1. Raft & Piled-raft analysis (Soil-structure interaction analysis)
23/04/2017
Raft & Piled-raft analysis (Soil-structure interaction analysis)

2. Raft & Piled-raft analysis
1. Introduction
2. Data required for raft & piled-raft analysis
3. How GSA raft & piled-raft analysis works
4. Running GSA raft & piled-raft analysis
5. Inspecting raft & piled-raft analysis results

3. Raft & Piled-raft analysis
1. Introduction
In theory, soil-structure interaction should always be considered in structure analysis
In practice, structure analysis and soil settlement analyses are sometimes done separately
If soil settlements are very small (its effects on top structure are negligible), structure can be analysed assuming supports are pinned or fixed
If soil settlements are relatively large, iterative procedures may be used, i.e. do structure analysis using spring support to represent soil , do soil settlement analysis to estimate soil stiffness
If soil settlements are relatively large and its effects on structure are significant, soil structure integrated model may need to be built and analysed
Including the whole structure in the soil-structure model (rare)
Including only the foundation parts of the structure model (common)

4. Raft & Piled-raft analysis
Introduction
3. Soil region need to be considered
The larger the better, but not unnecessarily too large
Assuming soil is rigid beyond some level when soil settlements are negligible (reducing soil region)

5. Raft & Piled-raft analysis
2. Data required for raft & piled-raft analysis
Raft & piled-raft data – structural model
Soil data – soil properties etc
Interaction data – define how soil & raft (pile) interact

6. Raft & Piled-raft analysis
2. Data required for raft & piled-raft analysis
1. Raft & piled-raft data - structural model
The same as other GSA models, but the top structure may be ignored in a raft or piled-raft model
The vertical direction should be free for raft-soil interaction nodes
All three directions should be free for pile-soil interaction nodes
The spring supports representing soil during the analysis will be generated and deleted automatically, so it is not necessary to define them in raft/piled-raft model

7. Raft & Piled-raft analysis
2. Data required for raft & piled-raft analysis
2. Soil data - soil properties etc
1. Raft analysis specification
Rigid boundary level – soil deformation below this level will be ignored
Maximum E ratio – sub-layer will be used if the E ratio of this layer is larger than this value
Global Poisson’s ratio (Boussinesq only) – used in calculating soil stress (note: Young’s modulus & Poisson’s ratio of each layer will be used to calculate the vertical strains and the settlements)
Mindlin analysis method (only used in pile analysis)
Use weighted average
Use greatest stiffness
Use stiffness at displacement point

8. Raft & Piled-raft analysis
2. Data required for raft & piled-raft analysis
2. Soil data - soil properties etc
2. Soil profiles – define soil properties in vertical direction
Top level
Etop m
Ebot
rigid level – deformation below is ignored
Layer 1
Layer 2
Layer 3

9. Raft & Piled-raft analysis
2. Data required for raft & piled-raft analysis
2. Soil data - soil properties etc
3. Soil zones – assign soil profiles (property) to areas in plan, soil profile defined latter will be used for overlapped areas
Zone 2 (profile j)
Zone 1 (profile i)
Zone 3 (profile k)

10. Raft & Piled-raft analysis
2. Data required for raft & piled-raft analysis
2. Soil data - soil properties etc
Pile-soil interaction properties (used by soil profiles), define:
Maximum soil stresses for each layer in different directions
Assign Pile Soil Interaction Coefficient (PSIC) curve

11. Raft & Piled-raft analysis
2. Data required for raft & piled-raft analysis
2. Soil data - soil properties etc
5. Pile-soil interaction coefficients (PSIC) curve, define:
the variations of pile-soil interaction forces along with the normalised relative displacement between pile & soil

12. Raft & Piled-raft analysis
2. Data required for raft & piled-raft analysis
2. Soil data - soil properties etc
6. Other data – normally not required and do not need to be defined
Load data – define the extra loads directly on the soil in addition to the loads from raft & piles
Displacement data – extra displacement output points, lines or grids
Non-linear curve – to define the variations of soil Young’s modulus along with strains (only used in Boussinesq analysis and ignored by Mindlin analysis)

13. Raft & Piled-raft analysis
2. Data required for raft & piled-raft analysis
3. Interaction data – define where soil & raft (pile) interacts
Node list – soil-raft interaction is through GSA nodes, the nodes list defines the interaction nodes
Dimension of interaction area – rectangular area defined explicitly by X & Y dimensions or generated automatically
Elevation of interaction – the level of the soil-raft (pile) interaction – it is relative to the elevation in soil profile or it can be generated automatically
Minimum pressure (raft interaction only)
= 0, soil will take compression only
< 0, defined as soil tensile strength
> 0, not allowed
Maximum pressure (raft interaction only)
define the maximum pressure that soil can sustain

14. Raft & Piled-raft analysis
3.1 How soil-raft interaction works
Add vertical support springs to the nodes on raft that interact with soil (default stiffness Ko = 1.0e7kN/m)
Do static analysis of the raft model only to obtain forces Fi & displacements draft at the interaction nodes
Calculate soil pressure load Pi for each of the interaction areas and apply them to soil
Pi = Fi/Ai
if Pi < Pmin, let Pi = 0
if Pi > Pmax, let Pi = Pmax
Do Pdisp (soil settlement) analysis to obtain soil settlements dsoil at the interaction points
Compare dsoil & draft, If the differences between them are smaller than the preset tolerance for all the interaction nodes, STOP, otherwise go to step 6
Update support spring stiffness according to the spring forces and the soil settlements Ki+1 = Fi/ draft Go to step 2

15. Raft & Piled-raft analysis
3.2 How soil-pile interaction works
Add three non-linear springs to each pile interaction node in x, y & z directions
The forces in these spring are calculated from the normalised relative displacements between soil and pile at the same points according to the PSIC curve, maximum soil stresses & interaction areas
The maximum forces in the nonlinear spring are controlled by the interaction area and the maximum soil stresses defined in pile-soil interaction properties table
In the converged state, the forces in the nonlinear springs make the pile in equilibrium with the rest of the structure.
See separate PPT file for details

16. Raft & Piled-raft analysis
4. Running GSA raft analysis
1. Analysis option
Mindlin – faster, no soil stress results, results are not sensitive to the number of soil layers, linear analysis only
Boussinesq – relatively slow, give soil stress results, results are sensitive to the number of soil layers (only for pure raft analysis), soil nonlinearity can be considered
2. Convergence criteria
1. Tolerance – the differences between soil settlements and raft displacements and the differences of the force in the nonlinear spring and the soil reaction force for pile interaction points
2. Absolute – define the absolute tolerance of displacement & force
3. Relative – define a percentage of tolerances and the program will calculate the tolerance automatically based on the largest soil displacements and pile-soil interaction forces

17. Raft & Piled-raft analysis
4. Running GSA raft analysis
3. Damping & initial soil stiffness
Damping is not normally required for raft analysis unless it is difficult to converge, e.g. there are heaves
Damping is normally required for piled-raft analysis and the damping ratio can be as high as 99%
Apply damping only affects the analysis speed, not the analysis results
Initial support stiffness – used only for raft interaction nodes, it only affects the analysis speed and not the results. If the given initial support stiffness is close to real soil stiffness, the analysis will converge quicker. Normally default value is used as we do not know soil stiffness beforehand

18. Raft & Piled-raft analysis
5. Inspecting GSA raft analysis results
Raft results – the same as the results from other analyses, e.g. nodal displacements, element forces & moments etc
Soil results
Soil settlements
Soil stress (Boussinesq analysis only)
Soil-raft interaction
Soil – raft contact pressure
Pile – soil contact pressures