How to seal bottom of rubble foundation - 1726 New England home

[dknight524]

Hi,
Our town recently did a smoke test of the sewer lines in the historic district. I observed smoke coming from the bottom of one part of our foundation, a section about 2 feet across. Our first reaction was an issue with our sanitary line, so we had a company come out and scope the line all the way out to the street sewer line. Inside the home it is cast iron, but just outside it is clay, which is common for most of the homes in the historic district. The scope came back fine, minor hairline cracks and a few joints loose, but no obstructions or clay pipe needing repair. So, what I'd like to do is seal the bottom of the foundation where the smoke came in (we do notice an odor in the home at times). The home sits on a rock shelf, with soil in the low spots. The basement is standup in most places, with the stone shelf visible. I have cleaned out the area where the smoke came in, and though it is mostly rock, there is some soil as well. I've linked to the pictures below, so you can see what I mean. The smoke came out from the dark area on the right side of the picture, from under the rubble foundation. You can see, to the right of the sanitary line, previous owners put in a concrete pad, right up to the line. That side is solidly sealed. What is the best approach to sealing the other side? You can also see the fresh water line comes in around that area too. I'm thinking any solution will need to extend to the rock shelf to the left. I most likely will hire someone, but wanted to get an idea on best solutions.

https://photos.app.goo.gl/mD4qKf3Nf9t88fW6A

[V-Man]

If the only area that you need to seal is the gap it seems that you could dig out some dirt and fill it with some more fieldstone and mortar. It doesn't appear (at least from the picture) that such a small repair would require any special footing or engineering brain power. The water seepage should probably be investigated as to the source and whether or not sealing the gap will cause the water to go elsewhere in the foundation and cause other problems. Just beware that most contractors will recommend a portland based mortar and I personally would be leery about that, because of the problems portland based mortar will cause to lime based mortar foundations and particularly the known water seepage. If you seal out the water seepage with portland based mortar, guaranteed it will go elsewhere and eventually cause other problems to the foundation at this location and/or elsewhere.