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Practical Considerations For The Application Of A Survival Probability Model For Rockfall
Rockfall fragmentation is a common and very complex phenomenon that is still inadequately understood and rarely modelled. When falling rock blocks break upon impact, their shape and size change and the kinetic energy is distributed amongst fragments. To efficiently design mitigation measures, it is necessary to adequately account for fragmentation when modelling rockfall trajectories. To do so, a better understanding of the fragmentation process, its occurrence and its likely outcomes is needed. The authors have recently proposed a novel model which can predict the survival probability (SP) of brittle spheres upon impact from the statistical distribution of material parameters, obtained by standard quasi- static tests (Brazilian tests and unconfined compression tests). The model predicts two Weibull parameters (shape parameter -m- and scale parameter – critical kinetic energy) that are used to define the SP. The model is based on theoretically-derived (from Hertzian contact theory) conversion factors used to transform the critical work required to fail disc samples in quasi-static indirect tension into the critical kinetic energy to cause failure of spheres at impact in vertical drop tests. The objective of this paper is to provide some practical insights into this model in relation of the analysis of the Brazilian test results and the number of Brazilian tests required to achieve an acceptable prediction. A first analysis highlights the importance of distribution of forces required to break the specimens in Brazilian tests and a common statistical based outlier removal methodology was applied to reduce the experimental error associated with the operator. After eliminating the outlier data, the quality of prediction is improved and, in particular, the influence of the specimen diameter used in Brazilian compressions to derive the model input parameter is significantly reduced. This latter point implies that the size effect is adequately captured. The second analysis reveals the highest variability for batches with low number of tests and a progressive reduction as the number of sampled test increases. Based on these results, it is suggested to use at least 30 Brazilian tests and remove outliers using the simple statistical approach presented in the paper (with of 0.5 or 1.0).
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Volume 36, Number 3 — Other
Table of contents, editorial and chairman’s column for Australian Geomechanics, Volume 36, Number 3.
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Embodied Carbon Assessment of Geotechnical Works
In the light of rising construction sustainability concerns, embodied carbon assessments are often one of the main engineering tools to identify the best “green” option. Embodied carbon assessments provide a simple way to quantify and measure the summation of all the greenhouse gases generated from the built environment. It includes a whole life carbon cycle assessment of a given project from the impacts of materials production, transportation, installation, maintenance, and any waste or disposals during and at the end of design life. This paper aims to allow geotechnical engineers to quickly determine the embodied carbon of their design, and more profoundly form the basis of an innovative and efficient design approach with the consideration of intelligent and alternate material choice to achieve the same performance. In this paper, the methodology of embodied carbon calculation will first be introduced, followed by a summary of carbon emission factors (CEF) that are applicable for geotechnical designs. The discussion herein will focus on the initial portion of the embodied carbon life cycle assessment which comprises of the “before use stage” only for a particular project. Case studies on the use of embodied carbon calculations were provided for a variety of geotechnical projects including foundation for road embankment, trench excavations, and tunnel design. These case studies will show the significance of carbon calculations during the initial design stages and its value in recognition of projects’ sustainability goals. Alternative real-life solutions in achieving de-carbonization will also be presented as a concluding remark, highlighting the possibility of sustainable design in geotechnical practice.
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Interaction of matric suction and net stress on a highly pressure grouted soil-cement interface
Nowadays, grouting pressure is considered as an important issue as it provides better interface strength than gravity grouting in case of cast in situ soil-cement grout interface, like soil-nail and soil-pile interface. In the present study, a series of interface direct shear tests is performed on highly pressure grouted soil-cement interface under different matric suctions and net stresses. The experimental results indicate that grouting pressure, matric suction and overburden stress have interactional significant influence on the interface behaviour of soil-cement interface. The shear strength increases with net stress at both saturated and unsaturated conditions for pressure grouted soil-cement interface. Similar to soil, the interface shear strength envelopes for a given suction are approximately linear. The apparent interface friction angle and adhesion intercept increase with matric suction. As compared with gravity grouted interface, the apparent interface friction angle decreases for higher pressure grouting at different matric suctions. Also, the strength of highly pressure grouted interface is greater at lower suction range, but lower at higher suction range.
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Seismic Analysis For Open Pit Mines
Open pit mines are of a dramatically different scale and nature to most civil works, so seismic analysis techniques required for open pit mines are also very different to those for civil works. This paper demonstrates two important aspects of the seismic behaviour of open pit mines; site response effects resulting from large scale man-made and natural topographical features, and the effect of earthquake ground motion on seismic stability assessments for steep and high slopes. An understanding of the first aspect is a prerequisite for understanding the second aspect.
The effect of topographical features is generally to amplify ground motion at the crests of pit slopes and ridges, and up through spoil piles. These effects will alter response spectra for infrastructure design. Slip mass scale effects mean that, during an earthquake, the maximum average acceleration within a slip mass is less than the maximum surface acceleration. The combination of these two effects means that the appropriate value of acceleration for use in slope stability calculations is highly dependent on topography, and the size and location of the potential slip mass under consideration.
Seismic analysis techniques for open pit mines are not well established. Two case studies, one of an open pit slope, and one of a large ore spoil pile on a ridge, demonstrate that meaningful insights, based on sound engineering principles and consistent with the literature, can be obtained with the use of appropriate and thorough seismic analysis techniques. It is suggested that dynamic analysis is required to assess seismic behaviour at open pit mines.
