Conventionally, rail tracks are positioned on ballast due to economy, rapid drainage and high shear strength. However, the ballast becomes fouledand degraded as track ages. In order to identify the risk associated with fouling, it is important to accurately assess the amount of fouling using volume based parameter, Void Contaminant Index (VCI). A series of isotropically consolidated drained triaxial tests using a large scale triaxial apparatus were conducted on both clean and fouled ballast with varying VCI to establish the relationship between the amountof fouling and the strength-deformation characteristics. Based on the laboratory findings, an empirical relationship between the peak deviator stress and VCI is proposed to assist in preliminary track assessment. Based on laboratory findings, a non-linear shear strength envelope for fouled ballast is presented.