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Case Study of Soil Slope Stabilisation Works at Banjo Paterson Park
An existing slope section overlooking Parramatta River in Banjo Paterson Park, Gladesville, NSW experienced a failure in 2017. The existing slope is on the south west side of the park. The unstable slope section is approximately 20 m to 25 m long, 5 m to 8 m height at a grade of approximately 1.5H:1V to 1H:1V.
This paper focuses on the stabilisation option study for the slope remediation and presents a case study of the use of sandstone riverbank wall to retain up to 7.5 m of existing soil slope. Sandstone riverbank wall was selected in preference to soil nail wall or retention system to increase the stability of the existing slope batter, expedited the construction process, reduce the construction cost and complexity, and for landscaping and wall appearance considerations. The detailed design considered both global and internal stability and confirmed adequate factor of safety for each failure mode.
This paper presents the assessment of the slope profile and cause of slip failure, comparison among the proposed stabilisation options, and remedial design undertaken for the selected sandstone riverbank wall. It also provides a discussion on the construction of sandstone riverbank wall.
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The mechanics of discontinua: Engineering in discontinuous rock masses
Rock masses are distinguished from most other engineering materials by their inherently discontinuous nature and by the range of scales on which discontinuities occur within them. The paper highlights a number of concerns that these factors pose in engineering practice. It reviews the basic mechanics of discontinua and the historical development of the characterisation, testing and analytical and numerical techniques available to the engineer working with discontinuous rock. The practical application of these techniques is illustrated by examples of their use in underground construction, caving methods of mining and hot dry rock geothermal energy exploitation. Despite the difficulties that still arise in engineering in discontinuous rock masses, it is shown that quite remarkable advances have been made in the last 40 years.
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Recent experiences in design and construction of high consequence temporary soil nail walls
This paper describes some of the authors’ recent experiences in the design and construction of high consequence temporary soil nail walls.
Various approaches to designing soil nail walls are discussed, including an approach recently adopted by the authors which is considered to be consistent with a limit state design intent.
However, it is reasoned that, regardless of which design approach is adopted, often “construction” aspects are far more significant than “design” aspects in determining how a soil nail wall performs. Three examples are used to demonstrate this point. By “construction” aspects the authors mean effects and deformations resulting from construction activities that are not typically explicitly accounted for in the analysis process, for example deformations from drilling and cleaning holes, vibrations, and small scale / temporary face instability e.g. slumping of interim unsupported excavation faces prior to shotcreting. By “design” aspects the authors mean the deformations of the soil nail wall due to stress relief resulting from progressive excavation of material in front of the wall, i.e. those that can be calculated using analysis.
The paper concludes that a complete soil nail wall design should consider not only the results of the analysis undertaken, but also address issues that may arise related to the construction of the wall, in order to achieve a successful outcome. Some observations and recommendations are made regarding how a design can address these “construction” aspects. It is concluded that the success of a high consequence temporary soil nail wall depends not only on the interaction between the soil and the structural elements, but also their interaction with construction aspects, and therefore a design should seek to address these aspects so far as practical.
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AGS Western Australia Symposium 2023
Geotechnical Engineering for a Sustainable Future
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Geotechnical mapping using a geophysical method of investigation
Site characterization procedures have hitherto overlooked areal mapping of sub-surface geotechnical properties of soil/rock and have instead emphasized conventional “point” testing processes to find and document these properties. On the other hand, the fast pace of construction and advancements in geophysical methods have led to the possibility of developing matching site characterization methods and geotechnical mapping is considered here as the way forward. In many countries around the globe, it is now argued that current lack of geotechnical maps, which could be used as support for planning in new areas, is one of the reasons for the development of areas with less favourable geotechnical conditions. Furthermore, contemporary developments in geotechnical engineering clearly exhibit an agreement with geotechnical mapping, seen as a helpful tool in rapid assessment of sub-soil conditions and hence suitability or otherwise of a particular site for a construction project. Current research is about geotechnical mapping of subsurface soil properties of a selected area in the vicinity of Islamabad with the help of the electrical resistivity method. The research is believed to be the first of its kind on a comparatively new subject.
