Bond strength

[Contraflexure74]

Hi,

I'm underpinning an existing strip footing to a single storey cottage with a series of micropiles. The footing is unreinforced. Does anyone know how to calculate the bond strength capacity between the existing strip footing and the grout of the new micropiles?

 

[dreber]

This sounds like a bad idea - consider replacing the foundations with reinforced concrete while you are at it.

Also, don't rely on the bond strength, epoxy dowel some bars into the existing foundation if you must. Use Hilti Profis Anchor software to determine anchorage strength.

 

[WARose]

Usually (in that situation) the contractor will have the wall footing (which I am assuming this is) bear on some type of seat at the top of the micropile. (This is assuming the load is all down. No uplift.)

In the cases where the micropile contractor does grout to something, it's his responsibility to come up with a connection that will transfer the force to your element. I've worked with them before and I just fed them the loads. The hardware was theirs.

 

[dreber]

Also, consider using helical anchors. I think that is the standard in the California Bay Area.

 

[Contraflexure74]

My problem is that the foundation is a very poorly constructed unreinforced raft, if you can believe that. So effectively I have to treat the raft thickening under the wall load as an unreinforced strip. The wall which has subsided is constructed very close to a site boundary wall, so access externally is non existent to install piles externally with those steel support brackets at the top of the pile. We have tried grout injection and it hasn't worked. The rock is too deep try traditional underpinning. I can install a cantilever needle to the front corner but I'm not sure what to do for the remaining gable due to close proximity to site boundary wall.