Based on the given description, it seems to suggest that Caterpillar bulldozers, specifically the D6R XL model, are capable of digging from the ground up, but it is a difficult task to accomplish. The word "admit" implies that this difficulty is acknowledged or accepted by Caterpillar as a manufacturer of bulldozers.

It is possible that the difficulty of digging from the ground up could be due to factors such as the hardness or composition of the soil, or the presence of obstacles or debris in the ground. Despite this challenge, the D6R XL model of Caterpillar bulldozers may still be able to achieve this task with the appropriate equipment, techniques, and skilled operators.