Groundworker Jamie North, 49, from Grimsby, sustained multiple leg fractures during the pile cropping work at Caistor Grammar School, Lincolnshire in March 2013.
North required two operations and had a steel frame and screws fixed on his leg.
He also developed a blood clot which required further treatment after a 21-day stay in hospital.
He was off work for a year and is still undergoing treatment on his ankle. He can no longer work in the construction industry.
An HSE investigation found that Topcon Construction Ltd failed to ensure that work equipment used for cropping piles was used only in suitable conditions, and failed to dismantle a reinforced concrete pile in a manner to prevent danger.
Lincoln Magistrates’ Court heard that North was working on the construction of the new school building which required the installation of pre-cast concrete driven foundation piles.
The six-metre long piles were driven into the ground using a piling rig, in an upright position, until they were set.
The excess part of the piles, which extends out of the ground, was then to be cropped using a hydraulic pile cropper.
HSE found the pile cropper hired by Topcon was only suitable for piles with a single, steel reinforcing bar running through the length of a pile.
Another cropping machine, a power cropper, had been recommended for the school construction job by the hire company but the advice had been disregarded.
Magistrates were told the pile cropper being used was not powerful enough to cut through the concrete and four steel bars so was used to nibble the concrete away to expose the steel bars, which were then cut through with a disc cutter.
The piles were then pushed to the ground in an uncontrolled manner.
As North was guiding the cropper over one pile, a colleague pushed another pile over but he had not cut through one of the bars. The pile twisted and fell onto North.
Topcon Construction Ltd of Louth Road, Grimsby, was fined £10,000 and ordered to pay £1,980 in costs after pleading guilty to safety breaches.
Topcon must also pay North compensation of £10,000 for his injuries.
Speaking after the hearing, HSE inspector Martin Waring said: “Topcon Construction Ltd failed to heed two warnings that the pile cropper they had ordered was not suitable for the job, hence the need to adopt the high-risk ‘tree-felling’ method of pushing the piles over.
“They should have foreseen that the felling of piles, in an area that workers could wander into, presented a high risk of injury.
“Mr North suffered very serious leg injuries in an incident that could have been prevented, had the work been better planned and managed.”