Not at all sure what your context is form the info given. A cajón can be translated as a caisson, which is watertight anyway, so putting impermeable is superfluous.

A caisson is a large case or chamber that enables dry working on a foundation. It can be built partly or wholly above ground and sunk below ground usually by dgging out the soil inside (Civil Engineering Dictionary by Penguin).

Another translation for cajón is box section, which can be used for transport infrastructure. This is usally jacked into position.

HTH