Learn English – Why are both “when” and “once” used in “… when it was once broken”

Transporting the stone when it was once broken was comparatively simple.

This sentence is by the English author George Orwell from the world-famous book Animal Farm. I am wondering the reason why the adverb once has been used as well as the word when there? It doesn't seem to make sense to me.

I do know the adverb once at this context means after.

Here is the paragraph that the sentence comes from:

The windmill presented unexpected difficulties. There was a good quarry of limestone on the farm, and plenty of sand and cement had been found in one of the outhouses, so that all the materials for building were at hand. But the problem the animals could not at first solve was how to break up the stone into pieces of suitable size. There seemed no way of doing this except with picks and crowbars, which no animal could use, because no animal could stand on his hind legs. Only after weeks of vain effort did the right idea occur to somebody–namely, to utilise the force of gravity. Huge boulders, far too big to be used as they were, were lying all over the bed of the quarry. The animals lashed ropes round these, and then all together, cows, horses, sheep, any animal that could lay hold of the rope–even the pigs sometimes joined in at critical moments–they dragged them with desperate slowness up the slope to the top of the quarry, where they were toppled over the edge, to shatter to pieces below. Transporting the stone when it was once broken was comparatively simple. The horses carried it off in cart-loads, the sheep dragged single blocks, even Muriel and Benjamin yoked themselves into an old governess-cart and did their share. By late summer a sufficient store of stone had accumulated, and then the building began, under the superintendence of the pigs.

One of the answers here thinks that this sentence is ungrammatical – but the poster isn't sure why. I wonder if it is because it comes from such a famous book. Can anyone give a definitive answer?

You could see the resource in the following site:

Animal Farm (1945) by George Orwell.