location and spacing of helical piers

[IsaacStructural]

I have a four unit residential building with significant settling in one corner. I'm thinking reinforcing with helical piers or another similar system. I'm not concerned particularly with the load on an individual pier as the loads should be low when compared to the pier capacities. My concern with is spacing the piers. The foundation wall wasn't designed as a beam to span between the supports. I don't even have existing drawings to confirm if it is reinforced.

my instinct was to space the piers at no more than about 8 or 10feet and to take the piers to one spacing beyond the extent that I observed the settlement. Is this overly conservative? I've got a contractor who wants to use half as many piers, by reinforcing a smaller area and using wider spacing.

Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[jayrod12]

Then let the contractor find another engineer willing to take the liability.

My design philosophy is generally "Can I sleep at night with this design going out?" if that answer is no then change the design until it is a yes.

I'm not sold on the helical piers yet. Every supplier of them that I talk to has different design requirements and philosophies. It has just scared me off of them.

 

[TLHS]

While helical piers don't have huge penetrations in some markets, they're reasonably well established at this point and are commonly used in some areas. I've used them on a large number of projects and they're pretty easy to work with. They're effectively just a steel pipe pile with an extra bearing plate. There's not a lot of mystery to it. Heck, there are design equations in the current version of the Canadian Foundation Engineering Manual (4th Ed).

There are also lots of vendor supplied design documents that you can get and a number of reasonable books. I'd suggest Perko's Helical Piles: A Practical Guide to Design and Installation. Depending on the vendor as long as you know what you're specifying you can get them designed by the vendor's specialty engineer in a lot of places, or you can just work it out yourself, especially with low loads like this scenario.

With regards to the problem at hand, it's really more an issue with the structure than the piers. If I couldn't confirm the capacity of the wall I'd likely run a header over the piers to provide continuous support to the wall somehow, especially if it may not be reinforced. Either that, or treat the whole thing as unreinforced and space based on pure compression in the concrete and the plain concrete stipulations of the code. Alternatively, you'd have to spend money and time investigating the wall.

If this is an existing building and you've got continuous settling (i.e. not just settling in the first period after construction) poke around a bit and make sure the whole building isn't settling with this corner just settling faster. You don't want to put this nice solid reinforcement in the corner and then find out in a year that the rest of the building was settling too and you made a nice solid high point.

 

[EngineeringEric]

As a standard, we don't space the piers over 6ft oc. we may nudge or change that if they are spanning a window so no real load but a start @ 8-10ft seems large. i would recommend also a minimum of 2' from the corner and under major jambs... But it all depends on what the condition is and the extent of actual settlement.

 

[EngineeringEric]

By we, I mean myself and my co-worker who also does some helical work; i do not mean as a field