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Managing the risks associated with acid sulfate rock in NSW road projects
Acid sulfate rock (ASR) is unweathered rock that contains metal sulfide minerals (commonly iron sulfides). When ASR is exposed to both oxygen and water, oxidation of sulfides leads to the formation of sulfuric acid, sulfates and salts. The probability of ASR being present, can to some extent, be predicted from the geological origins of the rock or later hydrothermal depositions of sulfides. An ASR risk map has been prepared to assist in the pre-design phase of road construction projects.
ASR has the potential to be problematic (depending on concentrations) with respect to environmental, structural and durability risks. It is becoming increasingly common for ASR to be encountered by roadworks in New South Wales where designs include deeper cuttings into unweathered rock that has generally not been the case historically. Examples are given of New South Wales where ASR has been encountered, together with an American example where significant environmental penalties and remedial costs occurred.
Other than low risk geological formations, site investigation for roadworks must include identification of ASR and, where present, screening, detailed testing and interpretation of the distribution of sulfide contents. The details of each aspect of this assessment need to be fully understood.
Where ASR is present, the design, specification and construction must include control measures to reduce environmental risks associated with exposing ASR and potentially releasing leachate into the environment. Control measures include dilution, encapsulation and treatment with crushed limestone. Control measures must also be developed to protect structures such as bridges, culverts and retaining walls, stormwater drainage pipes and pavements.
The locations of where ASR is placed within the earthworks formation must be limited with respect to environmental, structural and durability constraints. For successful management of ASR in construction projects, careful planning and staging of the earthworks is critical.
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GSI Adjustments for Directional Hoek-Brown Strength Calibrated by Step-Path Case Studies
Step-Path methods aim to quantify the negative rock mass shear strength impact of geological defects co-aligned with failure paths through rock slopes and the positive strength impact of intact rock ‘bridges’ between some or all of the co- aligned defects. In effect, Step-Path considers directional strength in rock masses.
Conceptually Step-Path may be reconciled with the Hoek-Brown Method by adjusting the Geological Strength Index (GSI) input to Hoek-Brown equations so that both Step-Path and Hoek-Brown yield the same shear strength outcomes. The GSI adjustment is a two-step process. GSI is first negatively adjusted for the relative portion of the failure path that is defined by geological defects co-aligned with the failure path. The GSI is then positively adjusted for the relative portion of the failure path that is defined by intact rock ‘bridges’ between co-aligned defects. For a ‘general’ rock mass, the adjusted GSI is derived via the following equations:
GSI design = GSI general rock mass – GSI defects adjustment + GSI rock bridges adjustment
Where
GSI general rock mass = GSI rating that would have been conventionally estimated for the ‘general’ rock mass
GSI defect adjustment = 0.4 x co-aligned defect occurrence (%)
GSI rock bridge adjustment = 1.2 x intact rock ‘bridge’ occurrence (%)
The above GSI adjustment factors of 0.4 and 1.2, respectively, are calibrated by reviewing 230 Step-Path case study models developed on projects over the last 20+ years. Key considerations impacting Hoek-Brown rock mass and Barton geological defect shear strengths and their influence on Step-Path strength are flagged. Challenges facing the reconciliation task are identified and discussed. In addition to GSI adjustment factors for the ‘general’ rock mass condition, adjustment factors are also suggested for rock masses partitioned by mi grouping and rock type. Limiting conditions are identified.
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Unbound Granular Pavements; Design With Understanding : A Revision And Extension
The original paper, Rallings (2018), presented a rudimentary behavioural model of the response of unbound granular pavements (UGPs), ones that are founded on compacted clay subgrades, to passages of single wheel loads. The following amends and expands the original model and includes new terms and concepts. The proposed model is based on the assumption that elevated subgrade shear strains in conjunction with a strain-controlled mechanism disturb the fabric of the overlying granular materials (OGM) causing reductions in their stiffness and in the UGP’s load carrying capacity, essentially its ability to maintain its surface shape. It is proposed that short term falls in the subgrade shear strength and/or increases in the frequency of the heaviest loads within the wheel load spectrum are the common major contributors to the degradation of a UGP’s load carrying capacity. The proposed model provides a direct and simple means to predict the response of a UGP to load passages, allowing designers and asset managers alike to distinguish between those wheel loads that pose a potential threat to the UGP from those that do not.
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Resilience And Vulnerability To Climate Change Through The Prism Of Soil-Water Interactions: Challenges Of Temporal And Geographical Scales For Geotechnical Engineering
The interaction between water and soil particles lies at the heart of the work of geotechnical and geo-environmental engineers. The water content of the subsurface is an important state variable influencing soil behaviour in relation to strength and stability, hydrologic and chemical insulation, sediment budgets and transport, and support for biological life. The capacity of many soils to maintain high shear strength and withstand loads applied to them without significant deformation, crushing or erosion; their ability to insulate contaminated sites and filter heavy metals and organic chemicals out of polluted water; and their effectiveness in supporting healthy biological life for food production and other ecosystem services, are all examples of vital, and sometimes conflicting services, that soils provide and which are critically dependent on water content.