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On the common criticism of initial moisture content bias in the shrink-swell test
The shrink-swell test, and its application in site classification and foundation design according to AS2870 (2011) has both advocates and critics. It has been used as the basis for many residential foundation designs in Australia for more than 30 years, and although examples of performance failures in residential structures are familiar to most practitioners, the incidence is relatively low compared with the number of structures that are constructed, and substantiated examples of widespread and systematic failures due to poor foundation design are few and far between. Nevertheless, there are those who advocate that the shrink-swell test is fundamentally flawed, generally on the basis that the reactivity index that results for any particular soil is affected by the water content of the soil at the time of measurement. Whilst there is anecdotal evidence to suggest this, the data usually put forward to demonstrate this assertion are seldom of high scientific quality, and not obtained under conditions that would allow the assertion to be rigorously and conclusively tested. In fact, scatter plots of shrink-swell index vs initial water content often put forward (eg Rogers and McDougall, 2020) as evidence do show a tendency for soils with higher water content to have higher shrink-swell index values, however, this paper demonstrates that this should actually be expected, and it does not, in itself, confirm that there is bias in the test. The paper discusses the alternative of basing foundation design on Atterberg limits, and recognises that whilst these give a good indication of the clay type and clay content of a soil, as they are determined on remoulded samples, they cannot account for soil structure which is a significant controlling factor in reactive soil behaviour. It goes on to consider what it would take to produce convincing data to demonstrate bias in the shrink-swell tests, and what would be needed to conclusively demonstrate that an alternative approach to site classification was more reliable.
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Level 1 Inspection And Testing: Reliability Or Liability?
In 1990, when the earthworks Guidelines were first introduced, adoption of Level 1 Inspection and Testing in accordance with AS3798 was seen as the panacea for standard practice in the industry for the determination of compliance with specification requirements for compaction of fill. Since then, the use of Level 1 responsibility has become widespread on large earthworks contracts where an engineered fill is the required product. Reliability of the engineering performance of compacted fill was seen as the main selling point for justification to principals.
Experience has shown there are many limitations associated with implementation of the concept outlined in the Guidelines, so that the result is a liability for many stakeholders in an earthworks contract. This paper explores some of the challenges associated with implementation of Level 1 Inspection and Testing, as it is now known, which can reduce the goal of ‘reliability’ to a ‘liability’ for the stakeholders in the earthworks contract. These challenges include:
- physical limitations of workload for the Geotechnical Inspection and Testing Authority/Geotechnical Testing Authority;
- whether sufficient testing is completed to reliably identify problem “hot spots”;
- whether sufficient data is recorded to enable audit of traceability of completed testing;
- the low relative cost of testing in relation to contract value;
- whether remuneration rates for Geotechnical Testing Authority staff are sufficient in relation to siteoperatives to resist temptations;
- what liabilities may arise for the stakeholders from defects or problems in the finished product;
- what remedies may be available to rectify the challenges;
- whether results of testing some time after completion are still representative;
- whether the GITA/GTA should be part of the contractors QA/QC system to enable better integration.
The authors do not intend to provide answers to these challenges, but to initiate some discussion.
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The GP sampler: a new innovation in core sampling
This paper introduces a new type of sampler called the GP sampler. It was designed to sample gravelly soils, but has proven to be successful in sampling soils ranging from dense sand, to gravel, as well as sedimentary rocks. The sampler is constructed of a single core barrel and uses a viscous polymer gel as its drilling fluid. The polymer plays a key role in obtaining high-quality samples, helping to preserve the soil structure. The polymer gel was also employed in more traditional style samplers, in an effort to improve the quality of samples obtained from silt, silty sand, and sand. In the field, GP samplers have been successful where other conventional methods have experienced difficulties or failed altogether. Although the GP sampler is not a perfect sampler, it is beginning to make a qualitative difference in the sampling of granular soils for engineering analyses.
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A Case Study Of Deep Excavation And Shoring Design For Sydney Metro Northwest
This paper provides an overview of the Sydney Metro Northwest (formerly the North West Rail Link) project and the underground station excavation retention design and construction works, including the key requirements set out in the scope of works and technical criteria (SWTC). Based on the assessment of the geological conditions a soldier piled wall shoring system was adopted for all five new underground stations and one of the two services facility shafts for ease and speed of construction. During the Castle Hill Station excavation a new planar wedge instability mechanism was considered to be credible based on the additional geological data, with the original three-dimensional block instability being no longer suitable. This led to redesign of the south wall stabilisation works based on the updated geological model and input parameters. The instrumentation and monitoring plan was also adjusted to ensure the required additional support provided would be adequate for the safety of the station box excavation. The monitored lateral movements at the capping beam and at the inclinometers were within the trigger values, indicating that the retention system constructed was robust.
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Laboratory Characterisation Of Non-Standard Pavement Base Materials
The vast majority of the road network in Australia is composed of unbound granular pavement layers. Given the annual growth in the use of granular materials in the construction and maintenance of pavements, there is a significant increase in energy and materials’ transportation cost and remarkable shortages/reduction of conventional quality materials. Therefore, there is an overriding need to employ locally available non-standard materials as a sustainable solution for pavement construction and maintenance which will result in reduced consumption of finite resources and a reduction in cost. Although non-standard materials do not generally meet the requirements of standard specification, they can result in a satisfactory performance when properly managed. Currently, there is no unified accepted test method to evaluate the performance of non-standard materials. The objective of this paper is to have a comprehensive evaluation of the physical and mechanical properties of non-standard materials using a range of laboratory experiments. For this purpose, seven non-standard materials sampled from the existing pavements together with one standard material were selected for the laboratory investigation. The adopted laboratory experiments included California Bearing Ratio (CBR), modified Texas triaxial, and wheel tracking tests in addition to the compaction test, particle size distribution, Atterberg limits measurements, and apparent particle density measurement. This study ranked and compared the performance of different tested materials under selected laboratory experiments. Lastly, the laboratory test results were compared against the materials’ in-service performance and the suitability of each adopted experiment for the characterisation of non-standard materials was accordingly investigated.