My builder who manages the project was worried about digging all the way down to the bottom of the foundation. There is a retaining wall about 6 feet away that has half ton boulders that go up 15 to 20 feet, holding neighbors house above it! The worry was that digging such a trench could undermine that and we would have far bigger problems. There was also some worry of the concrete wall holding our basement being undermined with soil now being loose around it.
My only choice is then to tear up the wall in our theater to look from inside. This wall was specially built, double wall on flexible channels for noise suppression. If we tear down that wall, it will be expensive to repair.
Maybe I wait until it fails again and then go after the drywall. I almost pulled it down during the flood but was too worried about the rest of the floor so spent our energy pumping that out.
With respect to the retaining wall, if excavating your foundation undermines it that means that retaining wall is putting lateral pressure against the foundation wall of your house. That would be a poor design, and should not have made it through any Code approvals/ inspections. If it was built without permits, etc. then who knows, though. In fact if that wall does load your foundation wall it may have caused some cracks.
Anyway, I thing you will get useful information removing at least some of the interior wall finish system. At a minimum make some holes in it and look around as much as you can with a borescope.
It is cast concrete. I was under the impression that it is always better to avoid water getting to the foundation than trying to block it. Are waterproofing measures effective? Is it as simple as slapping some compound on it?
Knowing it is a reinforced concrete wall means the water is either coming through cracks/ joints penetrations -- which can be sealed with about a 95% success rate using gel injection techniques from the interior -- or it is coming over the top of the wall. Water testing can help confirm this, but ideally you would have at least some of the wall covering system removed in the area where leaks were experienced. Concrete itself is effective waterproof (yes, it has some permeability but you can pool water on a concrete deck and not get any leaks through the concrete), so the concept of full exterior waterproofing is not nearly as critical as if the wall was reinforced CMU.
It is correct that it is best to avoid getting water against foundations, but that is rarely possible for the whole life of the structure. Waterproofing + foundation drainage + good surface drainage is the key to keeping out water for the long term. But properly installed waterproofing and foundation drainage should be able to make up for suboptimum surface drainage.
Think of it this way: Good surface drainage (which you may now have thanks to the contractor) + deficient foundation waterproofing + poor foundation drainage is like having a transparent source, but speakers that only score a 1.0 and there is no room treatment. You'll feel good about the DAC, but you will be on edge whenever the system is in use. Conversely good surface drainage + good foundation waterproofing and good foundation drainage is like having a transparent source with speakers that score >8 combined with optimized room treatment and properly integrated subwoofers. You will smile whenever the system is being engaged.
It is possible to waterproof structures that are below the water table, but it is not as simple as "slapping some compound on it". The proper membrane system needs to be selected (there are a LOT of them, and many used for residential applications are essentially Snake Oil), it needs to be installed by professional waterproofers, and it needs to be installed within the environmental constraints outlined by the membrane manufacturer. That being said, most submerged/ partially submerged structures have allowances for minor amounts of leakage without impacting interior finishes. But for structures that are not submerged adding in a functional foundation drainage system removes most of the hydrostatic pressure, which helps keep the membrane system well within it's performance envelope. Good surface drainage is just the icing on the cake.
But again, since the wall is reinforced concrete targeted repairs from the interior should be feasible (water test first though to make sure the leaks do not originate higher). The cost of interior finish replacement is usually at least an order of magnitude less than excavate the exterior.
