[Southengineer]

Darn river clay won't hold up for foundation pouring. I need 1500 psf, what do you guys recommend using to strengthen my soil or provide a good base for my concrete foundation? It's only holding up a modular office building, so it's a stemwall & concrete pier footings, but curious what options there are to strengthen the existing surface.

 

[SLSTech]

Your an engineer?

Larger footing to spread the weight out or dig deeper & hopefully find better material (probably not going to be there though)

 

[Southengineer]

Not a solid slab footing, or else making it bigger wouldn't be a problem. Also environmentally controlled soil, so we're avoiding extra digging at all possible. The options I've got available are basically

1. Digging down 2 feet, refilling those 2 feet with compacted DGA and moving forward.
2. Digging down 2 feet, refilling those 2 feet with flowable fill and moving forward.
3. Soil Cementing

I was just curious if anyone else had other ideas. This isn't my job on the design, I'm the project engineer, so the footings aren't mine to tweak in terms of size. I just have to make sure they don't end up in China.

 

[SLSTech]

Have you looked into these http://www.bigfootsystems.com/ for the piers?

The first engineer should either already know the numbers or modify the footings based on your soil tests if their assumptions are wrong

 

[CookeCarpentry]

Not a solid slab footing, or else making it bigger wouldn't be a problem. Also environmentally controlled soil, so we're avoiding extra digging at all possible. The options I've got available are basically

1. Digging down 2 feet, refilling those 2 feet with compacted DGA and moving forward.
2. Digging down 2 feet, refilling those 2 feet with flowable fill and moving forward.
3. Soil Cementing

I was just curious if anyone else had other ideas. This isn't my job on the design, I'm the project engineer, so the footings aren't mine to tweak in terms of size. I just have to make sure they don't end up in China.

Are these your options or the ones the engineer came up with? If you can't tweak the footing designs, why worry about it?

 

[Southengineer]

I don't know what the architect firm is going to come back with. There's a possibility they may resize footings. Or they may require the flowable fill. I'm simply trying to run some numbers on different options so that I can find the most economical solution without seriously overdesigning.

 

[Southengineer]

Have you looked into these http://www.bigfootsystems.com/ for the piers?

The first engineer should either already know the numbers or modify the footings based on your soil tests if their assumptions are wrong

Those are very similar to the "Sonotube" footing we're currently planning on installing actually.

 

[bconley]

How about a landscape fabric "geosynthetics" with spalls or other structural fill on top

 

[Southengineer]

Bconley, the only definition of "Spall" I'm familiar with is more affiliated with shrapnel than structural fill. And I don't think a geotextile will be of much assistance in this case. Could you explain a little more?

 

[bconley]

I'm not an engineer, but I have built some projects on fill spec'd out with a heavy fabric with rip rap on top, my understanding is that this acts as a system and distributes the load over a greater area.
It also creates a drainage plain.
They use it in road building all the time.

 

[bconley]

Or auger cast piling with grade beams

 

[ContrBob]

Helical Piers

There are several companies that make helical piers.

They look like an auger and are hydraulically screwed into the ground using an attackmet on a backhoe, trackhoe or Bob Cat. AB Chance and Fast Steel are 2 companies that sell these to contractors. I'm sure there are others.

They get screwed into the ground every 6, 8, 10 feet or so apart depending on the load they need to carry. After you screw a lenght into the ground, you bolt an extension onto it and screw it down further. This continues until the peir won't screw down anymore because you hit bedrock or until you get to an acceptable PSI in pressure.

There is a flat plate on top to sit inside your footer forms. You just pour your normal footer when done. Engineers recommend these when ground vibrations from pounded piers might disturb nearby structures. They work great.

 

[Southengineer]

I'll have to look into the helical piers, those could become useful on site. The architect has come back with a solution that I'm willing to comply with, so we're back at work this week. But I appreciate your suggestions guys, I'll be working in soil like this for a while, so I like having been able to look at a few other ideas for future work.

 

[mics_54]

you need 1500psf... what is it now?

 

[chrisbowles]

what was the architect's solution

 

[bconley]

Pin piles, might work as well

 

[Southengineer]

It's been 1100 at surface, 6", 12" and about 1200 @ 18", but we got 1500 about 24 in down. So architect has instructed we remove 24 in and retest, if it passes we'll fill the 24 in with flowable fill, and then after it sets come back in for foundation work.