PILE RAFT FOUNDATION THEORY

PILE RAFT FOUNDATION THEORY

A piled raft is a raft foundation that has piles to reduce the amount of settlement. The raft foundation and the piles would be designed to act together to ensure the required settlement is not exceeded. A major part of the bearing capacity comes from the raft rather than being dominated by the piles (as in a pile group). 

Theory

Between the geotechnical design parameters of bearing capacity and settlement, which are independent, if the design just satisfies bearing capacity, settlement will be over satisfied (i.e. settlement less than permissible), and vice versa, if the design satisfies settlement, bearing capacity will be over satisfied (i.e. factor of safety against bearing capacity failure higher than the minimum required value). The latter situation is well known to us in the case of rafts in sand. In either case, the design is not optimum, which is possible only in rare instances when both the requirements are optimally and simultaneously satisfied, i.e., the factor of safety against bearing capacity failure is at the minimum stipulated value, and settlement is just equal to the permissible value. When one requirement is over satisfied, as in a normal case, the resulting geotechnical design is partly over safe, and to that extent, conceptually uneconomical.The above points need for exploring methods by which the design can be made optimum. For example, in the case of rafts in sand, the question is whether it should be possible for us to design the raft satisfying bearing capacity and look for extraneous means to control settlement which would otherwise be excessive. One such solution available is to use piles in conjunction with the raft, the function of the piles being merely to control settlement. Such a system is called a piled raft.However, when such a system is provided, it becomes the combination of a shallow foundation such as the raft and a deep foundation such as the pile, both sharing in the process of load transfer to the soil.So in theoretical terms, there is a three-way interaction between the raft, the piles and the soil, making it a complex problem for any rigorous analysis, for which the best approach would be numerical analysis such as the ‘finite element method’. For design office use, however, one more often resorts to ‘approximate’ methods.

Abstract

Raft foundations are the favorite choice for most of the designers becouse of rapid industrialization & urbanization, there is growth in the vertical direction resulting in many high-rise buildings going up. This has resulted in heavy load, complicated stress conditions, limitation of bearing capacity of soil, and settlement of high-rise buildings. As a solution to the settlement problem of high-rise buildings, a number of piles are used and a new type of foundation - called a piled raft foundation - is coming up in a big way. In some designs, approach piles are used for reducing the settlement and a load is carried by raft only. Another design method still concentrates on providing adequate axial capacity from the piles to carry the structural load and the bearing capacity of the raft is neglected. In both the design approaches, the piled raft foundation becomes uneconomical as bearing capacity of raft and pile are not utilized in the single design approach. This shows that design rules and standards for piled-raft foundations are not well established. The interesting observation in the Poulos-Davis-Randolph (PDR) design methodology for piled raft, but using a simple stiffness formula, the piled raft foundation can be designed and analyzed. It is observed that an increase in pile length leads to increase in the settlement, which is contrary to the practical observation. It is because the stiffness of any structural member in our case pile decreases with increase in length for a given diameter. This can be avoided by using a length of pile up to depth of fixity. The length of pile below fixity, due to surrounding soi,l can be neglected. This length of pile above depth of fixity is also called free-standing length of pile. This free-standing length can be calculated using Indian standard on pile. By using simple stiffness formulae in PDR methodology, the piled raft design can be simplified.

PILE GROUP V/S PILE RAFT

Pile group is a set of piles that have a pile cap that means that they act together to carry the load. The pile cap would normally be in contact with the ground. The piles would be designed to share the pile load at ultimate state. The pile cap would be designed to link the piles together but the contribution of the pile cap to bearing capacity is not included in the design.

A piled raft is a raft foundation that has piles to reduce the amount of settlement. The raft foundation and the piles would be designed to act together to ensure the required settlement is not exceeded. A major part of the bearing capacity comes from the raft rather than being dominated by the piles (as in a pile group).

Though a piled raft and pile group serve similar purposes there are essential differences between them as follows :

• A piled raft will consist of at least four or more pile groups, because the piles in a piled raft are required to support columns in a superstructure and a minimum of 4 piles are required to support each column. The piles under each column are formed into a pile group that are held together by a pile cap.

• A pile cap that holds a pile group is never designed to support any ground pressure even though it may touch the ground. A pile cap is a thick and rigid slab and is designed primarily to resist shear forces. Horizontal Links are the main reinforcement. In a piled raft the raft is designed a portion of the superstructure load and bending moment may be the primary load effect. Horizontal top and bottom bars are the main reinforcement. Links are never provided in rafts. Where shear forces are high the raft is usually reinforced by beams interconnecting the columns with vertical links being provided in these beams. 

• The function of the piles in a piled raft is to reduce the foundation settlement and sometimes to increase the bearing capacity. Friction piles should never be used in clay otherwise the settlement of a piled raft in clay exceeds that of the raft used alone. End bearing piles penetrating gravel or bearing on rock should be used in clay. A pile group is essentially a tool for providing additional bearing capacity to enable a soil carry a column load because an isolated footing cannot develop sufficient bearing capacity.