Many homeowners will excavate several feet out of a crawlspace for either more headroom in the crawlspace or to make a full basement. This is one of the most dangerous things to both the house and the safety of the occupants a homeowner can do. Both the construction process and the finished product produce extremely unpredictable and unstable conditions. A soil failure during or after construction has the potential to put the entire house in jeopardy. Also, if you are doing this work, it is obviously being performed without proper building permits or inspections.
In almost every case, if a building inspector discovers this type of condition during or after construction, they will evacuate the structure until proper repairs are made.
The foundation of a house serves to transfer all of the loading on the house to the soil below. Most house foundations consist of a continuous footing under the foundation walls and possibly spread footings under basement or first floor columns. The load from the footings is distributed through the soil in a triangular pattern. To visualize, assume there is a line from the bottom of your basement wall going down into the soil and towards the interior at 45 degrees. Picture your wall resting on the top of a triangle. This is also true for column footings. In this case, picture the column resting on the tip of a cone. All of the soil within this triangle or cone is considered load bearing. Loaded soil exerts both a vertical pressure on the soil below it and a horizontal pressure. The horizontal pressure must be “contained”. This is generally accomplished by the mass of soil on all sides of the loaded soil.
When excavating a crawlspace, you are most likely cutting into this containment or into the loaded soil. This is a very complex issue, and some soil cuts may maintain themselves, but the risk is tremendous that the soil will cave in, leaving no support under the footings, and a potential structural collapse.
This type of work should not only be designed by an Engineer, but also supervised by an Engineer. The finished product needs to support all the loads on it and the construction process needs to insure that the soil and structure are stable during construction.