An assessment of the in-situ state and staged construction of a tailings storage facility in South Africa

Simjee, Yusuf (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: Due to the sizeable dimensions, in cases of failure, tailings dams represent high-risk not only to the mining operations but also to surrounding communities and the environment. The recent catastrophic failures of Mount Polley (Canada), Fundão (Brazil) and Brumadinho (Brazil) Tailings Storage Facilities (TSF’s) between 2014 and 2019, due to static liquefaction caused by the loss of containment of mine tailings, have placed the worldwide mining industry under even greater pressure to ensure that risks associated with TSF’s in their global portfolios are assessed according to industry best practice and sound engineering frameworks. Internationally detailed assessments of the in-situ state of tailings and staged construction are common in design and construction stages as well as post-construction monitoring of these facilities. Recent failures of TSF’s globally have prompted mining companies to review their TSF risk evaluation approach and design standards in terms of their current and future assets in their portfolios. Therefore, the need for more specialised studies is likely to gather pace and may possibly become a more frequent requirement in South African TSF design, operation and monitoring studies. This research covers an array of multifaceted aspects such as material (tailings) behaviour, in-situ and laboratory testing and numerical simulations. The main objectives of the research were to determine the static liquefaction potential of tailings by characterising its in-situ state using in-situ and laboratory test data available for the tailings dam and to perform a staged construction of a tailings dam under static loading conditions by using an appropriate soil constitutive model. This research describes the application of the Shuttle and Jefferies (2016) methodology to estimate the state parameter for silt-like materials. A comparison with more conventional approaches (developed for sand-like materials) namely, Robertson (2010) as well as Jefferies and Been (2016) was performed. The state parameters spatial distribution was determined by assessing each CPTu along its respective monitoring line. The following pertinent findings were observed: The state parameter using Shuttle and Jefferies (2016) procedure is systematically higher than the Robertson (2010) and Jefferies and Been (2016) methodologies. The Shuttle and Jefferies (2016) methodology shows that underflow tailings display predominantly dilative (strain-hardening) behaviour at most of the CPTu. Only a few CPTu show minor contractive layers within some portions of the underflow tailings. The Shuttle and Jefferies (2016) methodology shows that overflow tailings show contractive (strainsoftening) behaviour with interbedding of dilative layers in some portions of the overflow tailings. Good correlations were found between SBTn classifications and interpretation of state parameter using Shuttle and Jefferies (2016) methodology in that similar behavioural responses of the underflow (i.e. dilative) and overflow (i.e. contractive) were noted. In summary, the Shuttle and Jefferies (2016) methodology is based on a more fundamental understanding of the physics involved in cone penetration and uses a constitutive model built around the concept of state parameter, making it more reliable for silt-like tailings than empirically-based procedures (Sottile, et al., 2019). The staged construction of the tailings dam was numerically modelled using the commercial FEM package RS2. The software was used to generate a 2-D FEM model that involves defining type of materials and associated input parameters, phreatic surface and boundary conditions. Also, the software was used to calibrate the Softening-Hardening soil constitutive model with drained and undrained triaxial test data. As part of this research the pore pressure distribution, drainage conditions and location of phreatic surface were assessed at varying rates of rise using hydraulic gradients representative of hydrostatic and sub-hydrostatic pore pressure conditions. The following pertinent findings were observed: The RoR has a more noticeable impact on the location of the phreatic surface than the pore pressure regime. With an increasing rate of rise, there is a corresponding rise of the phreatic surface. The largest pore pressures are developed in the overflow zone that has the lowest permeability. Underflow tailings are unsaturated following construction due to its higher permeability. Phreatic surface and pore pressure trends observed in the analyses are comparable to the literature. Maximum bulbs/zones of pore water pressure form at the overflow-foundation interface. For RoR of 3.0 m/yr and 6.0 m/yr using hydraulic gradients representative of sub-hydrostatic pore pressure conditions, the lower portion of the overflow tailings demonstrate a drained-like behaviour during staged construction. This is reflected by the continuous increases in effective confining pressure (p’) that are notably higher than their corresponding pore pressure (pp). It can also be observed that drainage conditions become poorer into the TSF-A basin as the differences between p’ and pp progressively decrease from Points 1 to 4 (Figure 6-14). For RoR of 3.0 m/yr and 6.0 m/yr using hydraulic gradients representative of hydrostatic pore pressure conditions, the lower portion of the overflow tailings from Points 1 to 3 demonstrate a drained-like behaviour during staged construction. On the other hand, Point 4 demonstrates undrained-like behaviour. This is reflected by the continuous increases in pp that are notably higher than their corresponding p’ (Figure 6-15). The staged construction using fully coupled transient FEM analyses was found to be a valuable tool to understanding the pore pressure distribution, drainage conditions and location of phreatic surface within a tailings dam.

