###### Abstract  
The paper presents the results of research on the influence of compaction on the shear strength of fly ash, unburnt and burnt coal wastes, and a composite (a mixture of unburnt coal waste and 30% of fly ash). The tests were carried out in a triaxial compression apparatus on samples with a diameter and height of 10 and 20 cm, respectively. In order to verify usability of the tested waste materials for earthworks, stability calculations of the road embankment made of these materials were carried out. It was shown that the tested materials were characterized by high values of shear strength parameters, which significantly depended on compaction. The most favorable values of the angle of internal friction and cohesion were obtained for the burnt coal waste, slightly lower for the composite, and the lowest for the fly ash. Stability calculations for the road embankment model showed that the slope inclination and the load on the embankment have a significant influence on the factor of safety. It was also shown that a decrease in cohesion causes significant decrease in the factor of safety. The tests and the stability calculations showed that the tested waste materials are useful for earth construction purposes. f [topsep=0 pt,parsep=0 pt,topsep=0 pt,parsep=0 pt,topsep=0 pt,parse  
Coal waste contains significant amounts of carbon and pyrite, which oxidize on exposure to air (atmospheric oxygen). Then an endoenergetic reaction occurs, the consequence of which is self-heating and, as a consequence, self-ignition of coal waste [7]. The temperature contributes to changes in the mineral and petrographic composition, and consequently it modifies characteristics and parameters of the burnt waste, which becomes a cheap and valuable material in earth construction. The properties of this transformed material meet the requirements for aggregates used in road construction [5; 8]. Fly ash is another industrial waste generated in the process of burning ground hard coal or lignite in conventional or fluidized furnaces of power plants and combined heat and power plants. The ash structure is distinguished by its aggregate structure, with a complex surface, diverse porosity, and shear strength [9]. Fly ash is a secondary raw material used as a component of ceramic mass in the production of construction ceramics, as a component in the production of concrete and cement, for land leveling and land reclamation, and filling mining pits, as well as to mix with coal waste to prevent self-ignition. It is also used in mechanical stabilization of coarse soils to improve their granulation as well as a hydraulic binder that improves the mechanical parameters of road subsurfaces, especially in the case of forest roads in areas with high acidification [10]. Fly ash from brown coal combustion has binding properties and can be used as a concrete additive, but also as an independent hydraulic binder or as an additive in the production of these binders. Ash can also be used to stabilize some swelling soils, by reducing soil plasticity and swelling [11; 12; 13]. Mixing industrial waste in different proportions is a common way to increase the amount of wastes used in earthworks. By using waste mixtures, you can easily improve their chemical and physical properties, as well as increase their scope of use. By mixing coal waste and fly ash in fixed proportions, the parameters of the final product, such as impermeability, load-bearing capacity, or shear strength, improve compared to the initial ones [5; 13; 14; 15; 16]. Composites from ashes and coal wastes can be used in earthworks to build banks or rail and road embankments. Another application is surface hardening, land reclamation and land leveling, or using in underground mining techniques and ceramics industry [5; 17; 18]. [PERSON] [2], [PERSON] [6; 19], and [PERSON] [5] show that the geotechnical properties of coal waste depend on the grain size composition, retention time, and petrographic composition.