Several methods have been used over time to improve the mechanical properties of fine-grained soils. One of the recently introduced materials for soil stabilization is incinerated sewage sludge ash (ISSA). This material is a by-product of the wastewater treatment process that is usually disposed of during the treatment cycle. This paper investigated the effects of adding the optimum amount of ISSA and a mixture of ISSA with hydrated lime (IL) on the mechanical properties of dispersive fine-grained soil. The effects of curing time on the UCS was also evaluated. The Mohr-Coulomb failure envelope parameters of the mixtures were subsequently estimated based on the performed test results using the Consoli et al (J Mater Civ Eng 27(5):04014174, 2015) method which eliminates the need to perform triaxial tests. The results indicated that ISSA and IL can improve the mechanical characteristics of the dispersive soil effectively and that curing time was substantial for better performance of the treated soil. Finally, the application of the Consoli and others method to predict the failure envelope parameters of the treated soil was evaluated using triaxial tests. The comparison of the results proved the suitability of the proposed method to estimate the failure envelope parameters of the ISSA and IL-treated dispersive soil.