Comparing model calculated groundwater volumes with alternative methods in a mining enviroment
English: A new opencast coal mining operation is proposed in the Belfast region in Mpumalanga, South Africa. This proposed operation is the study site that was investigated in this thesis. The Belfast opencast operation is expected to be operational for 29 years and coal from mainly the number 2 and 3 seams will be mined. The inflow rate of the groundwater was determined by using both analytical and numerical groundwater methods. The rate at which groundwater flows into the mine voids are important to estimate before mining commence since this will determine at what rate groundwater needs to be pumped from the mining pits to ensure dry and safe working conditions. In order to obtain site specific data for the study area, several field investigations have been conducted. These investigations include a geophysical survey, drilling of monitoring boreholes and pump testing of the monitoring boreholes. These investigations are done to obtain a better overview of the aquifer conditions in the study area. For the study area a numerical groundwater flow model was constructed and the groundwater inflow was determined by making use of a water budget function. The analytical approach to determining the inflow included four different methods. A sensitivity analysis was done on the recharge with the numerical and analytical methods. The results from the numerical and analytical approaches were compared to determine whether the analytical approach is in fact a good way of obtaining values that relates with the numerically obtained results. If there is a good correspondence between the analytical and numerical results, the analytical approach can be regarded as a save and representative way to obtain groundwater related values. Especially during the early stages of mine planning analytical methods would be supportive to quickly determine mine related issues as this will assist in decision making and related cost estimates. From the results obtained in this thesis it can be concluded that the analytical approaches used during this study, although giving close to numerical answers, cannot be used in an effective manner in determining groundwater inflows during the early planning of mining. The fact that the analytical approaches did not reveal representative values for the groundwater inflows and also do not correlate with the numerical model results, does not mean that this will be the case at another site with different geohydrological characteristics. It is important to note that assumptions are always made in analytical methods. It is suggested that further research be conducted in relation with analytical and numerical modelling of opencast mines. Research should be performed at several mines to determine whether the relation between the numerical and analytical approaches display similar trends than was found during this study. These mines should preferably be on similar geological areas to compare with each other. The only way to determine whether the analytical methods can in fact be used to get a representative result is by repetition on several mining sites and also comparing these values with the numerical model results and also the actual inflow rates from the mine once mining has started.
Afrikaans: ‘n Nuwe oopgroefmyn word beplan in die Belfast omgewing, Mpumalanga, SuidAfrika. Hierdie beplande myn is die studie area wat tydens hierdie studie bestudeer is. Dit word verwag dat die Belfast projek operasioneel sal wees vir 29 jaar en Steenkool sal hoofsaaklik vanuit die 2 en 3 soom gemyn word. Die groundwater invloei tempo is bepaal deur gebruik te maak van beide analitiese en numeriese grondwater metodes. Dit is belangrik om die geskatte tempo van grondwater invloei na die oopgroef te bereken voordat mynbou begin aangesien hierdie geskatte waardes sal bepaal teen watter tempo grondwater vanuit die groef gepomp moet word om te verseker dat mynbou op ‘n veilige en droë manier plaasvind. Verskeie veld ondersoeke is in die studie area uitgevoer om data eie aan die omgewing te verkry. Hierdie ondersoeke sluit in ‘n geofisiese ondersoek, boor van moniterings boorgate asook pomptoetse op die nuwe moniterings boorgate. Hierdie ondersoeke word uitgevoer om ‘n beter oorsig van die akwifeer eienskappe in die studie area te verkry. ‘n Numeriese grondwater model is opgestel vir die studie area en die grondwater invloeie is bepaal deur gebruik te maak van die ‘water budget’ funksie. Vier verskillende analitiese metodes is gebruik gedurende die studie om die invloeie na die groef te bepaal. ‘n Sensitiwiteits analiese van die akwifeer aanvulling is uitgevoer deur gebruik te maak van die analitiese en numeriese metodes. Die resultate van die numeriese en analitiese metodes is vergelyk om te bepaal of die analitiese metodes wel ‘n goeie manier is om resultate te verkry wat in verhouding is met die waardes wat numeries verkry is. In die geval van ‘n goeie verhouding tussen hierdie twee metodes, kan die analitiese metode geag word as ‘n veilige en verteenwoordigende manier om grondwater verwante syfers te verkry. Veral gedurende die vroeë stadiums van beplanning van die myn, sal dit handig wees om vinnig mynverwante kwessies te bepaal en dit sal dus besluite en verwante koste beramings ondersteun. Vanaf die resultate wat verkry is tydens hierdie studie kan die gevolgtrekking gemaak word dat die analitiese metodes wat gebruik is tydens hierdie studie nie, alhoewel daar soms waardes verkry is wat naby aan die numeriese waardes is, nie gebruik kan word op ‘n effektiewe manier om die grondwater invloeie te bereken tydens die vroeë stadiums van myn beplanning. Die feit dat die analitiese metodes nie verteenwoordige syfers verskaf het nie en nie met die numeriese syfers vergelyk nie, beteken nie dat dit die geval sal wees in alle omgewings met verskillende geohidrologiese eienskappe nie. Aannames word altyd gemaak in analitiese berekeninge. Dit is dus die hoof aanbeveling dat die waardes wat tydens hierdie studie verkry is (veral numeries), vergelyk sal word met die feitlike invloeie wat deur die myn gemeet word wanneer mynbou in die operasionele fase is. Verdere studies in die analitiese en numeriese bepaling van grondwater invloeie na die groewe word voorgestel. Navorsing moet gedoen word op verskeie myne om te bepaal of die verwantskap tussen die analitiese en numeriese benaderings ook in ander omgewings eenderse resultate verkry. Die enigste manier om te bepaal of hierdie analitiese metodes wel verteenwoordigende syfers verskaf is deur die herhaling van die metodes op verkeie ander studie areas en ook die vergelyking met werklike pomp tempos verkry vanaf die myn wanneer mynbou alreeds begin het.