7_Pile Raft Foundation Analysis With Superstructure and Substructure

Pile Raft Foundation Analysis midas GTS NX

midas Soilworks

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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

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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

Pile-raft foundation

Project Application: Kingdom TowerConsultant: Langan International

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Need for Pile-Raft foundation1) Reduce the maximum Settlement2) Control Differential settlement3) Economical design4) Lateral loads

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Favorable Conditions for Pile-RaftAccording to Poulos, following Soil profiles are favorable for Pile-rafta) Soil profiles consisting of relatively stiff claysb) Soil profiles consisting of relatively dense sands

Unfavorable Conditions for Pile-RaftSituations which may be unfavorable to pile raft foundationa) Soil profiles containing soft clays near the surfaceb) Soil profiles containing loose sands near the surfacec) Soil profiles which contain soft compressible layers at shallow depthsd) Soil profiles which are likely to undergo consolidation settlements due to external causese) 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 procedureIt is to obtain detailed design informationa) Spacing of the Pilesb) Pile diameterc) 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 informationa) Spacing of the Piles- Hit & Trial Approachb) Pile diameter- parametric Studyc) Pile location/Arrangement of Piles- Hit & Trial Approachoach


Pile raft Foundation

Raft Foundation


With Raft With Pile Raft % Reduction in displacement

Displacement(mm) 3.96 2.38 39.89

3.3 2.15 34.84

3.96 2.4 39.39

3.3 2.16 34.67Maximum 3.96 2.40 39.14Minimum 1.46 1.08 26.57Differential Settlement 2.49 1.33 46.51

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


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

Advantages:

Response Spectra Analysis-Displacement contours

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

K=p/

Foundation responseSignificant differences in response between Winkler and Continuum models- Rigid foundations- Flexible Foundations

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Solution adopted:Approximate Approach1) 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!

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Generate Building with raft as plate element with fixed boundary condition

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

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

Run the analysis in GTS NX and obtain displacement at each node

K=P/

1

3

2

4

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Continuum model Settlements Structural model Settlements

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PL

P

y

P

y

P

y

P

y

Layer A

Layer B

Layer C

Layer D

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 segment2) Depending on the deflection, calculate soil resistance from the p-y curve 3) Deflections and soil pressures are interconnected4) Keep iteration to obtain the correct value5) Based on finite difference method

P-Y Non Linear Analysis

1-23

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

Learnings:

7_Pile Raft Foundation Analysis With Superstructure and Substructure

Documents

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