# Strength and Deformation of Pillars during Mining in the Shaft Pillar
## 4 Discussion
For this reason, the laboratory compressive strength results could not be influenced upwards. The published results of the effect of friction on the pushing surfaces of the test specimens only considered the same friction on both pushing surfaces. In reality, these frictions between the pillar and the overburden/bedrock are often different. It would be interesting to see how the different friction on the compression surfaces of the test specimen affects the measured compressive strength. Another influencing factor for laboratory-determined compressive strength is the laboratory conditions. The jaws of the loading mechanism have significantly different elastic properties than the rock under investigation [38; 39]. The degree of this influence will depend on the situation in the part of the mountain massif under consideration. If we assess the bearing capacity of a pillar that has rocks with the same mechanical properties as the pillar above and below it, then this influence may be more pronounced. In the case we are considering, rocks with a significantly higher deformation modulus and lower Poisson's number are above and below the coal pillar. For this reason, this influencing factor will not be as important, as the loading of the coal sample in the laboratory is partly similar to the loading of the coal pillar under mine conditions in this respect. When assessing the bearing capacity of rock pillars, the frequency of discontinuity surfaces is an important influencing factor [40]. This phenomenon is more evident in slender pillars, which are more severely affected by the presence of discontinuities than wider pillars. This study has also shown that as the slender ratio decreases below 0.8, the confining stresses within a pillar approach zero and brittle fracturing can occur throughout the unconfined pillar core. The fracturing and spalling failure mechanism is poorly understood, and it seems prudent to avoid designing pillars with slenderness ratio <0.8 that might fail in this manner. The parameter evaluating the frequency of discontinuity areas is directly included in the calculation of the bearing capacity of rock pillars. However, in the evaluation of coal pillars, the number of discontinuity surfaces is difficult to determine, and the slender ratio is commonly >0.8; therefore, it is usually not taken into account. In some cases, however, the effect of the size of the test specimens is considered, which indirectly takes into account the number of discontinuity surfaces. For the determination of the deformation of the pillars, we use the modulus of transformation--Equation (6). For the slenderness ratio \(t\) = 8.33, \(E_{p}\) = 5500 MPa. With a chamber width of 6 m and a pillar width of 25 m, the area of excavated space per chamber is 336 m\({}^{2}\). This excavated space constitutes 53.8% of the pillar area. The load on the pillar is increased by this percentage relative to the originally applied stress. The distance of room and pillar mining is mostly more than 100 m from longwall faces. Therefore, the additional load from the longwall faces is negligible. We can, therefore, assume an initial stress corresponding to a depth of 800 m, which is 20 MPa. After the chambers have been cut, the stress increases to \(\sigma_{p}\) = 30.75 MPa. The stress will, therefore, increase to = 10.75 MPa. The total vertical deformation of the pillar \(\Delta h\) is given by the equation  
Figure 7.1  
The entire 3 m pillar is lowered by 6 mm by excavating the surrounding chambers. The safety factor \(SF\) depends on the stress acting on the pillar. Figure 7.2  
The load capacity of the pillar according to Equation (2), which expresses the linear dependence of the strength of the specimen on the slenderness ratio w/h = 8.33, is 101.5 MPa, and according to Equation (3) for the power curve, it is 99.3 MPa. From these values, SF = 3.2. In reality, this value will be lower because we did not have the opportunity to investigate the effect of the increase in the volume of the test specimens on the resulting bearing capacity of the pillars.