The study aimed at the evaluation of the effect of moisture content and load on the California bearing ratio of the ash-slag mixture collected from the landfill of the combined heat and power plant “Kraków”. Determination of the bearing capacity index (CBR ratio) was performed at the load of 22 and 44 N and without load on the samples immediately after compaction and after 4 days of soaking in water. The test of the California bearing ratio of the mixture with a single geotextile insert (PPST 100) or geocomposite (geogrid ACGrid GG20-II + PPST 100 geotextile) was also carried out. Geotechnically, the ash-slag mixture was classified as multi-fractional silty sands with gravel (grsiSa). The conducted studies have shown the significant effect of soaking up water and load on the California bearing ratio. Increase in moisture content of the mixture after four days of soaking in water was big and caused a significant reduction in the California bearing ratio. Whereas the load increase from 0 to 44 N resulted in an increase in the California bearing ratio, especially in the tests after soaking in water. The use of geosynthetic liners resulted in a reduction of the California bearing ratio both in the tests immediately after compaction and after soaking in water in relation to the mixture without the inserts.