Robotic manipulation in a remote environment places a premium on the autonomy and reliability of the robot control system. A robotic excavator must be able to accept high-level operator commands and perform the excavation task without additional external input. An excavation robot must also be able to operate in all three states of manipulator motion: free-space, contact and exertion of a force against an environment. A position-accommodation control scheme allows the robot to effectively operate in all three states and transition smoothly between states. This study outlines the various models used to predict soil-tool interaction forces, as well as how soil changes under excavation various conditions. Excavation simulations compare the performance of a position controlled robot and a compliance controlled robot using position accommodation. Common excavation trajectories are used in the simulations.