Have you heard of steel prefabrication below-ground level, and want to learn more?

Through the industry, you may have heard about this method of constructing below-ground, and the term ‘prefabricated steel substructures’. I wouldn’t be surprised if you’d heard the term in passing through different construction professionals, such as engineers, architects or main contractors on a previous project.

After all, steel prefabrication has been around for over 35 years – but still, it’s nowhere near as common as more established methods.

As a subcontractor who specialises in prefabricated steel substructures, I’m going to explain exactly how steel prefabrication below-ground actually works, as well as the end result.

What is steel prefabrication below-ground level?

If you need a hole in the ground for whatever reason – such as a cellar, lift pit, machine base sump, vehicle maintenance pit, sump or tanks for an industrial launderette facility – or any other scenario which requires a sump or pit as part of a bespoke facility, you’re going to need a below-ground substructure.

Prefabricated steel substructures are manufactured entirely out of steel, off-site in a factory environment, and are a much more innovative and effective method of constructing below-ground. The manufacturing process involves the constructing of steel plates, and the necessary strengthening to build substructures in an off-site facility.

This facility must be certified to BS EN 1090, and must also be CE certified (i.e. a Construction Product Regulation hEN1090). This is a regulation managed by law for all structural manufactured items.

Once the prefabricated steel substructures have been completed, they are then transported to the site in question, before being fixed into position, then back-filled with concrete between the earth and the substructure.

What are the benefits of prefabricated steel substructures?

You may already know that the problem with more traditional methods of constructing below-ground can fail, and leak if the specification isn’t correct, work on site is of a bad standard or is missing the correct waterproofing applied in the correct environments.

Due to its unique approach of constructing the substructure out of steel plates before welding it together, this innovative method of below-ground construction completely eliminates the risk of leaking for up to 100+ years.

Another impressive thing about prefabricated steel is that it can be built to very, very high tolerances of +-3mm/4mm, whilst lowering long-term maintenance costs and reducing time spent on-site by 70 – 80%.

So, what’d usually take weeks – if not months to complete – could be done in just a matter of days. So, no more worrying about going over-time with your project, and less risk and issues for you and your team.

So, let’s break it down:

  • Jobs can be completed 70 – 80% faster (usually in a matter of days compared to weeks/months)
  • This method is guaranteed not to leak
  • It’s built to very, very high tolerances (+-3mm/4mm)
  • The long-term costs are much cheaper, due to reduced long-term maintenance issues
  • Prefabricated steel substructures can last for over 100 years in the ground
  • Operatives are not required to enter an excavation.

And what about the disadvantages?

Steel prefabrication may not be the right solution for every project.

To some, it can seem expensive, and it’s a more innovative method that not everyone’s fully aware of yet; this can lead to skepticism. In many cases, plans have already been designed for the likes of in situ poured concrete or precast concrete and changing those plans could be troublesome.

TIP: Prefabricated steel substructures usually come with their own full design package as part of the service.

So, to summarise:

  • It can seem expensive (with average costs at £1,000 – £3,000 per sq. metre)
  • It’s a lesser known method of below-ground construction 
  • It might not be ideal for every project – especially if plans have already been designed, or if the project is on a very large scale

When is steel prefabrication best used?

The best use of prefabricated steel substructures is when the structure in the ground is on the critical path of the programme, or when tight tolerances or precision engineering is a necessary requirement in the ground. They can also eliminate the risk when your project is located in a high water table area, especially if a long-term water ingress issue is a possibility.

It’s also best when you’re able to approach a subcontractor who specialises in steel prefabrication early on in the design process, before building this method of construction into the construction programme.

Below, I’m going to give you some more advice when considering steel prefabrication as a below-ground construction method.

Advice from a specialist subcontractor

The best advice I can give you is to approach a subcontractor who specialises in prefabricated steel below-ground very early on in the process. As early as possible, really. They will be able to explain more about the benefits of what they offer, and show you previous examples of their work.

Clients always seem to ask us: ‘Can you do it?’ or ‘Is it possible?’ in terms of constructing a substructure off-site. They send us a drawing of in situ or precast plans and ask if we can do it our way instead. The answer is usually ‘yes’.

However, the big question to ask yourself is; what is it going to be used for? Think carefully about its use – does it need to be tailored for a specific purpose? Will it be fully submerged in water? Will it need extra fittings or fixtures? Can it even be made more economically out of mild steel, then paint it in rubber-protected paint rather than using stainless steel?

These are all options a specialist will be able to talk you through.


I hope this blog has helped shed some light on steel prefabrication below-ground and how it works.

Although prefabricated steel substructures have plenty of benefits, it is a very different way of working, so I’d recommend contacting a specialist subcontractor well in advance – preferably in the early design stages. They should be able to talk you through your options.

Do you have any questions about steel prefabrication below-ground and how it might be right for your project?

Feel free to leave me a comment below – or alternatively, get in touch with me directly. I’d love to hear from you!