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Unsaturated free-standing mainline railway embankments – Part 2: An example of handling the awkward truth
The presence of negative pore pressures within cuttings and embankments, and the benefit of the consequent reduction of the likelihood of instability (also known as increased stability), have long been recognised by members of the profession. Negative pore pressures are usually a consequence of environmental influences upon clay soils in particular, and are frequently termed “soil suction”.
The recognition of suctions in the assessment of potential instability, by way of stability analyses, is less common, albeit that the tools are available to conduct such analyses, once the boundary conditions are understood.
Measurement of suction values in the field assists the selection of suction values appropriate for such analyses.
In the companion paper, the authors develop a philosophy and present a defensible model for analysis of free-standing embankments (Hull & Leventhal, 2019). Herein, a case history is presented that demonstrates one such analysis, being for Main Line Railway infrastructure. The results indicate the benefit accrued through recognition of suction in the estimation of potential instability of free-standing embankments.
The paper is intended to alert the profession to an improved assessment technique that incorporates these effects.
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Assessment of capillary ingress of water in stabilised pavement materials
Geomaterials such as pavement materials and soils are commonly stabilised with cementitious or other binders when it is necessary to upgrade the performance characteristics of the original material for a particular engineering application. Typical examples include construction or in situ rehabilitation of road pavements, formation of stabilised bases in soft or reactive soils and deep-mixing of soils. When these layers are placed at or close to the ground surface (or generally above the water table), they mostly operate under unsaturated conditions. When free water becomes available at or close to the edges of these layers, water ingress will take place predominantly by capillary action. A typical field situation idealizing the pathways for capillary water ingress with reference to a pavement layer is shown in Figure 1.
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The Guildford Formation re-evaluated
The Guildford Formation extends over a significant area of the Swan Coastal Plain from Cervantes north of Perth to Busselton in the southwest of Western Australia and forms an easily recognizable geomorphological unit — the Pinjarra Plain. The Guildford Formation was deposited as a series of coalescing alluvial fans at the foot of the Darling Scarp. These fans interfinger in the west with sandy fluvial and shallow marine sediments of the Gnangara Sand and Bassendean Sand. Along the middle reaches of the Swan River fluvial and estuarine sediments of a later alluvial complex infill a deep inset valley cut into and through the Guildford Formation. These deposits are defined as the Perth Formation. Along the middle and lower reaches of the Swan and Canning Rivers alluvial and estuarine sediments of a younger alluvial complex are found infilling a series of younger inset valleys cut into and through the Perth Formation and into the underlying bedrock. These sediments were previously considered to be lateral equivalents of the Guildford Formation but this correlation can no longer be accepted and they are defined as the Swan River Formation. Between Cervantes in the north and Bunbury in the south extensive drilling confirms a relatively simple model for the Guildford Formation. In contrast, the alluvial histories of the Perth Formation and of the Swan River Formation are complex and reveal alluvial regimes that have varied significantly over time. The complicated interrelationships between the sedimentary bodies of the Perth Formation and of the Swan River Formation are a result of both local-scale and regional-scale changes of river and channel morphology that were influenced by changes in sea level caused by climatic fluctuations during the Late Pleistocene.
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Guidelines for the applications of effective stress principle to shear strength and volume change determination of unsaturated soils
The application of the effective stress principle to shear strength and volume change in unsaturated soils is presented. Step by step guidelines are provided with relevant procedures for material parameter determination. A relationship is derived between instability index used in AS2870 and the basic soil properties.
