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The analysis of building structure in contact with soil involves an interactive process of stresses and strains developed within the structure and the soil field. The response of Piled-Raft Foundation system to the structure is very challenging because there is an important interplay between the component of building structure and the soil field. Herein, soil-foundation-structure interaction of buildings founded on Piled-Raft Foundation is evaluated through 3D-Nonlinear Finite Element Analyses using PLAXIS3D FOUNDATION code. The soil settlements and forces demand of the high-rise building structures and foundation is computed. The parametric study affecting the soil-foundation-structure response has been carried out. The parameters such as construction phasing, sequential loading, building aspect ratios, soil failure models and thickness proportion of soil field stiff layer, are considered. It is concluded that the interaction of building foundation-soil field and super-structure has remarkable effect on the structure.

The analysis of Piled-Raft Foundation is very challenging because the load in the piled-raft structures is transferred to the soil not only by the interaction between the soil and the piles but also by the interaction between foundation structure and superstructure. In this interaction, deformations in the soils are the key factor which will affect forces and deformation in foundation and superstructure. The soils below the ground level are heterogeneous and often found as layered system, i.e. layer wise varying properties below the ground. The combined piled-raft foundation penetrates deep into the foundation soil increasing its significant depth below the ground and affects the response of structure and soil. The method of analysis of foundation and structure also affects the response of structure and soil. The complex foundation system requires a reliable advance computational method that can simulate the 3D-non-linear soil behavior and structure-foundation system interaction. Considerable attention has been paid to analyze, design and construction of combined piled-raft foundation (CPRF) system. The survey of various analytical methods and numerical methods used to model the behavior of geomechanics has been presented by [

From the literature survey it is clear that the interaction of the superstructure in the soil-foundation analysis has not been taken into consideration in most of the research work and load from the super structure in considered acting directly on the raft as a uniform or concentrated load. The effect of construction phase and mode of superstructure loading on the response of structure and foundation has not been given due attention. In this paper, complete soil-structure interaction of combined Piled-Raft Foundation with the foundation soil and superstructure of the building is evaluated through 3D-nonlinear Finite Element Analyses using PLAXIS3D foundation code [

A 15-storey square/rectangular building having piled raft foundation in the two layered soil system is selected for the complete structure-foundation interaction analysis. The square building (aspect ratio = 1) has 4 bays in X-and Y-direction and the rectangular building (aspect ratio = 1.75) has 3 bays in X-direction and 6 bays in Y-direction as shown in ^{2}. Dimensions of columns are listed in ^{2} while concrete Poisson’s ratio and density is considered in structural models as 0.2 and 25 kN/m^{3} respectively. The typical floor plans and foundation plans having raft with pile location of square and rectangular shaped building are shown in

The slenderness ratio (L/D) of piles is taken as 26.7 and end tip of the piles are considered resting on the bottom surface of top soil layer having hardening soil model for different aspect of building, mode of application

Building Shape | Column Dimensions (m × m) | ||
---|---|---|---|

C1 | C2 | C3 | |

Square | 0.65 × 0.65 | 0.75 × 0.75 | 0.9 × 0.9 |

Rectangular | 0.65 × 0.65 | 0.75 × 0.75 | 0.9 × 0.9 |

of structure loading on foundation and for different failure model of soils. Modulus of elasticity of pile material is taken as 2.35 × 107 kN/m^{2} while its density is considered as 25 kN/m^{3}. The soil profile is of two layer systems with upper layer of loose sand and lower layer of dense sand (stiff soil). The different thickness of stiff soil is considered in the model to study the effect of stiff soil on the interaction analysis. Three thickness proportion of stiff soil namely 25%, 50% and 75% of total thickness are taken. The water level is assumed at the ground surface.

Soil is a complex material that behaves differently in primary loading, unloading and reloading. It exhibits non- linear behaviour well below failure condition with stress dependant stiffness [

The finite element method based on software PLAXIS 3D is used for three dimensional modelling of 15-storey building structure having piled-raft foundation in layered soil field. The columns and piles are modelled as frame elements with linear elastic properties. The interaction effect of pile and soil at the pile shaft is considered by means of Elasto-Plastic line-to-volume and point-to-volume interfaces [

Parameters | Soil model | Soil layer | |
---|---|---|---|

Loose sand | Dense sand | ||

Unsaturated weight (γ_{unsat}), kN/m^{3} | All models | 17 | 19 |

Saturated weight (γ_{sat}) | All models | 20 | 21 |

Stiffness (^{2} | HS [ | 20,000 | 60,000 |

Stiffness (^{2} | HS [ | 20,000 | 60,000 |

Stiffness (^{2} | All models | 100,000 | 180,000 |

Rate of increase of E with depth (ΔE) kN/m^{2} | MCI [ | 4720 | 31,470 |

Power (m) | HS | 0.65 | 0.55 |

Poisson’s ratio (ν) | All models | 0.2 | 0.2 |

Dilatancy (ψ), degree | All models | 2 | 8 |

Friction angle (φ), degree | All models | 32 | 38 |

Cohesion (c_{ref}), kN/m^{2} | HS | 0.1 | 0.1 |

The results of interaction of building foundation-structure with different aspect ratio of building and different soil models on the soil are given in

