Slope stability analysis using deterministic and probabilistic approaches for poorly defined stratigraphies
- Authors: Ghadrdan, Moshen , Dyson, Ashley , Shaghaghi, Tahereh , Tolooiyan, Ali
- Date: 2021
- Type: Text , Journal article
- Relation: Geomechanics and Geophysics for Geo-Energy and Geo-Resources Vol. 7, no. 1 (2021), p.
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- Description: The stability of slopes directly affects human lives, the environment, and the economy. Inaccurate geological profiles within numerical slope stability models can lead to potentially catastrophic consequences when model conditions do not appropriately reflect real-life stratigraphy. In cases where localised deposits are prevalent, probabilistic methods are often necessary to accommodate for unknown or poorly defined stratigraphies. Currently, there are no commercial geotechnical software packages that simulate probabilistic constitutive behaviour of materials within finite element methods for large-scale stratigraphic analysis. Instead, commercially available probabilistic methods such as the random limit equilibrium method are incapable of incorporating non-linear constitutive soil behaviour. For this reason, advanced constitutive models are seldom coupled with probabilistically varying soil layers or spatially variable soil parameters. The objective of this research is the implementation of a simplified method for probabilistic stratigraphic analysis within a commercially available FE environment, providing a technique to assess the effects of stratigraphic uncertainty on slope stability. The proposed method is presented, highlighting the impact of localised thin layers of soft material as well as their frequency and location on the slope of an operational open-pit mine. The significance of these stratigraphic effects is presented through a case study of Australia’s second-largest open-pit mine, at which the stability of a collapsed coal slope is analysed. To improve the reliability of the finite element method for slope stability assessment, the Monte Carlo approach has been incorporated to consider varying shear strength distributions for models incorporating advanced constitutive behaviour. Thicker probabilistically generated deposits of silty material resulted in increased slope Factors of Safety. Similarly, greater proportions of silty deposits within a predominantly clayey interseam produced larger safety factors than slopes without localised thin silty layers. Stratigraphic analysis indicated that the Factor of Safety was most sensitive to localised silt layers at depths greater than 83 m below ground level. © 2020, Springer Nature Switzerland AG.
Prediction and classification for finite element slope stability analysis by random field comparison
- Authors: Dyson, Ashley , Tolooiyan, Ali
- Date: 2019
- Type: Text , Journal article
- Relation: Computers and Geotechnics Vol. 109, no. (2019), p. 117-129
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- Description: This paper considers probabilistic slope stability analysis using the Random Finite Element Method (RFEM) combined with processes to determine the level of similarity between random fields. A procedure is introduced to predict the Factor of Safety (FoS) of individual Monte Carlo Method (MCM) random field instances prior to finite element simulation, based on random field similarity measures. Previous studies of probabilistic slope stability analysis have required numerous MCM instances to reach FoS convergence. However, the methods provided in this research drastically reduce computational processing time, allowing simulations previously considered too computationally expensive for MCM analysis to be simulated without obstacle. In addition to computational efficiency, the comparison based procedure is combined with cluster analysis methods to locate random field characteristics contributing to slope failure. Comparison measures are presented for slope geometries of an Australian open pit mine to consider the impacts of associated factors such as groundwater on random field similarity predictors, while highlighting the capacity of the similarity procedure for prediction, classification and computational efficiency.
Long-term dynamic behaviour of Coode Island Silt (CIS) containing different sand content
- Authors: Jamali, Hassan , Tolooiyan, Ali , Dehghani, Masoud , Asakereh, Adel , Kalantari, Behzad
- Date: 2018
- Type: Text , Journal article
- Relation: Applied Ocean Research Vol. 73, no. (2018), p. 59-69
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- Description: The foundations of nearshore and offshore structures are always subjected to long-term cyclic loading which is often a one-way, with low amplitude and a large number of cycles. Hence, the long-term dynamic behaviour of shoreline soils and sediments should be understood to avoid excessive deformation and liquefaction. As one of the most problematic soft soils in Melbourne, Coode Island Silt (CIS) at the northern shoreline of Port Phillip Bay contains a considerable but variable amount of sand. This paper explores the dynamic response of CIS containing different sand content subjected to a large number of cycles. To determine the liquefaction potential, and the effect of sand content on the resilient modulus and permanent strain of CIS, a series of long-term cyclic triaxial tests at a sinusoidal loading frequency of 1 Hz is performed. Based on the test results, it is found that CIS with varying sand content up to 30%, does not liquefy under the cyclic stress ratios and frequency applied in this study. Also, a sand content of 10% causes CIS to degrade more under cyclic loading. In the end, an empirical model to predict the permanent strain of CIS with a variable sand content is calibrated.
