Underpinning Old House

[Contraflexure74]

Hi All,

I've been asked to take a look at an alternative to traditional underpinning to an old semi detached house to increase the floor to ceiling height in an existing cellar.

Currently the floor to ceiling is 1.8m high and the client is looking for 2.3m. Typical underpinning will be slow and expensive and the attached sketch shows what is being proposed as a possible cheaper solution.

The building is 2 storey with a cellar under ground floor. The building is circa 100 years old and constructed in rubble stone with no foundations. The building is attached to another similar house on one side, is retaining a trafficked street to the front, has a yard to the rear (so yard will drop also) and a sloping access drive on the other side. All walls are 600mm thick with the exception of the front wall which is 1m due to retaining the street.

So basically what is being proposed is to construct a RC concrete ledge inside the load spread zone of the existing walls, reducing the internal floor space of the cellar.

I have the following concerns:
Will this actually work?
If ground water is present buoyancy could be an issue for the new dropped slab, but I guess a floor drain can be installed to relieve any water build up at slab level.

Any thoughts would be welcome, has anyone done something like this before?

regards,

John.

 

[bookowski]

This is very common in my area. Your excavation appears steep, I would try to get some offset from the footing to minimize disturbance and then a more favorable slope. We typically get info from the geotech on an acceptable slope, 2H:1V is typical, maybe 1.5H:1V in more aggressive cases. Depends on your situation but you'd want to look at temp bracing or doing the work in sections etc. but in general yes this can work.

 

[oldestguy]

First I would examine the existing wall to see if a "liquid" cement grout could be injected, especially near the base. Once set-up,the situation is much more secure. A temporary "dam" placed against old wall will help it to migrate in any voids. That dam can even be soil. Even if not, I'd do the excavation and new concrete in short sections.

 

[Contraflexure74]

Thanks guys. I assume we have to use traditional underpinning to the rear façade as the level inside the building will be the same level as outside the building?

 

[Sawbux]

I'd be concerned about the retained fill on the street side. You are removing the lateral restraint at the wall bottom. The new ledge does not appear to provide adequate compression tie between wall bottom and new slab.

 

[PEinc]

Even if you underpin the wall, you still need to check the wall and underpinning for sliding and overturning. The lighter the load on the foundation wall, the less safety factor for sliding and overturning. Calculate the load on the foundation wall and see if the load is sufficient. Bearing will not be the problem.

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