Three major sources of ecological and social change are reasonably certain in the 21st century:
- increased urbanisation with more demands placed on subsurface systems and structures, by the energy, transport, mining and environmental sectors;
- increased frequencies, magnitude and duration of droughts and floods as a result of anthropogenic climate change, with likely changes to patterns of precipitation and water retention; and
- significant rise in sea levels as a result of thermal expansion and melting of glaciers, leading to higher risks of erosion of coastal land and weakening of coastal foundations with possible damage to private properties and critical water, wastewater, telecommunications and transport infrastructure.
The paper‘s goal is threefold. First, different pathways for the impacts of climate change on subsurface systems are described through the lens of soil-water and land-ocean interactions. Second, a case study from Callala beach in Shoalhaven is presented to illustrate the complexity of making adaptation choices at the interface between land and water, especially as a result of uncertainty and unusual temporal and geographical scales of the problems. Third, the readiness of geotechnical education and practice to deal with these problems is discussed in the context of the difference between risk and vulnerability and the emerging distinction between incremental and transformational adaptation. The paper calls on the geotechnical community to engage more fully in the debate on adaptation to climate futures, going beyond the technical assessment of the integrity of infrastructure systems, and identifying long-term strategies for the conflicting demands we place on the subsurface. This will require innovations and possibly some extension of the spatial and temporal scopes of our experimental, analytical and theoretical methodologies.
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Geotechnical Engineering And Knowledge Transfer Through Academic-Industry Collaboration
There appears to be broad consensus among government, Academia and industry regarding the need to improve current low levels of academic-industry collaboration in Australia. In the authors’ view these low levels of cooperation, and the flow-on detrimental effects to innovation in the industry, are intrinsically tied to many of the discussion points raised in recent ISSMGE CAPG facilitated forums. While the problems are complex and varied, one of the main obstacles to enabling academic-industry collaboration in the geoprofession remains the inertia of industry and perceptions that academic-industry research fails to deliver tangible benefits to the company. The authors have highlighted some key issues relating to the debate regarding academic-industry collaboration while also reflecting on their own collective experiences to highlight the importance, and benefits, of academic-industry partnership and the role that learned societies can play in enhancing this collaboration.
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Subgrade ground improvement using in-situ stabilisation for track formation in Melbourne
The Southern Program Alliance (SPA) is one of the alliances formed in Melbourne to remove level crossings, construct track duplications and upgrade the rail network as a part of the Victorian government initiative to improve rail infrastructure. An integral component of a rail infrastructure project is the construction of track formation. The essential requirement for rail formation is to satisfy the bearing capacity (strength) and settlement (serviceability) requirements for rail loading. The strength and serviceability requirements are a function of track design, axle-load, speed, and, notably, subgrade characteristics. For the SPA projects to date, where existing subgrade conditions could not satisfy these requirements, either the weaker subgrade was excavated and replaced by structural fill material, or subgrade improvement using in-situ stabilisation was explored. Subgrade improvement typically minimises the excavation and earthworks required for the construction of track formation and provides significant sustainability, cost, and time benefits to the project, without compromising the functional requirements. Subgrade stabilisation methods using admixtures (lime and cement) were considered and/or adopted for differing surficial geological deposits, including alluvial deposits, residual soil of Silurian origin and Tertiary sediments of South-East Melbourne. The design strategy was to verify the applicability of admixture ratios through laboratory testing, whilst further undertaking quality assurance (QA) measures through the construction phase. To assess the depth of stabilisation required, both empirical and finite element analysis design methods were undertaken. This paper summarises key design, laboratory testing and construction considerations for subgrade improvement works undertaken for a rail track duplication between Diamond Creek and Wattle Glen in Melbourne’s North-Eastern suburbs.
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Recent advances in the application of vertical drains and vacuum preloading in soft soil stabilisation
Much of the world’s essential infrastructure is built along congested coastal belts that are composed of highly compressible and weak soils up to significant depths. Soft alluvial and marine clay deposits have very low bearing capacity and excessive settlement characteristics, with obvious design and maintenance implications on tall structures and large commercial buildings, as well as port and transport infrastructure. Stabilising these soft soils before commencing construction is essential for both long term and short term stability. Pre-construction consolidation of soft soils through the application of a surcharge load alone often takes too long, apart from which, the load required to achieve more than 90% consolidation of these mostly low lying, permeable, and very thick clay deposits can be excessively high over a prolonged period. A system of vertical drains combined with vacuum pressure and surcharge preloading has become an attractive ground improvement alternative in terms of both cost and effectiveness. This technique accelerates consolidation by promoting rapid radial flow which decreases the excess pore-pressure while increasing the effective stress.
