The contractor was refurbishing part of the Birmingham Metropolitan College in Kidderminster, known as The Piano Building, when the incident happened on 15 May 2012.
The 56-year-old woman from Kidderminster was walking past the site when two sections of the hoarding blew over and collapsed on her.
The impact knocked her down and left her unconscious.
The woman suffered substantial head injuries, including severe concussion, a gash on the head and fluid and bruising on the brain.
The injuries also affected her balance and senses of hearing, smell and taste. She spent 16 days in hospital and was unable to work for seven months.
Kidderminster Magistrates’ Court heard that the woman’s injuries were sustained two weeks after another part of the perimeter fence had blown over in the wind. No one had been hurt on that occasion.
Magistrates were told Thomas Vale had put up solid fencing around the site to stop dust and debris from escaping.
The firm discounted using concrete foundations for the fence as they said too many underground services would be disrupted and the fence needed to be moved at intervals for deliveries.
After the first incident which blew a section of fence out, Thomas Vale didn’t seek expert advice on fixing the problem and left the securing and re-building of the fence to inexperienced workers without the correct training.
HSE found the company failed to recognise the potential vulnerability of the whole perimeter fence and only strengthened and stabilised the section that collapsed.
Thomas Vale pleaded guilty to a breach of the CDM regulations and was fined £20,000 and ordered to pay costs of £10,250.
Speaking after the hearing, HSE inspector Jo Anderson said: “Thomas Vale Construction had a duty to its workforce and to members of the public to ensure the hoarding around the site was safe.
“This fencing was constructed using guesswork. The company failed to seek expert advice in order to ensure the hoarding was designed correctly and did not consider the substantial force which strong wind can impart on solid hoardings.
“This woman, who had parked and was on her way to the shops, suffered life-changing head injuries in what was a preventable incident. Without doubt, though, we could easily have been dealing with a fatal incident.”