Trench Footings 2

[tolind]

I'm looking for some general do's and don'ts when it comes to trench footings. Any information in regars to stability issues and any difference in strength and capacity would be greatly appreciated.

Thanks in advance.

 

[Zambo]

well what a question! there are so many does and dont's! I suppose the most important is to follow the design/drawing. You should excavate. find the soil conditions as expected by the designer then construct the foundations accordingly. THAT'S IT.

Zambo

 

[tolind]

Sorry I didn't clarify. I meant from a design standpoint. All the information I have is for the design of formed spread footings. I know that it is a lot cheaper to trench but I don't have any experience with it.

 

[JAE]

We use trench footings quite a bit - their behavior is very much like typical spread footings - just deeper. With the extra depth, you have a number of differences that should be considered:

1. The trenched footing has much more concrete cross-section and therefore, to meet ACI requirements, higher reinforcing is required. You could possibly assume that the trenched footing concrete is over and above that required for shear and use the same rationality that ACI uses for its architectural column exception to the minimum 1% column steel limit.

2. The trenched footing has generally irregular sides that worry some with regards to frost heave. With the freezing of the ground it is thought that the adjacent ground could grab the footing and drive it upwards. I know a small city near Chicago that explicitly rejects the use of trenched footings for just this reason. I don't believe that this really happens. We've used them before and have no problems.

3. The trenched footing will give you a much stiffer footing in the longitudinal length along the wall it supports. This is generally a good thing, especially when there is the potential for soft spots, liquifaction in seismic zones, or sink-hole issues. This also has the advantage of spreading out any concentrated loads.

4. Finally, with columns supported by the trenched footings, we generally just widen out the trench at the column and form a large, thick mat for the baseplate. This is one big blob of concrete - and we reinforce it as though it was a 12" or whatever thick spread footing and ignore the "blob" of concrete above the theoretical spread footing thickness. But be careful with this for exterior column pads as the top outside portion of the pad may be very close to the exterior grade and peek out with long term settlements and erosion. We typically ask the contractor to slope this top face of the column pad downward about 3 to 4 inches and away from the building to direct moisture away from the building and avoid the concrete from becoming exposed.

 

[dirtsqueezer]

One small detail for when it comes to actual construction: The irregularity of your earth-formed walls will also mean irregular clearance around your reinforcing steel. I don't think I've ever looked at steel that didn't need to be dug out or re-placed due to proximity to the walls. It's probably typical for your plans to specify three inches in this case, for the inspection that you'll hopefully have. It's a good, cheap method- just one that takes a little more care.

 

[JAE]

Yes - 3" clear is required except at the top of the footing.