founding or piling through ground made up

[oneintheeye]

hi, we have a site and on part of that site we have to place found. The contractor has starting excavating the area prior to us completing the design. The ground is made up of lots of loose bricks etc. So made ground but with lots of obstructions. I don't want to found on it, they say they may have trouble piling through it. Its a bit of a catch 22. Its a lightly loaded structure, so we may get away with ground bearing. Any other ideas out there.

 

[ishvaaag]

Propose them some more costly solution and will be able to drill.

 

[CTW]

I don't see the catch 22 here. It's your design and liability not theirs. Regardless of how lightly loaded the structure is, you need a stable subsurface to support a foundation.

Surely they can excavate through the brick and replace with engineered fill.

Is this an isolated area that wasn't picked up during the soil sampling or does it cover the entire site?

 

[dik]

Other than the foundations, is the existing fill going to interfere with the new building?

Dik

 

[BAretired]

I don't believe loose bricks will hamper the drilling operation but large chunks of concrete will. I had the same problem a couple of years ago in a very deep fill. A small percentage of piles had to be re-located because they ran into either boulders or concrete chunks at a depth of about 3m. The piles were 400 diameter x 10m long friction piles.

It was not a major problem in my case, but if in doubt you might want to try a few test holes before committing to a particular design.

BA

 

[oneintheeye]

its a local problem to this area. The overall site covers a huge area, this is one location. Its not for a major structure. They will be limited to how deep they can dig out and replace due to adjacent structures (without spending vast money and time). Think I'll have to stick to insisting on piling though, I'm fed up with bending over backwards to help out on this job!

 

[phuduhudu]

One possibility is to improve the ground insitu. For this type of site they often do this basically by dropping a large block of concrete from a crane about 10m high repeatedly in a pattern on the made up ground to precompact it. It has some technical sounding name that I forget but it's a pretty unsubtle form of treatment. Doesn't work well if you have neighbouring buildings for obvious reasons.

 

[oneintheeye]

dynamic compaction? Sounds impressive doesn't it?

 

[BigH]

phuduhudu - dynamic compaction when excavation is hampered by adjacent buildings - don't think so unless you want a law suit on your hands. Again, as with all foundations, please provide us a few details - how deep is the fill? What type of structure are you placing on the fill? You might consider, so long as you are not going to raise the ground level - to dig out about 2x the foundation width and backfill with crushed stone and place a smaller footing on top so that stresses are not introduced into the underlying fill. Might work for a light structure. You could also dig out a few metres (if possible), sort out the debris and recompact (assuming you are not dealing with a clay). There are a number of options - but it would be helpful to know the details.

 

[oneintheeye]

yes sorry I don't have a full picture yet. They are digging about down there now! There may be an issue with existing founds. depending on how things pan out I think I know the way to go. When I get more info I'll post on here to keep you updated. What i've seen so far there is quite a mixture to around 2m then there appears to be a very hard layer of compacted material (stone, gravel). So as you say I may be able to replace. There is not much room there for a ground bearing base which is why I was going piles.