The BBS Construction Ltd employee, who does not wish to be named, smashed his right hip in the incident at Benenden School for Girls on January 18 2011.

The face of an excavation which was four metres in depth collapsed on top of him.

Maidstone Magistrates’ Court heard that the collapse occurred during work to build a new multi-storey science block.

The injured worker, then aged 44, was excavating a basement to support the new building.

But concrete to fix the posts for a supporting wall had been overpoured and the excess had to be chipped away with a manually-operated hydraulic breaker.

It was this work that triggered the collapse.

A quick-thinking excavator driver managed to stem the flow of earth by placing the bucket of his machine into the face of the weakened earth, but his colleague was still caught and trapped.

Another worker managed to jump clear.

An HSE investigation established that the risks of manually breaking the excess concrete were not properly assessed.

An inspector also found that earlier in the project a waste water drain had been installed by BBS less than half-a-metre from the face of the basement excavation at a depth of two metres.

Magistrates were told that this may have weakened the earth around the basement and made it more susceptible to collapse.

BBS Construction Ltd, of Tovil, Maidstone, was fined £8,000 and ordered to pay £10,536 in costs after pleading guilty to a single breach of the Construction (Design and Management) Regulations 2007.

After the hearing HSE Inspector Kevin Golding said: “This prosecution demonstrates the need to properly plan all aspects of excavation activity, and to reassess systems and methods of work if a problem arises – as happened here when the concrete for the supporting posts was overpoured.

“The retaining wall system itself was sound, but the decision to manually chip away the excess was flawed.

“The injured worker was dangerously exposed, and could have been more seriously hurt, or possibly even killed, but for the instinctive action of his colleague in using the excavator to shield him.

“BBS Construction Ltd could and should have done more to prevent workers from needing to work so close to the unsupported vertical face of a four metre deep excavation.”