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Piles 

GEO5

Geotechnical software

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Vertical bearing capacity - Spring method

This analysis provides the limit loading curve and distributions of forces and displacements developed along the pile.

The main advantage of this module is availability of the required input parameters of soils around the pile – the user is asked to specify the angle of internal friction, cohesion, unit weight and deformation modulus of a given soil.

The solution procedure is based on a semi-analytical approach. The pile is represented by standard beam elements. The response of surrounding soil follows from the well known solution of layered subsoil as a generalization of the Winkler-Pasternak model. The elastic rigid plastic response in shear is assumed along the pile-soil interface in view of the Mohr-Coulomb failure criterion. The normal stress acting on the pile is determined from the geostatic stress and soil (concrete mixture) pressure at rest.   

The influence of water in the vicinity of pile is not only introduced into the shear bearing capacity of the pile skin, but also affects the depth of influence zone below the pile heel. 

The pile may reach incompressible subsoil, which substantially influences its response. This effect is also taken into account in the program. The pile settlement can also be influenced by the settlement of the surrounding terrain. In particular, settlement of soil may reduce the pile bearing capacity. The pile settlement increases without increasing load. This phenomenon is modeled in the program as so called negative skin friction.

The analysis may also account for the influence of technological process of pile construction on the stiffness of pile foundation.

The solution procedure consists of several steps:

1) The pile is represented as a member composed of several beams. Subdivision into individual element complies with the condition that the ratio between the pile length and its diameter should be approximately equal to 2,5.  The minimum number of beams, however, is 10.
2) Each element is supported at its bottom node by a spring. The spring stiffness serves to model both the shear resistance of skin and at the pile heel the stiffness of soil below the pile heel.
3) For each element the limit value of shear force transmitted by skin Tlim is determined.
4) The pile is loaded at its top end by increments of the vertical load. For each load increment the magnitude of spring force for each element is determined. This value is then compared with the value of Tlim for a given element. If a certain spring force exceeds the value of Tlim its magnitude is set equal to Tlim.
Analysis for this load increment is then repeated so that the force is redistributed into other springs. Such an iteration within each load increment proceeds as long as each currently active spring does not transmit force that is less than its corresponding Tlim. Gradual "softening" of individual springs results in deviation of the limit loading curve from linear path. It is evident that for a certain load level all springs will no longer be capable of increasing its force and the pile begins to settle in a linear manner supported only by the heel spring that has no restrictions on the transmitted force.
5) As a result the analysis provides the limit loading curve, forces developed in the pile and a graph showing variation of shear as a function of deformation at a given location.

Language: english