AFRIKAANSE OPSOMMING: Vanweë die aansienlike groote, in die geval van faaling, hou slikdamme 'n hoë risiko vir nie net mynbedrywighede nie, maar ook vir die omliggende gemeenskappe en die omgewing. Die onlangse katastrofiese faalings van Mount Polley (Kanada), Fundão (Brasilië) en Brumadinho (Brasilië) se opbergingsfasiliteite tussen 2014 en 2019, weens statiese vervloeiing wat veroorsaak word deur die verlies aan behoud van mynafval, het die mynboubedryf wêreldwyd onder nog grooter druk geplaas om te verseker dat die risiko's verbonde aan slikdamme in hul wêreldwye portefeuljes, volgens beste praktyke en vertoubare ingenieursraamwerke beoordeel word. Internasionale geditalleerde assesserings van die in-situ toestand van die slikdamme en gefaseerde konstruksie van ‘n numeriese model is algemeen in ontwerp en konstruksie fases. Dit word ook toegepas op post-konstruksie monitering van hierdie fasiliteite. Onlangse faalings van slikdamme wêreldwyd het mynmaatskappye aangespoor om hul slikdam-risikoevalueringsbenadering en ontwerpstandaarde in terme van hul huidige en toekomstige bates in hul portefeuljes te hersien. Dus, die behoefte aan meer gespesialiseerde studies is dus geneig om meer in aanvraag te wees, en kan moontlik 'n vereiste in die Suid-Afrikaanse slikdamontwerp bedryf en moniteringsstudies wees. Hierdie navorsing dek 'n verskeidenheid van aspektes aan soos materiaal (slik) gedrag, in-situ en laboratoriumtoetsing, en numeriese simulasies. Die belangrikste doelwite van die navorsing was om eerstens, die statiese vervloeiing potensiaal van slik te bepaal deur die eienskappe van die in-situ toestand van die slikdam te gebruik. Dit word met behulp van in-situ en laboratoriumtoetsdata wat beskikbaar is vir die slikdam bepaal. Tweedens om 'n gefaseerde numeriese model konstruksie van n slikdam te analiseer onder statiese lading toestande. Die numeriese analise was gedoen met behulp van 'n toepaslike grondkonstitutiewe model gebruik te maak. Hierdie navorsing beskryf die toepassing van Shuttle en Jefferies (2016) se metodologie deur die staatparameter vir slik-agtige materiaal te beraam. 'n Vergelyking was ook getrek deur van meer konvensionele benaderings (ontwikkel vir sand-agtige materiaal) gebruik te maak naamlik, Robertson (2010) asook Jefferies en Been (2016). Die staatsparameters se ruimtelike verspreiding is bepaal deur elke CPTu langs die onderskeidelikke moniteringslyn te assesseer. Die volgende bevindinge is waargeneem: Die staatsparameter wat bepaal is deur behulp van Shuttle en Jefferies (2016) se prosedure, is hoër as die wat bepaal word deur Robertson (2010), en Jefferies en Been (2016) se prosedures. Die Shuttle en Jefferies (2016) metode toon dat ondervloeislik oorwegend dilatiewe (spanning-verharding) gedrag toon by meeste van die CPTu . Slegs 'n paar CPTu vertoon klein kontrakktiewe lae binne sommige gedeeltes van die ondervloeislik. Die Shuttle en Jefferies (2016) metode toon dat die oorloopslik kontrakktiewe (spanning-versagting) gedrag toon, met dilatiewe interbedingslae in sommige gedeeltes van die oorloopslik. Goeie korrelasies is gevind tussen SBTn klassifikasies en die interpretasie van Shuttle en Jefferies (2016) se prosedure van die staatsparameter, deurdat soortgelyke gedrag met