Fieldstone and limestone foundation walls are the most common foundations found in homes that are approximately 100 years old or older. They were constructed by using mortar to bond the stones together as the stones were stacked. The type and strength of mortar used at the time the wall was constructed is always questionable.
As these foundations age the mortar deteriorates (turns to powder) and the walls lose strength, possibly causing them to develop cracks and bow inward. The inward bowing is caused by the lateral or horizontal earth pressure pushing the weakened foundation wall inward. Lateral earth pressure increases significantly as the soil becomes saturated with water. With these older stone foundations the problem is made worse as perimeter drain tiles were rarely installed. This is usually evident as the basement area may be wet.
If the mortar joints are deteriorated but the foundation wall is not cracked or bowed, and the objective is to simply extend the serviceable life of the foundation walls, then the solution may be as simple as cleaning and tuck pointing the mortar joints. If water penetration is an issue, then the exterior may have to be excavated and a perimeter drainage system installed. At the same time this is done, the exterior of the stone foundation walls should also be cleaned, tuck pointed, parge coated (covered with a layer of mortar), and water proofed. Care must be taken when back filling stone foundation walls so as to not damage them with the impact of the back fill material.
If the mortar joints are deteriorated and the wall has developed cracks or is bowed inward, then the foundation may have to be replaced or reinforced with a new concrete wall in front of it. Unfortunately bracing the foundation wall with vertical steel beams is not a viable option as the wall will continue to bow and crack between the vertical braces.