Roger Daw, 58, was operating a fully loaded front tipping dumper on his employer’s site in Liskey Hill, Perranporth.
He drove the dumper down an incline where it became imbalanced and overturned.
Daw, who appears to have not been wearing a seatbelt, was thrown from the vehicle, which landed on his legs and crushed him.
Truro Crown Court heard that there were a number of failings that led to the incident.
The specific type of truck being used by Daw was not appropriate for the task but no-one on site had assessed the plant equipment’s limitations.
An HSE investigation found the company had also not carried out an assessment for any of their drivers or their competence in using the plant equipment.
Daw, from Plymouth, was airlifted to hospital where they had to amputate both of his legs about the knee.
MJL Contractors Ltd of Helston Cornwall, pleaded guilty to safety breaches and was fined £200,000 and ordered to pay costs of £12,312.56.
HSE inspector Jo-Anne Michael, said “Roger Daw’s life has been changed forever. If MJL Contractors Ltd had planned the work properly, assessed the equipment and the drivers this incident would not have happened.
“Companies must learn that risk assessments are there to protect their workers from the real risk that mobile plant can become unstable.”