Analytical and numerical approaches to evaluate the effect of time-dependent and time-independent soil characteristics on the stability of deep excavations
- Authors: Ghadrdan, Mohsen
- Date: 2021
- Type: Text , Thesis , PhD
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- Description: Excavating the ground for different purposes, such as extracting valuable materials or undertaking urban construction, may cause concerns regarding the stability of the formed slopes, which can affect the environment, the economy, and human lives. Slope stability analysis in large-scale and deep excavations such as open-pit mines is challenging due to uncertainties regarding varying material parameters, complex field conditions and lack of or insufficient data such as pore water pressure distribution, in-situ stress conditions, and discontinuities. Despite different advanced analytical and numerical slope stability techniques having been developed, slope stability analysis may produce unreliable conclusions due to these uncertainties and challenges. This study’s objective is to investigate the effect of different factors associated with slope stability through a case study of the Yallourn brown coal open pit mine in Australia. In this study, the two most common slope stability methods—the Limit Equilibrium Method (LEM) and the Finite Element Method (FEM)—were employed. A comprehensive study was conducted to determine how the generation and dissipation of Negative Excess Pore-Water Pressure (NEPWP) affect slope stability assessments. Additionally, due to the complex geological stratigraphy of the site, different scenarios for geological layering were defined and investigated for the slope stability analyses. Moreover, the sensitivity of the slope stability assessment to not only different material characteristics but also different formulations and assumptions of LEM and FEM are presented.
- Description: Doctor of Philosophy
Application of ground penetrating radar (GPR) to detect joints in organic soft rock
- Authors: Tolooiyan, Ali , Dyson, Ashley , Karami, Mojtaba , Shaghaghi, Tahereh , Ghadrdan, Mohsen
- Date: 2019
- Type: Text , Journal article
- Relation: Geotechnical Testing Journal Vol. 42, no. 2 (2019), p.
- Full Text: false
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- Description: The detection of joints and discontinuities is of particular importance to the stability of a broad range of geostructures, including slopes and underground and open-pit mines. As a common example, the mechanical response of soft rocks observed within open-pit mines is significantly influenced by the existence of joint networks, resulting in a complex stress distribution that governs the stability factor of safety as well as the failure mechanism. In this article, surface geophysics scanning by ground penetrating radar (GPR) is presented for the detection of vertical joints at one of the largest open-pit coal mines in Australia. The optimum soil velocity, point interval, and antenna frequency for joint detection in Victorian Brown Coal (VBC) are presented in comparison with electromagnetic properties of known organic soils. Furthermore, the performance of an assorted set of post-processing signal filtering techniques to successfully identify the underground coal fractures are detailed, along with obstructions affecting the feasibility of GPR vertical joint discovery in this light organic soft rock.
Design and optimisation of drainage systems for fractured slopes using the XFEM and FEM
- Authors: Shaghaghi, Tahereh , Ghadrdan, Mohsen , Tolooiyan, Ali
- Date: 2020
- Type: Text , Journal article
- Relation: Simulation Modelling Practice and Theory Vol. 103, no. (2020), p.
- Full Text: false
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- Description: The reliable and optimised design of a drainage system for saturated slopes is often a challenging geotechnical task. Such a task becomes even more challenging when a slope contains pre-existing joints and discontinuities. In saturated and semi-saturated conditions, the existence of joints may lead to a complex distribution of pore water pressure within the slope, affecting the effective stress distribution and the stability of the slope. This paper aims to study the effect of horizontal borehole drainage systems with different arrangements on pore water pressure distributions within a saturated fractured slope. In this study, several coupled pore fluid diffusion and stress-strain analyses were conducted using the e-Xtended Finite Element Method (XFEM) in conjunction with the Finite Element Method (FEM) to simulate the efficiency of a drainage system of a deep slope at the second largest open-cut mine in Australia. As one of the objectives of this study, the effect of water flow inside a joint and normal to the joint surface (normal flow) is considered as an essential simulation component. The results show that the pore water pressure distribution at the vicinity of the joint is considerably influenced by the magnitude of normal flow. Such influence should be taken into account when designing a drainage system, as the magnitude of normal flow and the performance of the drainage system may affect each other directly. © 2020 Elsevier B.V.
