TY - THES

T1 - The role of pillars in raise caving

AU - Gams, Patrick

N1 - embargoed until 12-09-2027

PY - 2022

Y1 - 2022

N2 - The demand of minerals increases with the growing world population and the growing economic development. As mineral deposits at shallow depths become depleted, deeper deposits must be extracted to meet the needs of the commodity markets. With greater depths, rock pressure related problems occur, due to the increasing stresses. LKAB’s Kiruna mine has experienced those rock pressure related problems in the past years and plans to combat those problems with a novel mining method called “raise caving. The raise caving method is based on an active stress control approach. In terms of this approach slots are developed by drilling long raise bore holes which are extended in the strike direction of the mineral deposit by means of drilling and blasting to form a wide narrow slot which provides a stress shadow for the subsequent large-scale mineral extraction. These slots are separated by massive pillars, which are crucial for the success of raise caving. In the early, so-called “de-stressing phase” when the de-stressing slots are created, the pillars need to control the stresses, to control the seismicity, and to prevent hanging wall caving. In the production-phase large-scale mineral extraction is conducted in the volume of rock de-stressed by the slots. Infrastructure is developed delayed in de-stressed ground and stopes are mined in de-stressed ground behind the de-stressing slots. To increase the extraction ratio, it may also be necessary to extract at least some of the ore situated in and behind the massive pillars. As the massive pillars are highly stressed after the de-stressing phase, they must be de-stressed before they can be extracted. Hence, studies on the behavior of pillars and their role in raise caving are of great importance. First, the importance of pillars and their role is outlined and discussed on a conceptual basis. With the help of numerical simulations, different layouts are compared to investigate the stress redistribution in the raise caving layout, the pillar stresses and the stability of infrastructure with the help of the RCF value. There are two different approaches of pillar simulation, namely infinitely strong pillars which do not fail and pillars with a defined load-deformation behavior, which are allowed to crush, when the pillar strength is exceeded. All simulations are done with a linear elastic material behavior. The overloading of pillars is approximated with the implementation of pre-calculated pillar stress-strain curves which are based on available studies on pillar strength and behavior. Results show that pillars are the crucial point for the success of raise caving. Pillar crushing in the de-stressing phase leads to considerable stress redistributions. Abutments on top of the raise caving layout and side abutments are highly stressed as a consequence. The development of infrastructure, which is needed in the de-stressing phase, such as slot raises, drifts and caverns, is difficult due to the high stresses and their impact on infrastructure stability. As pillars crush the spatial extent of the stress shadows increases. Pillar crushing in the de-stressing phase is critical and must be prohibited. This can be done with an adapted mine layout of pillars and slots in combination with the extraction sequence of the de-stressing slots. To prevent crushing the pillar behavior plays an important role for the pillar design. Generally, pillars are highly stressed after de-stressing phase. It is important to de-stress the pillar in the production phase to be able to commence mineral extraction behind them and to develop the need infrastructure. For the de-stressing of pillars, the strength of the pillar is stepwise reduced by the decrease of the pillar width to height ratio due to stope extraction. Methods such as pre-conditioning can support this de-stressing process. Unstable crushing of pillars must be prohibited. The results of this thesis provide input for the design of layouts and sequences in raise caving. Further investigations are recommended and they should especially address the pillar behavior.

AB - The demand of minerals increases with the growing world population and the growing economic development. As mineral deposits at shallow depths become depleted, deeper deposits must be extracted to meet the needs of the commodity markets. With greater depths, rock pressure related problems occur, due to the increasing stresses. LKAB’s Kiruna mine has experienced those rock pressure related problems in the past years and plans to combat those problems with a novel mining method called “raise caving. The raise caving method is based on an active stress control approach. In terms of this approach slots are developed by drilling long raise bore holes which are extended in the strike direction of the mineral deposit by means of drilling and blasting to form a wide narrow slot which provides a stress shadow for the subsequent large-scale mineral extraction. These slots are separated by massive pillars, which are crucial for the success of raise caving. In the early, so-called “de-stressing phase” when the de-stressing slots are created, the pillars need to control the stresses, to control the seismicity, and to prevent hanging wall caving. In the production-phase large-scale mineral extraction is conducted in the volume of rock de-stressed by the slots. Infrastructure is developed delayed in de-stressed ground and stopes are mined in de-stressed ground behind the de-stressing slots. To increase the extraction ratio, it may also be necessary to extract at least some of the ore situated in and behind the massive pillars. As the massive pillars are highly stressed after the de-stressing phase, they must be de-stressed before they can be extracted. Hence, studies on the behavior of pillars and their role in raise caving are of great importance. First, the importance of pillars and their role is outlined and discussed on a conceptual basis. With the help of numerical simulations, different layouts are compared to investigate the stress redistribution in the raise caving layout, the pillar stresses and the stability of infrastructure with the help of the RCF value. There are two different approaches of pillar simulation, namely infinitely strong pillars which do not fail and pillars with a defined load-deformation behavior, which are allowed to crush, when the pillar strength is exceeded. All simulations are done with a linear elastic material behavior. The overloading of pillars is approximated with the implementation of pre-calculated pillar stress-strain curves which are based on available studies on pillar strength and behavior. Results show that pillars are the crucial point for the success of raise caving. Pillar crushing in the de-stressing phase leads to considerable stress redistributions. Abutments on top of the raise caving layout and side abutments are highly stressed as a consequence. The development of infrastructure, which is needed in the de-stressing phase, such as slot raises, drifts and caverns, is difficult due to the high stresses and their impact on infrastructure stability. As pillars crush the spatial extent of the stress shadows increases. Pillar crushing in the de-stressing phase is critical and must be prohibited. This can be done with an adapted mine layout of pillars and slots in combination with the extraction sequence of the de-stressing slots. To prevent crushing the pillar behavior plays an important role for the pillar design. Generally, pillars are highly stressed after de-stressing phase. It is important to de-stress the pillar in the production phase to be able to commence mineral extraction behind them and to develop the need infrastructure. For the de-stressing of pillars, the strength of the pillar is stepwise reduced by the decrease of the pillar width to height ratio due to stope extraction. Methods such as pre-conditioning can support this de-stressing process. Unstable crushing of pillars must be prohibited. The results of this thesis provide input for the design of layouts and sequences in raise caving. Further investigations are recommended and they should especially address the pillar behavior.

KW - Raise-Bruchbau

KW - Gebirgsfesten

KW - Tiefer Bergbau

KW - Numerische Simulationen

KW - raise caving

KW - deep mining

KW - pillars

KW - numerical simulations

M3 - Master's Thesis

ER -