RMC Building and Civil Engineering Ltd was prosecuted by the HSE following an investigation into the incident at the Longleat Estate in Warminster.
Swindon Magistrates’ Court heard that the company had been hired to install fencing around the estate in January 2014.
Peter McGrellis, who was 48 and living in Westbury at the time, was one of three employees carrying out the work, which involved using a digger to push wooden fence posts into the ground.
The posts were held by hand whilst the operator of the digger rested the bucket on top of the post and applied downward pressure to it.
On 16 January, McGrellis was holding one of the posts ready for the digger to push it down.
The top of the post split, causing the bucket to slip and hit McGrellis on the shoulder, knocking him to the ground.
He suffered significant injuries including a broken vertebra. McGrellis was in hospital for over a week and he still suffers with pain.
An HSE investigation found that the company failed to plan, manage and monitor the work.
The method statement prepared by the company for the operation indicated a piece of equipment called a post driver was going to be used to drive the posts into the ground but that the digger may be used to position the posts.
A post driver had been ordered along with a smaller digger but it arrived after the incident happened.
The investigation also found that while a risk assessment had been produced, it made no reference to the risk of working close to the digger.
RMC Building & Civil Engineering Ltd of Greenford, London, was fined £1,500 and ordered to pay costs of £1,117 after pleading guilty to a single safety breach.
Speaking after the hearing, HSE Inspector Ian Whittles said: “The use of excavator vehicles in such a manner is dangerous and is known to cause injury.
“The serious failure of RMC Building & Civil Engineering in not managing this job properly led to this avoidable incident and unfortunately Mr McGrellis suffered as a result.
“Workers have a right to expect that the equipment they use is appropriate for the task – on this occasion the equipment used was clearly not suitable for the job.
“Anyone in control of construction projects must ensure the work is properly planned and thoroughly risk-assessed to avoid such incidents.”