction boom contributing to major transport and infrastructure projects across Victoria, South Australia, and Western Australia. The diversity of geological conditions and project types broadened my technical capabilities and deepened my understanding of region-specific standards and construction methods. Notable projects include site investigation for the Dubai Metro Red Line (52 km), foundation design and reviews for over 40 tall towers and deep basements, the Doha Metro Gold Line (35 km), and key roles on the North-East Link, Suburban Rail Loop, Torrens to Darlington, and Armadale level-crossing removal projects. This reflection highlights key lessons from some of the projects I was involved in. These experiences reinforce the value of adaptability, continuous learning, and multidisciplinary collaboration. As a woman in engineering, navigating these environments has also meant challenging the norm and finding space for leadership in traditionally male-dominated settings. I offer this reflection to inspire inclusive, diverse voices to shape the next generation of geotechnical practice.
Over the last two decades, significant investment in major road and rail infrastructure has occurred across the Greater Melbourne area. Many of these projects have involved widening of existing arterial roads, untangling of freight and suburban rail lines and lowering new road or rail alignments beneath existing infrastructure. These activities have often required deep excavations or encroachment into existing slopes, embankments and spill-through batters for which a soil nail wall solution has provided an efficient and cost-effective means of retention that is often favoured by contractors. Notwithstanding this, soil nail walls present a riskier form of construction than other common retention methods such as soldier pile and sheet pile walls. This risk is primarily associated with the excavation face being unsupported at various stages throughout construction of the soil nail wall, amongst other challenges. If these challenges are not managed well, it can result in increased wall movement, increased settlement behind the walls, legacy issues and potentially wall collapse. It is incumbent upon designers to be aware of these risks and challenges and to be prudent in the way they are considered in the design and documentation of soil nail walls. This paper summarises the authors’ experiences across various projects and includes results of pullout testing to failure conducted in different soil units, the performance of various soil nail wall case studies and commentary on the applicability of available design methods and construction practices. A recent innovation to reduce wall movement is also discussed, along with recent experiences in residual basaltic clay.
