Micro Pile Underpinning

[Contraflexure74]

Hi All,

I need to underpin a subsiding single storey house on one side. My problem is I have limited access to underpin from the outside. The house is founded on what appears to be an unreinforced raft. One thing that has always bothered me is if I drill micro piles at a slight angle down through the slab thickening which act as friction piles (from the inside) how does one make a positive connection between the top of the pile and the raft thickening. In other words what stops the raft to continue to slip down the pile shaft? This solution has been proposed by a piling contractor.

Thanks.

 

[oldestguy]

Before going further investigate how the problem can be resolved with compaction grouting. In that case the earth just below the slabs and for several feet below transfers the increased support up to the slab. The pips injecting the grout more economically can be inserted under the house at various angles from outside. If that is impractical of course they can be putdown in sections something like you would have to do it with piles. More than likely inserting these pipes would require less reaction resistance than for piles. The main need for the injecting bulbs of grout is that they are quite deep so as to distribute their lift over a wide area. Only an experienced grout contractor should be used and some reference from past jobs should be given.

In answer to your question on piles, by providing holes in the upper parts of the piles and injecting grout you might be able to do it, but you also need then to fill the pipes with grout. Some thinking even then has to be done as to being able to develop full load to the pile and not damage the slab,etc. I'd look at other fixing methods first.

 

[Contraflexure74]

Hi Oldestguy,

Thanks for your input.

We have previously run with a grout injection solution but it hasn't worked. The house is still subsiding. The soil below the raft appears to be a soft clay. In my experience with grouting, it doesn't penetrate the clays enough to increase the soil bearing.

I suppose my questionis; is micro-piling down through the house footing a ground improvement method or is it meant to act as an actual piled solution. If it is the latter then how can one be satisfied with the connection at the top of the pile and the footing?

 

[BridgeSmith]

An alternative, similar to compaction grouting but possibly more effective, may be the deep injection polyurethane expanding foam, such as the Uretek system. I suggest contacting them for an assessment of your situation.

 

[oldestguy]

It would help in our review if there is a test boring on site to review. Also to what bearing strata will these micro piles extend?
You are right, compaction grouting might be the wrong name since they do not necessarily compact clays, but shove the soil upward taking along the foundations. I'd more likely call them jacking grouting. If the problem is due to continued compressing of soft clays, by-passing the soft material seems to be the best option. Well, maybe not. If you previously did compaction grouting and there are bulbs of hardened grout;t everywhere down there, how will your micro-piles by-pass those zones. Stopping on them may help but unlikely stop the settlement.

 

[Contraflexure74]

Hi Oldestguy,

What we seem to have on site is 1.6m+ of soft clay above rock. The micro pile head would bore through any grout for sure, I've seen them go through concrete. I would normally go with traditional underpinning down to rock under the raft but access to physically do this is limited on the outside due to close proximity of boundary wall. This is why it was proposed to install micro piles.

I think you maybe missing my point, my initial question is how you get a positive connection between the top of the pile and the raft slab??

 

[oldestguy]

yes that connection is a difficult thing to accomplish. I stabbed at it, but not very positive a thing. providing holes in the upper parts of the piles and injecting grout you might be able to do it,
I am assuming the slabs are thin and a significant load to each mini-pile might result in a shear failure around that connection. If so, then MANY mini piles, each lightly loaded..

 

[Contraflexure74]

Okay thanks oldestguy.

 

[structSU10]

See the link for a product I have seen used in the past. Pile is designed for eccentricity from the bracket.

http://www.williamsform.com/Header_Pages/whats_new.html