It's translated as "soup duty" in a couple of places:

"The work of Jean Droit, which certainly has a pedagogical dimension-La route est repérée (The Road Has Been Located, 1914) and Un 210 éclate (A 210 Explodes)-is similar to Pierre-Albert Leroux's Corvée de soupe (Soup Duty)."
http://www.cheminsdememoire.gouv.fr/en/la-peinture-et-la-gra...

But I think "soup fatigue", used in this text (also translated from French, a text by Henri Barbusse), is probably the authentic term:

"Here, lying across our path, which we are following upwards like a disaster, like a flood of debris beneath the dense sadness of the sky, lies a man who seems to be sleeping; but he is flattened against the ground in the way that distinguishes a dead body from a sleeping one. He was a man on soup fatigue, with his rosary of loaves threaded into a belt and a bunch of his comrades’ mess tins held to his shoulder by a tangle of straps. He must have been hit the previous night, his back holed by a piece of shrapnel. We must be the first to find him: an obscure soldier who died in obscurity."
https://gerryco23.wordpress.com/tag/leslie-norris/

Here it is again, in a contemporary account:

"It was in the Thiescourt Woods, I remember, that I saw Alan on his return from convalescent leave. My section was in first line trenches and his, in reserve, in the second line. I was on soup fatigue and was going to the Chalffour Quarry when I saw him in front of me, walking along alone."
True Stories of the Great War, 278
https://archive.org/details/truestoriesofgre05mill

"3 (fatigues) Menial non-military tasks performed by a soldier, sometimes as a punishment."
https://en.oxforddictionaries.com/definition/us/fatigue

It's not necessarily a punishment (and presumably isn't here); it's a menial but necessary task soldiers have to perform (when it's their turn).

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Note added at 1 hr (2017-02-22 08:23:12 GMT)
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That account I cited on the death of the American poet Alan Seeger is actually also translated from French; it is by his companion Bif Bear, a young Egyptian. However, the translation is contemporaneous and probably uses authentic terminology; it was published in the New York newspaper The Sun in 1916:
http://nyshistoricnewspapers.org/lccn/sn83030431/1916-10-15/...

Here's the expression used in an account by an American combatant:

"The strafing of infantry was first thought to involve too great a risk, but eventually it was practiced by both sides. In the American forces the first exponent of this kind of low-altitude fighting was Maj. Elmer Haslett, who broke up a German soup fatigue in a communication trench. This, from a German point of view, was almost as tragic as the battle of Chateau Thierry."
https://archive.org/stream/cu31924030744688#page/n37/mode/2u...

And here's another account in English, by a British PoW in the Second World War, indicating that "fatigue" is probably the term (both for the task and for the party performing it):

"It was hard work, peeling potatoes from 7.30 in morning to 4.30 in the evening but it was worth it as we received a double soup ration and were up to all the dodges of the day sneaking away with a few exlra patatoes. That, however, only lasted a few days for when the
German's [sic] heard that the English were on potato fatigues, they stopped it." (p. 19)
"To make the daily soup, fatigue parties were detailed who used to
gather snow, which was then melted to provide the necessary water for the soup." (p. 65)
http://www.lamsdorf.com/uploads/6/4/2/7/6427590/e._j._lees_2...