Rock is a part of rock mass, and its engineering geological characteristics generally do not directly determine the stability of rock mass, but they are one of the important factors affecting the stability of rock mass. In the rock mass and soft rock with complete block structure, the structural plane does not play a leading role in the deformation and failure of rock mass, and the characteristics of rock mass are not essentially different from those of rock mass. The engineering geological characteristics of rock mass include physical properties, hydraulic properties and mechanical properties, but mechanical properties are the most important. The basic mechanical properties of rocks are elasticity, plasticity, hardening, strength, stiffness and toughness. Different kinds of rocks, different deformation degrees and different mechanical properties. Some properties are relative, such as brittleness and toughness. At present, it is generally measured by residual strain after fracture or total strain before fracture. For example, the maximum strain before fracture is less than 3%, which is defined as brittleness, while greater than 5% is toughness, and it is transitional between 3% and 5%. Therefore, it is necessary to study the deformation mechanical properties of sedimentary rocks when evaluating the quality and stability of engineering geology and roof rock mass.

The mechanical properties of rock mainly refer to the deformation and strength characteristics of rock. In order to study the strength and deformation characteristics of rock and the development process of rock fracture, uniaxial or triaxial compression tests on cylindrical rock specimens are one of the basic means. The most intuitive way to express the characteristics of rock deformation is through the stress-strain relationship curve (Figure 6. 1). The stress-strain curve of rock obtained on the rigid experimental machine well reproduces the strain strengthening and strain softening characteristics of rock. For most rocks, when the stress exceeds the compressive strength of the rock, due to the propagation of internal microcracks, the rock shows gradual failure, and its strength gradually decreases (strain softening), accompanied by volume expansion (expansion) until it reaches a residual strength value. Therefore, the period from peak strength to residual strength can be regarded as the process of rock development from integrity to fragmentation.

The deformation process of an ideal rock can be roughly divided into three stages (Figure 6. 1): elastic deformation stage, plastic deformation stage and failure stage. In the elastic stage, there is a linear relationship between stress and strain. When the external force is removed, the deformation can be completely recovered. In the plastic stage, the strain increases sharply with the increase of stress, and there is a convex curve relationship between them, so the deformation can not be completely recovered after removing external force. When the external force increases to a certain limit, the specimen will be destroyed.

Fig. 6. 1 ideal rock stress-strain curve

However, the actual rock has different mineral composition and structure, and even some tiny cracks, and its deformation process is far more complicated than that of ideal rock. After a large number of uniaxial compression tests on 28 kinds of rocks, R.P. Miller summed up six types of stress-strain relationships (Figure 6.2).

Class I: Elasticity, the stress-strain curve has the deformation characteristics very close to a straight line, mainly elastic deformation, and suddenly breaks when the deformation is not large, mostly belonging to brittle rocks. Basalt, quartzite, diabase, dolomite and hard limestone all belong to this type.

Type ⅱ: elastic-plastic, the stress-strain curve is a simple function, and it shows large residual deformation when unloading. Soft limestone, siltstone and tuff belong to this type.

Figure 6.2 Typical stress-strain curve of rock under uniaxial compression until failure.

Type ⅲ: plastic-elastic, the stress-strain curve begins to bend slightly from concave to upward, and then gradually turns to concave to slightly bend downward, and fails in the form of unyielding brittle fracture. Such rocks include sandstone, granite, schist and some diabase.

Type Ⅳ: plastic-elastic-plastic, the stress-strain curve begins to bend upward, with a section near a straight line in the middle, and then bends downward. Generally speaking, the curve is steep S-shaped. Metamorphic rocks, marble and gneiss all belong to this type.

Class ⅴ: elastic-plastic-elastic, and the stress-strain curve is gentle and S-shaped. Belonging to this type is schist which is stressed perpendicular to the direction of schist.

Class ⅵ: elastic-plastic creep, that is, the deformation of rock samples increases with time after reaching a certain stage. Stress-strain has a short initial straight part, and then enters the plastic deformation stage, which can produce large plastic deformation. Rock salt, potash mine and other evaporite all belong to this type.

Among these six types, Class III, Class IV and Class V curves are all concave and curved at the initial stage. Experiments show that this is because the experimental rocks have large porosity, micro-cracks or foliation. With the increase of stress, micropores and microcracks are closed or compacted, and the stress-strain curve in the initial stage reflects this process.

The deformation characteristics of rocks can be expressed by a series of deformation parameters. For tensile or compressive deformation, the most important deformation parameters are deformation modulus (E) and Poisson's ratio (μ).

The No.3 coal seam of Shanxi Formation and its roof strata, which are mainly mined in Yanzhou Coalfield, are all formed in shallow water delta deposits, and the sedimentary rocks are composed of sandstone, siltstone, silty mudstone, argillaceous rock, clayey rock and terrigenous clastic rock coal seams. Because of the different lithology and combination of roof, the stability of roof is also different. Production practice shows that sandstone roof has high stability and large initial caving step, while mudstone roof has poor stability and small initial caving step.

Table 6. Physical and Mechanical Properties of1Three-layer Coal

Table 6.2 Physical and Mechanical Properties of Direct Roof, Main Roof and Floor with Different Lithology

From the mechanical point of view, the influence of sedimentary rock properties on roof stability mainly depends on the mechanical strength of the rock. The test shows (Table 6. 1, Table 6.2) that the mechanical properties of any kind of rock are quite different, and they intersect with other rocks in a wide range. For example, the uniaxial compressive strength of sandstone in Yanzhou coalfield is 48.7~ 76.8MPa, siltstone is 34.0 ~ 57.0 MPa, and mudstone is 29.5 ~ 40.1. The coal seam is 1 1 ~ 18 MPa, and other parameters have similar characteristics, reflecting that the mechanical properties of the same type of rocks are quite different, which also shows that there are many factors affecting the mechanical properties of sedimentary rocks, such as the composition, structure, cementation composition, cementation type and support type of sedimentary rocks. Although the rock mechanical properties of the same lithology change greatly, it can still be seen that the uniaxial compressive strength, tensile strength and rock quality grade are the largest in sandstone, followed by siltstone, mudstone and coal seam. Therefore, lithology types have an important influence on rock strength and quality.