Since they're building an apartment complex up the street, I'm getting a view of how the construction progresses, as well as what the soil subsurface looks like in the neighborhood. I had never thought much about ground stabilization where it's critical. In the photo, you can see the concrete ledge they've created. First they dug down to that level. Then they drilled holes all around the edge. They poured concrete into the holes and to create the ledge, but apparently had forms to make the columns hollow - as you can see wires protruding from each "pillar" which was underground when it was poured. Once that concrete hardened, they dug below the surface concrete, which now appears as the ledge. The columns (which were holes with forms inside) are now visible, and they ran what looks like electrical conduit through them to supply the parking garage lighting system. I recall hearing about a project in Indianapolis (USA) that didn't use adequate stabilization, and a nearby building settled and cracked as a result.
As you can see, the soil is pretty solid clay. We're basically on a large hill, so when it rains the water drains off pretty quickly. Our back yard has dirt that seems much more fertile, but from the construction site I can see our soil is probably just a thin layer that may have been brought in. We've had a couple of heavy rains since they started digging, and the clay accumulated surprisingly little water. I think they have some deeper holes on site that catch the water or may connect into subterranean drainage.
