## 2 Test and Calculation Methods  
The range of tests included determination of the grain size distribution, specific density, and compaction parameters using Polish standard (PN) or European standard (EN) (PN-EN, PN-EN ISO) methods [36, 37, 38, 39]. Shear strength parameters, i.e., the angle of internal friction and cohesion, were determined using the UU (unconsolidated undrained) method in a triaxial compression apparatus. During the test the sample was compressed axially until the limit state was reached while maintaining a constant lateral pressure and constant deformation speed. Test specimens with a diameter of 10 cm and a height of 20 cm (Figure 1a) were formed in a three-part cylinder in a Proctor apparatus at optimum moisture content and unit compaction energy of 0.59 J-cm\({}^{-3}\) until the compaction indexes (values of relative compaction) \(\mathrm{I_{S}}\) = 0.90 and 1.00 were reached. The value of Is parameter (also called relative compaction) expresses the ratio of the dry density of soil determined in the field tests (or for moulded samples in laboratory tests) to the value of the maximum dry density determined in a laboratory test (Proctor's method). After compaction, the samples were consolidated for 30 min and then sheared at the velocity of 0.33 mm min\({}^{-1}\) and at the following normal stresses: 50, 100, 200, 300, and 400 kPa. The shear criterion was the maximum value of the stress deviator in the range of up to 20% of the vertical deformation of the sample. Knowing the values of shear strength parameters of materials used in earthworks is important, especially regarding construction and stability analysis of embankments. Stability analysis of road embankments is one of the main tasks that has to be done in order to verify their safe operation. The value of the factor of safety may vary depending on the method and above all on the adopted geotechnical parameters of soils in the embankment. Eurocode 7 [40] requires checking the GEO (geotechnical) or STR (structural) limit state, and thus showing that the design effects of the actions (turning moments) are smaller than the corresponding design soil resistance (stabilizing moments) determined based on design geotechnical parameters. With this approach, the minimum factor of safety should not be less than 1. The GEO limit state is associated with damage in the soil massif, e.g., in the form of a landslide. On the other hand, the STR limit state concerns cases of damage or large displacements in the soil massif with construction elements. The limit equilibrium methods that are used in stability calculations allow determining the connection between the slope inclination, the load capacity, and the stress state in the entire massif. It is assumed that a limit stress state occurs along certain slip surfaces. Based on the premise that the deformation or destruction mechanisms occur along the slip surface, the force system associated with this mechanism was analyzed. Stability analysis led to the determination of the minimum value of the factor of safety, while taking into account the design values of geotechnical parameters as well as actions and resistances obtained as a result of using partial coefficients. Stability calculations were carried out for a road embankment model with a height and crown width of 10 m at four slope inclinations: 1:0.5, 1:1, 1:1.5, and 1:2 (Figure 2). The calculations were made with the load on the embankment crown (evenly distributed vertical pressure of 10, 25, and 50 kPa) and without any load on the crown. It was assumed that the embankment was built from the fly ash, unburnt coal waste, and the composite (the compaction index was \(\mathrm{I_{S}}=1.00\)). Shear strength parameters and bulk density were adopted on the basis of obtained test results. In order to show the influence of cohesion on the factor of safety, it was assumed that the value of cohesion will correspond to the one obtained from the tests, but it will also be reduced to 50% and 10% of its initial value.