Up to the present, conventional pile-group design practices are still largely concentrated on providing adequate axial capacity to carry structural loads, and estimation of pile-group settlement is generally treated as a secondary issue. It is necessary to reverse these priorities, and to develop new analytical methods that allow simple estimation of the load-deflection behavior of pile-group foundation systems, and hence permit design studies to focus more on settlement issues rather than pile capacity issues alone. To achieve this aim, it is impossible to confine the research just in the linear elastic response of pile-groups. The manners in which the nonlinear elastic stress strain response of soil affect single pile response and interaction between piles must be explored, and the importance of the nonlinearity of pile-group response by comparing with a single pile, and the average shaft and base response must be examined in a reliable and acceptable manner.
This thesis concentrates on developing a practical and simple semi-analytical approach for estimating the load-settlement performance of large size pile-group and piled-rafts system. In order to attain the aim of the research, a semi-analytical model will be developed that will provide a base for interpretation and anticipation of the load settlement behaviors of single-piles and pile-groups under both linear and nonlinear loading ranges. The model is mainly intended to incorporate the three important pile-soil-raft interaction aspects: the interaction between pile and soil, the interaction between pile to pile in a pile group, and the interaction between the soil reaction pressure developed under the raft with the surrounding pile. The fiamework is essentially a theoretical model based on existing mathematical hypothesis and some observations, made from laboratory shear ring test results and field pile-load test results.
The first step to achieve this aim is to develop a simplified nonlinear load-transfer function to simulate the shaft shear stress and shaft displacement relationship for a single pile and the corresponding algorithm. The key feature to this load-transfer function is the resulting displacement induced by the shaft shear stress, which can be separated into two parts. The first part is a nonlinear localized shear displacement, which is developed in a thin disturbed layer around the interface of the pile shaft. In this thesis, a simple hyperbolic model is adopted to simulate this nonlinear displacement. The second part is a linear elastic displacement field outside the disturbed zone, which can by simulated by well-established elastic solution. For the second part, the principle of superposition is assumed to hold. The following steps are examined in order to verify the formulation concept and the procedure adopted in this study:
1. Comparison between the laboratory ring shear test results and the proposed hyperbolic load-transfer function to describe the localized shear displacement and shear stress relationship;
2. Propose a back-analysis method to obtain the coefficients required for the description of the hyperbolic load-transfer function around a vertically loaded single pile and to verify the reliability of the proposed nonlinear model;
3. To study the result from nonlinear FEM analysis, is based on a complex constitutive relationship to model the load-transfer relationship around a pile shaft; and
4. A number of field pile-load test results are compared with the results predicted by the proposed analytical method.
All the results as mentioned above seem to agree with the fundamental concept of the proposed nonlinear load-transfer function, which leads to the conclusion that the localized shear displacement zone around the pile shaft dominates the load-displacement behavior of single piles. In similar concept, two additional simplified load-transfer functions for nonlinear analysis of pile-groups are formed. The first load-transfer function can simulate the interaction of pile-group systems, and the second transfer function can simulate piled-raft systems in which the raft is fully contacted with the ground.
Although the proposed load-transfer functions involve large simplification and idealization of soil response to pile load, the proposed simplified load-transfer function captures the essential load-displacement behaviour of pile-groups. By adopting the proposed load-transfer functions, a highly efficient and stable algorithm is presented. Parametric studies are performed in order to study the basic nonlinear behaviour of single piles and pile-groups. Emphases are placed on following effects of large-scale pile-groups:
1. Pile load distribution among the pile group;
2. Soil reaction pressure distribution;
3. Mobilization of shaft shear stress along the pile depth;
4. Pile slenderness effect on the pile load distribution and soil pressure distribution;
5. Pile base nonlinear responses.
The applicability and limitations of the proposed analytical model solution will be discussed. Some pile interaction effects of large pile-group system were obtained in this study, which were never discussed in previous research studies, are observed in the present study. These findings provide valuable insight in the understanding of the fundamental load-displacement mechanisms of large pile-groups under linear and nonlinear load conditions. It is believed that the new findings in this thesis can assist the industry to better understand the load-settlement behaviour of pile-group foundations and to provide engineers with reasonable recommendations and judgements.
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