I have a home with a block wall foundation and adjacent garage (built on a slab) on the left side facing the house. The house sits on a grade with right side to left side drop of 3/4" to 1" per foot. In November of 2015, we had our crawl space encapsulated and installed an interior french drain (perforated corrugated pipe laid on gravel wrapped in landscape fabric with a sump pump).
The drain works great in bringing water down the front and back walls due to the natural slope, but we are having issues bringing it from the back of the house to the front on the garage side where the sump pump is located. Four months after it was installed, we discovered water was standing against the block wall foundation in the middle of the wall adjacent to the slab.
The encapsulation company surmised the original trench running along that wall did not have enough drop so they re-dug it. Both times they "eye balled" the slope rather than using any type of measuring device.
In the past year, the garage slab has cracked, dry wall cracks have appeared in multiple places in the garage, and the garage entry to house (which is built on the slab) noticeably dropped. All this seems to suggest the drain is affecting the garage slab (the home is 34 years old and the soil is not expansive).
Since 90% of the water entering the crawl space travels down the back wall, I suggested replacing the corrugated pipe running from the back wall to the front (sump location) with a solid PVC pipe, which will prevent water loss and facilitate achieving a consistent drop. My theory is that water is flowing out of the corrugated drain wherever there are low spots (since trench was "eye balled") and that water is compromising either the block or slab foundation. The encapsulation company agreed, but wants to put a 20 mil barrier between the drain and foundation which will extend to the bottom of the drain. They also want to leave the corrugated drainage pipe in place to assist in absorbing water and directing it to the sump pump (the solid pipe would be installed over the corrugated pipe). I am going to pay them to do this, but they are doing it for their cost. Does this sound like a reasonable solution? If not, what would you suggest? Any insight you can provide will be appreciated. Thanks!