13-08-2013, 04:07 PM
Optimal arrangement of piles in a pile group
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ABSTRACT
Piled raft foundations provide an economical foundation option for circumstances where the performance of the raft alone does not satisfy the design requirements. Under these situations, the addition of a limited number of piles may improve the ultimate load capacity, the settlement and differential settlement performance, and the required thickness of the raft.
Piled raft foundations utilizes piles support for the control of settlements with piles providing most of the stiffness at serviceability loads, and also the raft element providing additional capacity at ultimate loading.
Piles are mostly designed with equal or similar lengths and diameter where loads are not varying significantly, otherwise the can be designed differently.
Besides strength and structural capabilities of piled raft foundations, the arrangement of piles also plays a major role in optimizing the performance of the foundation, reducing the differential settlement, enhancing sharing of loads among all the piles etc. therefore the study of arrangement of piles is of significant importance.
This project presents an optimal pile arrangement scheme for minimizing the differential settlements of piled raft foundations. A raft is modeled as a plate based on the Mindlin theory, and soil and piles are modeled as Winkler and coupled springs, respectively. The stiffness of piles is obtained by the approximate analytical method proposed by Randolph and Wroth, and the modulus of the subgrade reaction is adopted to evaluate the Winkler spring constant.
GEO 5 and GSA soil pile interaction computer programs are used to evaluate different arrangements and their effects on the differential settlement, stiffness, and stability of piled raft foundation.
INTRODUCTION
GENERAL
Pile foundation fall into the category of deep foundations which are used by structures of heavy loads when shallow foundations cannot provide adequate capacity due to size and structural limitations.
Pile foundation relies on end bearing resistance, frictional resistance along their length, or both in developing the required capacity.
High rise buildings are usually founded on some form of pile foundation which is subjected to a combination of vertical, lateral and overturning forces. Combined pile-raft foundations can be a particularly effective form of foundation system for tall buildings because the raft is able to provide a reasonable measure of both stiffness and load resistance.
Geometrical arrangement/configuration of piles on a raft is among the most significant factors that control the overall performance of the piled raft foundations
OBJECTIVE OF THE PROJECT
Piled raft foundations are widely adopted to limit settlements in cases where a simple raft foundation can induce excessive settlements even though it provides an adequate bearing capacity.
Therefore, accurate estimation and control of settlements are an important aspect in the design of a piled raft foundation. In particular, differential settlements that have numerous negative effects on a superstructure as well as on a foundation should be restricted to allowable limits
This project presents an optimal pile arrangement scheme for minimizing differential settlement of a piled raft foundation based on optimization The location of each pile is determined through optimization, while the sizes, the number of piles and the loading condition are assumed to be predefined/constant
(1) Steel H-Piles
Steel H-piles have significant advantages over other types of piles. They can provide high axial working capacity, exceeding 400 kips. They may be obtained in a wide variety of sizes and lengths and may be easily handled, spliced, and cut off. H-piles displace little soil and are fairly easy to drive. They can penetrate obstacles better than most piles, with less damage to the pile from the obstacle or from hard driving. The major disadvantages of steel H-piles are the high material costs for steel and possible long delivery time for mill orders. H-piles may also be subject to excessive corrosion in certain environments unless preventive measures are used. Pile shoes are required when driving in dense sand strata, gravel strata, cobble-boulder zones, and when driving piles to refusal on a hard layer of bedrock.
(2) Steel Pipe Piles
Steel pipe piles may be driven open- or closed end and may be filled with concrete or left unfilled. Concrete filled pipe piles may provide very high load capacity, over 1,000 kips in some cases. Installation of pipe piles is more difficult than H-piles because closed-end piles displace more soil, and open-ended pipe piles tend to form a soil plug at the bottom and act like a closed-end pile. Handling, splicing, and cutting are easy. Pipe piles have disadvantages similar to H-piles (i.e., high steel costs, long delivery time, and potential corrosion problems).
OPTIMIZATION CRITERIA
Depending on the type and significance of the superstructure, the optimization criteria can be to maximize the overall stiffness and/or to minimize the differential settlements. The former is more common when a pile group is supporting a column or relatively small structure, where the pile cap is stiff or is assumed to be rigid. The latter criterion usually applies to heavy structures which may be supported by large pile groups or piled rafts, where differential settlements are the primary concern in foundation design. Therefore, reduction of differential settlements will be the focus of this section
The following sections describe the optimization analyses for square pile groups and piled rafts, where the benefits of the optimized configurations relative to a uniform length pile group are presented both in terms of increased average stiffness and reduced differential settlements.