Optimally designed pile foundations

Abstract eng:
The foundation of a structure is defined as that part of the structure in direct contact with the ground, which transmits the load of the structure to the ground. There are a number of types of piles, in this work bearing piles will be implemented. Bearing piles are required where the soil at normal foundation level cannot support ordinary pad, strip, or raft foundations or where structures are sited on a deep filling which is compressible and settling under its own weight. Piled foundations are a convenient method of supporting structures built over water or where uplift loads must be resisted. Inclined or raking piles are provided to resist lateral forces. Combinations of vertical and horizontal loads are carried when piles are used to support retaining walls, bridge piers and abutments, and machinery foundations. Pile-supported structures are known to have existed in pre-historic times, and references to cedar timber piles in Babylon can be found in the Bible. In the Middle Ages, pile foundations supported a wide assortment of structures in Venice and in Holland. Today, the main types of pile in general use are driven piles, driven and cast-in-place piles, jacked piles, bored and cast-in-place piles and composite piles. The first three of the above types are sometimes called "displacement piles" since the soil is displaced as the pile is driven or jacked into the ground. In all forms of bored piles, and in some forms of composite piles, the soil is first removed by boring a hole into which concrete is placed or various types of precast concrete or other proprietary units are inserted. The basic difference between displacement and non-displacement piles requires a different approach to the problems of calculating carrying capacity. Having decided that piling is necessary, the engineer must make a choice from a variety of types and sizes. Usually, there is only one type of pile which is satisfactory for any particular site conditions. Given the magnitude and the importance of piled foundations that support high-performance structures such as bridges, high-rise buildings, power stations and offshore platforms, it becomes a necessity to find the best pile foundation design in terms of economy and performance. The foundation cost, for this type of structural systems, can vary from 2% to 20% of the construction cost of the structure while the number of piles required might exceed several thousands. In the first part of this study the modelling of the soil-pile structure interaction using the finite element method is described while in the second part a formulation of an optimization problem is proposed, aiming at achieving the most economical (optimum) design of the pile foundation layout. Two different design procedures are adopted to assess the performance of each candidate design obtained during the optimization process. The German Foundation Code DIN 4014 and the Eurocode 7 design procedures. Because of the nature of the problem, a mesh generator is used in order to create automatically the finite element mesh both for pile members and soil. Two real-world structures are considered for testing the proposed formulation. A sixteen storey and a thirty-one storey building are employed, for the comparative study and a significant reduction of the pile foundation cost is achieved in both cases.

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