Numerical and analytical modelling of friction pile group settlement performance in clay

Abstract

Over the past couple of decades, the focus of foundation design has shifted from ultimate limit state to serviceability limit state, particularly where ground conditions are challenging. Nowadays, the serviceability of a deep foundation system can be considered using advanced constitutive laws within computational software packages that are capable of considering the foundation system as a continuum. However, these methods can become both computationally-intensive and time-consuming if non-linear pile-soil-pile interaction and large groups with non-standard geometries are to be modelled. Simplified methods, generally empirical in nature, can be used in preliminary foundation design, alleviating the computational effort associated with more rigorous analyses. However, some of the assumptions therein are over-simplified. The goal of this thesis is to develop new simplified methods which are relatively easy to implement, yet founded where necessary in parametric studies using rigorous continuum analyses. Both absolute and differential settlement performance of pile groups are considered using PLAXIS 2-D and PLAXIS 3-D Foundation finite element software packages and primarily the Hardening Soil and MIT-S1 constitutive models. Previous empirical solutions from linear elastic work had identified a significant dependence of stiffness efficiency on pile group size and group spacing. In this study, the effect of the pile length-to-diameter ratio, the compressibility of a stiff bearing stratum beneath the pile group and the depth below ground level to the stiff bearing stratum are also considered as variables. As a first stage of this thesis, a set of empirical equations has been formulated which has been shown to predict the stiffness efficiency of pile groups in a database very well. For the purpose of developing a more comprehensive semi-empirical solution (analytical in large parts), the load transfer (t-z) method is employed to describe the nonlinear behaviour of a single pile. The Interaction Factor Method (IFM) was then used to extend this analysis to pile groups. A number of important features associated with single pile and pile group behaviour were included herein that have not been considered in previous studies: (i) Pile-to-pile interaction effects were considered within a non-linear framework. In the course of this exercise, the author distinguishes between alternative definitions of interaction factor and shows that settlement predictions using IFM match continuum analyses very closely for friction piles and reasonably well for end-bearing piles. (ii) A numerical study on single pile and pile group installation effects was undertaken using advanced constitutive models. A variation on the traditional cavity expansion method (CEM) was adopted for the analysis of single pile effects while group effects were investigated using a new method based on volumetric expansion of tunnels in PLAXIS 2-D; these effects were subsequently incorporated into the analytical approach by means of a simplified model. (iii) In addition, the reinforcing effects of additional group piles on the soil continuum and pre-failure pile-soil slip has also been taken into account; this is the first study to include these effects in a nonlinear analytical approach. On the subject of differential settlement, an extensive numerical study of the angular distortion of piled foundations, which has been documented as the most influential settlement characteristic in the cracking of buildings, has also been undertaken and formulated into a set of fully-normalised trends. A consistent trend between angular distortion and stiffness efficiency for corresponding pile cap rigidities was evident. The aforementioned simplified finite element-based approach was developed using a similar framework and is thus a compatible way to estimate angular distortion for the group. Through comparisons with previously published field test data and measurements from buildings in addition to numerical simulations, the results presented in this thesis indicate that the proposed approaches provide a viable framework for the serviceability design of pile groups while alleviating the computational effort and expense associated with rigorous continuum analyses.