## 3 Test Results
### Shear Strength
The increase in compaction from \(\rm I_{S}=0.90\) to \(1.00\) increased the angle of internal friction of the fly ash by nearly \(10^{\circ}\) (39% relative), of burnt coal waste by just over \(4^{\circ}\) (12% relative), and of the composite by nearly \(9^{\circ}\) (34% relative) (Figure 6). However, along with increase in compaction, the cohesion increased by 25 kPa (126% relative) for fly ash, by 50 kPa (73% relative) for burnt coal waste, and by 8.5 kPa (16% relative) for the composite. By analyzing the influence of the fly ash addition to unburnt coal waste (Figure 7) it was found that when compaction index was \(\rm I_{S}=1.00\) the angle of internal friction of the composite was higher by \(2^{\circ}\) (6% relative) compared to the value for unburnt coal waste and lower by \(0.5^{\circ}\) (1.5% relative) compared to the value for the fly ash. On the other hand, the cohesion of the composite at this compaction increased by 2.1 kPa (3% relative) and by 17.4 kPa (28% relative), respectively, in relation to the values for coal waste and the fly ash. When the compaction index was \(\rm I_{S}=0.90\) the value of the angle of  
Figure 5: Shear strength line of the tested materials as a tangent to Mohr’s circles. Figure 6: Influence of compaction on the angle of internal friction and cohesion. internal friction of the composite was greater by 0.5\({}^{\circ}\) (2% relative) and cohesion by 34 kPa (63% relative) compared to the values for fly ash. Tests on coal wastes from the Upper Silesian Coal Basin (Poland) (among others: [4; 5; 19; 43]) which were carried out using the direct shear method indicate that these wastes are characterized by high values of shear strength parameters. The highest values of the angle of internal friction (about 40\({}^{\circ}\)) were usually obtained for wastes from current coal production. This results from the sharp-edged shape of rock pieces and their surface roughness. In the case of weathered waste stored on landfills, the angle of internal friction was smaller by 5\({}^{\circ}\) compared to the value for waste from current production. In the case of treated wastes, the values of the angle of internal friction were different and ranged from 35\({}^{\circ}\) to 39\({}^{\circ}\). Triaxial testing on coal wastes [32] also confirmed high values of shear strength parameters. The obtained results indicate that the values of these parameters depend significantly on the shear velocity and waste compaction. The authors showed that the increase in compaction from I\({}_{5}\) = 0.90 to 1.00 increased the value of the angle of internal friction from 1\({}^{\circ}\) to 5\({}^{\circ}\), which is similar to the relationship obtained in the presented tests on burnt coal waste. [PERSON] et al. [44] also showed that the increase in shear velocity from 0.05 to 0.10 mm\(\cdot\)min\({}^{-1}\) increased the angle of internal friction from 1\({}^{\circ}\) to 10\({}^{\circ}\) and the cohesion from about 45 to nearly 130 kPa. Coal wastes from the Lublin Coal Basin (Poland) are characterized by slightly different geotechnical properties. Results of tests on these wastes with different storage time (from fresh waste to wastes stored in heaps for 5 and 7 years), using the direct shear method, presented by [PERSON] and [PERSON] [45] confirmed the influence of the progressing weathering process on the values of the angle of internal friction and cohesion. As storage time increased, the content of coarse fractions decreased, which reduced the angle of internal friction. On the other hand, higher cohesion values, in relation to the tested waste and the waste from the Upper Silesian Coal Basin, were caused by a higher content of fine fractions, particularly silt and clay. The values of the angle of internal friction of these wastes ranged from 27\({}^{\circ}\) to 55\({}^{\circ}\), and cohesion from 21 to 40 kPa.