Subterranean stack of plastic crates in house construction.

I wallked past a building site today - of a type that has become very common in the UK - where property developers have bought a house with a fairly big garden, razed it to the ground and are constructing six or more individual houses in the plot.

Anyway, there was something going on in the construction that I had not seen before - a really big hole in the ground, being filled with a neat blocked stack of (what appeared at first to be) plastic crates. On second glance, it became apparent that the objects are crate-like, but are not actually crates - they are skeletal cuboids of plastic - about two feet or so on a side - obviously strong and rigid, but markedly more void than material. Uniform, new-looking and presumably purpose-made - but for what purpose?

They were being placed into a neat, rectangular stack with similar dimensions and volume as a small/medium room, but this was happening within a deep and broad trench. At one end, the stack was being covered with thick black plastic sheeting and was being buried with soil, coming back to ground level.

What are they for? What would be the purpose of creating a large, long, porous plastic underground cuboid?

The soil in that area is predominantly heavy clay Could this construction be intended to allow an existing underground stream to flow unimpeded (perhaps percolating through a seam of gravel)?

It’s more than likely a storage system for floodwater. It’s considered a form of sustainable urban drainage. Run-off from the development will be collected by various gulleys, routed through the plastic crates, and discharged into a watercourse. The rate of discharge will be limited by the local authority or the Environment Agency, so the storage system allows for flood water to back up. There will be a hydrocarbon separator between the storage and the outfall.

Are they building a form for pouring a concrete foundation? I’ve seen TV shows here in the US where a system of pieces of foam insulation held apart by something like rebar are assembled to form the walls for a house, then cement or concrete is poured in to the form. You get structural strength from the concrete/cement plus rebar, and you get insulation from the foam.

I like Dave.B’s guess better, but I wanted to put this possibility out there.

More than just a guess… I design these things for a living! From the description given, I’d be 90% certain that’s what they are.
This is one proprietry system:

Thanks Dave - that sounds spot on - especially considering that the site is being changed from mostly grass and soil to mostly roofs and paved paths and parking - the surface drainage properties of the site are going to be radically altered.

I think there are new planning rules in place to ensure that surface drainage is not impaired by building development. Even if you just want to pave over your front-lawn or drive-way it has to be designed in such a way that’s it’s permeable to rain-water. This is to prevent street drains from being overloaded by the run-off from hard surfaces in streets which originally were lawns or gardens,

Do these things store runoff? My fantasy green house has a grey-water cistern under the house and/or yard.