Comparison of 2D and 3D slope stability factors of safety considering spatial variability

Mutede, Tawanda Blessing (2022-04)

Thesis (MEng)--Stellenbosch University, 2022.

Thesis

ENGLISH ABSTRACT: Historically, slope stability analyses have relied on two-dimensional (2D) models. This is due to the relative ease of implementing 2D slope stability analysis both from a theoretical and computational stand-point. However, with the emergence of powerful computers, the use of three-dimensional (3D) methods is increasing. While 3D slope stability analysis methods model reality better, site investigation methods often do not provide adequate information to create detailed 3D slope models. As a result, the extrapolation of 2D cross sections to generate 3D models is employed. This simplification gives rise to the question of whether these 3D extrapolated analyses are safe, as this typically results in higher factors of safety compared to the corresponding 2D analyses. Extrapolation may hide the spatial variability along slopes leading to an unsafe analysis. This research aimed to determine whether it was safe to conduct 3D slope stability simulations using extrapolated 2D sections or whether it was safer to analyse slopes using simple 2D methods. This research was done with reference to tailings dams. A total of 816 different slopes were generated and analysed using Rocscience’s Slide 2 and Slide 3 software programs. Three different models were used to conduct slope stability analyses, namely: 2D models, 3D extrapolated models from 2D extrapolated sections and 3D composite models which consisted of three merged 2D extrapolated slope sections. The research found that when analysing slopes, if the weakest slope sections are sampled and analysed using 3D extrapolated models, the results that are obtained are safe. However, if the strongest slope section is unfortunately sampled and analysed using 3D extrapolated models, the results obtained are unsafe. In such instances it is safer to rely on normal 2D modelling for analysis of slopes. The reason for this is that 3D extrapolated models’ factor of safety (FOS) do not consider heterogeneity along a slope. With relation to tailings dams, the research concluded that unless extensive sampling is done during the site investigation phase, it is safer to rely on 2D slope stability results especially when the tailings dam has thin embankment walls and high levels of heterogeneity. If good sampling procedures are done during the soil investigation phase, chances of identifying the weak and critical slope sections are increased. This makes the extrapolation of 2D slopes to conduct 3D slope stability analysis safer.

AFRIKAANSE OPSOMMING: Histories het hellingstabiliteitsontledings staatgemaak op tweedimensionele (2D) modelle. Dit is te wyte aan die relatiewe gemak van die implementering van 2D-hellingstabiliteitsanalise vanuit 'n teoretiese en berekeningsoogpunt. Met die opkoms van kragtige rekenaars, neem die gebruik van driedimensionele (3D) metodes toe. Alhoewel 3D-hellingstabiliteitsanaliseringsmetodes die werklikheid beter weerspieël, bied werfondersoekmetodes dikwels nie voldoende inligting om gedetailleerde 3D-hellingmodelle te skep nie. As gevolg hiervan word die ekstrapolasie van 2D- dwarssnitte gebruik om 3D-modelle te genereer. Hierdie vereenvoudiging laat die vraag ontstaan of hierdie 3D-geëkstrapoleerde ontledings veilig is, aangesien dit gewoonlik hoër veiligheidsfaktore in vergelyking met die ooreenstemmende 2D-ontledings tot gevolg het. Ekstrapolasie kan die ruimtelike veranderlikes van die hange verberg, wat tot ‘n onveilige ontleding kan lei. Hierdie navorsing was daarop gemik om vas te stel of dit veilig is om 3D- hellingstabiliteitsimulasies uit te voer met behulp van geëkstrapoleerde 2D-gedeeltes of dat dit veiliger is om hange te analiseer met behulp van eenvoudige 2D-metodes. Hierdie navorsing is gedoen met verwysing na uitskotdamme. 816 Verskillende hellings is gegenereer en ontleed met behulp van Rocscience's Slide 2 en Slide 3 sagteware programme. Drie verskillende modelle is gebruik om hellingstabiliteitsanalises uit te voer, naamlik: 2D-modelle, 3D-geëkstrapoleerde modelle van 2D-geëkstrapoleerde gedeeltes en 3D-saamgestelde modelle wat bestaan uit drie saamgevoegde 2D-geëkstrapoleerde hellingafdelings. Navorsing het met ontleding van die hange bevind dat as die swakste hellingafdelings met behulp van 3D-geëkstrapoleerde modelle gemeet en ontleed word, die resultate veilig is. Maar wanneer monsters van die sterkste hellinggedeeltes ontleed word met behulp van 3D- geëkstrapoleerde modelle, is die resultate onveilig. In hierdie gevalle is dit veiliger om op normale 2D-modellering staat te maak vir die ontleding van hellings. Die rede hiervoor is dat 3D-geëkstrapoleerde modelle se “factor of safety” (FOS) nie heterogeniteit oor 'n helling in ag neem nie Met betrekking tot uitskotdamme, het die navorsing tot die gevolgtrekking gekom dat tensy uitgebreide monsterneming tydens die terreinondersoekfase gedoen word, dit veiliger is om op 2D-hellingstabiliteitsresultate staat te maak, veral as die uitskotdam dun damwalle en hoë heterogeniteit het. As goeie monsternemingsprosedures tydens die grondondersoekfase uitgevoer word, is die kans om die swak en kritieke hellinggedeeltes te identifiseer groter. Dit maak die ekstrapolasie van 2D-hange om 3D-hellingstabiliteitsanalise te doen, veiliger.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/124672
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