Stability analyses were conducted with the finite element method to study the stability of the oxidized ore stope of Songshujiao Tin Mine, China. The most important feature of the deposit is that the oxidized ore is weaker than the country rocks because it behaves like a soil. Results of the numerical simulation have shown that ore excavation does not influence the stability of the country rocks. The main threat to the working safety during ore extraction from the stopes is the risk of falling blocks created by discontinuities. When joints are well developed In the roof, long bolts are thus needed to reinforce the roof.
Songshujiao Tin Mine is located in the mountainous region of the Yunnan province, southwest of China. In this mine, the depth of the present excavating stopes with respect to the surface is around 700 meters, yet the main development openings of the Mine are drifts and some of the horizontal openings communicate directly to the surface. Although these are richer than the sulphide deposits that were excavated in the past, the dimensions of the oxidized ore bodies are usually much smaller. The deposits were formed by Weathering action in the cavities and both roof and floor of the stopes are formed by marble and/or dolomitic limestone. The oxidized ore is weathered as a soil and its strength is extremely low, although there are some compact blocks of iron nodules existing in the ore. In fact, the compressive strength of the ore is below 1MPa and it is approximately the same as that of the cover soil near the surface. Country rocks have moderate strength and are not very stable. Due to weathering action, the direct roof is less stable than the remaining part of the roof. However, no evident boundary exists between the direct roof and the immediate roof, which makes excavation conditions very difficult. The mining system adopted in the oxidized ore is by sublevel stoping methods with previous bolt supports in the roof. In effect, the first step of excavation is to cut a slot with an height of about 2 meters at the top of the ore, to remove the unstable direct roof. Then the roof is reinforced with metal bolts of 1.8m long and after that excavation of the ore is possible. Inside the ore body, stope size is 40m wide and about 8m high, which is the natural height of the ore body. The stopes are divided into rooms along the strike of the deposit and the rooms are perpendicular to the strike. The width of a room, or the size along the strike is 10m, and so the stope is divided into 4 rooms. The length of the rooms (or the stope size along the dip) is about 50 to 60m. When the length of an ore body along the dip is greater than 60m, it is divided into sections at the inclination direction and different sections would be excavated separately.