Soil improvement techniques are frequently necessary for soils that do not possess sufficient performance under specific conditions. With growing concerns related to environmental and economic aspects, there has been a growing interest in utilizing waste products to stabilize problematic soils. The purpose of this study is to improve the residual granite soil collected from the granite quarry of Lena Agregados, located at Lugar Couto da Travanca, Escalos de Baixo, Castelo Branco, Portugal, by adding tyre fibres. There were performed geotechnical tests to determine the index properties and respective classification of the soil. The soil resistance before and after the mixture with tyre fibres, in the percentages of 1, 2 and 3%, was determined through the CBR test. The results were compared with the specifications of soil to use in capping layer of road pavement provided by Estradas de Portugal and with two works previously developed at Higher School of Technology, Polytechnic Institute of Castelo Branco (ESTCB). It was found that the maximum dry density decreased with the increase in the percentage of fibres added, which may be due to the difficulty in homogeneously mixing the tyre fibres into the soil. The bearing capacity of the residual granite soil decreased with addition of fibres. The resistance of the soil also decreases by adding fibres to clayey sand studied in work previously developed at ESTCB. Hence, it could be concluded that the addition of tyre fibres does not lead to an improvement in soil-bearing capacity.