The foundation does not fail by sinking into the oblivion. In fact, assuming the structural part of the foundation is strong and healthy, the resistance capacity of a foundation increases as the foundation sinks deeper into the ground. Whilst the settlement of foundations is well understood by the general public, the ultimate failure of foundations is a lot more complicated to understand - This is the focus of this blog.
The failure of a foundation can happen either in the foundation structure itself (e.g. piles break), or the ground. The ground failure mechanisms of a foundation can largely be categorised into two:
Excessive settlement. This is the Serviceability Limit State failure – the foundation has incurred so much settlement that it defeats the function of the structure (e.g. the deck of a bridge no longer levels with its approach road, or a house heavily leaned to one side), or results in damage of the structure (e.g. differential settlement causing cracks). Another controlling factor in a built-up area could also be excessive ground heave causing damage to third party assets. Under this failure mode, the structure does not collapse or overturn, but the excessive movement of the foundation has rendered itself unusable, or damaging to other structures nearby.
Plastic failure. This is the Ultimate Limit State failure. The foundation has reached a plastic state where the stiffness of the ground supporting the foundation is effectively close to zero. In this instance the structure topples over in a disastrous manner.
The Ultimate Limit State failure of a foundation
The ultimate failure of a foundation is the result of the soil automatically finding a shear failure path of least resistance. As you will see later, the path is not necessarily is straight line and can be curved. As this previous blog explained, soil fails mainly in shear, which is the sliding between interfacing particles of the soil. As this other blog explained, the triaxial test simulates the failure of the soil under the load from a foundation. In a triaxial test, although the soil is subject to a vertical pressure larger than the confining pressure, the soil does not fail by compression, but by shear (sliding) along a diagonal plane, relative to the direction of the major principal stress.
The ultimate failure of a foundation can be categorised into three:
Global shear (sliding) failure
Local shear (sliding) failure
Punching failure
The global shear failure of a foundation
The soil directly underneath the foundation gets trapped and can not escape due to the squeezing action from all direction. The trapped soil largely takes the shape of a triangle, and works like a wedge extension of the foundation itself, prying out the soil next to it following a line of least resistance.
The zone immediately next to the ‘no-escape zone’ is a what I call a ‘shear pivotal zone’. In this zone, the soil automatically works out the line of least resistance, which has to take the shape of an upward turning curve, because it is the only possibility for the soil to fail – any downward pointing failure lines would require an infinite amount of energy. The curve is very similar to the curve formed by a landslide, because it naturally takes the least effort to break.
The last zone is ‘passive resistance zone’, because soil within this zone is simply pushed away by the other two zones, serving as a passive resistance. Upon reaching this zone, the general direction of shear failure is already confirmed by the previous two zones, and this zone has to do little to steer the shear plane again – it simply follows through.
When other failure modes may happen first
In a loosely compacted soil, there is a lot of void space. It allows a lot of room for the soil to compact before shear failure happens, i.e. the soil is ‘contractive’ under shear. A local shear failure is more likely to happen ahead of a global shear failure; in other words, the soil close to the foundation has already started failing in shear, whilst the soil further away from the foundation is still undergoing compaction.
The situation for dense or stiff (overconsolidated) soil is the reverse. The soil is already very well compacted with little voids in between soil particles. When under shear, the already densely packed soil particles have to break up and roll over each other in order to result in shear failure. Therefore these types of soil dilates when subject to shear, and the movement of the ground transmits to a very far distance immediately compared to loose soil.
Therefore the global shear failure mode is more likely to happen in dense or stiff soil, whereas local shear failure is more likely to happen in loose and not well compacted soil.