Over the past 15 years, the Author and his co-workers have developed numerous experimental, analytical and numerical approaches that simulate the mechanics of prefabricated vertical drains (PVDs) and vacuum preloading, including two-dimensional and three-dimensional analyses, and more comprehensive design methods. These recent techniques have been applied to various real life projects in Australia and Southeast Asia. Some of the new design concepts include the role of overlapping smear zones due to PVD-mandrel penetration, pore pressure prediction based on the elliptical cavity expansion theory, and the rise and fall of pore pressure via PVD under cyclic loads. These recent advances enable greater accuracy in the prediction of excess pore water pressure, and lateral and vertical displacement of the stabilised ground.
This E.H. Davis Memorial Lecture presents an overview of the theoretical and practical developments and salient findings of soft ground improvement via PVDs and vacuum preloading, with applications to selected case studies in Australia, Thailand, and China.
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Resilience and vulnerability to Climate Change: Challenges of temporal and geographical scales for geotechnical engineering
The interaction between water and soil particles lies at the heart of the work of geotechnical and geo-environmental engineers. The water content of the subsurface is an important state variable influencing soil behaviour in relation to strength and stability, hydrologic and chemical insulation, sediment budgets and transport and support for biological life. The capacity of many soils to maintain high shear strength and withstand loads applied to them without significant deformation, crushing or erosion, their ability to insulate contaminated sites and filter heavy metals and organic chemicals out of polluted water and their effectiveness in supporting healthy biological life for food production and other ecosystem services, are all examples of vital, and sometimes conflicting services, that soils provide and which are critically dependent on water content.
Three major sources of ecological and social change are reasonably certain in the 21st century:
- increased urbanisation with more demands placed on subsurface systems and structures, by the energy, transport, mining and environmental sectors
- increased frequencies, magnitude and duration of droughts and floods as a result of anthropogenic climate change, with likely changes to patterns of precipitation and water retention and
- significant rise in sea levels as a result of thermal expansion and melting of glaciers, leading to higher risks of erosion of coastal land and weakening of coastal foundations with possible damage to private properties and critical water, wastewater, telecommunications and transport infrastructure.
The paper’s goal is threefold. First, different pathways for the impacts of climate change on subsurface systems are described through the lens of soil-water and land-ocean interactions. Second, a case study from Callala beach in Shoalhaven is presented to illustrate the complexity of making adaptation choices at the interface between land and water, especially as a result of uncertainty and unusual temporal and geographical scales of the problems. Third, the readiness of geotechnical education and practice to deal with these problems is discussed in the context of the difference between risk and vulnerability and the emerging distinction between incremental and transformational adaptation. The paper calls on the geotechnical community to engage more fully in the debate on adaptation to climate futures, going beyond the technical assessment of the integrity of infrastructure systems, and identifying long-term strategies for the conflicting demands we place on the subsurface. This will require innovations and possibly some extension of the spatial and temporal scopes of our experimental, analytical and theoretical methodologies.
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Experiences with post-construction retesting of engineered clay fills
A considerable amount of disputation, both legal and informal, arises where compacted clayey fills are retested some time after completion of the works and is predicated on two assumptions. Firstly, that once compacted, clayey fills remain unchanged thereafter and secondly that the results of post-construction retesting are more credible than the results of control testing carried out at the time of construction. The authors have been exposed to a number of cases, including legal proceedings, where earthworks having apparently been properly carried out and reputably tested during construction were retested some time after completion and assessed to be below specification. The simple conclusion often drawn by owners and their experts in such instances is that the earthworks were inadequately carried out at the time of construction. However, in the authors’ view there has developed a body of factual evidence which does not support that simple conclusion. This evidence has arisen from a variety of sources involving actual construction works where multiple testing and/or retesting by a range of reputable authorities often under conditions that were less than ideal. Nevertheless they have provided a series of experiences or case histories to which geotechnical engineers, earthworks contractors, lawyers and owners should have regard and from which valuable insights and lessons may be drawn. This paper deals with the factual aspects of seven cases which, in the authors’ view, challenge the simple conclusion based on the assumptions of essentially inert compacted clayey fills and the primacy of retests over tests during placement.
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Geotechnical support for open pit coal mining
Geotechnical considerations for support of coal mining are described in this review, which is an updated version of an earlier review published in 1995. A thorough working knowledge of the geological environment and of modern coal mining operations is required to provide specialist geotechnical advice. The mining industry operates within a range of constraints and drivers that are quite different in some respects to those encountered in the wider geotechnical community that supports civil projects. Fundamental geotechnical requirements remain the same: a sound working knowledge of applicable geotechnical parameters and groundwater conditions and reliable analytical tools. Opportunities for data gathering are limited and much reliance is placed on experience, judgement and consideration of mine slope forming processes and the operating requirements of equipment. Principles for stability assessment of both rock and spoil slopes are outlined. The implications of modern risk management procedures are discussed along with future developments that are anticipated.