Truro Crown Court was told that Roger Daw, 58, was operating a fully-loaded front-tipping dumper on MJL Contractors' site in Perranporth, Cornwall.
As he drove the vehicle down an incline, it became unbalanced and tipped over. Daw, who is not thought to have been wearing a seatbelt, was thrown from the dumper which then landed on his legs and crushed them.
He was airlifted to hospital where both legs were amputated.
The Health and Safety Executive (HSE) said the dump truck was unsuitable for the task and no one on site had assessed its limitations. The executive's investigation found the company had not carried out an assessment for any of its drivers or their competence in using the plant.
MJL Contractors, which describes itself as "one of the largest and most prestigious groundwork contractors in Cornwall and Devon" pleaded guilty to breaching s 2(1) of the Health and Safety at Work Act. It was fined £200,000 plus £12,312 in costs.
HSE inspector Jo-Anne Michael said: "If MJL Contractors had planned the work properly, assessed the equipment and the drivers, this incident would not have happened."