Underpinning and Grade Beam Design

[AlpineEngineer]

I have a structure with a below grade basement all supported on spread footings. Due to soil expansion we wish to underpin the structure with helical anchors and hydrovac soils from below the footer and install void form below the footer. We are esentially turning a reinforced concrete basement wall and footing into a caisson and grade beam system. My question is, how far can we space the helicals?

The factored gravity loads are minimal, less than 2KLF, the helicals are capable of 35kips. It seems to me my limiting factor is the capacity of my reinforced concrete basement wall that I am turning into a grade beam. How do you analyze this wall? Do I analyze it as a deep beam? I've only got #4 horizontal and vertical at 24" o.c .. I've got no horizontal steel in my footer.

Can I just analyze it as a normal beam? The helical guys suggest prescriptive spacing but it makes me nervous having never done this before.

Thanks

 

[hokie66]

The wall will act as a deep beam, so just use a truss analogy (or strut and tie method). My first guess would be to space them to match the wall height.

 

[dm3415]

Like Hokie66, I would go with a strut & tie model, (check ACI for height/thickness limits as you may run into those) but since you don't have horizontal steel in your footing (weird) I would base my analysis on keeping tensile loads in the wall below the modulus of rupture of your concrete, with some phi factors thrown in for good measure. At the minimum I would apply this concept to the bottom of the wall if you are sure there is adequate steel in the top of the wall.