2008 Higher Degree Theses

The following postgraduate students were awarded higher degrees for their theses in 2008.

Doctor of Philosophy

Helen Chow, Analysis of Piled-Raft Foundations with Piles of Different Lengths and Diameters

Helen Chow
Analysis of Piled-Raft Foundations with Piles of Different Lengths and Diameters
Supervisor: John Small


In recent years, there have been an increasing number of structures using piled rafts as the foundation to reduce the overall and differential settlements. For cases where a piled raft is subjected to a non-uniform loading, the use of piles with different sizes can improve the performance of the foundation. Extensive research work has been performed in the past to examine the behaviour of piled rafts. However, most of the research was focused on piled rafts supported by identical piles, and the use of non-identical piles has not received much attention.

In this thesis, the behaviour of piled rafts supported by non-identical piles is examined by the use of a computer program APRILS based on the finite layer and finite element methods. The finite layer method is used for the analysis of the layered soil system. The application of this method to different shapes of loadings is presented and has been shown to be in good agreement with the theoretical solutions. The finite element method is used for the analysis of the raft and piles. Full interaction between raft, piles and soil which is of major importance in the behaviour of piled rafts is considered in the analysis. Among the four different types of interaction present in the piled raft foundation, the interaction between piles plays an important role.

Interaction between non-identical piles subjected to both horizontal and vertical loadings is examined. It is found that for a pair of piles under consideration, if the diameter and length of one of the piles is a multiple of the diameter or length of the other pile, the reciprocal theorem holds.

Program APRILS can be used for the analysis of piled rafts subjected to horizontal and vertical loadings in which the base of the raft is treated as being rough and slip along the pile-soil interface is allowed in the analysis. By having a rough raft base, there is a significant effect on the vertical movement of the raft due to the lateral resistance of the soil and the piles. For piled rafts embedded in a non-homogeneous layered soil, the modulus of each layer of soil is used in the computation and accurate solutions are obtained without the use of an averaging technique.

The performance of piled rafts is affected by several factors such as the soil conditions, pile dimensions and arrangements. Detailed examinations of these factors have shown that the displacement of the raft and the proportion of load carried by piles are significantly affected by them. For piled rafts subjected to non-uniform vertical or horizontal loads, the use of non-identical piles can improve the performance of the piled rafts. For vertically loaded piled rafts, longer piles are preferred to be used underneath the heavily loaded region while for horizontally loaded piled rafts, larger diameter piles are more preferable.

Several field cases are studied in this thesis, and they show that the solutions obtained from program APRILS are in good agreement with field measurements. The use of the insitu test results and back-analysis is used to obtain the correlation between the modulus of the soil and field test results for laterally loaded piles.


Download Helen Chow's thesis (3.0 MB PDF)