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Technical Note: Regression Analysis of Soil Nailing Parameters Using Finite Element and Limit Equilibrium Methods
With rising urbanization and growing transportation needs, stabilization of soil slopes has become extremely vital to ensure adequate, satisfactory performance and safety. Soil nailing is amongst the most cost-effective methods used for slope stabilization by inserting relatively slender reinforcing elements, mostly reinforcement bars, into the soil slopes at predetermined intervals, followed by finishing up with shotcrete and mesh installation. This study is based on the determination of the most economical nail length pattern for different soil conditions and establishes a trend using a multiple regression technique with nail length as a factor. In this study, conventional design analysis using relevant charts and tables published by the US Federal Highway Administration (FHWA) has been used to design a 6m high soil nail wall which has been further analysed using Finite Element method and Limit Equilibrium method for a range of soil parameters. Based upon the obtained results, a relationship between soil deformation, soil parameters and nail length using multiple regression technique has been derived. The results obtained from the derived relationship were then compared to the relevant literatures and case studies and were found to be in good agreement.
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Young Geotechnical Engineers’ Evening Seminar & 17th Dr Baden Clegg Award
Jarrad Coffey, Andrew Lim and Sam Stanier
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Probabilistic risk assessment of mine subsidence
Instability of coal pillars within disused underground mines is an important cause of mine subsidence affecting surface developments. Uncertainty and variability of materials, dimensions and loads affects our ability to assess the stability of these coal pillars. Traditionally, these parameters have been quantified using a deterministic approach. Variables affecting coal pillar stability were probabilistically analysed to determine the factor of safety and probability of failure. Two case studies in the Newcastle region were undertaken in order to simulate real situations. There are two types of uncertainty that need to be modelled: aleatory and epistemic. Aleatory uncertainty is ‘random’ uncertainty or natural variability. Epistemic uncertainty is uncertainty due to the lack of information. The probabilistic risk assessment method used in the case studies provided an assessment of aleatory and epistemic uncertainties. In the assessment of the stability of pillars in old abandoned workings, both types of uncertainty are encountered. Hence, the UNSW Pillar Design Method under-estimates the probability of failure as it ignores epistemic uncertainties of old pillars in abandoned mines. It was also found that there is no direct relationship between factor of safety and probability of failure. It was revealed that the level of uncertainty can have a significant impact on the probability of failure of a section of workings. The results of this study may lead to a requirement that developers use probabilistic methods to verify that proposed developments are subject to an acceptably low risk of subsidence damage.
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A settlement hazard risk management framework for the development of backfilled open-cut quarries
Surface mining operations typically lead to the disturbance of large areas of land over the life cycle of mines. With growing pressures on land use, particularly in urban areas, brownfield sites such as closed open-cut mines are being considered more and more for redevelopment opportunities. Naturally, these areas require reinstatement to some workable final surface level which is then suitable for the future development. Nevertheless, the business case that supports the decision to develop these complex areas should aim at balancing and optimising the investment required in the rehabilitation of the original site itself, as well as the cost of subsequent infra- and superstructure development. Controlled backfilling of these disused operations is often complex, including considerations of variability and engineering characteristics of the available backfill materials and the influence of groundwater recovery. Variations in the properties of the backfill material may not have any significant impact on some after-use activities (e.g. when the area is intended for conservation or agricultural land use after rehabilitation), but subsequent surface infrastructure development could be severely affected by settlement of backfilled land. The strategic planning and closure process for open-cut mines typically involves a number of different stages and a detailed consideration of the potential behaviour of the backfill is crucial in ensuring successful development. The purpose of this paper is to present a Risk Management Framework (RMF) focused on addressing excessive post-construction settlement. This RMF needs to form part of a more comprehensive geotechnical, and ultimately overall risk management and mine closure plan.
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Listening To The Earth: An Unconventional Scientific Approach To Understanding Sub-surface Ground Conditions
Digitally recorded background ambient noise can be used to extract details regarding subsurface soil conditions. This unique methodology has been implemented to provide comprehensive assessments of geotechnical site conditions. Ambient noise is the persistent vibration of the ground in response to anthropogenic and natural causes. In many contexts, these background vibrations are classified as noise, and efforts are made to remove these signals from recorded data. However, these background vibrations also contain valuable information regarding the materials they travel through. The refraction microtremor (ReMi) technique separates these waves from noise recordings to determine soil shear-wave velocities. Interpolation of the closely spaced one-dimensional velocity-depth profiles along linear arrays allow two or three-dimensional velocity-versus-depth representations to be produced, thereby mapping lateral variations and extending subsurface characterisations between more expensive spot borehole measurements. ReMi technique provides a non- invasive and cost-effective way of estimating vertical soil/rock shear-wave versus depth profiles. This paper examines the contribution ReMi shear-wave velocity assessments can make towards enhancing subsurface geological and geotechnical models to mitigate risk from unforeseen ground conditions.
