Several experimental results in the literature demonstrate that tillage and excavation forces increase dramatically as soil accumulates ahead of a cutting blade, however these effects are not always modeled; when they are added to classical models, they are included as a surcharge force without consideration of the soil properties of the piled soil, other than mass. This work introduces a modified McKyes excavation cutting model, that takes into account the friction angle and cohesion of the piled soil when computing the failure plane of a cut. Simulation results show that this new model replicates the decreasing failure plane angles, as a cut proceeds, observed in experimental and Discrete Element Model (DEM) results in the literature. Simulations at 1g and 1/6g gravity, with various levels of cohesion, show that excavation forces due to soil accumulation are especially sensitive to cohesion in planetary excavation. These results elaborate on prior work in the literature showing that cohesion plays an important role when soil accumulation is not considered. Modeling such aspects of excavation without resorting to computationally-intensive DEM is especially useful in autonomous terrestrial excavators that must model and plan cutting in real-time or in planetary excavators where computing power is limited.