7_Pile Raft Foundation Analysis With Superstructure and Substructure

Pile Raft Foundation Analysis

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midas GTS NX midas Soilworks

Index • Introduction : Pile raft foundation,Need, Favorable/unfavorable conditions Design Approach

• Numerical Analysis: Need, Preliminary/Complete Analysis • Case Study: Modelling of SuperStructure & SubStructure in GTS NX, Advantages

• Structural Soil Model : Rigid & flexible foundation,Determination of Soil Spring • Introduction to P-Y non linear Analysis: Soilworks

Pile-raft foundation

Project Application: Kingdom Tower Consultant: Langan International

Foundations which make use of raft and piles to reduce differential settlements leading to considerable economy are referred to as pile-enhanced raft or Pile raft Foundations www.midasuser.com

Need for Pile-Raft foundation 1) 2) 3) 4)

Reduce the maximum Settlement Settlement tlement Control Differential settlement Economical design Lateral loads

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Favorable Conditions for Pile-Raft According to Poulos, following Soil profiles are favorable for Pile-raft a) Soil profiles consisting of relatively stiff clays b) Soil profiles consisting of relatively dense sands

Unfavorable Conditions for Pile-Raft Situations which may be unfavorable to pile raft foundation a) Soil profiles containing soft clays near the surface b) Soil profiles containing loose sands near the surface c) Soil profiles which contain soft compressible layers at shallow depths d) Soil profiles which are likely to undergo consolidation settlements due to external causes e) Soil profiles which are likely to undergo swelling movements due to external causes

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Two Stage Design process 1) Approximate preliminary Stage It is just to assess the feasibility of Pile-raft foundation for the project and estimation of the required number of piles to satisfy design requirements

2) Complete analysis procedure It is to obtain detailed design information a) Spacing of the Piles b) Pile diameter c) Pile location/Arrangement of Piles

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In preliminary analysis one need to check the feasibility of pile raft and at least predict with the minimum number of piles meeting the design requirements

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It is to obtain detailed design information a) Spacing of the Piles- Hit & Trial Approach b) Pile diameter- parametric Study oach c) Pile location/Arrangement of Piles- Hit & Trial Approach

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Pile raft Foundation

Raft Foundation www.en.midasuser.com

With Raft

With Pile Raft

% Reduction in displacement

3.96

2.38

39.89

3.3

2.15

34.84

3.96

2.4

39.39

3.3

2.16

34.67

Maximum

3.96

2.40

39.14

Minimum

1.46

1.08

26.57

Differential Settlement

2.49

1.33

46.51

Displacement(mm)

Pile raft Foundation helps in reducing maximum Settlement and has considerable effect in reducing differential settlements

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Advantages: 1) Real ground Simulation considering Sub surface Soil profile variation 2) Considering SSI to evaluate Pile-raft Analysis 3) Dynamic analysis of whole structure considering damping of Soil

Response Spectra Analysis-Displacement contours

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Spring Stiffness The K-value(Winkler Spring or modulus of Subgrade reaction) is the ratio of pressure over displacement

K=p/͌ Foundation response Significant differences in response between Winkler and Continuum models - Rigid foundations - Flexible Foundations

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Solution adopted: Approximate Approach 1) Stiffness of outermost springs should be increased to account for increase in soil rigidity. - For “Stiff” Soil, outer springs stiffness may increased by 30-40% - For “Soft” Soil, they may need to be increased by 2-3 times!

2

1

Generate Building with raft as plate element with fixed boundary condition

Run the analysis and obtain reaction at each node of the plate element

3

4

Generate the raft with same meshing and soil mass in GTS NX and apply the reaction on each node as obtained in previous step www.midasuser.com

Run the analysis in GTS NX and obtain displacement at each node K=P/ɷ

Continuum model Settlements

Structural model Settlements

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P-Y Non Linear Analysis PL P

Layer A y P

Layer B y

Layer C

P y

Layer D

P

y

Points to be noted: 1. Different p-y curves have been developed for different soil types. 2. Reflects nonlinear behavior of soil. 3. Analysis summary -Pile : divided beam -Soil : nonlinear spring ( p-y curve : input parameter) 1) Calculate the deflection of each pile segment 2) Depending on the deflection, calculate soil resistance from the p-y curve 3) Deflections and soil pressures are interconnected 4) Keep iteration to obtain the correct value 5) Based on finite difference method

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Learnings: 1) Design Approach of Pile-raft using numerical analysis 2) Consider of Superstructure model from midas Gen to midas GTS NX 3) Determination of Spring Stiffness for Structural model 4) 3D Modelling of Pile-raft foundation in GTS NX 5) Effectives of pile-raft in reducing differential Settlement

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