Victorian cellar conversion. Alternative to underpinning works.

[Annker]

My house has a cellar which I am considering converting to a habitable space.

Currently headroom is only ~1.8m, to achieved sufficient headroom and compliant floor build-up I would need to excavate the floor sub-level to ~400mm below the base of exsisting footings.

 

Typically that would mean underpinning the footings however I think there is an alternative method where a concrete ring beam is cast at the base of the footings.

This onits the need for underpinning works albeit at the cost of floor space.

 

Is anyone familiar with this method and if so can you point me in the direct of a technical resource outlining the method?

[George]

You may as well traditionally underpin with mass concrete. There's no less risk with this 'side pinning' ( and possibly more) and the reinforcement is a significant complicating factor.

 

Underpinning is not rocket science, but it does require some technical understanding and good procedural control, which is why it is often done badly.

[saveasteading]

If you live in a nice area of a big city, this could be commercially viable. 

Elsewhere I suspect cost will far exceed value or benefit.

 

I'm not giving any advice or even thoughts on the best procedure here. 

It could be very specific to the building and surroundings.

You are deliberately undermining a, presumably,  perfectly good building, so there is risk.

You need an SE from an early stage.

 

Your annual insurance renewal will have a box tick for ' has your house been underpinned'. The insurer will assume there is a continuing risk.

But an SE will design a scheme that will be a construction project, not underpinning as such.

 

I hope you have allowed a lot of money for this and are prepared for earth and rubble going out of the hallway.

 

 

 

[Gus Potter]

On 29/08/2023 at 14:01, Annker said:

I think there is an alternative method where a concrete ring beam is cast at the base of the footings.

I think I can see where you are coming from. Below is a simple model / sketch.

 

 

 

 

9 hours ago, George said:

You may as well traditionally underpin with mass concrete. There's no less risk with this 'side pinning' ( and possibly more) and the reinforcement is a significant complicating factor.

 

Underpinning is not rocket science, but it does require some technical understanding and good procedural control, which is why it is often done badly.

Agree.. and it is often done badly.. and dangerously.. to do it well costs money.. with basements some contractors take the "who dares wins" approach, the first things that gets compromised is safety and quality of workmanship.

 

6 hours ago, saveasteading said:

Your annual insurance renewal will have a box tick for ' has your house been underpinned'. The insurer will assume there is a continuing risk.

But an SE will design a scheme that will be a construction project, not underpinning as such.

What a good point, it's these things you would not probably know about.

 

@Annker The theory around the sketch is well tested. This cantilever slab method is used when you have say a gap site and can't pile up against the adjacent buildings for one reason or another. The above I think results in an independent floor that doesn't impact too much on the existing founds etc. Looks simple as it's a foundation within an existing foundation... but.. the underbuilding will maybe have cross walls etc so it's not as simple as it looks as a sketch!

 

The new strip found might cause a bit more settlement due to the extra stress on the soil but most buildings tend to fail / protest when the walls move outwards rather than inwards.

 

In a domestic application you may just have a configuration where this is best solution, not often but sometimes.