What is this (at construction site)?

There is an 8-unit condo going up next door. I took this picture this morning of something they’re doing at the rear of the property.

Basically they excavated some dirt and laid down some kind of barrier, then put these orange plastic curved things in the hole (i.e. they’d be hollow inside), and covered it with gravel. The area is about 20 feet on each side.

At the top and right of the picture are parts of the underground parking area, which have cement sides and roof.

I think someone mentioned a sump pump but would this sort of thing be constructed for that purpose? It’s not for a septic tank because we have city sewer and it’s not for oil because they have natural gas.

If the building has an elevator it may have an emergency generator. Or if the power is flaky in your part of the country.

In either case, if the building has a generator it may have diesel tanks for the generator.

Dry well.

My guess, without seeing the surrounding, is that this is an underground infiltration chamber. Rainwater from the roof and/or parking lot gets piped into the chamber(s) under the yellow plastic, then drains into the ground.

Yep that would be my guess as well.

All the crushed stone implies it’s intended for leaching. If it was fuel storage or such they won’t want permeable material.

I don’t think they’re underground fuel storage either. But it has nothing to do with the backfill, and what you say about it is incorrect as far as my experience goes.

I’ve spent much of the last year developing plans for the replacement of large underground fuel storage tanks (both for emergency power generators and fueling facilities) and we only use pea gravel for backfill on the plans we’ve produced. Similar plan sets I’ve seen also use pea gravel. None of the plans I’ve seen or guidance I’ve consulted or pictures on the internet of such things have used anything but that type of material.

Furthermore, the liner we use is permeable and only serves to keep the pea gravel from migrating into the surrounding soil. It’s intended to let ground water drain away. Nothing outside the tank (and piping) is intended to prevent liquid (water or leaked fuel) from draining away. You don’t want ground water accumulating around the tank trying to float it up out of the ground.

That said, I see no sumps/ports or anything along the top like a fuel storage tank would have and none of the piping.

Also, they would be way too big for fuel storage for an 8 unit condo, if such a small residential building would even have emergency power generators. These tanks are about the size of what we’re replacing for a large office building for hundreds of people, and they supply fuel to half a dozen generators, each the size of a small bedroom. (They would also be the right kind of size for a gas station.)

The picture at the top of this page shows a couple of fuel tanks before they have been completely backfilled with gravel. This is before all the stuff on top is installed, but the access ports on top for that are visible, unlike the pic in the OP.

http://www.tercenter.org/pages/usts.cfm

The picture at the top left of this page shows some tanks that have been backfilled with gravel and the sumps, piping, etc. have been installed. (This one is for a fueling facility as opposed to storage for generators but they look much the same.)

http://kesslertank.com/services.html

I don’t know what the tanks in the OP are, but they don’t look like fuel storage to me. If I had to guess, I’d go along with you and those above suggesting underground storm water detention.

This is correct- that looks like a (quite small) stormwater management chamber field. This appears to be the manufacturer, although there are several companies who provide similar systems.

A larger version of this was required when my business expanded in New York- if I recall, by law the runoff from 100% of new impermeable surfaces (roofs, parking lots) had to go to some sort of stormwater management system. Many projects use retention ponds (hence why most new commercial developments in my area include ponds), but if space is limited (as it was on our project), these chambers can be buried, and paved over. Here’s a picture of a small part of ours, under construction.

In our situation, the requirement for the system was particularly absurd, as we are within line of sight to a large, level-controlled body of water. But in general, I believe the intent of the laws that require these systems is to prevent municipalities from having to upgrade their storm water systems in response to the increase in stormwater from new developments, and to instead shift that cost burden onto the developments.

I was going to suggest a septic tank or sewage/greywater buffering tank of some sort, but those tend to have access ports in the top so they can be pumped out or cleared of solids.

In the UK, anyone who is covering ground with impermeable material like concrete for example, has to provide a means of returning rainwater to the subsoil.

Over the years, as more and more land gets built over, the rainwater gets diverted straight to drainage and then to rivers, thus depleting the aquifer. This measure is intended to stop that getting worse.

When I replaced my shabby tarmac drive (a mere 50 sq metres) I used concrete blocks, which both look good and allow water to soak through. If I had used concrete, I would have had to put in some kind of soakaway to take the runoff.

The other facet to that is that draining rainwater into the sewer system (as is the setup for some older houses) can overwhelm it and cause foul water to emerge in the wrong places.

Thanks everyone for the replies. This video on the site provided by mikedb explains it very well. It is definitely for rainwater / sump pump and the orange units are the same configuration as shown on that web site. Also thanks to sich_hinaufwinden for explaining that the liner is to prevent the gravel from migrating.