Hey Paul,
I'm new to the forums and just reading through, so I'm resurecting an old post, but maybe I can shed some light here. The reason that a french drain works with the configuration as you described in your previous posts, is by the priciple of hydraulic pressure. When soil becomes saturated, it loses it's structural stability and acts in a fluid way, much as a mudslide. The weight of the saturated soil above the bottom of the drain"s "grade" exerts pressure on the water in the soil. The drain, being an area of lower resistance, allows this hydraulic pressure to force the water in the soil into the void of the stone and further, the pipe when the volume is sufficient, regardless of where the holes in the pipe are located. Sort of like osmotic pressure and cells in the body.
Thus the drain allows the soil to maintain some amount of structural strength, keeping it from caving in your basement wall or allowing your concrete footings to subside into the goop. Let's say your footings for your house are 36'' deep and your french drain is 60'' deep. the soil at 60'' deep won't be able to contain a lot of water because of the weight of all the soil above compacting it and forcing the water upwards. Since your footing are 36" above the french drain they rest on 36'' of stable soil, allowed to drain in the french drain. Even if the soil at 60'' deep is saturated, that dry 36'' of soil above that level supports your footing and distributes that force over a very large area and prevents that footing from sinking. This functions much the same way in a basment wall situation, except those forces are exerted laterally, by the soil against the foundation wall. If you are unlucky enough to have a concrete block wall with open cells, they are not designed to withstand lateral load; Only the dead load exerted by the weight of the home. I've had to repair several of these.
This is why you would install a french drain above a structure on a slope. It allows the soil uphill to drain and maintain it's structural strength and to intercept water running down to saturate soil under the structure. Unless the drain runs around the structure allowing the water to emerge somewhere below the structure, it really doesn't function properly. Unfortunately these systems only work up to some reasonable theoetical limit and I doubt any system will handle massive (i.e. 4'' rainfall per hour) for a sustained period. In this situation a swale uphill of the french drain would be of great benefit. I do construction and remodeling for a living.
Picture attached shows what we ideally like to install. Item 1 in the drawing would be some type of EPDM rubber membrane around 1.5' deep, allowing landscape plants to set down roots but channelling large volumes of water to the french drain. Hope this helps!