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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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Modelling brittle fracture in weak rock with the bonded block method, from laboratory to tunnel scale
This paper presents results from a series of 3D bonded block models of weak rock with properties based on the Ashfield Shale in Sydney, NSW. The bonded block method is a subset of the discrete element method, where intact rock can be represented using an assembly of 2D polygons or 3D polyhedra with strong, stiff contacts. Results from a series of numerical UCS, triaxial, and direct tension tests confirm that the micro-scale breakage of block contacts can accurately reproduce laboratory scale brittle fracture behaviour. The laboratory simulation results are used to develop a tunnel scale bonded block model that explores the Voussoir beam analogue for flat roofed tunnels, with focus on the role of brittle fracture in progressive roof yielding. A simplified synthetic rock mass is constructed by embedding horizontal, cohesionless bedding discontinuities into a bonded block assembly. The bedding discontinuities promote delamination of shale beds and shear failure of bonded block contacts.
By adding rockbolts to the roof, several discrete beds can be stitched together to behave as a thicker equivalent beam. Rockbolt reinforcement inhibits fracture initiation and propagation, helping the rock mass to retain its inherent cohesion and tensile strength and establish a stable compression arch in the roof. The results demonstrate that the bonded block approach can help us to better understand the influence of ground support on progressive brittle fracture for tunnels in weak, horizontally bedded rock like the Ashfield Shale.
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BIM to Numerical Modelling Interoperability for Geotechnical Design of Underground Metro Station
Building Information Modelling (BIM) is one of the important processes being adopted by the construction industry as it provides a collaboration platform in conjunction with technical standards for interoperability over the lifecycle of an asset. However, geotechnical analysis engaging numerical tools has yet to leverage the BIM benefits due to the lack of effective interoperability means, which not only results in unnecessary remodelling and rework at a cost of labour and computational waste, but also with possibility of errors, misinterpretation and omission of information. Using a trinocular underground station as an example, a workflow underpinned by heuristic techniques is proposed to enhance the interoperability between a BIM design authoring tool (Revit) and a numerical modelling tool (FLAC3D) for geotechnical analysis. A BIM-based multiple LoD (levels of detail) model framework is proposed to represent different information requirements for different purposes of BIM use throughout the project lifecycle. Leveraging the associated geometry and semantics, techniques of parametric modelling, data manipulation via visual and traditional programming are engaged to bridge BIM and numerical modelling for geotechnical analysis on different design scenarios. The simulated results are visualised through a backward automation cycle to Revit for design optimisation. The presented solution offers and automates an error-free design-to- design workflow solution and therefore enables efficient exploration of design scenarios and design optimisation.
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Tasmanian Rock Stress
The first rock stress measurements undertaken in Tasmania formed part of an exhaustive and innovative study to create Tasmania’s first underground hydro-electric power station at Poatina in the central north of the State in 1960 (Endersbee and Hofto, 1963). Those were two-dimensional measurements undertaken using flatjacks. The first three-dimensional stress measurements in Tasmania were undertaken at the Dolphin Mine on King Island using CSIRO hollow inclusion (HI) Cells in 1975 (Worotnicki and Walton, 1976). Since then, the majority of stress measurements in Tasmania have been undertaken using HI Cells at mines and civil construction projects on the West Coast. However, innovation has remained a theme, and many techniques have been trialled for various reasons over the years, and these trials continue today.
The purpose of writing this paper is to collate the results of Tasmanian rock stress measurements and present them in a usable form for the majority of readers. A key requirement of the presentation was to be able to compile the results into a single data set. This was achieved by reprocessing all data to a common reference. Results are reported relative to true north. Where possible, results were reprocessed from the original measurement data. To the best of the authors’ knowledge, this paper documents all Tasmanian two- and three-dimensional virgin rock stress measurements, extant at the time of writing.
Not discussed in this paper, the World Stress Map includes a dozen or so stress observations derived from petroleum wells in the Bass Basin east of King Island. These represent the only records from within the Tasmanian jurisdiction included in the World Stress Map released by Heidbach el al. (2016).
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Insights Into Geotechnical Project Success
Nine experienced ground and underground engineering professionals from Aurecon in ANZ were asked to provide details of successful projects where collaboration had occurred. The responses are presented in differing ways, but the insights highlighted several recurring reasons why collaboration had occurred. The factors that were considered to have a positive influence on the outcome of the project included trust, necessity, delivery framework and involvement. The insights gained about these factors were pitched against ideas and hypotheses from accepted business and management theory and found to correlate well. It is concluded that setting up a team for success by having the frameworks in place that will facilitate collaborative mindset and practices will result in team alignment and better project outcomes.