Best way to tie a floating slab to main slab?

[solid7]

I have a project that involves tying a floating slab (approximately 4" thick by 4' X 12') back into the main slab of a home. The main slab is 6-8" thick, and is the foundation of the home. The home is in a beachside community, so it is built on sand. Apparently, the cause of the float was twofold:

1) no initial underpinning and tie-in
2) no gutters on the home

The lack of gutters appears to have caused undermining of the sand around the base of the slab, resulting in a shift in the entire slab. (and a complete break-away from the foundation)

I have already had the slab underpinned, to prevent any further movement in the vertical direction. However, because there had been some repair on the main walls, (this was hurricane damage) it was decided not to pressure grout, so as not to cause damage to the walls or roof. (as the repairs were made in the "sunken" condition)

So my question is: what would be the best way to tie back into the foundation? While the front of the slab is supported, the back half is not, and should, I believe, be tied in properly.

My idea was to put in Z-bent rebar every 12-15", and run it half the depth of the floating slab. Is this sufficient? Or should the slab be also underpinned on the back side? Additionally, there is a 1/4" - to 3/8" gap running the full length of the slab, full depth. Should this be filled with and expansion joint, or a full tie-in repair be made with the existing slab?

thank you in advance.

-----------------------------------------------------------
Catia Design|Catia Design News|Catia V5 blog

 

[msquared48]

Is the floating slab for mechanical equipment, such as a heat pump, serving as an isolator or additional mass to counter vibration?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[solid7]

Nope. It is a actually a bay window.

-----------------------------------------------------------
Catia Design|Catia Design News|Catia V5 blog

 

[msquared48]

I would put 2 or 3 pin pile under the edge of that slab too, and epoxy grout some rebar into the existing slab at the centerline of the existing slab. I would not cantilever it. That way any settlement due to the pile system, which should be very minimal, would be similar. I assume no keyway was provided for the connection

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[solid7]

No keyway.

I had suspected that perhaps another piling was the way to go. Not an ideal situation, but do-able. What should the dimensions of the pilings be? and for the center (inside) piling, can I drill a hole in the slab, and work from the inside? My assumption is just that I go down to stable soil? (which is about 3' in this location)




-----------------------------------------------------------
Catia Design|Catia Design News|Catia V5 blog

 

[msquared48]

2" to 3" diameter pin pile work well driven to refusal with a 90# jack hammer. Depending on the recommendations of the geotech involved, and I trust you have one, they should be able to support 2000 to 4000 pounds each.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[solid7]

I am actually considering now whether (for the size involved) if it might not be better to skeletonize the slab, (leave it about 10" thick to the outside wall") and put rebar into the slab, and repour the inside area. Would you think this a better option? It would let me pour proper footers, and tie into the slab at the same time.


-----------------------------------------------------------
Catia Design|Catia Design News|Catia V5 blog

 

[Ron]

First of all...why? Are you concerned about lateral drift, vertical faulting or both? Your loading on this slab is probably significantly less than the adjacent slab. Your idea of removing the inside of the slab and letting that serve as the tie, is probably the better one unless you take Mike's suggestion of the pin piles.

 

[solid7]

Why? Because it has already happened, and once it is fixed I don't ever want to have to deal with it again. The original work was not mine, but when I do it, I want it to be right, and as good as new.

As mentioned previously, this is in a coastal area, built on sand, and the slab has already drifted and sunk. (significantly) It isn't so much about the bearing load, as the instability of the ground it's built on. The structure faces an easterly direction, therby making it prone to driving winds and rains from that direction. (think hurricane or tropical storm) This is what was mostly to blame for the original damage. So my fear is that with soil saturation, further settling will occur behind the current pinning. (which is only on the front of the slab) I just want to reliably tie the slab back into the main slab.

The idea of putting in the pins is sound, but the work has to be performed from inside, and it almost seems easier to cut old, and put new slab. I could be wrong.

-----------------------------------------------------------
Catia Design|Catia Design News|Catia V5 blog

 

[Ron]

solid7...no actually I think you're probably right; however, you still have to correct the unstable soil issue. You might consider some compaction grouting, expansive foam injection and/or the pin piles.

Tieing the new slab to the exist fdn will be easy, then you can dowel the perimeter of the old slab to the new.