Civil Engineering Association

FEM Modeling of Pile-Raft Foundation

Abstract
In conventional design of pile foundations, all loads are taken by the piles, i.e. the contact pressure between the raft and the soil is neglected. In the last decades geotechnical engineers have started to take this pressure into account in design of pile foundation. Such a foundation, where the raft and the piles interact to transfer the loads to the ground, is in this dissertation called piled raft foundation or piled raft.

Analysis of piled rafts requires numerical methods, due to complex soil-structure interaction. In this dissertation four different modelling approaches for analysis of piled raft foundation are compared; a full three dimensional finite element method model (FEM model) and also three plane strain FEM models (i.e. two dimensional models). All models are carried out by using programs developed by Plaxis, i.e. Plaxis 3DFoundation and Plaxis 2D, respectively. The plane strain models are similar but differ in the way of modelling the interaction between the piles and the soil. The first plane strain model is introduced in Chapter 3.4.2. Since this model produce questionable results, due to too weak modelling of the pile-soil interaction, two alternative models are introduced in Chapter 8.4 and 8.5 (called AM1 and AM2). Piled raft foundations are three dimensional problems, in a two dimensional analysis one has to introduce simplifications and thereby inaccuracies. However, it could still be convenient to use this method since it is faster and the software is less expensive. The inaccuracies in a 2D model compared to a 3D model will vary depending on the characteristics of the problem. The object of the work is to study the inaccuracies and how these changes as the characteristics of the problem change.

In conventional design of pile foundations, all loads are taken by the piles, i.e. the contact
pressure between the raft and the soil is neglected. In the last decades geotechnical engineers
have started to take this pressure into account in design of pile foundation. Such a foundation,
where the raft and the piles interact to transfer the loads to the ground, is in this dissertation
called piled raft foundation or piled raft.
Analysis of piled rafts requires numerical methods, due to complex soil-structure interaction.
In this dissertation four different modelling approaches for analysis of piled raft foundation
are compared; a full three dimensional finite element method model (FEM model) and also
three plane strain FEM models (i.e. two dimensional models). All models are carried out by
using programs developed by Plaxis, i.e. Plaxis 3DFoundation and Plaxis 2D, respectively.
The plane strain models are similar but differ in the way of modelling the interaction between
the piles and the soil. The first plane strain model is introduced in Chapter 3.4.2. Since this
model produce questionable results, due to too weak modelling of the pile-soil interaction,
two alternative models are introduced in Chapter 8.4 and 8.5 (called AM1 and AM2).
Piled raft foundations are three dimensional problems, in a two dimensional analysis one has
to introduce simplifications and thereby inaccuracies. However, it could still be convenient to
use this method since it is faster and the software is less expensive. The inaccuracies in a 2D
model compared to a 3D model will vary depending on the characteristics of the problem. The
object of the work is to study the inaccuracies and how these changes as the characteristics of
the problem change.
The work starts with a study of previous master dissertations and other literature. To get a
better understanding of Plaxis, a sheet pile wall is then modelled in Plaxis 2D. Subsequently,
a hypothetical piled raft is analysed in Plaxis 2D and 3DFoundation to illustrate the different
modelling approaches. The piled raft is square with the piles distributed uniformly, and it is
loaded vertically and uniformly. Finally, a parametric study of the different models is
performed, where two parameters for the hypothetical piled raft are varied, i.e. pile spacing
and the shape of the piled raft (rectangular, with different base to length relations).
When comparing the results, the 3D model is considered “true” and the maximum values are
examined. In general for the piled rafts analysed, the two dimensional models (AM1 and
AM2) overestimate the settlement (~30%), the raft bending moment (~30%) and the pile
force (~10%). As the pile spacing decreases, the pile force from the 2D models resembles the
3D models more. While the settlement and the bending moment coincides less as the pile
spacing decreases. As the length of the piled raft increases, the settlements and pile force from
the 2D model converge towards the 3D results, which is expected since the problem get more
two dimensional. However, a problem occurs as the raft gets non quadratic. The maximal
differential settlement will then take place in the longer direction, and to calculate it,
calculations have to be made for sections in the longer direction, were the plane strain
condition is less satisfactory. This problem is off course less significant when the differential
settlements are small as for a piled raft with a very stiff raft.

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Raft foundation covers the entire area of the structure, transmitting the entire structural load and reduces differential settlements whereas piles are relatively long, slender members that transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock strata having a high bearing capacity. In recent years, a piled-raft foundation which is a composite structure consisting pile and raft has been proved to be an appropriate alternative instead of conventional pile or mat foundations. In this paper, analysis of piled raft foundation has been carried out by using finite element software ANSYS. For understanding the behavior of piled raft foundation, parametric studies has been carried out in medium sand by varying pile diameters and pile lengths in different combinations. It has been found out that Pile diameter has significant influence on the ultimate capacity of piled raft foundation whereas the pile length has not of much significance. It is concluded that an optimum combination of pile diameter of 0.5 m at the center of the raft with 0.4 m at the edges of the raft is giving ultimate load of 4.45 MN with settlement of 26.76 mm which is in acceptable limits

Direct Link to Posts #1 & #2:
FEM Modelling of piled raft foundations in two and three dimensions