The features of weak rock in Chinese coal mines is the subject of this paper. An optimum entry layout and support system are proposed. The importance of putting the roadways in the destressed zone and the necessity of using closed supports are emphasized.
There are many urgent problems of construction in weak rock which remain to be solved for mining, tunneling, dam construction and other civil engineering applications. These problems are even more serious in those coal mines where the underground geo- 0glcal and operating conditions are complicated, some of which seriously threaten the construction of new mines and the routine operation of the existing mines.
The following is a brief account of three aspects.
Weak rock possesses many features different from that of, hard and medium hard rocks, the most prominant ones eing: low strength, high porosity, strong water absorption, strong weathering Characteristics, high clay constituents, such as montmorillonite, illite, etc., and a certain degree of Swelling characteristics. The principle features of weak rock are:
Low uniaxial compressive strength. he mostly commonly met rocks in Chinese Coal mines are argillaceous shale. mudstone, sandy shale, etc., and the uniaxial compressive strength of most strata is below 200 kg/cm2, with the lowest figure of the average uniaxial compressive strength being only 33.8 kg/cm2 (Table 1), which is 5–20 times less than that of medium hard and hard rock.
(Table in full paper)
The strength of some rocks may reach 500–600 kg/cm2. However, depth and slacking and swelling upon wetting make them very low in strength. As soon as the stress reaches the limit of strength plastic deformation occurs.
High natural moisture content and strong water absorptivity have a great influence on rock strength. Rock, in general, has a given amount of moisture content and will be softened upon absorbing water. However, the natural moisture content and the degree of softening upon wetting vary with different rocks. Therefore, the natural moisture content and the water absorbability of the rock have a great influence on the rock strength. Even the hardest granite exhibits the character of softening upon absorbing water, and the influence on weak rock is quite obvious. For example, the uniaxial compressive strength of granite is reduced by 13% after water immersion, while that of sandstone in the Datong mine area is reduced by 32–35%, and that of argillaceous shale in the Qiantun Mine, Shengyang Mines Administration is reduced by 80% when the moisture content is increased from 2% to 12%, as shown in Fig. 1.
(Figure in full paper)
The porosity of the weak rock is generally higher than that of other rocks, that is one of the reasons why it has higher moisture content, strong water absorbability and hydration. The natural moisture content and porosity of the weak rock is far higher than that of the hard rock (Table 2). Therefore, in weak rock water drainage and water-sealing measures are very important to reduce the moisture content and to halt
