Geotechnical Software - Pile Group Settlement Analysis

Pile Group Settlement Analysis

Under the design load, the pile behaviour will be nearly elastic except for some shaft length near the top of the pile, where the ultimate interface friction may be reached and the pile may slide through soil. The parameters that are required for the group settlement estimate under the design load are the pile head displacement, load carried by the pile tip, pile tip displacement and the radius rm which may be approximated as equal to pile length. The software makes use of Randolph and Wroth (1978) approach along with Mylonakis and Gazetas (1998) procedure for including the diffraction effect.

A three step approach is followed to obtain the group vertical settlement.

  1. Based on the soil profile, pile dimension and properties, the ultimate pile capacity is estimated. Making use of the factor of safety, design load Pd for the pile is obtained.
  2. Pile head stiffness and the pile tip stiffness under the design load are obtained by carrying out axial pile analysis based on either
    1. t-z curves based on elastic properties of soil layers
    2. t-z curves based on API recommendations.
  3. Pile Group settlement is computed using the RWMG (Randolph, Wroth, Mylonakis and Gazetas) model using the results of axial pile analysis under design load, group geometry, cap conditions and pile group loading data.

Pile Capacity Estimation

The pile capacity estimation is based on the sub-soil layer properties and the methods chosen for the assessment of shaft friction and base capacity. The design load is computed from the pile capacity taking into account the design factor of safety.

Procedures available in the software for pile capacity estimation

Clay Rock Sand
Side Friction
  • API-2011
  • α method (IS-2911)
  • Semple & Rigden method
  • Kolk & Van-der-velde method
  • β method (API-2011)
  • K-δ method (API-2000)
  • K-δ-Zc method (IS-2911)
  • Meyeroff SPT method (IS-2911)
  • Approach based on unconfined strength is adopted
Base Capacity
  • Nc = 9
  • Nq-qlim method (API-2011, API-2000)
  • Nq - Zc method (IS-2911)
  • Nq-Berezantev-Zc method
  • Meyeroff SPT method (IS-2911)
  • Approach based on unconfined strength is adopted

There are options available in the software to prescribe user defined parameters.

A distance of 3D is used for developing full base resistance in strong layers. A safe distance of from pile tip of 3D is adopted to preclude punch through underlying weak layers.

Axial Capacity Estimation

The Axial pile deformation analysis is performed to determine the pile head and pile tip stiffness under the design load.

Pile is modelled as an elastic structural member having the cross section of the pile and the elastic properties of the pile material. The soil support providing the shaft friction is modelled by a set of side springs based on t-z curves. The tip resistance provided by the pile base the base is modelled by a spring based on q-z curve.

The software supports both ‘Elastic Bi-linear’ and ‘Non-Linear’ approaches for modelling the soil layers and any one of them can be selected for analysis.

In the ‘Non-Linear’ approach’ for the soil layer, based on the tmax and qmax values calculated , non-linear t-z curves (interface shear stress- vertical pile movement at that point) and q-z curve (bearing stress and toe displacement) are developed based on API-2011 guidelines. API based methods, also account for reduction in post peak adhesion in clay layers through a factor R.

In the ‘Elastic Bi-linear’ approach, for the soil layer, t-z and q-z relationships are modelled by bilinear elastic – plastic curves based on the elastic modulus, Poisson ratio, tmax and qmax for the layer.

In the case of rock layers, using the tmax and qmax values, t-z and q-z relationships are modelled by a bilinear elastic – plastic curve based on the elastic modulus and Poisson ratio of the rock layer.

The axial pile analysis follows a non-linear finite element model using the axial rigidity of the pile and the nonlinear soil support based on the t-z curves and q-z curve. . The analysis uses an Iterative approach to achieve convergence.

The analysis provides displacement of the pile head and pile tip under design load, and the load transferred at the pile base.

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