How can I prevent interior French Drain from further eroding soil under garage slab?

Question

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!

Answer 1

Using perf pipe allows water out just as it allows water in. The water stain half way along the garage wall could be from water backing up and out of the perf pipe. I like the solid pipe idea for "transporting" water, if you're not trying to "gather" excess water. Here we can buy one-half perf and one-half solid pipe. We install it with the solid side down and when the water table gets high enough to reach the mid-point of the pipe, it flows into the pipe. We just install the mid-point of the pipe at the level we want to keep the water table at.

How about moving the sump away from the garage to the other end of the house? Then, when it starts pumping, it won't erode (suck) all the soil from the surrounding area out the drain. Also, maybe a sump pump that moves water slower or comes on more frequently so as not to impact the drainage system so severely each time. (It could cut down on the washing of the "fines" (soil or sand) out of the french drain and from under garage slab.)

If you don't have a perf pipe along side the garage, you'll run the risk of water seeping into the garage.

Also, look at separating your water. Remove all your downspouts from this collector system and use it strictly for subterranean de-watering. Rain tends to come quickly and thus flood the system...perhaps backing up and out of the perf pipe.

Erosion is caused by water "washing" across a surface. If the material under your garage slab is dirt (or sand), then it'd be easy to erode. We use crushed rock. Can crushed rock be placed back under the slab, or is that too difficult?

Last thought: hopefully your installers used washed round rock (or crushed rock WITHOUT the fines) for your french drain. The key here is without the fines. For material to wash away from under the garage slab, it has to have a place to go. If it's confined then it won't wash away. However, if the french drain was installed using fines, then the fines in the french drain would wash into the perf pipe allowing the material under the slab to "sluff" away. Dumb!!! Are these guys licensed to do this kind of work? At least you have filter fabric encapsulating the entire french drain, right?

I'm not sure what the solid sheeting between the garage and house is suppose to do.