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

Abstract: 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.