The evolution of mining-induced stress field in longwall panel is closely related to the fracture field and the breaking characteristics of strata. Few laboratory experiments have been conducted to investigate the stress field. This study investigated its evolution by constructing a large-scale physical model according to the in situ conditions of the longwall panel. Theoretical analysis was used to reveal the mechanism of stress distribution in the overburden. The modelling results showed that: (1) The major principal stress field is arch-shaped, and the strata overlying both the solid zones and gob constitute a series of coordinated load-bearing structures. The stress increasing zone is like a macro stress arch. High stress is especially concentrated on both shoulders of the arch-shaped structure. The stress concentration of the solid zone in front of the gob is higher than the rear solid zone. (2) The characteristics of the vertical stress field in different regions are significantly different. Stress decreases in the zone above the gob and increases in solid zones on both sides of it. The mechanical analysis show that for a given stratum, the trajectories of principal stress are arch-shaped or inversely-arched, referred to as the “principal stress arch”, irrespective of its initial breaking or periodic breaking, and determines the fracture morphology. That is, the trajectories of tensile principal stress are inversely arched before the first breaking of the strata, and cause the breaking lines to resemble an inverted funnel. In case of periodic breaking, the breaking line forms an obtuse angle with the advancing direction of the panel. Good agreement was obtained between the results of physical modeling and the theoretical analysis.

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Measurements have been made of the ratio of ferric to ferrous iron in CaO-Al 2 O 3 -SiO 2 and MgO-CaO-Al 2 O 3 -SiO 2 slags at oxygen activities ranging from equilibrium with pCO 2 /pCO Ϸ 0.01 to as high as air at temperatures of 1573 to 1773 K. At 1773 K, values are given byfor melts with the molar ratio of CaO/SiO 2 ϭ 0.45 to 1.52, 10 to 15 mol pct Al 2 O 3 , up to 12 mol pct MgO (at CaO/SiO 2 Ϸ 1.5), and with 3 to 10 wt pct total Fe. Available evidence suggests that, to a good approximation, these redox equilibria are independent of temperature when expressed with respect to pCO 2 /pCO, probably from about 1573 to 1873 K. Limited studies have also been carried out on melts containing about 40 mol pct Al 2 O 3 , up to 12 mol pct MgO (at CaO/SiO 2 Ϸ 1.5), and 3.6 to 4.7 wt pct Fe. These show a strongly nonideal behavior for the iron redox equilibrium, with 0.37 3+ Fe pCO 2 ϰ ͑ ͒ 2+ Fe pCOThe nonideal behavior and the effects of basicity and Al 2 O 3 concentration on the redox equilibria are discussed in terms of the charge balance model of alumino-silicates and the published structural information from Mössbauer and NMR (Nuclear Magnetic Resonance) spectroscopy of quenched melts.