Indeed, Structural Collapse Due To A Weak Foundation Can Be Classed As

Indeed, structural collapse due to a weak foundation can be classed as foundation failure, but this is not the only mode of foundation failure. A foundation failure is said to occur when the foundation has failed to fulfil its intended purpose. As discussed in chapter 3, the purpose of a foundation is more than to merely keep the structure standing.
Structural Collapse A complete or partial collapse of a structure as a result of inadequate foundation is definitely the worst case of foundation failure. Because the whole structure lies on its foundations, a foundation failure can result is total destruction of everything above. Although this is rare, is does occur, especially during construction.
Excessive Settlement Once constructed, the building continues to settle under its own weight and other imposed loads. This settlement has to be predicted and accounted for in the structural design. The foundation will be said to have failed if the settlement occurs much faster than the predicted rate or to a higher extent than the predicted settlement. Excessive settlement leads to all sorts of problems such as cracks, drainage system failure and failure of other services.
Differential Settlement If a building undergoes settlement that is not equal at all the footings, it is said to undergo differential settlement. Differential settlement is the main cause of cracks in buildings. These cracks can be only on the finishes or can include the cladding, leaving the building exposed to air and water leaks. It can be more serious and be structural cracks in structural components and it would then pose a direct threat to the integrity and safety of the building.
Reasons of Failure
A foundation is the interaction between the building and the ground. The building is not likely to change significantly over its lifespan. However, the ground is constantly undergoing change. The change can be a result of the building itself or its surroundings. The weight of the building is constantly compressing the soil underneath. The properties of the soils change as it gets compressed.
As explained in section 3.1.2, the properties of soils are largely dependent on water content. So any change in water content is likely to cause a change of properties. When water present between soil particles is removed, the soil particles fall back together and the soil shrinks. When water is added to the soil, the latter can be expected to swell. When a soil looses water and shrink, it gains strength and when it gains water and swells, it becomes weaker. Clays absorb and give up large amounts of water. So they are more prone to swell and shrink bay large volumes as a consequence of change in water content.
Vegetation and Trees
The effect of trees in the immediate vicinity of a house can be direct or indirect. The foundation and more often the floor of a house can be damaged by strong roots of big trees.