On the long-term behaviour of tension loaded piles in natural soft soils

Doctoral thesis, 2016

The complexity and scale of new infrastructure projects have challenged the current geotechnical design practice. Urban areas are growing at a fast pace in the horizontal and vertical direction, with taller buildings and deeper underground constructions in already densely populated areas. The West Link project in Gothenburg city is a good example of the latter. The geotechnical challenges in this project include deep excavations and deep foundations in soft sensitive natural clays. An important aspect in this case is the large buoyancy load arising from the ground water pressure, the stability of the soil mass in the excavation vicinity and the unloading heave from the soil. Typically, these loads are counterbalanced by the superstructure self-weight and by bedrock anchors. The very deep clay deposits in Gothenburg, however, require traditional floating piles to sustain the permanent tension loads from these processes. Little is known about the long-term behaviour of pile foundations in deep soft soil deposits under permanent tension loads. The need for a reliable foundation system for the West Link tunnel and the limited data available on permanently loaded tension piles in soft clays motivates further theoretical and experimental investigation of this pile type in natural soft structured clays. As a result this Thesis presents new findings on the long-term behaviour of tension loaded piles in natural soft structured clays. The unique results from the field tests on six pile elements incorporate all significant stages in the pile cycle, i.e. pile installation, set-up and long-term loading, yet are sufficiently short to link the pile response to the soil behaviour of one particular layer. Furthermore, a novel cost-effective loading rig using gas springs and remote logging based on open-source software and freely available cloud storage is developed for execution of the field tests. The results indicate that the measured long-term bearing capacity is smaller than the short-term reference capacity. The difference is in the order of 20 – 30 % smaller. This reduction is attributed to the on-going creep deformations in the soil surrounding the pile shaft. These deformations cause relaxation of the effective stresses due to the kinematic constrains at the pile-soil interface. In addition to an analytical system level interpretation of the measured pile head displacement that showed only benign maximum final pile head displacements after 100 years, an advanced numerical analysis that incorporates a state-of-the-art rate dependent soft soil model is performed. The measured data and simulation results are in good agreement and corroborate previous investigations, however, for the first time the physical mechanisms underpinning the measured response are generalised and tertiary creep failure is reproduced. The long-term pile response is directly related to the behaviour of the soil adjacent to the pile shaft. Further work should focus on the evolution of the stress field and soil properties under long-term pile loading. Deviatoric creep deformations should be studied in more detail by means of element level laboratory test on natural and remoulded soft clays.