Dig out the leak from the outside your foundation walll (hopefully the leak is not under a sidewak or something like this). After the leak is exposed, seal it with some sort of caulk that is rated for damp conditions. I do not know precisely what sealant to use, but someome else who replies probably will. You might need pure silicone for this. Make sure that the place you apply the caulk is free of debris. If you caulk a disintegrating mess, you will be caulking again in a few months. If there is concrete breakage around the pipe and perhaps radiating down from it, you might want to consider hydraulic cement.
If you can get to the pipe from outside your house, do not waste your time messing around with the leak from the inside. The trick to stopping water from getting in your house is getting to it before it hits the foundation, not after the water has penetrated it.
You should also check to see if the area around the leak has sufficient drainage. Grade should be sloped away from the house, and make sure a gutter downspout is not dumping water in the suspect area.
When you backfill the hole, do not dump in clay (if you removed clay in the first place...)
mh