Ground substance with no void
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How Does Clayboard Compare to Alternative Void Formers?


When it comes to building a really solid foundation, we don’t all have the luxury of building on stable ground – and ground movement can be a huge, costly headache for millions around the world. But that’s where a well-grounded void former strategy becomes a lifesaver.

You’re likely aware of the reasons for ground movement, so the question is how to stop it most effectively.

Types of Void Formers

There are different ways to make a foundation void, but not all methods are made equal. When pouring the foundation, there needs to be something that can support it while it sets but that will allow the void to actually be formed later on. 

Other than creating a slab off the floor via the precast block and beam method or via removable jacks (requiring enough space to be able to get in and remove them later), the main types of cast-in void formers can be categorised as collapsible or compressible.

As the name suggests, collapsible void formers offer a temporary, sacrificial formwork, which will yield under subsequent heave, allowing soil to move without transferring stresses to the structure. In the case of Clayboard, this is achieved by ‘switching off’ its compressive strength after the slab has cured.

Compressible, polystyrene void formers, on the other hand, have a much narrower safe load-fail load window and require the designer to significantly thicken the slab in order to resist the uplift.

While polystyrene is lightweight and impermeable – its different void-forming properties means it has to be considerably thicker than Clayboard, so it’s considerably bulkier than collapsible void formers. So again we have a benefit outweighed by a significant set of downsides: extra transportation, extra excavation time, and extra muckaway. All these, really, add up to “extra cost”.

Dufaylite clayboard used for New York subway
Less overall cost, easier installation, lighter slabs

With our Clayboard paper honeycomb system, you can form rock solid foundations on dependable voids. It’s 30% slimmer than polystyrene alternatives and comprises recycled paper and recyclable facings (so better for the planet!). This also means that significant savings can be made on the amount steel required to stiffen the slab.

Our Clayboard system uses Voidpak pipes to allow you to soak the Clayboard core once the foundation slab has achieved self-supporting strength. This weakens the honeycomb core and reduces its 30kN/m2 compressive strength to a nominal 2-3kN/m2. Essentially, any heave will be accounted for and you’re not going to get an unpleasant surprise from subsequent ground heave.

Let’s talk

We’ve helped create great foundations for projects as varied as small plots on residential developments, through the whole gamut of commercial, industrial, and infrastructure applications. Right the way up to landmark sites including football stadia, the New York subway, and the Queensferry Bridge in Scotland – if you’ve got a project that needs great foundation voids, let’s talk.

3 September 2021

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