Raft analysis : Piled-raft analysis steps

The following are the main steps required for doing a piled-raft analysis.

1. Build up a piled-raft model, the raft can be modelled by a grillage using beam elements or by a horizontal plate using 2D elements. A pile should be model by a number of vertical beam elements, since pile-soil interaction is defined by the nodes on the pile, the number of beam elements used to model a pile depends on the length and section sizes of the pile. Normally, 10 beam elements may be required to model a single pile.
2. Define loads on the raft in the same way as for doing other GSA analysis. If there are loads acting on pile directly, they can also be defined.
3. Open Raft analysis specification dialog box from the Data explorer or from menu Model > Data modules > Specification > Raft analysis specification... to define rigid boundary level and other soil settlement analysis parameters.
4. Open Soil profiles table from the Data explorer or from menu Model > Data modules > Raft > Soil > Soil profile to define soil properties. One soil profile defines the soil properties in a vertical line from the top surface of the soil to the rigid boundary level below it. One soil profile may include soil properties of many layers depending on the real soil conditions. There is no limit for the number of soil profiles to be defined and the number of soil profiles depends on the actual site requirements. Soil profiles are then assigned to rectangular areas called soil zones.
5. Open Soil zones table from the Data explorer or from menu Model > Data modules > Raft > Soil > Soil zones to assign soil profiles (properties) to relevant rectangular areas called soil zones. If soil zones are overlapped, the overlapped areas will use the soil-profile defined later in the soil zone table.
6. Open Pile-soil interaction properties table from the Data explorer or from menu Model > Data modules > Raft > Soil > Pile-soil interaction properties to define the pile-soil interaction properties. The properties include the maximum soil stresses (strengths) at the top and bottom of each soil layer in X, Y & Z directions as well as at the pile base. The interaction in X, Y & base are normal contact strength and the interaction in Z is shear strength between pile surface and soil in vertical direction. A Pile-soil interaction property is referred to by one or more soil layers in soil profile table. The pile-soil interactions are non-linear and the non-linear relationships are defined by Pile-soil interaction coefficients which is a curve defining the relationship between normalized relative soil-pile displacements and the reduction factor of the soil strength. Which Pile-soil interaction coefficients (curves) to be used for this pile-soil interaction in X, Y, Z & base need to be defined in this table as a reference to the Pile-soil interaction coefficients table.
7. Open the Pile-soil interaction coefficients table from the Data explorer or go to Model > Data modules > Raft > Soil > Pile-soil interaction coefficients to define the pile-soil interaction coefficients (curves). These coefficients (curves) define the relationship between the normalized relative soil-pile displacements and the reduction factor of the soil strength. It is used (referred to) by the pile-soil interaction properties.
8. Open Raft interaction table from the Data explorer or from menu Model > Data modules > Raft > Raft interaction to define the nodes on raft to interact with soil. The interaction areas of the raft interaction nodes can be defined on this table or choose Automatic to allow GSA to calculate the interaction areas automatically. The elevation of the interaction can also be defined here or choose Automatic to take the nodal z coordinate as the interaction elevation. Minimum and maximum soil pressure can also be defined in this table. Zero minimum soil pressure means that soil will not take any tensile stresses. If a negative minimum soil pressure is defined, it will be used as the soil tensile strength. The maximum soil pressure is the compressive strength of soil. If this pressure is reached during the analysis, the soil-raft contact pressure will not increase any further to allow soil local yield effect to be considered.
9. Open Pile interaction table from the Data explorer or from menu Model > Data modules > Raft > Pile interaction to define the nodes on piles to interact with soil. The interaction areas of the pile interaction nodes can be defined on this table or choose Automatic to allow GSA to calculate the interaction areas automatically. The elevation of the interaction can also be defined here or choose Automatic to take the nodal z coordinate as the interaction elevation. Normally the elevation of soil-pile interaction should be automatic as pile interaction nodes are not at the same level.
10. Now we are ready to do piled-raft analysis by going through the analysis wizard start from menu Analysis > New analysis task... in the same way as doing other GSA analysis.

See also the section on raft analysis steps and the iteration scheme in section Raft analysis (theory)