Piles - How do they Work?
Deep bearing piles are most often built of concrete or steel – both materials are stiffer than the soils they are embedded in by an order of magnitude, and most piles have comparatively high stiffness relative to the loads applied to them.
Consequently, there is just a small amount of deformation that occurs in the pile shaft under the applied load, meaning that any significant movement occurring at the head of the pile must be reflected as significant movement at the toe of the pile.
So how do piles work? Unless special measures are taking to avoid transfer of load in some portion of the pile, piles always transfer load through a combination of shaft and base resistance.
This is the key. The maximum capacity that can be developed in shear along the pile shaft – what we describe as the ultimate shaft resistance – tends to develop at a downward pile movement equal to around 1% of pile diameter. The maximum available base resistance on the other hand is developed at a downward movement nearer to 10% of pile diameter.
Simply put, piles which benefit from an accurately estimated available shaft resistance which is in excess of the applied load will move very little, and piles which are overly reliant on base resistance to support the applied loads might move more than is hoped, possibly more than is tolerable for the structure. This behaviour is reflected in the adoption of lower partial factors in EC7 for pile shaft capacity, comparative to the partial factors applied against base resistance.
Perhaps counterintuitively, piles founded in rock do not always behave counter this this expectation. Highly concentrated stresses at the pile toe can compress bedding layers in fractured rock masses, and the difficulties in achieving a perfectly clean joint between pile toe and rock head can lead to the presence of a thin compressible layer between the two, and an unusual behaviour during loading.
At Suoloconsulting we have many years of experience designing bearing piles of all shapes and sizes, in a range of ground conditions, and using a range of methods.
Come and speak to us at suoloconsulting@gmail.com
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