After what depth is a subsurface drain (e.g. French drain) no longer practical?

Question

We currently have a building underway and it's foundation consists of individual concrete footings extending 3m (~10 ft) below ground. It's also worth noting that the structure does not have a basement.

I thought it would be a good idea to install a subsurface drain (e.g. French drain) around the building as a safety/preventative measure. However, it seems to me that maintaining and replenishing a subsurface drain drain at the depth of the footings (3 m / ~10 ft) would be quite impractical and costly. Also, the sump into which it drains would be equally deep and hard to maintain.

What are my options in this scenario?

• Do foundations as deep as mine usually implement subsurface drainage?
• Is a subsurface drain even worth having in my scenario? (Recall there is no basement.)
• Should I split the difference and install the drain at a shallower depth above the footings?

Answer 1

It sounds like you are using a pier foundation. Piers should be deep enough that they are hitting undisturbed, solid soil or bedrock. As such, there's likely little reason to have a french drain in that situation as the footings should be well below the depth that would be effected greatly by surface water.

Answer 2

I take it you are somewhere flat, since you refer to a sump that is (or would be) deep.

I can't imagine what sort of drain maintenance/replenishment you would do at any depth that would involve digging the drain up. If built correctly they should not need any maintenance.

In a non-flat area, I would always and have always chosen to run a drain line around the base of the footing, run out to daylight (no sump, gravity drainange to the surface at a lower point on the slope.) It costs practically nothing when the hole for the foundation is open, and stops ground water problems before they start. Yes, even on footings without a basement. However, that math might change if you do not have a complete trench excavated when placing the "individual footings" (i.e., if you are more or less drilling holes for them.) In that case, you might as well skip it if you'd be incurring major expense to add it.

In a flat area you could do the same but run to a (probably exterior) sump/drywell - don't install a pump in the sump, just provide conduits and pipes to support installing a pump if one ever became advisable. You can monitor the water level (if any) in the sump.