Split level garden + extension

[Graham Jones]

Hello,

I'm rather worried about the extension our builders are doing. The house and garden are on two different levels, the garden being about

650mm lower than the house level. The extension juts out from the back of the house with the footings for the back wall of the extension dug down from garden level. The other two sides of the extension have stepped footings.

It is a block - cavity - brick extension and I noticed the builder filling the cavity below the level of the DPM with concrete. I asked him about this and he said it is because of the split level. There is obviously a lot of earth against the back wall.

My worry is how does the builder know that a block - concrete filled cavity - brick structure will retain that amount of ground? Should the architect have got structural calculations done for this? Is this something that can really be left to the builders? The building inspector has not questioned it but I am still worried?

Can anyone put my mind at rest?

Thanks,

Graham

[Phil L]

This is normal procedure regardless of whether the footings are split level or not, and it's to stop the two leaves of masonry being pushed together by surrounding soil

You've got the wrong end of the stick, the wall isn't retaining anything, it's all going to be underground, unless you mean that it's going to be retaining the infill under your floor?

[The Natural Philosopher]

I build a retaining wall just like that of similar height. Only thing Id say is if he didn't put rebar in the concrete or ties in all the bricks, then I'd query it.

But talk with BCO.

[Graham Jones]

yes the infill under the floor.

[Graham Jones]

No rebar in the concrete, but he did use wall ties.

[Hugo Nebula]

The rule of thumb for the height of an unreinforced retaining wall with loading above is 4 x T, T being the thickness of the wall; in this case probably 100mm + 100mm. Therefore a difference in level of

800mm is acceptable. If the wall above is not restrained at floor level, then it should be no more than 3 x T, but 650mm doesn't seem excessive.

As others have posted, is the wall actually retaining anything? If you have >600mm between ground level and floor level, it is feasible (and probably preferable) to construct it as a timber joisted floor.