Effect of negative excess pore-water pressure on the stability of excavated slopes
- Authors: Ghadrdan, Mohsen , Shaghaghi, Tahereh , Tolooiyan, Ali
- Date: 2020
- Type: Text , Journal article
- Relation: Geotechnique Letters Vol. 10, no. 1 (Mar 2020), p. 20-29
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- Description: Generation of negative excess pore-water pressure (NEPWP) due to the excavation of saturated soils under undrained conditions and the dissipation that follows over time may result in different short- and long-term slope instability. The NEPWP generated due to excavation gradually decreases towards equilibrium or, in some cases, steady seepage. Hence, total pore-water pressures immediately after excavation are lower than the ultimate equilibrium values, leading to a reduction of the average effective stresses in the slope and subsequently threatening stability in the long term. In this research study, the stability of three benchmark civil and mining excavations has been studied, considering the effects of the generation and dissipation of NEPWP. A series of numerical simulations are conducted to determine the role of in situ stresses and time in NEPWP dissipation as well as the consequent effects on the stability of the excavated slopes. To conduct a realistic time-dependent transient analysis, fully coupled hydro-geomechanical formulation has been employed. Results show that in general, higher removal of stress levels lead to higher NEPWP generation and higher factor of safety values in the short term. Thereafter, the dissipation of NEPWP threatens the long-term stability of the excavation.
Effect of rock mass permeability and rock fracture leak-off coefficient on the pore water pressure distribution in a fractured slope
- Authors: Shaghaghi, Tahereh , Ghadrdan, Mohsen , Tolooiyan, Ali
- Date: 2020
- Type: Text , Journal article
- Relation: Simulation Modelling Practice and Theory Vol. 105, no. (2020), p. 1-13
- Full Text: false
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- Description: The reliable assessment of the stability of saturated slopes becomes a challenging task when slopes are consisting of discontinuous materials and containing pre-existing joints. The discontinuous nature of the slopes' material could increase the overall permeability of the slope, while existing joints facilitate groundwater leakage through the joint surfaces into the slope which subsequently exerts a major impact on deformation and the effective stress distribution. This paper aims to study the Pore Water Pressure (PWP) distribution changes in a saturated fractured slope by conducting advanced coupled pore fluid diffusion and stress-strain analyses, while investigating the sensitivity of results to the variation of permeability and leakage properties of fracture surfaces. Modelling of jointed slopes is carried out using the e-Xtended Finite Element Method (XFEM) in conjunction with the Finite Element Method (FEM). In this study, the fluid flow inside the joint is the major focus at which the constitutive response of the fluid inside the joint considers both tangential and normal flows. To demonstrate the state-of-the-art simulation technique presented in this paper, simulation of a fractured slope at the second largest open-pit mine in Australia is performed as a case study. This study shows the effect of a variable leak-off coefficient of the joint surfaces and the permeability magnitude on the pore water pressure distribution.
- Description: This research has been supported financially by the Earth Resources Regulation of the Victorian State Government Department of Economic Development, Jobs, Transport and Resources. The first and second authors are funded by the GHERG LV Batter Stability Project Scholarship and Faculty Tuition Scholarship of Federation University Australia.
Investigation of an Australian soft rock permeability variation
- Authors: Tolooiyan, Ali , Dyson, Ashley , Karami, Mojtaba , Shaghaghi, Tahereh , Ghadrdan, Mohsen
- Date: 2020
- Type: Text , Journal article
- Relation: Bulletin of Engineering Geology and the Environment Vol. 79, no. 6 (2020), p. 3087-3104
- Full Text: false
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- Description: In this study, permeabilities of Victorian Brown Coal (VBC) as an Australian soft rock are determined for a range of depths of a continuous coal seam located at the batter crest of the Yallourn brown coal open-cut mine in Victoria, Australia, by implementing a Lugeon packer testing procedure. Permeability values are determined both analytically and by numerical simulation and are compared with laboratory test results. Field testing resulted in permeabilities several orders of magnitude higher than laboratory testing, suggesting the existence of fractures common to lignite structures on a greater scale than can be observed in the laboratory. The variation of depth-based field and laboratory permeabilities is discussed, as well as the necessary conditions required for the numerical modelling of packer testing within VBC. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature.