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Reuse of Iron Exploitation Waste as a New Binder for Tailings Stabilisation in Dry Stacks: Circular Economy Approach
The dried allocation of the tailings, rather than the disposal in a slurry form, appears as an alternative to attend new legislation and improve the safety of mines operation. Also, the use of cementing agents in dry stacking facilities can enhance aspects of operations such as guaranteeing dilatant behaviour at the base and increasing tailings’ strength. The present research assesses the technical and environmental viability of a new alkali-activated cement (AAC) in iron ore tailings stabilization. The mechanical response of compacted tailings-AAC specimens was evaluated through strength and shear modulus tests while Life Cycle Assessment (LCA) was performed to verify the sustainability of this new binder when compared to conventional AAC. This new binder is derived from the residues of iron exploitation and is intended for use in new disposal schemes, such as dry stacks. The AAC is mainly composed of metakaolin (MK), produced from the residual soil removed during the mining activity, and sodium silicate (SS), produced with sandy tailings. Using tailings and waste in AAC production aligns with sustainable practices, minimizing resource consumption and promoting waste recovery. Also, LCA demonstrates a lower impact for tailings AAC when compared to conventional AAC. In addition to environmental and mechanical aspects, using this AAC supports the application of circular economy in mining since it enables the reuse of waste produced in mine operation as a substitute for conventional cement (that involves another industry and raw materials).
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Groundwater And Deep Excavations
The presence of groundwater in deep excavations in soil and rock has the potential to profoundly impact both the stability of the excavation, and the impacts that the excavation can have on the areas surrounding the excavation. This paper is intended as a guidance based on the author’s experience to some of the key considerations and pitfalls that designers of deep excavations should take into account when designing deep excavations where groundwater is present. The paper focuses on the influences of Melbourne geology and hydrogeology on the design of deep excavations and provides a brief summary of some of the important groundwater features of the central Melbourne area, but the principles discussed have wider application to projects in similar geology. A series of generalised examples are presented to demonstrate some of the important impacts that groundwater can have on the stability of excavations and structural loads on buried structures, as well as some of the impacts that dewatering activities can have on areas surrounding the excavation.
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Internal Compression Of Fill Material Originating From Bringelly Shale
A rule of thumb handed down from senior to junior geotechnical engineers in Australia is that internal compression of embankment fill is 0.1% of embankment height and it has been widely used in performance specifications for major infrastructure projects around Australia. It is not clear where this rule of thumb originated from or on what basis it was developed. In Western Sydney this rule of thumb has been adequate for many years because the scale of earthworks in terms of fill thickness has been relatively minor. Large scale earthworks have started to occur over the past decade or so as more significant road and rail development has occurred. Recent experience with higher fills constructed in Western Sydney shows that internal compression strain rate can be greater than 0.1% per log cycle of time. Embankments to about 10 m height on a rail infrastructure projects on Western Sydney were constructed from Bringelly Shale. The fill materials and compaction were compliant with the relevant engineering standards. Comparison of topographic survey about 5 years after construction with design profiles indicated that they had settled between 0.05 and 0.25 m, subject to construction tolerances, and the internal compression strain rate varied between 0.5% and 6.3% per log cycle of time, adopting 1 year as the starting time of post construction settlement. Anecdotal evidence from road embankments of up to 14 m have identified similar magnitudes of settlement response from fill formed of the same Bringelly Shale materials. These values are much higher than the rule of thumb of 0.1% per log cycle of time. Though Bringelly Shale-based fill material has shown such a significant settlement issue, to the authors knowledge, there are no references found in the technical literature that provides some guidance on assessing the internal compression of Bringelly Shale based fill material. Therefore, a series of laboratory tests have been conducted to understand the settlement behaviour of compacted Bringelly Shale. Compaction tests, particle size distribution, Atterberg limits, and small and large-scale compression tests of compacted fill material have been conducted. This paper summarises some of the findings of the laboratory tests and authors’ view on the performance of Bringelly Shale fill material and future studies.
