Is it normal for an exterior block wall to just sit on the slab edge?

You are looking at a classic case of thrift-building that was surprisingly common from the 1930s thru the late 1960s, especially in regions where codes were lax or non-existent. Builders back then would often just pour a slab and stack the CMU right on the edge, or maybe do a thickened edge that was barely thicker than the floor itself. It isn't necessarily that they were trying to scam anyone; materials were expensive and money was tight, so they did what they could to put a roof over a family's head. Those horizontal cracks along the bottom courses are a major red flag for **hydrostatic pressure** or soil movement. Since there is no footer to distribute the load, any change in the moisture content of that packed soil under the slab edge is going to cause immediate shifting. When you started digging, you likely relieved the lateral pressure that was holding the soil in place, wich is why the wall is starting to show its age now. If you keep digging out the perimeter all at once, you're looking at a serious risk of the wall blowing out or dropping several inches.

To fix this properly, you need to look into **underpinning** the wall in sections. This involves digging out small three-foot wide sections at a time, several feet apart, and pouring a proper footer beneath the existing slab edge. You let those cure, then move to the sections in between. It is back-breaking work involving alot of time in the dirt with a shovel and a rebar drill, but it is the only way to stabilize a masonry wall that is essentially floating. Whatever you do, don't just patch the cracks with mortar and walk away. Without a solid base, the patch will just pop back out or the blocks will shear further next season. The priority has to be getting a solid foundation under that masonry and then managing the water runoff so the soil stops shifting under the weight.

You should also verify the local frost depth because a three-inch slab edge is utterly incapable of resisting the vertical displacement caused by frost heave, which likely exacerbated those horizontal fissures as the frozen moisture in the soil expanded against the unreinforced masonry. While the previous poster correctly identified the need for underpinning, I would suggest specifically considering a series of helical piers if the soil composition reveals a high clay content, as traditional poured-in-place concrete underpinnings might still succumb to the same localized subsidence if the underlying strata aren't sufficiently load-bearing. It is precisely the lack of a proper frost-protected shallow foundation design that makes these vintage builds so precarious once you disturb the surrounding grade.

I dealt with this exact mess on a garage addition a few years back. The wall started sagging the second we hit a rainy week with the dirt exposed. What acutally worked for me was drilling rebar pins into the side of that thin slab before pouring the new footer sections. If you don't tie the new concrete to the old stuff, they’ll just move independently and the cracking won't stop.

I also used a non-shrink grout to pack the last inch under the wall once the footer cured. It’s miserable work, but my wall hasn't moved a fraction of an inch since.

Honestly, I'm surprised it didn't settle sooner. If you've got clay soil around there, you’re in for a real headache once it gets wet. Just a heads up—don't leave that trench open if there's a storm coming. If the water gets under that slab edge now that the dirt's gone, it'll wash out what little support you've got left in a heartbeat. You ever thougt about helical piers? They're way faster than hand-digging footers, though they'll definatly cost you a bit more.

If you're going to follow the advice about underpinning in sections, you need to make sure the concrete you pour can actually take the load quickly. I'd definitely use Quikrete 5000 for this. It has a much higher compressive strength than the basic mix and sets up fast, which is critical when you've got a floating masonry wall hanging over your head. Also, when you're pinning that slab, don't just shove rebar in a hole. Use Simpson Strong-Tie SET-3G anchoring adhesive. It’s the only way to ensure those pins actually bond the new footer to the old slab so they don't just pull apart when the ground gets saturated again.