For a cohesive soil, the majority of load will be carried in shaft friction. In reality due to the design factors of safety, it is quite likely that in a cohesive soil the design working load for the pile will be carried almost entirely by shaft friction. Conversely, for a non-cohesive soil, it is likely that the majority of the load will be carried in end bearing. For the end bearing to be mobilised, the pile will 'bed' into the founding material. Where load is carried in shaft friction, the stress within the pile shaft dissipates with pile depth. Thus the elastic shortening of the pile will be less than for a free standing column.

There will however be some pile settlement as the shaft frictional resistance is taken up. Hence for a pile founding in cohesive soils, settlements and recovery for loads up to working load could be expected to be dominated by elastic behaviour. Where load is carried by majority end bearing, the shaft frictional resistance may be less than the piles safe working load. Thus the concrete stress will not dissipate as rapidly - the elastic shortening of the pile may therefore be similar to that for a free standing column. In addition, with all the shaft friction mobilised, pile settlement is greater as the contribution from end bearing is attained. Hence for a pile founding in a cohesionless soil, settlement and recovery will not be dominated by elastic behaviour.

The settlement at working load for a pile in a cohesionless soil could generally be expected to be greater than that of a pile in a cohesive soil. Recovery upon unloading would be expected to be less for the pile in a cohesionless soil. Consequently, when specifying pile performance criteria, it is necessary to have an understanding of how the pile is carrying its load. It is not sensible to have a standard settlement and recovery criteria at working load and 50% overload since this would result in more onerous a criteria for a pile in a cohesionless soil type. It is intended that this guidance note will assist the Engineer in specifying pile performance criteria for various ground conditions in an informed and logical manner whilst also explaining the reasons for the differences in pile settlement behaviour.

The performance of a pile is determined by the following criteria:

Pile Load:	The applied load will affect the pile settlement. The greater the load, the larger the settlement. The usual loads designed to are compression, horizontal and tension.
Pile Diameter & Length:	The recorded movement at the pile head will be as a result of both elastic shortening and pile settlement. Elastic shortening will influence pile head deflection to a greater extent in a long slender pile than in a short, large diameter pile.
Ground Conditions:	The ground conditions and method of pile installation, determines the proportion of applied load carried in shaft friction and end bearing.

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