An investigation of correlation factors linking footing resistance on sand with cone penetration test results
- Authors: Gavin, Kenneth , Tolooiyan, Ali
- Date: 2012
- Type: Text , Journal article
- Relation: Computers and Geotechnics Vol. 46, no. (2012), p. 84-92
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- Description: Significant research effort has led to improvements in our ability to estimate the ultimate bearing resistance of footings in sand. These techniques often estimate the footing resistance at relatively large displacements, typically 10% of the footing width, q b0.1. Cone Penetration Test (CPT) design methods typically link q b0.1 and q c through a constant reduction factor, α. A range of α factors for shallow footings have been proposed, some methods suggest that α is constant and while others that it varies with footing width and depth (or stress level). There is a dearth of field data with which to compare these correlation factors, in particular where foundation width and depth have been varied in the same ground conditions. For this reason finite element analyses have proven to be a useful tool for performing the parametric studies required to asses factors controlling α. This paper describes the results of numerical analyses performed to investigate α factors for soil profiles which were calibrated using the results of the CPT tests performed at a dense sand test-bed site. The numerical model was first used to perform parametric analyses to consider the effect of footing width, B and footing depth, D on the α factor mobilised in dense Blessington sand. In order to assess the effects of relative density, footing tests in a range of natural sands with variable in situ densities were modeled. The results of the finite element analyses suggest that a direct correlation between q b0.1 and q c can be established at a given test site which is independent of footing width and depth and is relatively weakly dependent on the sands relative density if the zone of influence of the foundation considered is large enough. © 2012 Elsevier Ltd.
Probabilistic investigation of RFEM topologies for slope stability analysis
- Authors: Dyson, Ashley , Tolooiyan, Ali
- Date: 2019
- Type: Text , Journal article
- Relation: Computers and Geotechnics Vol. 114, no. (2019), p. 1-15
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- Description: The Random Finite Element Method (RFEM) is an increasingly popular tool in geotechnical engineering, especially for analysis of spatial variation and uncertainty in slope stability. Although the method has gained prominence in recent years, topological effects of strong and weak zones and the impact of their locations remain largely unknown. Although numerous potential slip surface realisations can be generated with RFEM, probabilistic failure statistics are often governed by several representative slip surfaces (RSS). In this research, random field similarity methods and clustering techniques are coupled with RFEM slope stability simulation to determine the impact of shear strength spatial patterns on slope failure mechanisms and safety factors. Regions of significance are highlighted within a case study of a Victorian open-cutbrown coal mine, with particular attention given to the effects on the slope failure surface as well the factor of safety. Results are presented of Factor of Safety distributions when particular slip surfaces and clustering constraints are imposed, providing further understanding of the impacts of shear strength characteristics on probabilistic simulation results.
Numerical and finite element analysis of heat transfer in a closed loop geothermal system
- Authors: Hemmingway, Phil , Tolooiyan, Ali
- Date: 2014
- Type: Text , Journal article
- Relation: International Journal of Green Energy Vol. 11, no. 2 (2014), p. 206-223
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- Description: Analysis of the thermal regime created by a geothermal borehole heat exchanger is performed using a closed form radial heat flow equation, a geothermal borehole heat exchanger design tool, and a finite-element model. Climatic, heat exchanger construction, and building load data are entered into the heat exchanger design tool to create a theoretical model along with thermal parameters from a number of geological formations. Output data from the design tool model are used in conjunction with the closed form radial heat flow equation to calculate the predicted temperature with respect to time and distance from the heat exchanger for the modeled ground formations. The output data from the design tool are also used to create a number of finite-element method models against which the predictions calculated using the closed form radial heat flow equation can be compared. A good correlation between the temperatures predicted by the finite-element models and the closed form equation calculations is observed. However, when used within its recommended limiting conditions, the closed form equation is shown to slightly underestimate the temperature of the ground when compared with the finite-element model predictions. The limiting conditions associated with the closed form equation are discussed in the context of the output from the finite-element method models.
Geophysical and geotechnical assessment of a railway embankment failure
- Authors: Donohue, Shane , Gavin, Kenneth , Tolooiyan, Ali
- Date: 2011
- Type: Text , Journal article
- Relation: Near Surface Geophysics Vol. 9, no. 1 (2011), p. 33-44
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- Description: A geophysical investigation was carried out after the failure of an important railway embankment in the south-east of Ireland, The embankment, which had a long-term history of stability problems, was studied using a combination of ground-penetrating radar (GPR), electrical resistivity tomography (ERT), multichannel analysis of surface waves (MASW) and geotechnical testing. A significant thickening of the ballast layer around the failure location was observed using GPR, which confirmed the existence of an ongoing stability problem in the area. ERT profiles determined the presence and spatial extent of a significant layer of soft clay both beneath and to the east of the embankment, which could have a major impact on its long-term stability. ERT also detected steeply sloping bedrock close to the failure zone that is likely to have contributed to the long-term settlement of the embankment, which necessitated frequent re-ballasting. MASW confirmed the presence of the steeply sloping bedrock in addition to determining the low stiffness (Gmax) values of the embankment fill. High quality sampling of the soft clay deposit was undertaken and strength and compressibility tests revealed the importance of this layer to both the on-going serviceability problems evident for the original embankment and the stability problems encountered by the remodelled section. © 2010 European Association of Geoscientists & Engineers.