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Distribution of ‘laterites’ and lateritic weathering profiles, Darling Range, Western Australia
This paper summarizes the distribution and characteristics of the lateritic regolith of the Darling Range and presents models for its formation and evolution. Typical, complete, weathering profiles on granite average about 20 m in thickness and consist of gravelly soil, lateritic duricrust, saprolite and saprock. Lateritic duricrusts occupy gently sloping to horizontal upland areas and are either residual, or locally transported and recemented. In much of the Darling Range, the lower part of the duricrust, especially on hill slopes rather than crests or valley floors, is highly aluminous and forms an extensive resource of bauxite. Fragmental, fragmental-pisolitic, pisolitic and vesicular types can be identified on the basis of secondary structures. Fragmental duricrust largely consists of gibbsite, hematite, goethite and quartz and has resulted from direct gibbsitization of saprock or bedrock without forming the kaolinite-rich deep saprolite. Outcrops of duricrust with relict bedrock textures are common. In contrast, pisolitic duricrust with hematitemaghemite and χ-alumina rich mineralogy have a more complex history than fragmental duricrust with simple mineralogy. Vesicular duricrust is goethite-rich and is formed by the ferruginization of sandy detritus and quartz pebbles. The profiles show no condensed sequences, and individual rock types are traceable geochemically and mineralogically, but with increasing difficulty, from bedrock to the surface. The concentrations of Fe, Al, Si, Ti, V, Cr and residual quartz, particularly in fragmental duricrust, can be used to identify bedrock. Deep weathering profiles at Jarrahdale and Boddington yield late Tertiary palaeomagnetic ages and it appears that modification of these profiles to form bauxite is continuing today.
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Geotechnical aspects of the Narrows Bridge duplication
Aspects of the Narrows Bridge Duplication Design are presented including a general introduction to the Geology and seismic risk of the Perth City area. An overview of the original bridge geotechnics and design (late 1950s) is given, in addition to a discussion of what effects the site reclamation works for the original bridge have had on the design of the Duplication. Characterisation of the subsurface profile (strata and materials) for the Bridge Duplication Design is described, together with comment on the site investigation. Soil analysis models used for design and typical foundation details are presented, together with consideration of pile design loads including earthquake and liquefaction. Details of the pile load testing are given together with pile load test results. Comparison is also made between the geotechnical investigations for the original bridge and Duplication.
One of the geotechnical issues addressed for the Duplication is the execution, reporting and review of the geotechnical monitoring before and during the project construction period. Concern had been expressed about possible ground movements and potential detrimental influence on the existing bridge foundations, associated with the proposed piling and bridge construction works. The contract required monitoring of settlements and lateral movements, as well as land survey of the existing bridge, to ensure that the new bridge was constructed in a manner that did not have any significant impact on the existing structure. Inclinometers, settlement gauges, and Sondex settlement monitoring systems were surveyed over a period of nearly two years to enable continuous assessment of the impact of the various phases of construction on the existing structure. The paper discusses issues concerning the nature and performance of the monitoring systems, the data obtained and an overview of the ground and bridge deflections that were observed.
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Dynamic embedment of projectiles in clay
This paper provides an overview of the work of the Australian Research Council-funded Centre for Geotechnical Science and Engineering on free falling projectiles that have applications as seabed characterisation tools and as anchoring systems for floating facilities. These projectiles are released in water and dynamically embed into the seabed through the kinetic energy they gain during freefall. The high penetration velocity, which can be up to 25 m/s at impact with the seabed, induces shear strain rates in the soil that are up to eight orders of magnitude higher than in a typical laboratory test. The difficulty in quantifying the soil strength at these very high strain rates, together with hydrodynamic aspects including pressure drag and potential water entrainment along the projectile-soil interface, complicates assessment of the penetration response. A large database of centrifuge and field data has been collated by the Centre and is used in this paper to quantify embedment potential and to examine the merit of a simple analytical framework that captures the dynamic response of free-falling projectiles. Aspects of the dynamic embedment process that cannot be predicted by the analytical framework, including potential hole closure during installation and pore pressure generation are investigated in finite element analyses that model the dynamic penetration of projectiles in soil. Example results from these analyses are provided.