So we briefly learned what shallow foundations are and how they react and how they are sized. Now we will see what Deep foundations are and how they act in different soils. As the word suggest Deep Foundations go deeper into the soil. Some examples of deep foundation structures are concrete, steel or wooden piles. One of the main reason deep foundation are used is may be because the top layers of the soil are weak and compressible and cannot support the load properly. Or the load is pretty heavy and the piles need to carry the loads to bedrock.
For example, imagine a layer of soil as weak as a sponge extending 10ft deep and as wide as a football field at a location where an office building is going to be built. It is too big of an area to completely excavate and refill with better soil or engineering fill or use a Mat foundation. In situations like this deep foundations come in handy.
By nature, anything that needs digging deep adds more expense to a project. Deep piles counteract the loads on it by end bearing capacity and skin friction.
End bearing load capacity is the load carried by the bottom tip area which rests on bedrock. Allowable bearing force x the area of the bottom will give the end bearing capacity. These type of piles are called “End Bearing piles”Sometimes the end bearing piles have to be too long to hit bedrock and too unpractical.
Skin Friction capacity of the pile is based on the surface area of the pile and the friction the pile experiences from the soil. These type of piles are called ” Friction piles”
For anyone who would like to see an animation of the many different types of piles and their methods of construction, here are some youtube videos (courtesy of Hayward Baker Inc)