Characteristics of ground pressure in open field mining method (2)

The roadway near the stope and the roadway arranged under the pillar, cracking and serious damage.

At the same time as the large area of ​​the well fell, the ground cracked and sank, forming a surface subsidence basin, and there was a development process over time. The surface subsidence period in the central area is about one year, while the eastern area is as long as 7 to 8 years. The surface subsidence in the central and eastern regions is shown in Table 3.

Table 3 Surface cracking and sinking in the Middle East

When the underground falls, the landing area expands year by year, causing the ground pressure in the lower mining stage to increase, making the mining work difficult and the ore recovery rate decreasing. Before the big fall, the average recovery rate of 51 ore blocks in the 5th stage of the Eastern District was above 75%. However, after the formation of the caving area, when the mining stage of the lower 7 stages is recovered, the recovery rate is reduced to 50% to 60%. There are similar situations in the two regions of China and the West.

Jiangxi tungsten mine has a mining history of 50 to 60 years. Since the 1960s, 31 large-scale ground pressure activities have occurred in 10 mining areas of 9 tungsten mines such as Dajishan and Pangushan. The ground pressure activity of the Pangushan tungsten mine is a representative example.

Pangushan tungsten based high-temperature gasification - hydrothermal fissure filling type deposit tungsten, bismuth, from the south, middle and north group consisting of veins, mineralized area of 1.2km ^2, three groups of veins with surface away from the 220m, 250m. Among them, the southern group has the largest scale, with 66 veins, mainly 19, with a strike length of 1350m, a depth of 800-1000m, a pulse width of 250-300m, an average pulse thickness of 0.35m, a tendency to SW, and an inclination of 85°.

The ore body is a quartz vein. The surrounding rock is mainly composed of quartz sandstone and metamorphic sandstone, which is hard and brittle. The geological structure of the mining area is relatively complex, and the faults are most developed after mineralization, and can be divided into four groups according to the occurrence, and the NEE is most developed to the fault. The joints are also very developed. There are five interpenetrating veins in the mining area.

The mine is mined using shallow hole retention. The length of the mine is 50-60m, the height is 42-70m, the top column is 2~3m thick, and the bottom column is 2.5~3m high, leaving no column.

Before the ground pressure activity, the depth of mining was 260-320 m, and the total area of ​​the goaf was 1.789 million (m) ³ . No treatment was done.

The first rock mass movement occurred on June 29, 1966, mainly between the stages 899 and 831, but had little effect on the 831 stage, and the upper 970 stage had little effect. Before the ground pressure activity, the pavilion fell off the stone, the lower roadway support in the lower part of the stope was broken, the piece was helped, and the rock wall was peeled off. During the ground pressure activity, the sound of cracking and breaking of the rock can be heard in the 899 stage, and the thunder of the friction when the rock body moves.

After the first rock movement, there were many ground pressure activities in the six stages, such as the closeness of the roadway, the breakage of the cross bracing, the crushing of the bottom column, the collapse of the wall, and the leakage of the floor.

The second large-scale rock movement occurred on September 24, 1967, affecting 696 to 1012 m6 stages, all of which suffered varying degrees of damage. Within a few hours, the tens of thousands of meters of roadway sank, 57% of the stope fell, the surface cracked and collapsed (Figure 1), the maximum crack width of 0.8m, the collapse area of ​​80,000 (m) ² , resulting in resource losses The value reached 24.8 million yuan. From 1967 to 1970, the production capacity decreased by 45% for four consecutive years, and the direct economic loss of enterprises was 7.35 million yuan.

Figure 1 Photograph of surface cracking and collapse

The basic characteristics of the geotechnical activities of the tungsten deposits in Jiangxi are that the veins of the veins along the weak geological structure face the slip of the goaf and collapse of the wall. The large-scale weak surface cuts the rock mass, destroying the continuity of the rock mass and reducing the stability of the rock mass. The size of the goaf is expanding with the mining work, and the stability of the rock mass is more vulnerable. The ratio of the empty solid is large, and the thinner the wall is, the easier it is to collapse. The top and bottom columns are subjected to unidirectional pressure on the upper and lower plates, resulting in stress concentration and being easily crushed or slipped along the weak surface. If there is water, it will reduce the strength of the fractured rock mass.