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Modelling installation effects in geotechnical engineering

Final Report Summary - GEO-INSTALL (Modelling installation effects in geotechnical engineering)

Executive Summary:

The aim of the proposed project was to develop new numerical tools for modelling large deformation problems in geotechnics, as these affect all constructions. Infrastructure construction all over the world involves installation of structural elements, such as piles, ground improvement and reinforcement into natural geomaterials, such as soils. The installation process itself can be quasi-static (for example jacked piles) or dynamic (e.g. stone columns and driven piles). The installation involves very large deformations and causes changes in the pore pressures and the structure of the soils, changing both the apparent stiffness and the strength of the soil. These issues were explored in the project by developing new models and techniques, both within the context of large deformation finite element analyses and the so-called Material Point Method (MPM), which is a meshfree numerical method based on FEM.

As the IAPP scheme is fundamentally a knowledge-exchange and secondment scheme, most of the actual research had to be funded by other sources. As part of the project, the material modelling capabilities for cyclic and dynamic loading have been extended, and these are now increasingly applied to practical context. This involved development of user-defined models for clays and sands. The former incorporate features such as anisotrotropy, bonding and destructuration, rate effects and ability to model cyclic loading/unloading behaviour including hysteresis. The former include void ratio dependency and ability to model sand liquifaction. Under the sub-theme of enhanced finite elements, new approach for modelling periodic ground improvement using volume averaging technique was developed and is currently being validated against instrumented field structures. Similarly, the embedded pile concept was further validated and developed during the project and these advances have been included in the latest release of the Plaxis finite element code. With regards of soil modelling, the work continues to some extent as part of FP7 IAPP project CREEP (Creep of Geomaterials), coordinated by NTNU, in which Deltares (GDI), NGI and USTRAT are partners (USTRAT has since the start of the project been replaced by CHALMERS).

For modelling large deformation and impact problems, associated with installation both quasi-static and dynamic MPM codes were developed as part of the project. These codes are still in rather early stages of development to allow for large scale commecial use, and this work continues as part of IAPP project MPM-DREDGE (PIAP-GA-2012-324522) focussing on modelling fluid-soil interaction.