Instability of low-lying saturated subgrade soil contributes to 'clay-pumping' that is responsible for contaminating the overlying track materials. The clay pumping occurs under the action of cyclic loading (train loading) due to the build-up of excess pore water pressure. The work contained in the paper is focused on monotonic loading response of fouled ballast and is a crucial first step towards a more complete analysis of the clay pumping effect. In this paper, a bounding surface plasticity model is presented for clay-fouled ballast within the framework of critical state soil mechanics, adopting isotropic hardening and a non-associated flow rule. The role of the constitutive parameters and their dependence on various levels of fouling is discussed. The size and shape of the bounding surface is influenced by the extent of fouling. The model is calibrated against the results of consolidated drained triaxial tests conducted using a large scale cylindrical apparatus designed and built at the University of Wollongong. The model was successfully validated against triaxial testing on fouled ballast for an array of confining pressures, and the model predictions were found to be in good agreement with the laboratory data.