Structural reinforcement & crack repairs in +100y old Greek house

First time here so hello everyone!

We have a house in Greece that is at least 100y old and its built from big (and small) stones with what looks like red sand in between the stones. The sand is very soft and can be rubbed away with the finger very easily. The roof is "new", maybe like 40 years and made from concrete and in good condition, so is the floor. We plan to make an extension to the house and that required excavations on one side of the house. It was made with a rock drill attached to a tractor… The excavation happened very close to the house and naturally removed some plastering from the lower part of the wall, exposing it to rain. And most likely the vibration and changes in support caused some cracks and movement of some stones on the corner of the house.

Now the plan is to fix the cracks, renew the plastering and perhaps most urgently to add some more support to the corner of the house, it looks like it might collapse with more rain and minor earthquakes that happen here occasionally.
What kind of support would you suggest and what kind of materials?

We have an engineer who suggest to drill stainless steel rods to the base rock and the big corner stones, glue them with epoxy and to make a some sort of concrete support to the corner… For the cracks he suggests lime based mortar in a quite liquid form to reach deep in the cracks… Any thoughts on this?

Im very conflicted the engineer, who is an experienced builder, but has very limited experience in this kind of renovation is insisting on using concrete to fix the corner. His logic is that to prevent the outward movement of stones, something very strong is needed. From what I read, natural stone buildings and old sand/claymortar (if its even mortar thats between the stones) don't go well together, but is it really a crucial issue in this case.

I have also been using ChatGPT's "Heritage Sage" gpt (attached), which at least to an untrained eye seems convincing. But as always in important matters it needs a human eye to verify the results. So any comment on this would be valuable.

Thank you in advance!

It could be repaired in a traditional way by surrounding the wall stones with large stones, use mortar with similar properties and recompact the area around the stones VERY WELL. The original foundation was probably built in a trench of virgin ground, so it was/is surroundad by nature-compacted solid substrate.
Concrete and nice stainless/epoxy would be good too, but at the same time it feels like some incompatible patch. Tieing the old stone with bars and concrete will surely reinforce them, but it may shift the problem to other parts of the wall.

The addition being built will probably have concrete foundation that will reinforce the old wall on entire exposed length. My suggestion would to use the new foundation as the support wall for the old foundation. This way it will mimick the support of the compacted soil that was removed. The less intrusion into a building that is otherwise fine, the better. Also, it's not the building's fault that its surrounding ground was disturbed and damaged something that survived 100 years.
For the cracks I would suggest to use some similar mix. Lime is a wonderful material, but the original mortar is more compatible with the wall.

At the same time I understand that the wet season has started and my suggestion may not work due to time constraints.

Greetings,
I have not worked with stone such as you have (I have done lime-mortar work in soft brick), but I can confirm that Portland-based cement is not compatible with soft lime-based cements, which are typical in century-plus old construction. The Roman pozzolan cements put all modern materials to shame, in spite of being "weaker". Injecting a fine lime/sand glue mortar can be effective, but you do not want to inject at a high pressure (lest you float the joint), and you will get better penetration into the crack with modest vibration (but again, gently).  If the rock is lime-based, which seems likely enough in Greece, lime is more compatible. If the rock is granitic (which I hear happens in some places), then a more aggressive cement might be OK, but I think in all cases, lime would be an acceptable choice.

Epoxy + stainless will indeed create a new load path, which might help, but as Cristobal noted, this is somewhat likely to just shift load paths into unexpected places - as in, it will take a system that relied only on compression and cause some of it to rely on tension and bending (which stone does not like). Gravity compression of adjacent materials is going to develop better distribution of load against the wall. To avoid outward motion of the foundation, you need pressure, not strength, and you want it to be distributed evenly over the older materials. This could be done with concrete buttresses with a backfill against the wall, if you like how they look, or it could probably be done with rock-filled gabions, or other things, but in any case you probably do need to key some reinforcement into the ground/bedrock, so you get shear resistance, whether a footer trench or vertical pins (of steel or concrete), I can't guess from a distance. Cristobal's comment about using the new foundation as the retaining wall to buttress the old is quite reasonable.

I am a PE over here in the US, and for that reason, I urge you to take your engineer seriously, but not too seriously. Don't be shy about asking questions, doing your own research, finding other case studies, and talking them over in a conversational, curious manner. There are very few experts in this kind of thing, but remarkably, old buildings have survived quite a lot of expertise over the years. Keep clear in you head the important distinctions between pressure (distributed loads), forces (point loads), and strength (material properties), and aim for a gradual load path change - ie, a very strong material with high forces embedded in a weak material that requires pressure loads is risky. This sort of thing has a lot in common with putting pins in bone or putting nails in wood than with putting rebar in conventional concrete. The act of drilling for the reinforcement rods could itself cause unexpected problems. The sooner you can protect that exposed footing, though, the better. Godspeed! Kali dynami!

Mark


"Et facta est lux."