Post-long-term cyclic behaviour of Coode Island Silt (CIS) containing different sand content
- Authors: Jamali, Hassan , Tolooiyan, Ali , Dehghani, Masoud , Asakereh, Adel , Kalantari, Behzad
- Date: 2018
- Type: Text , Journal article
- Relation: Applied Ocean Research Vol. 80, no. (2018), p. 11-23
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- Description: It is widely accepted that the post-cyclic stiffness and shear strength of marine clays may alter as a consequence of experiencing a large number of deviatoric load cycles. Most marine clays in bay areas are already undergoing long-term one-way and low amplitude ocean and wind waves either because of the seabed topography or existing infrastructures. For the engineering of such clays, particular attention should be given to the post-cyclic behaviour of the material when investigating the effect of alteration in loading regimes exerted by human-made or natural phenomena such installation of new infrastructure, earthquake, tsunami and port upgrade construction, in which the magnitude of the applied load changes. As one of the most sensitive soft soils in Melbourne, Coode Island Silt (CIS) at the northern shoreline of Port Phillip Bay comprises a considerable and variable amount of sand. This paper explores the post-cyclic constitutive behaviour of CIS containing variable sand content. To investigate the stiffness and shear strength of CIS subsequent to experiencing a large number of low amplitude cycles, a series of post-cyclic triaxial tests are performed on CIS specimens with varying sand content ratio up to 30%, immediately after applying 30,000 semi-sinusoidal load cycles. Based on the test results, it is found that the undrained shear strength of CIS, does not alter considerably as the results of long-term cyclic loading. However, a significant increase in the secant stiffness of CIS followed by very brittle yielding is observed. In the end, it is intended that results be summarised in a form applicable by industry. Hence, the possible effects of such alterations in the constitutive behaviour of CIS on the design of monopile foundations are discussed.
Unconfined Expansion Test (UET) for measuring the tensile strength of organic soft rock
- Authors: Tang, Zhan , Tolooiyan, Ali , Mackay, Rae
- Date: 2017
- Type: Text , Journal article
- Relation: Computers and Geotechnics Vol. 82, no. (2017), p. 54-66
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- Description: An Unconfined Expansion Test (UET) is presented for measuring the tensile strength of Intermediate Geotechnical Materials (IGM). The test is performed by generating radial cavity expansion inside a cylindrical specimen. Pressure redistributes evenly around the cavity wall during the test. Tensile failure initiates on the weakest plane around the cavity and radially propagates across the specimen. By also capturing the stress-strain relationship and deriving the shear modulus, a UET is also capable of measuring the tensile elastic modulus of the material. An eXtended Finite Element Method (XFEM) analysis using Abaqus/Standard has been carried out to verify the UET test results. © 2016 Elsevier Ltd
Modelling the cone penetration test in sand using cavity expansion and arbitrary lagrangian eulerian finite element methods
- Authors: Tolooiyan, Ali , Gavin, Kenneth
- Date: 2011
- Type: Text , Journal article
- Relation: Computers and Geotechnics Vol. 38, no. 4 (2011), p. 482-490
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- Description: The paper considers two techniques to model the Cone Penetration Test (CPT) end resistance, qc in a dense sand deposit using commercial finite element programmes. In the first approach, Plaxis was used to perform spherical cavity expansion analyses at multiple depths. Two soil models, namely; the Mohr-Coulomb (MC) and Hardening Soil (HS) models were utilized. When calibrated using simple laboratory element tests, the HS model was found to provide good estimates of qc. However, at shallow depths, where the over-consolidation ratio of the sand was highest, the relatively large horizontal stresses developed prevented the full development of the failure zone resulting in under-estimation of the qc value. The second approach involved direct simulation of cone penetration using a large-strain analysis implemented in Abaqus/Explicit. The Arbitrary Lagrangian Eulerian (ALE) technique was used to prevent excessive mesh deformation. Although the Druker-Prager soil model used was not as sophisticated as the HS model, excellent agreement was achieved between the predicted and measured qc profiles.