betreking tot die ondervloeislik (dilatiewe) en oorloopslik (d.w.s. kontrakktiewe), gevind is. In opsomming, die Shuttle en Jefferies (2016) metodologie is gebaseer op 'n meer fundamentele begrip van die Fisika wat betrokke is by kegel penetrasie, en gebruik 'n konstitutiewe model, gebou rondom die konsep van die staat parameter. Dit maak dit meer betroubaar vir silt-agtige slik as empiries-gebaseerde prosedures (Sottile, et al., 2019). Die fase numeriese model konstruksie van die slikdam is gemodelleer met behulp van die kommersiële sagteware pakket FEM-pakket RS2. Die sagteware is gebruik om 'n 2-D FEM model op te stel deur die tipe materiale te definieer asook gepaardgaande inset parameters, watertafel vlak, en grenstoestande. Die sagteware is ook gebruik om die versagting-verharding grond konstitutiewe model te Kalibreer met behulp van gedreineerde en ongedreineerde triaksiale toets data. As deel van hierdie navorsing word die porieëdrukverspreiding, dreineringstoestande en ligging van die watertafel vlak teen wisselende tempo van verhooging (RoR) geaseseer. Die assessering was uitgevoer deur middel van die hidrouliese gradiënte wat hidrostatiese en sub-hidrostatiese porieëdruktoestande verteenwoordig te gebruik. Die volgende tersaaklike bevindinge is waargeneem: Die “RoR” het 'n meer merkbare impak op die ligging van die watertafervlak as wat die porieëdrukregime het. Soos wat die “RoR” toeneem is daar 'n ooreenstemmende toeneeming van die watertafel vlak. Die grootste porieëdruk word ontwikkel in die oorloopsliksone wat die laagste deurlaatbaarheid het. Ondervloeislik is onversadig na konstruksie as gevolg van die hoër deurlaatbaarheid. Die Watertafel vlak en porieëddruk tendense wat waargeneem is in die ontledings, is vergelykbaar met die literatuur. Maksimum sones van porieëwaterdruk vorm tussen die oorloopslik en fondasievlak. Die onderste gedeelte van die oorloopslik toon 'n gedreineerdagtige gedrag tydens die fasekonstruksie van die numeriese model. Dit vertoon spesifiek by ‘n tempo van verhooging van 3,0 m/jaar en 6,0 m/jaar deur gebruik te maak van hidrouliese gradiënte verteenwoordigend is van sub-hidrostatiese porieëddruktoestande. Dit word weerspieël deur die deurlopende verhoging in effektiewe beperkende druk wat veral hoër is as die ooreenstemmende porieëdruk. Swakker dreineringstoestande is waargeneem in slikdam A as gevolg van die verskille tussen beperkendedruk en porieëdruk wat progressief afneem van punte 1 tot 4 (Figuur 6-14). Vir RoR, van 3,0 m/jaar en 6,0 m/jaar met behulp van hidrouliese gradiënte verteenwoordiger van hidrostatiese porieëdruk toestande, word die volgende waargeneem: Die onderste gedeelte van die oorloopslik, van punt 1 tot 3, toon 'n gedreineerd-agtige gedrag tydens die fase konstruksie van die numeriese model; punt 4 toon ongedreineerde-agtige gedrag. Dit word weerspieël deur die deurlopende styging in porieëdruk wat veral hoër is as hul ooreenstemmende behoudendedruk (Figuur 6-15). Die fasekonstruksie van n numeriese model wat ten volle gekoppel is aan “transient” FEM-ontledings, is 'n waardevolle hulpmiddel om die porieëdrukverspreiding, dreineringstoestande en-ligging van die watertafelvlak binne 'n slikdam te verstaan.

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