» عنوان : FEM Modelling Ofpiled RAFT Foundations Intwo AND Three Dimensions
تعداد صفحات : ۸۳
In conventional design of pile foundations, all loads are taken by the piles, i.e. the contactpressure between the RAFT and the soil is neglected. In the last decades geotechnical engineershave 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 dissertationcalled 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 foundationare compared; a full three dimensional finite element method model (FEM model) and alsothree plane strain FEM models (i.e. two dimensional models). All models are carried out byusing 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 betweenthe piles and the soil. The first plane strain model is introduced in Chapter 3.4.2. Since thismodel 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 hasto introduce simplifications and thereby inaccuracies. However, it could still be convenient touse this method since it is faster and the software is less expensive. The inaccuracies in a 2Dmodel compared to a 3D model will vary depending on the characteristics of the problem. Theobject of the work is to study the inaccuracies and how these changes as the characteristics ofthe problem change.The work starts with a study of previous master dissertations and other literature. To get abetter 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 differentmodelling approaches. The piled raft is square with the piles distributed uniformly, and it isloaded vertically and uniformly. Finally, a parametric study of the different models isperformed, where two parameters for the hypothetical piled raft are varied, i.e. pile spacingand 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 areexamined. In general for the piled rafts analysed, the two dimensional models (AM1 andAM2) overestimate the settlement (~30%), the raft bending moment (~30%) and the pileforce (~10%). As the pile spacing decreases, the pile force from the 2D models resembles the3D models more. While the settlement and the bending moment coincides less as the pilespacing decreases. As the length of the piled raft increases, the settlements and pile force fromthe 2D model converge towards the 3D results, which is expected since the problem get moretwo dimensional. However, a problem occurs as theraft gets non quadratic. The maximaldifferential 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 straincondition is less satisfactory. This problem is off course less significant when the differentialsettlements are small as for a piled raft with a very stiff raft.