Define Piles

Overview

Modelling piles using piles elements in RS2 is more accurate as it accounts for that fact that piles are inside the surrounding material. Using structural interface/composite liner to model piles will be limited because those elements are cutting the surrounding material and are more appropriate to model walls and the properties will have to recompute to get the equivalent parameters.

Pile element include a liner and a joint element representing the pile and soil/pile interaction, respectively.

Pile Properties

CONNECTION TYPE

• Free: Pile head will not connect to raft/beam
• Hinge: Only force will be transfer from pile head to raft/beam
• Rigid: Pile head will have rigid connection to raft/beam (fully force and moment transfer)
• Semi Rigid: Full force will be transfer but only part of moment will be transfer from/to raft/beam. User will also be require to input MMax value for maximum moment capacity of the connection

Length: Pile length

Out of plane Spacing: The spacing between piles in the out-of-pane direction

Interface: To model the pile/soil interaction behaviours

SKIN RESISTANCE:

There are 5 types of skin resistances provided. Except for Elastic, the pile will slip of the shear stress on a interface exceeds the shear strength as defined by the skin resistance parameters. Please note that unlike joint where the shear strength was determine by the normal stress in joint, the shear strength in pile was determine by the confining stress of the surrounding material which calculated as a normal component of the stress to the pile direction.

All the skin resistances except “Material dependent” will require to input normal and shear stiffness for the interface between pile and surrounding material. This stiffnesses should be determine by curve fitting the pile modelling results with experiment data.

Elastic:

• The pile will not slip

Morh Coulomb:

• Cohesion
• Friction Angle
• Residual values of cohesion and friction angle: The residual strength parameters will be in effect if pile slip
• Use shear resistance cut off: if the option is selected then the shear stress on the interface will not exceed the cut off value. It is useful for come material such as clays where the skin resistance value is capped at some certain depth.
• Perimeter: pile perimeter, it is used to calculate the strength of the interface around the pile. The shear strength per pile unit length will be the yield stress multiple to the pile perimeters.

Linear:

• The shear strength per unit length of a pile will be calculate based on the Max traction at the top and Max traction at the bottom.

Multi Linear:

• The shear strength per unit length of a pile will be calculate based on the Max traction input along the pile. The max traction input can be defined by 2 methods: Distance from the top or by Elevation (y coordinate)

Material dependent:

The shear strength per unit length (and optional stiffness) will be derived from the soil or rock materials through which the pile passes. The Material Dependent option is applicable if the soil or rock materials use the Mohr-Coulomb or Hoek-Brown strength options. It is not applicable for the Cam Clay strength options.

The material dependent joint strength is defined in terms of an “Interface Coefficient” which is used as a multiplier for the cohesion and friction angle of the surrounding material. The Interface Coefficient ( Ci) is applied as follows to determine the interface cohesion and friction angle from the material cohesion and friction angle:

Cjoint = Ci * Csoil

Phi = ArcTan (tan(Phisoil)* Ci)

Residual joint strength parameters are similarly determined from the residual soil/rock strength parameters, if the surrounding material has failed. For Hoek-Brown materials, equivalent values of cohesion and friction angle are first determined, and then the interface coefficient is applied to determine joint strength.

The Material Dependent joint stiffness coefficient is optional, and can be defined by selecting the Material Dependent Stiffness option and entering a Stiffness Coefficient (Cs).

The stiffness coefficient is applied as follows to determine the normal and shear joint stiffness based on the material modulus:

Ks = Cs * Young's Modulus soil;

Kn = 10 * Ks

These values were found adequate for most applications; however, material dependent joint stiffness is optional, and the user can input the stiffness values manually if required.

Use shear resistance cut off: if the option is selected then the shear stress on the interface will not exceed the cut off value. It is useful for come material such as clays where the skin resistance value is capped at some certain depth.

Perimeter: pile perimeter, it is used to calculate the strength of the interface around the pile. The shear strength per pile unit length will be the yield stress multiple to the pile perimeters.

Base resistance

With the “Use Base Resistance” checkbox selected, specify the Base Normal Stiffness taken from a pile load test to account for the interaction between pile tip and the soil at the tip.

BASE FORCE RESISTANCE

With the “Use Base Resistance” checkbox selected, specify the Base Force Resistance. To remain in the elastic region, the normal force, Fn, at the base of the pile must be less than the Base Resistance value: Fn < Base resistance value

Note that base only works when the Fn > 0 (compression)

BEAM

Application:

• Constant: The pile will have only one section and use the liner specified in “Liner properties”
• Define beam segment by length: Pile can consist of multiple sections defined by “Define Beam Segment” table

FORCE / DISPLACEMENT

If you wish to apply a force/moment directly to the pile/forepole you can do so in the the Force/Displacement tab, select appropriate type (Force/Displacement) and location (Top/Bottom) and enter the appropriate value of force or displacement in X and Y direction.