Am I undermining my house?

[mediocremartin]

Hi, can someone please advise.

I am replacing my old wood frame garage with new brick garage (single skin with piers). The floor level is about a metre below floor level of house. Whilst digging out the new base I am concerned that I may be undermining the house. The concrete path adjacent to the house and brick retaining wall have sagged a little as you can see from the photos. Not worried about the path - I was going to replace that anyway but worried about house.

The white bars show where the new floor level will be. I was going to dig 600mm trenches for the footings, but am worried this will undermine the path (and possibly house) further. (house appears to be on strip foundations - ex council house built in 1950s)

Does anyone know firstly if I will need to underpin the house or put in some other form of reinforcement?

Also should I continue to dig trenches or should I use a raft foundation (depth of oversite is currently 300mm from floor level - I would then probably need to go down another 100-150mm). Again do you know if this would be suitable. Perhaps an edge beam raft 450mm thick with cage of reinforcement bars around the perimeter?

Many thanks for any help or advice.

Untitled

• mediocremartin
• 29 Aug 2011
• 1

Untitled

• mediocremartin
• 29 Aug 2011
• 1

 

[JohnD]

your path is going to fall, mind you are not near to it when it does

you really need a trial hole near to the house to see where its footings reach.

have you looked under a floor and does it seem to be on a raft?

if you are on clay, the house and garage will move separately, this will be especially obvious in drought years.

 

[Blagard]

As JohnD has indicated you are just guessing and taking risks. Since the path is going anyway, take out a section now and check where your house foundations are by doing the trial pit. Once you find out, you can get on with making informed decisions. Given that the ground levels vary a fair bit around you house I would not expect it to be a raft. If you have a suspended floor inside, the depth of the void may be useful in getting an idea on the minimum depth of the foundations.

 

[^woody^]

You are not undermiing the house - the excavation to too far away

Just get a move on to get the base in to prevent any more of the ground coming out from beneath the path wall

Or put something like a scaffold board up against the soil and some 2x2 pegs into the ground to retain the soil for now

 

[Blagard]

You are not undermiing the house - the excavation to too far away

I don't agree, the OP has not started to dig for his 600 footings yet. His excavation is currently 300mm below finished floor level. While he is not yet deep enough to be a problem, he is right to stop now and check what he is doing.

 

[mediocremartin]

thanks all for your advice. we are on clay. i have started digging a trial hole next to house as suggested. i am down to 300mm below the level of the garage excavation and still have not found the house footings - it's brickwork all the way down so far. Floors are all concrete so no chance of finding level from inside.

At the risk of sounding even more stupid than I have been already, do I still need to keep digging? Is it possible there is no concrete foundation below the brickwork? Alternatively if there is a concrete base would it be reasonable to assume it would be at least 300mm thick? If so, that would mean the base of the house would be at least 600mm below the level to which I have excavated for the garage so far. So if I dug a 600mm deep trench for the garage footings, the trench would be no deeper than the footings of the house. If the footings of the house are no deeper than my trial hole then my 600mm trench would be 300mm below bottom of house. I don't want to take chances, but just wondering if this sounds reasonable.

Many thanks again.javascript:emoticon('')

 

[mediocremartin]

Sorry - also forgot to mention. The path does not appear to have sagged yet, just the retainaing wall. What appears to be a crack/sag in the middle of the path is how it has always been. The brickwork has probably dropped by about 5-10mm in the 3 days since i dug out that side, although difficult to say.

cheers

 

[Blagard]

It seems to me that you have established the house foundations are not shallow. That was the real risk. If your garage foundations are only a little deeper than your house, say 300mm more, I would not be worried.

If it was me I would now be thinking about doing the trench for footings although depending on ground conditions do you really need them 600mm ?

You also need to take measures to ensure that brickwork to the path does not collapse on you. It may not hit you on the head but a broken leg or ankle will still cause pain and inconveniance as a minimum! Also remember to isolate the area with a temporary fence to stop idiots falling into your excavations

 

[Blagard]

I just thought a post script might enlighten the need to keep an open mind about the dangers of excavations near foundations. You can't just rely on 45degree angles etc.

I had specified on a project that an engineers report was required to some wall movement (vertical) on a property I inspected. I was in total agreement with the engineer to underpin and so that is what was scheduled.

Now most will understand the builder needs to do this in 1m sections in an organised sequence as in fact detailed by the engineer.

What happened is that unknown to me or the engineer the builder decided to "prep" by exposing the entire length of the wall foundation, not yet excavating below, just a trench alongside. It rained overnight.

I received a telephone call from the builder in the morning telling me the wall had collasped, the foundation had literally slipped sideways. Perhaps the most amazing thing was that of the two storey structure its was only the ground floor wall that went leaving the second storey in a remarkable example of a massive brickwork corbelled cantilever, because the wall had also taken out the corner of the building at ground floor level.

After suitable propping it was then possible to do a new foundation in one go and rebuild to back to the underside of the stucture above. Incredibly lucky, and a very good lesson for the builder.

 

[^woody^]

It looks like the OP would have to be digging down about 1500mm before he gets into the influencing depth of the house foundation

 

[mediocremartin]

all very helpful and reassuring - thank you very much

 

[Nige F]

Hi, can someone please advise.

- ex council house built in 1950s

That`ll never move - not like the shyte social housing thrown up in the 70`s and later