Characteristics of ground pressure in open field mining method (1)

When the level of the mining rock is stabilized and the ore body is gently inclined, the comprehensive method and the room and column method are generally used.

The Hunan tin mine antimony ore with nearly a hundred years of mining history is a typical example of large-scale ground pressure activities after mining of this type of ore body.

The deposit is mainly hydrothermal filling after the magma period, and the single sulfide sulphide deposit accompanying the replacement has a layered output with a length of about 1500 m, a thickness from thin to medium thickness, a gentle slope, and a dip angle of 10 to 35 °. 250m. The roof of the ore body is shale , ƒ=3~5, unstable, and the bottom plate is stable silicified limestone, ƒ=10～18. Faults and joints in the mining area are relatively developed.

Mining by room and pillar method, the mine is 8~10m wide (bolt top), or 10~15m (0.8~1.0m for retaining top mine), 3×4m (bolt top) for intermittent pillar, or 4 ×5m (retaining mine protection roof), the inclination distance between the pillars is 5m, and the slope length is 40~60m. It is divided into three mining areas of east, middle and west according to natural conditions. By 1972, the mining situation is listed in Table 1.

Table 1 Mining situation of tin mines in southern mines

In order to protect the surface, the old goaf was filled with waste rock in the Western District before 1965, and a small amount of new empty space was filled with waste rock in the central part, while the goaf in the eastern area was not treated.

Due to the long-term existence of a large number of goafs in the east and central regions, the upper part of the western area has been filled, but with the expansion of the lower mining area, the ground pressure has gradually increased. From 1965 to 1971, large-scale ground pressure activities occurred in three areas. (Table 2).

Table 2 Ground pressure activities of tin mines in South Mine

The obvious phenomenon during the period of earth pressure activity is firstly the sound of the rock formation, called “rock sound”, the loud sound (such as the bombing) is the deep layer and fault of the rock formation, and the small sound (such as firecracker sound) indicates that the shallow rock is damaged by the pressure. . The sound is extended from the deep to the shallow and the pillar, and the frequency increases from low to high. A few days before the big fall, the sound of the sound increased sharply, and more than 70 times per minute on the eve of the fall. Stope case of roof and pillar layers bearing damage hair ring, continuous, large caving indicating imminent.

The second phenomenon of ground pressure activity is that the top plate of the stope is loose and falling off. The top plate of the stope in the middle area is in the first three months before the big fall, and the number of falling blocks is 1.5-6 times/hour. Before approaching the big fall It increased to 15 to 17 times per hour in about 10 days; on the eve of the big fall, it increased dramatically to 27 to 30 times per hour.

The destruction of the pillar is an important basis for causing the roof to fall. According to the actual investigation of 84 pillars in 21 stopries in the central area, it was found that at the first month before the big fall, the pillars with cracks and spalling accounted for 12% of the total; the cracks and spalling were more, and the supporting capacity was weakened. Accounted for 28.5%; cracks staggered, exfoliation of more than 1/3 of the loss of support capacity accounted for 59.5%. As the pressure of the pillar increases, the bottom drum and cracking appear in the floor of the stope.

At the same time as the pillar collapses, the roof of the stope sinks, cracks and partially falls. According to the actual measured data, the average sinking speed of the roof in the middle area of ​​the three months before the big fall (October 1965) was 0.3-0.5mm/d, but it was 3~ on the eve of the big fall (late December 1965). 5mm / d, up to 7mm / d. Before the fall of the Eastern District, 128 plots were surveyed, and 69 occurred in different degrees.