Abstract
Deep foundations are essential for the soft soil deposits that are ubiquitous in the Nordic countries. Precast driven displacement piles are commonly used, due to the ease of installation and their low cost. The stiffness and strength of the clay surrounding the piles will change during and after the pile installation process. This paper presents an effective stress based numerical framework that elaborates the pile set-up and pile ageing mechanisms observed empirically. For the trial case studied, it is demonstrated that the magnitude of the increase in axial bearing capacity over time is strongly linked to the pile installation stage. Furthermore, it is found that classic effective stress based soil mechanics concepts readily describe the observed behaviour.
Highlights
Deep foundations form a necessary foundation solution in regions with soft soils, such as the sensitive clays that are common in the Nordic countries
The data starts directly after pile installation and is normalised with the far field value for the mean effective stress p0. This value represents the mean effective stress at rest at a depth of 7 m is not influenced by the pile installation
The increase in the shaft capacity is best described as a strength recovery process towards the original intact strength before the pile is installed into the soil
Summary
Deep foundations form a necessary foundation solution in regions with soft soils, such as the sensitive clays that are common in the Nordic countries. Pre-cast driven piles remain economical due to controlled (mass) production and ease of installation. The large shear distortions from pile driving combined with the low hydraulic conductivity of clays, lead to significant soil disturbance and excess pore water pressures that subsequently dissipate. The generation of excess pore water pressures during constant volume mass-displacement and their subsequent dissipation lead to additional volume change over time. Additional creep might be triggered in the soil, especially in sensitive clays that have a large degree of initial bonding [1]. Within the discipline the increase in pile capacity over time after pile installation is referred to as ‘setup’
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