We're considering have a french drain system installed in our unfinished basement to collect water that comes in every Spring where the foundation walls meet the cement floor. Our foundation walls are cement block and we've been told that due to the age of our house it's quite likely there is no footing supporting the walls. Will putting in a french drain system, with a trench as much as 15" deep that's back filled with irregularly shaped washed stone, impact the integrity of the walls? Any comments or suggestions you may have would be greatly appreciated. Thanks.