Earthwork construction is the process of compacting the soil on a particular site for the purposes of building a road, house, warehouse or bridge, etc. While the natural ground usually doesn't require compaction, one cannot know if a particular site is really the natural ground and in most urban areas it most certainly is not. Also different soils take different loads so that the soil may require removal and compaction in order to reduce subsidence once a load is placed upon it.
When speaking about earthworks for construction, it is critical to determine the layer where the natural ground begins. The natural ground has had thousands of years to settle. The load from the upper soils have been pressing down and compacting them. The upper layers are suspect because they are often disturbed due to previous construction, farming, animal burrowing, or floods.
If it floods every 50 years, a particular site might look flat and natural but along with floods come debris often consisting of wood and other organic material. Those organic materials will decompose leaving voids in the soil. For farming those voids are inconsequential but if you are pouring a foundation, those voids will eventually collapse so that the building will settle unevenly. Thus your footings should be placed into properly engineered and compacted soil or placed into the natural soil.
If one is experienced, one can get a sense of where the natural ground is by looking at the general lay of the land. If the general lay of the land is sloping but you see a bump or dip, it could be the location of an old trash heap or an abandoned septic tank or well. Approach with caution. Abandoned wells can drop 30 feet or more. There are also abandoned mine shafts in California that are left over from the Californian Gold Rush of 1848 that are simply a hole going straight down with no way to climb back out even if you survive the fall.
Sometimes one can determine the natural ground by digging sample holes and looking for a change in color or soil composition. Try digging a fencepost hole. Take a screwdriver and knock a little bit of the soil from the walls of the hole so that you can see the real color of the soil without the smearing that a fencepost digger produces. If you see a distinct change in color, that lower layer may be the natural ground. Measure the depth to the point where the soil color changes and plot a map of the holes and the depths. If you see radical changes in depth then the first layer is definitely artificial fill and the second layer is disturbed soil that may or may not be natural soil. As you knock off bits of soil from the hole, look for indications of artificial fill such as bits of glass, asphalt or crumbing plaster. A beer can would be a real good indicator that the layer you are digging in was placed there by man.
Soils Engineering Report
In a developed area, soils engineering reports may exist for sites near you. They are sometimes submitted along with the building permits and plans for commercial buildings. Soils conditions can change radically within a few miles but seeing a soils report from sites nearby can be helpful in figuring out the issues in your area. Building codes may also give you a clue. Building codes grow out of local problems people have had in the past. That doesn't mean you will have the same problems, but it does mean that you should follow the local building codes and you should try to figure out what the problem is that the city of county is trying to solve with a particular building code.
For most single-story housing construction, deficiencies in the soil can be made up using a better foundation design. Even a two-story wood-frame house is reasonably light. In earthquake country, building requirements will demand better construction. Multistory construction such as an apartment building and tilt-up concrete construction will require formal engineering due to the heavier loads. A soils engineering report will be helpful. This is NOT a report on the chemical composition of the soil, although that may be needed if the site was once used as a cow pasture.
Issue with Corrosive Soil
Old pastures also means you must use a different type of concrete that is resistant to the urine and cow dung in the soil. It tends to eat away at normal concrete. You may want to have the soil tested for such a condition.
Old cow pastures are a notorious problem in construction. Farmers have few qualms in using a pasture as a place to dump natural waste such as the unusable parts of a butchered cow. Thus you may find old burial pits filled with decomposing bones and such. These must be removed from the building area. You cannot incorporate them into compacted soil. Bones will eventually decompose and leave voids.