Summer stack creep, photographed on our own yard

The experiment

Creep is the slow, permanent deformation of a polymer under sustained load. It is entirely normal, entirely predictable, and almost impossible to notice day to day — which is the problem. We set up three test stacks in June on our own slab and measured overall stack height daily for 21 days.

Look at row two. A three-high stack lost 1.9 inches in three weeks, almost all of it out of the bottom container. That is the bottom bottle spreading sideways under the load above it, and it is permanent — the stack did not recover height after being broken down.

What 1.9 inches actually means

It means the bottom bottle got shorter and wider. Volume is conserved, so a container that loses two inches of height has gained roughly an inch of girth, and it has gained it against the inside of a steel cage that has not moved. That is the failure mechanism: the cage bars begin to bear into the bottle wall, and the bottle wall thins locally where they do.

The visible warning sign, if you look for it, is bulging in the bottom third of the lowest container and the beginning of an impression where the horizontal cage bars sit. By the time you can see a clear indentation, that container is finished for anything except ballast.

The rules, and why they are what they are

• Two high filled, maximum. Row one of the table shows what that costs: 0.4 inches over three weeks, which is negligible and recovers most of the way.
• Three high empty only. An empty container weighs 127 lb; three high is a trivial load. This is fine and we do it constantly.
• Never three high filled. Row two is why. It is not that the stack falls over — it is that you quietly destroy the bottom container.
• Check the data plate stacking load, especially for dense products. Row three shows what SG 1.4 does even at two high.
• Level ground only. A stack on a slope concentrates the entire load onto two corner posts and the arithmetic gets much worse.
• Break down long-term stacks. If containers will sit for months, store them one high or empty. There is no rush.

None of this is new information — it is on every data plate and in every handling guide. What the measurements add is a sense of the timescale. Creep is not a dramatic event and there is no moment where it goes wrong. It is three weeks of nothing visibly happening, and then a container that will not sit square.