How far out to install French drain so water can't pass underneath it and still get into house?

Question

We have Bilco doors in our backyard that go down into our basement. Our basement is cinder block foundation. Unfortunately our entire back yard slopes downward toward the house, and so during heavy rainfall we end up with water pooling around the Bilco doors. This water seeps into the ground and eventually presses up against the outside of the cinder block foundation, and over time has damaged the cinder blocks enough to allow the water to weep through and start pooling in our basement:

I believe a French drain system is the proper solution here, perhaps in conjunction with grace ice & water shield. Essentially I would trench out all the earth around the Bilco doors, going down several feet. I would slope the earth away from the Bilco doors and lay down grace ice and water shield. The bottom of the shielding would feed into a French drain (drain tile) that carries the water away from the house and into the back yard:

So rain water hits the Bilcos, slides of and onto the grass in front of the Bilcos. Seeps down into the ground, hits the ice shield, and follows along the top of the ice shield until it drips down into the French drain. From there is travels through the drain some 20 - 30 feet out into the back yard (the drain will be sloped slightly) until it falls into a big pit (that I will dig out and cover with grass) full of stone where it diffuses and spreads out.

I really think this will work. But one concern.

If the rain fall is heavy enough, I'm thinking it may still be very possible for the water that has seeped into the ground to still make its way underneath the drain and still find its way into my house:

So my question is: How far out should I be placing my drain so that this isn't a concern? Remember, the further out away from the house I go, the more rows of grace ice & water shield I will need to use, but that's OK as long as it works. Thanks in advance!

Answer 1

French drain below foundation level, problem solved. Anything less than that, problem potentially remains.

And crip659 s also correct - you should fix the grade - my rule of thumb is that you want at least 12 feet around the house to slope away from the house by at least 1%, 2% better. i.e. the part 12 feet from the house should be 1-1/2" to 3" (or more is fine) lower than the grade at the house.

Either way, or both, some major landscaping.

Answer 2

Update at the 24-month mark:

Well, I followed this solution and have taken in no water whatsoever for 24 months now. Bone dry! If it can work for me I think it is a good emergency solution for n e 1.

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Since this is Do It Yourself SE and not How to Hire A Contractor SE here is the trigonometry-based solution I'm going with, based on an assumption that rain water, once it hits the ground, will not "wick" or seep at a slant greater than 45°. I base this on a question I found off of Physics SE. It may not hold true in extreme case such as if we have a persistent torrential downpour for many hours, without relent. But if that only happens once or twice a year, I can live with this as the "permanent, temp solution".

My foundation goes down 7'. If I draw a right triangle we will see that a 45° angle is formed 7' away from my bilco doors. This means that rain water hitting the ground 7' away from my bilcos should almost never wick/slant all the way to my basement:

So then, all I have to do is provide grace ice and water shield all the way down from the right angle to the hypotenuse, and place the drain on the hypotenuse, which is 3.5' out and 3.5' down.