Soil Models | Location | Maximum Soil Settlement (cm) | |
---|---|---|---|

Hardening Model (Aspect Ratio = 1.0 & 1.75) | Below the raft, 2 m below GL | Aspect Ratio = 1.0 | 23.19 |

Aspect Ratio = 1.75 | 22.0 | ||

10 m below GL | Aspect Ratio = 1.0 | 11.74 | |

Aspect Ratio = 1.75 | 11.30 | ||

Below the pile, 20 m below GL | Aspect Ratio = 1.0 | 3.22 | |

Aspect Ratio = 1.75 | 3.05 | ||

Mohr-Coulomb Incremental Model (Aspect Ratio = 1.0) | Below the raft, 2 m below GL | 26.83 | |

10 m below GL | 14.37 | ||

Below the pile, 20 m below GL | 4.16 | ||

Mohr-Coulomb Incremental Model (Aspect Ratio = 1.0) | Below the raft, 2 m below GL | 12.97 | |

10 m below GL | 4.43 | ||

Below the pile, 20 m below GL | 1.23 |

the increase of aspect ratio of building. This may due to the reason that the load is distributed on a larger area in one direction of the building. The use of different soil failure model for soil field has also predicted dissimilar soil settlement. The behavior of soil at various levels also varies under different failure models of soil. The soil settlement is predicted highest using Mohr-coulomb failure criteria (MC) and predicted least by Mohr-coulomb incremental stiffness model (MCI). The soil settlements of a square building (aspect ratio = 1) at raft level are 26.83 cm, 12.97 cm and 23.19 cm respectively in Mohr-coulomb (MC) model, Mohr-coulomb incremental stiffness (MCI) model and hardening soil (HS) model while, the settlements at pile end are 4.16 cm, 1.23 cm and 3.22 cm. For the rectangular structure (aspect ratio = 1.75), the soil settlement is concentrated in the shorter direction of the building structure. The contours of soil settlements along vertical cross-section of the soil field for building aspect ratios and soil failure criteria are depicted in

Soil Models | Location | Maximum Soil Settlement (cm) |
---|---|---|

Top Loose Sand―−25% of total depth | Below the raft, 2 m below GL | 18.46 |

10 m below GL | 5.49 | |

Below the pile, 20 m below GL | 2.76 | |

Top Loose Sand―−50% of total depth | Below the raft, 2 m below GL | 23.19 |

10 m below GL | 11.74 | |

Below the pile, 20 m below GL | 3.22 | |

Top Loose Sand―−75% of total depth | Below the raft, 2 m below GL | 23.95 |

10 m below GL | 13.89 | |

Below the pile, 20 m below GL | 6.0 |

Soil Models | Location | Maximum Soil Settlement (cm) |
---|---|---|

Loading through super structure | Below the raft, 2 m below GL | 23.19 |

10 m below GL | 11.74 | |

Below the pile, 20 m below GL | 3.22 | |

Loading without super structure | Below the raft, 2 m below GL | 23.45 |

10 m below GL | 11.6 | |

Below the pile, 20 m below GL | 3.25 | |

Sequential loading through super structure | Below the raft, 2 m below GL | 22.74 |

10 m below GL | 11.47 | |

Below the pile, 20 m below GL | 3.16 |

Building Aspect Ratio | Settlements/Max. Moments/Max. Shear Force/Vertical Load | Soil Models | ||
---|---|---|---|---|

Hardening Model | Mohr Coulomb (MC) Model | MC Incremental Model | ||

Aspect Ratio = 1.0 (Square) | Differential Settlement (cm) | 7.3 | 7.8 | 6.37 |

Positive Moment (kN∙m/m) | 4850 | 5092 | 4455 | |

Negative Moment (kN∙m/m) | 130 | 121 | 120 | |

Shear Force (kN) | 4852 | 4986 | 4916 | |

Total Vertical Load (kN) | 138,801 | 138,801 | 137,014 | |

Aspect Ratio = 1.75 (Rectangular) | Differential Settlement (cm) | 6 | ||

Positive Moment (kN∙m/m) | 2562 | |||

Negative Moment (kN∙m/m) | 213 | |||

Shear Force (kN) | 1401 | |||

Total Vertical Load (kN) | 138,812 |

with Mohr-coulomb incremental stiffness (MCI) failure criteria. The computed value of raft maximum positive moments are (4850, 5092, 4455 kN∙m/m), raft maximum negative moments are (130, 121, 120 kN∙m/m), and raft maximum shear force are (4852, 4986, 4916 kN) using hardening soil (HS) model, Mohr-coulomb (MC) model and Mohr-coulomb incremental stiffness (MCI) model respectively. The maximum positive moment, maximum negative moment and maximum shear force in the raft obtained from the interaction analysis are 2563 kN∙m/m, 213 kN∙m/m and 1401 kN respectively for building aspect ratio of 1.75.