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Designing stiffened raft footing on non-engineered fill
This paper presents the results of numerical analyses of a stiffened raft footing on non-engineered fill (NEF) located in the Melbourne suburb of Chelsea. Analyses have been undertaken using a simplified approach with linear elastic parameters in a commercial Finite Element (FE) program (STRAND6) followed by a more comprehensive analysis using a non linear constitutive model in a commercial Finite Difference (FD) program (FLAC3D). The results of the numerical analyses and findings of a simplified empirical design approach known as the “Soft Spot” method are compared with the field settlement behaviour. The results indicate that the “Soft Spot” method is unable to predict the field behaviour of the raft, in particular the behaviour of the raft slab panels, while the FLAC3D program predicts the raft behaviour more closely. Furthermore, the efficiencies of using additional concrete (such as the haunching, increasing slab thickness) are evaluated and it is found that an increase in slab panel thickness is a more efficient alteration which reduces differential settlement of the raft footing. The use of haunches is observed to reduce the shear stresses compared to no haunches; however the developed shear stresses in both the cases are well within the requirements of AS 2870 (1996). An aspect ratio (length/width) of unity for the raft footing is also found to be more effective in reducing the surface deflection and total settlement of raft over NEF.
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Probabilistic techniques in geotechnical modelling – which one should you use?
Predictions of performance are at the core of geotechnical engineering design. Predictions based solely on deterministic analyses suffer from unquantifiable uncertainties and the implied absoluteness of the prediction. On the other hand, probabilistic estimates suffer from being vague, which is unsettling to most geotechnical engineers. Combining deterministic and probabilistic analyses offers synergies that are best utilized only if the geotechnical engineer appreciates the relative role of each type of analysis. This paper describes an overview of the hierarchy of probability-based analyses in geotechnical engineering predictions. The aim is to provide geotechnical engineers with a framework that integrates analytical and probabilistic analyses. The available probabilistic analyses, their level of complexity, applicability and limitations are considered, in order to enable the geotechnical engineer to choose correctly the optimum analysis that best suits their specific project and circumstances.
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The Wall That Moved
A multi-row anchored soldier piled wall was designed to support the sides of a deep basement excavation adjacent to a railway embankment. As part of the approval process, the contractor’s consultants had to carry out detailed analyses to demonstrate the suitability of the wall and agreed to a monitoring plan of regular site inspections by a geotechnical engineer including regular surveying of the wall to check on the wall’s performance during the construction phase. Construction began, the soldier piles were installed, and excavation and anchoring commenced in stages. Surveying of the wall also started, but the contractor neglected to get the geotechnical inspections required by the monitoring plan. All seems to be going well until unacceptable lateral movements of the retaining wall were detected from the survey. This paper describes the design of the wall, a review of the construction phase information and offers an explanation for the higher than expected wall movements.
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Can the Shrink-Swell Index be predicted in the Wagga Wagga Region based on Atterberg Limits?
Relationships between the shrink-swell index and other clay tests, such as Atterberg limits have been investigated by others however, there is not any available information for the Wagga Wagga region in NSW Australia. This study’s objective is to establish some relationships between the shrink-swell index and Atterberg limits of the soils in the Wagga Wagga region. A total of 27 samples at different locations and depths was tested for shrink-swell index, linear shrinkage, liquid limit, plastic limit, plasticity index and soil particle size distribution, using Australian Standard test methods. The results and data analysis indicated that shrink-swell index correlated with Atterberg limits. The shrink-swell index can be estimated based on a single Atterberg limit test or the combination of linear shrinkage + liquid limit and linear shrinkage + plasticity index. In conclusion, linear shrinkage and liquid limit are the reasonable prediction factors for the estimation of shrink-swell index for the soils in the Wagga Wagga region.
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Mapping expansive soils from space
This paper describes a new approach to detecting motion related to soil expansivity from ground motion time series data. Interferometric Synthetic Aperture Radar (InSAR) is a remote sensing technique used to detect and monitor ground and surface infrastructure motion with millimetric precision using SAR satellite imagery.
A total of 74 high resolution TerraSAR-X SAR images were used to produce ground motion time series covering a large urban area (450 km2) in Auckland NZ between August 2019 and May 2022. Sixense’s Atlas InSAR processing chain produced 7.6 million ground motion time series. Data science techniques were used to locate motion related to expansive clays swelling mechanisms.
Climate data from NIWA weather stations and soil data layers from Landcare Research NZ have been included to complement the analysis. 450k building footprints from Land Information New Zealand (LINZ) have been used to map those buildings potentially affected by expansive clay seasonal movements. Finally, ground instrumentation data from the Mount Eden City Rail Link (CRL) station construction site is used to validate the observed motion precision of Sixense’s Atlas InSAR.
The end goal of this study is to provide the Auckland city council cost effective added value information in the context of climate change adaptation, where erratic weather patterns pose a serious risk to building foundations.