Eric Wilson, 62, was controlling the piling hammer, suspended from an excavator, during work to renew a sewage outfall when the incident happened on 16 September 2012.
He was standing in seawater one metre deep, using the hammer to drive timber piles into the beach. But as the hammer was moved from one pile to the next, the sling supporting it broke and the hammer fell. He suffered fractures to his back, hip and leg.
The HSE found that Wilson's employer Southbay Civil Engineering had failed to properly plan, supervise and carry out the lifting operation in a safe manner. The authority also found that principal contractor Costain had failed to properly manage and monitor this phase of the work.
The court was told a smaller hammer, which was easily lifted into position by the excavators on site, had been identified when the work was planned. However, the smaller hammer was found not to be powerful enough so a larger hammer was brought in.
The additional weight was not an issue for the excavators, but the extra length led to problems when lifting the hammer over some piles, which led to unsafe working practices being used, in which the slings were stressed over the sharp edges of excavator buckets.
This caused the sling to break, and the hammer to fall, according to HSE inspector Martin Smith.
"Had the work plan been changed when the circumstances changed and a different hammer brought into use, [the accident] could easily have been avoided by using a suitable lifting machine," comments Smith.
"The failures by both companies to look properly at the risks involved and then organise the lifting operation properly put staff at needless risk."