This research has shed light on the capability of the HASP model in reproducing the dilatant behaviour of OC clays in drained and undrained conditions. The model is formulated by employing a combined hardening rule and uses the void ratio as a state variable while maintaining the simplicity of the MCC model. However, a sensitivity analysis has revealed that the model is sensitive to some input parameters which when varied slightly can largely affect the outcome of an analysis. This has led to the formulation of the PLAXIS OC clay model while maintaining the framework of the HASP model but replacing the void ratio with volumetric strain as the state variable. Thus resulting in the use of the modified compression (λ*) and swelling (κ*) indexes which are used in obtaining the soil stiffness parameters used as model inputs. The PLAXIS OC clay model features the use of real soil stiffness parameters instead of soil indexes, the addition of small stain stiffness by T. Benz to improve model prediction in the small strain region and the elimination of the sensitivity issues noticed when using the HASP model. The PLAXIS OC clay model is validated for boom clay (BC) at single stress points by simulating CU test and comparing with the available experimental data for the BC. Good agreement is found with experimental data as shown in the stress strain, pore water pressure and stress path plots obtained from the analysis. Furthermore, the model is used to simulate boom clay in a trial excavation. Piezometers and extensometers are installed into the BC layer prior to the excavation to monitor the changes in porewater pressure and vertical displacement (heave) on the BC during the excavation. A comparison of the numerical and experimental data shows that good agreement is observed in porewater pressures and vertical displacement in the BC layer.