## 4 Results of Stability Calculations  
The factor of safety of the road embankment, calculated using the Janbu method, was in a fairly wide range, from 0.51 to 2.43. All the factors used in the calculations had an influence on this range, i.e., the inclination of the slope, load on the embankment, and the assumed cohesion value. It should be clearly emphasized that the slope stability was also influenced by the angle of internal friction and bulk density of the material that was used to build the model. In these calculations, the values of the angle of internal friction were in a fairly narrow range, from about 33\({}^{\circ}\) to 35\({}^{\circ}\). Therefore, it was assumed that this parameter will not be taken into account in further analysis. The next parameter was the bulk density, whose values for coal waste and the composite were similar. There was a significant difference only for the fly ash (reduction by nearly 3.5 kN\(\cdot\)m\({}^{-3}\)). In the further part of the analysis, it was decided not to take into account the influence of this parameter on the factor of safety. The analysis of the test results showed that the slope of the road embankment would be unstable if the inclination was 1:0.5 and 1:1 with and without load on the embankment and assuming that the cohesion would be equal to 10% of the value from the tests. In these cases, the factor of safety ranged from 0.51 to 0.95 (Figure 8). In the other ones, the values of the factor of safety were much higher than 1. So, according to Eurocode 7, it can be stated that the slopes of all calculation models were stable. It should be emphasized that as the cohesion increased, the range of the slope factor of safety increased for the adopted inclinations and loads on the embankment. When the cohesion was the same as the value obtained from tests in a triaxial compression apparatus the factor of safety ranged from 1.38 to 2.43, at 50% of its value from 0.96 to 2.01, and at 10% of its value from 0.51 to 1.51. Figure 8: Slip surface at different load and cohesion values when the inclination was 1:0.5. Increasing the load on the road embankment from 0 to 50 kPa resulted in a decrease in the factor of safety from about 14 to 39% (Figure 9) (adequately when cohesion was 10% of its test value and the maximum value obtained from the tests). For example, when cohesion was 10% of its test value there was a decrease in the factor of safety from about 0.65 to 0.52 (when the inclination was 1:0.5) and from 1.51 to 1.31 (when the inclination was 1:2). However, when the full consistency value was used, there was a decrease in the factor of safety from 1.92 to 1.38 at 1:0.5 slope and from 2.43 to 1.92 at a 1:2 slope. The ranges of changes in the factor of safety along with an increase in the load on the embankment corresponded to the full range of changes for all three materials at a given inclination of the slope. The analysis of Figure 9 shows that these changes are similar for all materials at the adopted cohesion value and inclination of the slope. However, the range of the factor of safety depends on the inclination of the slope and increases with its decrease. Figure 9: Changes in the factor of safety with increasing load on the embankment depending on the inclination. As indicated previously, reducing the slope inclination from 1.05 to 1:2 resulted in a significant increase in the factor of safety (Figure 10). The range of these changes depended mainly on the cohesion value used in the calculations. At its full value, i.e., obtained from tests, the factor of safety increased on average by 0.51, at 50% of its value on average by 0.63, and at 10% of its value on average by 0.80 with decreasing inclination for all materials and all embankment loads. The values of increase of the factor of safety given above are an average value of the difference between its maximum and minimum value for a given material and load. Figure 10 shows an example of the relation between the factor of safety and inclination (while using the full consistency value, obtained from the tests).