Settlement reducing piles are used to reduce differential settlement of piles in basement slab and shaft slab construction. Different aspects of settlement reducing piles in piled shafts and basements are discussed in the following sections.
Fig.1: Settlement Reducing Pile
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Settlement Reducing Piles
What is settlement reducing pile in piled shafts and basements?
As it may be understood from its name, pile reducing settlements is applied to solve the problem of differential settlement of shafts and basements slabs.
Fig.2: Use of Piles to Reduce Differential Settlements
The differential settlement is prevented by constructing longer and consequently stiffer piles at the center portion of the basement or shaft slabs and shorter and more ductile piles around the perimeter of the slab.
The ductile piles are designed to yield to avoid or decline differential settlement between central region and perimeter of the slab. As the outer or perimeter columns yield the exerted loads will be distributed to the central piles by the stiff slab.
Apart from the above arrangements of piles to decrease differential settlements, there are other pile configurations to reach the same goal. For example, the same pile size can be used to obtain differential settlement reduction provided that the distance between perimeter piles are increased and the configuration of loads due to superstructure permits such redistribution of loads.
Another arrangement of piles to reduce differential settlement is by changing the geometry of the piles. For perimeter area, piles with soft toes or short expanded base piles can be used and long straight shafted pile can be considered for the central region of the slab.
Soft toe can be obtained by placing loose sand or soil at the base of bore hole prior to the pouring of concrete. When the load acts on the slab and for the same loading, short enlarged based pile or soft toe pile would settlement more than that of long straight shafted pile.
Since the outer piles are permitted to yield, then the stability of the structure may be questioned. That is why it is required to guarantee the stability of the entire structure when settlement reducing piles are considered. So, the load should be shared between the raft and settlement reducing piles.
It is claimed that, the ultimate potential and benefits of the settlement reduced piles can be used if the underlying soil is stiff and compressible.
Why is the purpose of a pile reducing settlements in piled shafts and basements?
When piles with the same length and diameter used to support basements and shafts slabs that carry uniform distributed loads, the deflection of the slab would be like dished shape.
At the beginning, the slab would support almost all the applied loads, but as the loading increases, the quantity of loads imposed on the piles increases progressively.
The loads exerted on the pile influence those located at the center and consequently central piles would experience greater settlement.
Due to the central pile settlement, the applied load would be redistributed through the slab to the stiffer piles around the perimeter and eventually greater loads will be exerted on these piles.
It is demonstrated that, after the redistribution of loads, around sixty percent of loads is supported by perimeter piles whereas nearly forty percent of applied loads are carried by central piles.
This differential settlement is a serious problem and need to be tackled properly. The utilization of settlement reducing piles is an appropriate answer for such problem. Not only does pile reducing settlements would solve differential settlements but also would result in a more economical construction.