Stability of sandy silt soils is important to increase the resistance of the foundations of the buildings and hydraulic structures in civil engineering projects. It seems that adding some materials with different percentages of addition can raise the values of the compressive strength. Present study carried out a series of experimental tests by using the unconfined compressive strength (UCS) and California bearing ratio (CBR) in low plasticity sandy silt soils due to different additive materials such as cement, pozzolan cement, Confix Pr50, cement + one percent of the polypropylene fibers, Confix Pr50+25 percent of bentonite, Emaco S45, Galena, Galena+1 percent of lime, Galena+3 percent of lime. In total, 299 experiments have been performed in the soil mechanics laboratory of Science and Research Branch, Islamic Azad University (SRBIAU (. In order to investigate the effects of different additive percentages on compressive strength and strain variation, three values of additive materials were considered as 3, 5 and 7 percent. Furthermore, the UCS values and CBR numbers were measured during the time steps as 3, 7, 14, 21 and 28 days from the start point of the tests. Also, the values of the specific weight, porosity, humidity percent and unconfined compressive strength were measured over the considered time steps. In order to measure the mentioned values, the scaled volumes of experimental soils weredried in the oven and the values of specific weight, and humidity percent were calculated. Furthermore, the values of the unconfined compressive strength were measured by the related apparatus. The results indicate that the mixed soils with cement enhance the higher value of unconfined compressive strength and California bearing ratio such as solo cement, pozzolan cement and cement with one percent of the polypropylene; however, adding the Confix Pr50+25 percent of bentonite decreases the values of CBR marginally. Experimental sample soils with 3, 5 and 7 percent of the confix attained less values of UCS and CBR which can be deduced that confix material is not appropriate to raise the stability of the foundations in sandy silt soils. Finally, by employing thesoil properties over the time, an equation has been proposed to estimate the unconfined compressive strength by using the non-regression technique. The accuracy of the proposed equation was calculated and the results indicated that the predicted values had a good agreement with observed data.