How Deep Is Too Deep For A Shallow Footing?

[enigma2]

I have a project with columns of a drive-thru canopy designed to be set on shallow, isolated footings. However, there is a 10'-0" high utility tunnel that runs directly beneath the canopy. The top of the tunnel is 15'-0" below grade. Even though the canopy columns are fairly lightly loaded (max. 45k on 5ksf soil) I have no data on the tunnel design so I cannot assume it can take the surcharge load from the soil. Since there are no deep foundations on the project, I'd like to avoid that route. However, if I drop the shallow foundations down to minimize influence on the utility tunnel, it looks like I need about a 20'-0"+ pier on top of the footing. I'm a bit uneasy with this. It seems this is beyond the realm of a "shallow" footing. Any thoughts?

 

[oneintheeye]

too deep to be economical. Just pile it. Consider how much it would cost to excavate that far including protecting the tunnel and then cast concrete that thick including ptotection against the thermal effects of concrete that thick.

 

[CarlB]

Does the canopy have a grade slab? The increased load on the tunnel from that(~75 psf) will be about the same as the canopy/footing weight, and together about the same as adding 1' of soil. Check with the utility company, they may have no concerns. I would have thought the utility company would not allow any structure over it, protected by an easement...

 

[enigma2]

It's a private utility on government property, so everything's a little different. They don't allow any enclosed space over the tunnel but they accepted an open, detached canopy. There is a drive slab that does go over the tunnel. Again, it's 15 feet down to the top of the tunnel, so this load is pretty well dispersed. The center of one of the canopy columns comes within 4 feet of the tunnel.

herewegothen makes a valid point. It is a stand alone canopy. I'll have the contractor run some numbers. Even with mobilization of equipment, it may be more economical to go to piles. They may be willing to go this route for a small portion of the project

 

[BigH]

Not sure I am catching it all - but for 45k on 5ksf soil, this implies a footing of about 3ftx3ft. Stresses from the footing, say founded at 3 ft spread out over 12 ft would be low. The tunnel shouldn't really feel any more "stress". If you are worried, I might suggest the following as compared to piles. Dig out say 5 to 6 ft of soil and backfill with a foamed grout such as Elastizell - which weighs in the order of 30 pcf compared to soil's 125 pcf. It has more than enough strength. This will reduce the actual stresses imparted to the soil thereby lowering even more the stresses at the top of the tunnel. I've used Elastizell before - effective.

 

[Ron]

Agree with BigH...besides 45k on a canopy??? Seems a bit high. I've done a lot of canopies and that kind of loading is extremely rare.

One caution on the Elastizell...it has a high enough compressive strength that is can act like a footing block, so while it reduces the total stress, it does "throw" it lower in the ground. You can balance the effect, though.

 

[oldestguy]

On the piling alternative,do not go for driven piles. You may cause damage to that tunnel.

For this alternative, I'd use a grout type pile on either side of the tunnel and strap the tops together with a concrete beam upon which you set your column.

By the way do you know exactly where that tunnel is? No point in drilling into it.

 

[enigma2]

The owner has agreed to look into the economy of deep foundations. They have a specialist on board who is working with the soils engineer to determine the best installation for the subgrade--but now it's looking like augered piles. The tunnel location is fairly well documented.

I like the idea of adding straps to provide some additional stability

 

[dirtydude]

With the relatively light loads applied to deep foundations, perhaps helical piles might be a more economical approach for your solution.