Effects of plastic fines content on the engineering properties of cement-stabilized sands

Cement-stabilization of soil is a common and useful ground improvement method to increase the strength and stiffness of weak soils. The engineering properties of cement-stabilized soil can be affected by the amount of fines in the soil, but there exist controversial views on the effects of fines in the literature, indicating that the underlying mechanism in such cemented soils is not fully understood yet. This paper presents a laboratory study to investigate the effects of fines content on the strength and the wave velocities of cement-stabilized soil using specimens with well-controlled packing densities. The effects of void ratio and cement content are also examined. Experimental results show an interesting finding that the strength and the wave velocities of the cement-stabilized sand first increase and then decrease as the fines content increases from 0 to 15%. The contact and bonding characteristics at the particle level indicate that the strength of the sample is affected by the bonding area and the bonding strength. For a given cement content, the addition of fines increases the bonding area but decreases the bonding strength. Such controversial effects of fines enable to better understand the observed trend of strength variation. A similar mechanism can also be applied to the effects of fines on the wave velocity. Based on the particle-level mechanism, an empirical equation is proposed to characterize the effects of fines. The unconfined compressive strength of the samples can be correlated with the ultra-sonic pulse velocity and shear wave velocity for field monitoring and quality control. The correlations are affected by the fines content.