## 3 Test Results
### Shear Strength  
Analysis of the connection between the shear stress and vertical deformation showed that in most samples there was a brittle shear (Figure 4). However, it should be clearly indicated that in the case of samples where compaction index was \(\mathrm{Is}=0.90\), it was a shear that can be defined as ductile. In these tests, after reaching the maximum value, the shear stresses stabilized without a clear break in the stress-strain curve. Although at higher compaction, i.e., \(\mathrm{Is}=1.00\), in the case of burnt coal waste and \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline  & & \multicolumn{6}{c}{**Value for the Waste Materials**} \\ \cline{3-7} \multirow{2}{*}{**Parameter**} & \multirow{2}{*}{**Fly Ash**} & \multicolumn{3}{c}{**Coal Mining Wastes**} \\ \cline{3-7}  & & \multicolumn{2}{c}{**Unburnt**} & \multicolumn{2}{c}{**Burnt**} & \multirow{2}{*}{**Composite**} \\ \cline{3-3} \cline{5-7}  & & **Before** & & **After** & & **Before** & **After** \\ \cline{3-7}  & & \multicolumn{6}{c}{**Test**} \\ \hline Fraction content [\%]: & & & & & & \\ \(-\)gravel, Gr: 63 \(\div\) 2 mm & 0.0 & 84.1 & 73.8 & 71.0 & 60.4 & 60.6 \\ \(-\)sand, Sa: 2 \(\div\) 0.063 mm & 17.1 & 9.6 & 10.2 & 25.9 & 29.9 & 14.5 \\ \(-\)sil, Si: 0.063 \(\div\) 0.002 mm & 78.4 & 4.8 & 13.0 & 2.6 & 8.0 & 22.8 \\ \(-\)clay, Cl: \(<\)0.002 mm & 4.5 & 1.5 & 3.0 & 0.5 & 1.7 & 2.1 \\ \hline Uniformity coefficient [-] & 7.5 & 33.5 & 741.7 & 84.3 & 25.0 & 1000.0 \\ \hline Coefficient of curvature [-] & 1.5 & 13.2 & 114.7 & 2.3 & 2.5 & 0.2 \\ \hline Name of soil acc. to [42] & MSi & MGr & clMGr & saMGr & siMGr \\ \hline Density of solid particles [g\(\cdot\)cm\({}^{-3}\)] & 2.40 & 2.25 & 2.78 & & 2.33 \\ \hline Optimum moisture content [\%] & 34.00 & 6.65 & 17.60 & 13.40 \\ \hline Maximum dry density of solid particles [g\(\cdot\)cm\({}^{-3}\)] & 1.160 & 1.760 & 1.715 & 1.690 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Geotechnical characteristics of tested waste materials. fly ash (Figure 1b), a brittle shear occurred. In this case, the shearing of the samples occurred when their vertical deformation was from 5% to 8%. A ductile shear was noticed only for burnt coal waste and the composite (Figure 4). The values of the maximum shear stress depended on the type of soil and the main stress \(\sigma_{1}\). When the main stresses ranged from 50 to 300 kPa, the shear stress values ranged from 315 to 990 kPa (compaction index \(\mathrm{I_{S}}\) = 0.90) and from 320 to 1356 kPa (\(\mathrm{I_{S}}\) = 1.00). For instance, when the main stress was 150 kPa and compaction index \(\mathrm{I_{S}}\) = 0.90, there were similar values of shear stress for the fly ash and the composite and 2 times higher for burnt waste. However, when the compaction index was \(\mathrm{I_{S}}\) = 1.00, the fly ash, unburnt waste, and the composite had similar values of shear stress, and burnt waste had values about 35% higher. Figure 5 shows the shear strength line as a tangent to Mohr's circles. The analysis of the drawings allowed us to state that the accuracy of the tests was satisfactory. This is particularly important considering that the majority of tested materials should be classified as coarse-grained soils. The highest values of shear strength parameters were obtained for burnt coal waste, and the lowest for fly ash (in case of both compaction indexes). The tests showed that compaction has a significant influence on the tested parameters. In case of unburnt coal waste, the analysis of this influence was not carried out. This was due to the fact that the sample after forming (the compaction index \(\mathrm{I_{S}}\) = 0.90) crumbled while putting it in the chamber of the triaxial compression apparatus. Figure 4: Changes in shear stress along with increasing axial deformation of samples.