I. Introduction
Excavators are versatile earth-moving machines that are widely used in mining, agriculture, and construction. Excavator operation requires highly skilled operators and often-times happens in hazardous environments that could cause injuries [1]. This has led to an increasing amount of effort in excavation automation [2], but one of the key challenges of excavation automation is that it is impossible to model the machine-earth interaction accurately [2], especially for complex, non-homogeneous terrains such as fragmented rocks.