[maccoinnich]

What I mean is that the soil probably has nothing to do with the winter garden, and is likely being used to support the structure of the building.

Most buildings don't rest on the bedrock; their weight is at least partially born by the soil. For a lightweight building such as single family house the soil in most parts of the city is strong enough to bear the entire weight of the building. For heavier buildings, or in areas of the city with soils known to be poor, the city will require a licensed geotechnical engineer to provide a soils report, which provides guidance to the project's structural engineer. In the report there will be recommendations about whether the existing soils should be used; or not. In an area like the Pearl—a former swamp with a long industrial past—I wouldn't be surprised if poor soils were encountered. If so, they might have over excavated, and then replaced the existing soil with "structural fill" (the new soil) to support a mat foundation at the basement level.