We cleaned out our first pond in August of 2015 the next day after we drained it with a trash pump. The dozer started at the edge and scraped down to solid clay as he went across, pushing into a breach dug into the dam. A backhoe was used to help dip out the muck as it was pushed into the breach.
The dozer was able to maintain traction by making sure the muck was all scraped away as it proceeded. The clay under the muck was undisturbed and solid. YMMV, depending on the substrate under the muck.