## 3 Test Results
### Shear Strength
In comparison, the results of tests on coal wastes from Great Britain [19; 46] indicate that their angle of internal friction was in a fairly wide range (from 22.5\({}^{\circ}\) to 50\({}^{\circ}\)), and the cohesion was from 0 to 80 kPa. The results of the presented fly ash tests showed that the values of the angle of internal friction ranged from 25\({}^{\circ}\) to 35\({}^{\circ}\), which is a much wider range of values than those obtained by [PERSON] and [PERSON] [47] for fly ash from the 'Leg' Heat and Power Plant in Cracow (35.0\({}^{\circ}\)-36.5\({}^{\circ}\)), however it falls within the range of values obtained for the type 'F' fly ash, studied by [PERSON] et al. [48]. On the other hand, values of cohesion presented by [PERSON] and [PERSON] [47] were from 16 to 30 kPa, which can be considered as similar to those obtained in this work. [PERSON] et al. [20] studied fly ash from a  
Figure 7: Changes in the angle of internal friction and cohesion depending on the fly ash addition. coal power plant and they showed that at optimal moisture content the angle of internal friction and cohesion were 32.7\({}^{\circ}\) and 26.4 kPa. The authors of these studies indicate that fly ash is highly useful in civil engineering. [PERSON] et al. (2019) also showed high values of shear strength parameters of this type of waste (36\({}^{\circ}\) and 32 kPa). Composites of unburnt coal waste with fly ash in the amount of 10%, 20%, and 30% by weight were the subject of [PERSON]'s research ([PERSON], 2018). Studies have shown that the angle of internal friction was about 32\({}^{\circ}\) for the fly ash and it was slightly lower than the value for coal waste (just over 33\({}^{\circ}\)). For composites, the values of the angle of internal friction were higher and with the increase of ash content from 10% to 30% they decreased from 39\({}^{\circ}\) to 34\({}^{\circ}\). Cohesion of fly ash was 33 kPa and was lower than for coal waste (47 kPa). In the case of composites, [PERSON] and [PERSON] (2018) stated that as the ash content increased, cohesion decreased from 49 to 27 kPa. [PERSON] et al. (2018) indicate that the values of shear strength parameters of coal waste and fluidized fly ash mixtures significantly increased with the addition of ash. However, they indicate that the values of the angle of internal friction were in a small range (from 35\({}^{\circ}\) to 41\({}^{\circ}\)), but with a significant increase in cohesion (from 37 to 55 kPa) when the ash addition increased from 0 to 40%, which was confirmed by the test results presented in this paper. As shown above, the tested materials are characterized by high values of shear strength parameters, especially cohesion. This is consistent with previous results of test on industrial wastes (among others: ([PERSON] et al., 2018; [PERSON] et al., 2019; [PERSON] et al., 2020)). However, it should be clearly emphasized that the tested materials, with the exception of fly ash, should be considered as coarse-grained, sharp-edged soils. In these types of materials, cohesion is not the result of intermolecular interaction, but interlocking of grains. Using such high cohesion values in design calculations may raise some concerns because some elements might be overestimated, for example, inclination of slopes at a given load on a road embankment. Therefore, in the further part of this work, stability calculations of a road embankment were carried out with reduced values of cohesion and unchanged values of the angle of internal friction.