Moles come to mind right off and although their tunneling can uproot plants they do not eat them. Moles primary source of food are earthworms, which is an indication of a healthy soil. Gophers also will tunnel, but you will find entry and exit holes in those tunnels that will not be there if it is moles. Gophers will eat your plants as will voles, Field mice, that will use the tunnels of others. The most effective means of control is a good, tight fence around the garden with some of the buried at least 1 foot deep with a leg, at least 1 foot long pointing outward from the garden, a backwards "L". There are numerous poisons that have varying degrees of success as well as various repellants that have varying degrees of succes, all of which can be very expensive to purchase.

I once bought a semi-detached three-storey 1890s home that featured a lowered basement floor. When I gutted the home, I found that there was no concrete floor and the lowering had been accomplished by digging beneath the footings the entire way around. I had an engineer design a supporting wall all way around the interior, complete with rebar parallel and at right angles to the wall. You don't say how "small" your "spot" is. At the least, it should be dug out and underpinned for that distance--as long as it is less than 4 feet.

I've put pipe underneath sidewalks and retaining walls by hammering through a piece of larger pipe, pulling it out (not exactly easy), and then inserting the pipe after taping off the end to keep dirt out. That's still a lot of digging. I'd also be concerned about damaging the basement wall by chiseling a hole through it. I'd use a core drill to bore a neat hole through the wall. They can be rented for about $150 to $250 a day, including the bit, depending on the size.

Good morning Anthony, our firm specializes in this type of work in California. We suggest you consult with a geotech firm to speak with a soil engineer and it is essential you consult with a structural engineer. This will help you learn more about the soil you referenced. Any good contractor and knowledgeable structural engineer can address your concerns and tell you what they are doing to fix the things you may worry about. We also suggest you find a reputable design-build firm, they will be able to coordinate consultations for you with their trusted resources. Less stress on you and well-rounded field of experts.