- Description: Department of Economic Development, Jobs, Transport and Resources, DSDBI The second and third authors are funded by the Australian Government Research Training Program (RTP) and the GHERG scholarship programme.
Maximising the efficiency of Menard pressuremeter testing in cohesive materials by a cookie-cutter drilling technique
- Authors: Tolooiyan, Ali , Dyson, Ashley , Karami, Mojtaba , Shaghaghi, Tahereh , Ghadrdan, Mohsen , Tang, Zhan
- Date: 2021
- Type: Text , Journal article
- Relation: Engineering Geology Vol. 287, no. (Jun 2021), p. 106096
- Full Text: false
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- Description: Menard pressuremeter testing has been widely used in geotechnical engineering applications for 40 years and is an important technique in determining in-situ horizontal stress distributions. In this study, Menard pressuremeter testing is combined with a "cookie-cutter" insertion technique to determine horizontal stresses for a soft-rock in an operational Australian mine. The method presents an alternative to the Self-Bored Pressuremeter, with cookie-cutter drill rods allowing for sample recovery and further laboratory testing. The method accommodates for the presence of gravel and hard layered materials that present a risk of damage to cutting shoes of Self-Bored Pressuremeter devices. The combination of a sonic drill rig, coupled with the cookie cutter rods produces a close tolerance pocket resulting in "pseudo self boring pressuremeter tests". The undrained shear strength, unload-reload shear modulus and in-situ horizontal stress are presented from pressuremeter tests conducted in the region for the first time. The undrained shear strength was observed in the range of 0.47-0.57 MPa, the unload-reload shear modulus between 17.43 and 18.25 MPa, the lift-off pressure in the range of 0.35-0.61 MPa. The K-0 of coal was equal to 1, with interseam materials ranging from 2.1 to 3.5. Results of the cookie-cutter insertion method are compared with conventional drilling methods, with the cookie-cutter insertion test providing results in good agreement with both advanced triaxial laboratory tests and FEM numerical analysis. Cookie-cutter pressuremeter tests were conducted on cohesive soils at Australia's second-largest open-pit mine, with pressuremeter test results presented for Victorian brown coal for the first time.
Sensitivity of the stability assessment of a deep excavation to the material characterisations and analysis methods
- Authors: Ghadrdan, Mohsen , Shaghaghi, Tahereh , Tolooiyan, Ali
- Date: 2020
- Type: Text , Journal article
- Relation: Geomechanics and Geophysics for Geo-Energy and Geo-Resources Vol. 6, no. 4 (2020), p.
- Full Text: false
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- Description: Due to the spatial variability of material characterisations in deep and large scale excavations, stability assessment is often a challenging task. Numerous slope stability analysis methods based on a range of assumptions and principles are implemented in commercial software packages to ease the process of stability assessment of non-homogeneous and multi-layered slopes. However, the selection of a suitable method remains crucial as the application of an unrealistic or unsuitable method may lead to catastrophic consequences. Besides material shear strength parameters, and analysis methods, non-strength characterisations such as permeability and creep can affect the result of slope stability analysis significantly. In this study, the sensitivity of the stability assessment of a deep excavation in Australia to material characterisations such as friction angle, cohesion and permeability and creep is investigated by the use of different formulations and assumptions of the Limit Equilibrium Method (LEM) and the Finite Element Method (FEM) as the two most common slope stability methods. The results show that the stability assessment is highly sensitive to the applied method and assumptions. Moreover, the role of material strength and non-strength parameters and the selection of a suitable constitutive model in slope stability assessment is presented. © 2020, Springer Nature Switzerland AG.