###### Abstract
Weathering, compaction, and moisture content have a great influence on the value of the angle of internal friction and cohesion. On the other hand, in the case of fly ash, its geotechnical properties will be related to the properties of the energy source, technology, and method of combustion as well as storage time [5; 20; 21]. Shear strength parameters of industrial wastes are usually favorable. Nevertheless, there are cases where waste embankments failed [22], which indicates that the use of industrial waste for construction purposes requires proper recognition of their geotechnical properties, mainly their shear strength parameters. As [PERSON] [19] and [PERSON] et al. [23] indicate, the basic problem in the case of coal waste testing is its grain size composition, and thus a significant content of large rock pieces. The presence of coarse grains makes it necessary to use medium- or large-size apparatus. In this regard, it has to be noted that application of anthropogenic soils to construct earth-made structures requires two things: achieving suitable geotechnical parameters and fulfilling all environmental requirements. Unburnt colliery spoils are prone to a self-ignition process [19; 24; 25; 26], which poses a threat to the structures made of them and contributes to emission of harmful chemical compounds to the atmosphere. Studies of leachates from colliery dumps indicate that these materials can contain sulphur and chloride compounds and some heavy metals [19; 27; 28; 29], which can pollute the environment. An analysis of the leachate from structures built from colliery spoils in the area of southern Poland showed [4] that the amount of leachate can be reduced through proper execution of earthworks, mainly by high compaction of the structure. Consequently, high compaction of the earth-made structure reduces the amount of leachates. Fresh mine stones are usually neutral or alkaline, but during the weathering process an oxidation of pyrite occurs and they become acid. Fly ashes from ground coal, which are usually alkaline, show slightly distinct properties. Tests on the content of basic and trace elements, pH, and conductance of energy wastes confirm their high usefulness in earthworks [30]. [PERSON]'s [31] test results indicate that mixtures of colliery spoils and fly ashes can provide a composite whose pH value can be neutral. Studies of fuel ashes from a few heat and power plants [5,32,33] indicate that their chemical composition depends on the type of coal, the amount and type of unburnt parts, and technical parameters of the power plant equipment. Studies of the leachate from industrial wastes indicate that the amount of leached substances did not exceed the limit values [30,33,34,35], so these wastes should not pose a threat to the quality of surface and underground waters. It can, therefore, be assumed that they won't endanger the natural environment around the structure. However, because of the high variability of their chemical parameters, an evaluation of their usefulness based on geotechnical tests and evaluation of their influence on the natural environment based on physicochemical tests must be carried out every single time. The purpose of this study was to determine the influence of compaction on the shear strength parameters of selected industrial wastes. The following materials were used in the study: Fly ash from the electrostatic precipitator in Cracow Combined Heat and Power Plant, unburnt coal waste from current coal production and burnt coal waste from a heap in Myslowice-Wesola coal mine with a grain size smaller than 20 mm, and a composite (a mixture) of unburnt coal waste from Myslowice-Wesola and fly ash from Cracow Combined Heat and Power Plant (30% of fly ash in relation to the dry matter). Slope stability calculations were also carried out for a road embankment model made of the tested materials. The influence of cohesion changes on the factor of safety was taken into account during these calculations.