The analysis results of piled-raft foundation model, developed without the superstructure and loading directly to structure based on tributary area of columns and loading through super-structure with or without phasing of construction, is shown in the

Settlements/Max. Moments/Max. Shear Force/Vertical Load | Proportional Depth of Loose Sand (HS) Layer | ||
---|---|---|---|

25% Loose Sand | 50% Loose Sand | 75% Loose Sand | |

Differential Settlement (cm) | 9.26 | 7.3 | 7.26 |

Positive Moment (kN∙m/m) | 5129 | 4850 | 4809 |

Negative Moment (kN∙m/m) | 117 | 130 | 129 |

Shear Force (kN) | 2390 | 4852 | 2295 |

Total Vertical Load (kN) | 138,759 | 138,801 | 138,767 |

Settlements/Max. Moments/Max. Shear Force/Vertical Load | Mode of Super Structure Loading | ||
---|---|---|---|

Single Phase Super-Structure Loading | Without Super-Structure-Direct Loading | Sequential Loading | |

Maximum Settlement (cm) | 23.19 | 23.45 | 22.74 |

Differential Settlement (cm) | 7.3 | 6.76 | 7.32 |

Positive Moment (kN∙m/m) | 4850 | 3916 | 4856 |

Negative Moment (kN∙m/m) | 130 | 170 | 131 |

Shear Force (kN) | 4852 | 3626 | 4992 |

Total Vertical Load (kN) | 138,801 | 138,808 | 138,804 |

the super structure.

Top Deflection (mm)/Axial Load (kN) | Mode of Super Structure Loading | ||
---|---|---|---|

Super-Structure Loading | Sequential Loading | Without Super-Structure Model | |

Top Deflection (x-dir.) | 23.19 | 23.45 | 22.74 |

Top Deflection (y-dir.) | 7.3 | 6.76 | 7.32 |

Axial Load (max.) | 4850 | 3916 | 4856 |

Axial Load (min.) | 130 | 170 | 131 |

Storey Level | Max. Slab Deflection/Max. Moments | Mode of Super Structure Loading | |
---|---|---|---|

Single Phase Super-Structure Loading | Sequential Loading | ||

First Floor | Deflection (mm) | 34.6 | 38.8 |

Positive Moment (kN∙m/m) | 64 | 64 | |

Negative Moment (kN∙m/m) | 97 | 97 | |

Eighth Floor | Deflection (mm) | 37.5 | 24.0 |

Positive Moment (kN∙m/m) | 65 | 63 | |

Negative Moment (kN∙m/m) | 95 | 100 | |

Roof | Deflection (mm) | 38.6 | 16.1 |

Positive Moment (kN∙m/m) | 66 | 61 | |

Negative Moment (kN∙m/m) | 94 | 106 |

building are respectively (66 kN∙m/m, 94 kN∙m/m), (65 kN∙m/m, 95 kN∙m/m) and (64 kN∙m/m, 97 kN∙m/m) due to loading through building super structure and while these forces respectively are (61 kN∙m/m, 106 kN∙m/m), (63 kN∙m/m, 100 kN∙m/m) and (64 kN∙m/m, 97 kN∙m/m) due to sequential loading of the super structure.

The analysis of combined piled-raft foundation of multi-storey building is very challenging because of complexities involved in the interaction between the components of building structure and soil field. The analysis of the tall building structure with complex foundation system in non-uniform (layered soil) soil field should include the interaction of structure-foundation-soil. In this study, the finite element 3D interaction analysis of building structure having piled-raft foundation in two layered non-cohesive soil field is carried out using PLAXIS 3D foundation code. The complete interaction among the soil field depth, soil layer type with foundation and foundation with super-structure with different aspect ratio and loading mode has been evaluated. The available literatures on soil-piled-raft foundation analysis are based on direct loading of superstructure on raft and without considering interaction of superstructure and foundation. The foundation soil in piled-raft foundation-soil models without including super structure will be stiffer than models with the one-phase super structure loading or sequential super structure loading.

The foundation structure and soil field response is significantly affected by different building structure shape and soil failure models. The foundation soil settlement and raft differential settlement is highest using Mohr- coulomb (MC) failure criteria of soil field among the HS, MC and MCI failure criteria. The soil field response in layered soil is also affected by presence of lesser stiff layer below the raft. The soil behavior in piled-raft foundation is not much affected by lesser stiff layer having thickness more than the pile length. A clear foundation- structure interaction effect is observed on the building superstructure components behavior with application of construction loading sequentially. The wide variability of deflection and moments of the floor slab is also observed due to loading of super structure applied in sequential manner which is not observed when super structure loading is applied as a single phase. The deflection and moments of the first floor slab is observed highest due to loading of super structure applied in sequential manner but observed decreasing on upper floors of building structure.