Strain softening for micropiles under tensile loading

Abstract

In March 2016, an addendum to the Dutch design guideline for micropiles, CUR 236, was
published. This addendum elaborated on the ‘axial stiffness of micropiles’. The softening
behaviour was here first mentioned as an aspect relevant for the axial stiffness of the
micropiles. Due to the slender geometry of the micropiles, this further translates into axial
capacity. Up to this moment very little research was done into this phenomenon and to be
conservative a ‘best guess’ of 50% of the peak shear stress was made.
The goal of this thesis is to have a more profound understanding about this (strain) softening
behaviour, leading to the main research question: “How does strain softening manifest for
micropiles under tensile loading?”. This strain softening behaviour can be further subdivided
into the mobilisation from peak to residual shear stress and the reduction of the residual
shear stress relative to the peak. Furthermore, the installation effects play a big role in the
bearing capacity. Also, some thought is given to the large scale measurements of the
behaviour.
Three different paths are chosen to research the strain softening behaviour. Numerical
modelling, small scale tests on sand and large scale pile tests are performed. In case of the
numerical modelling, the large scale situation is simulated in the numerical software Plaxis,
using an axisymmetric model and the hypoplastic constitutive model. Direct shear tests are
assumed to represent a small part of the micropile, simulating the local softening behaviour.
Numerous tests are performed varying relative density and top pressure to assess the local
strain softening behaviour. Last, a method is developed to quantify the occurring ratio
between peak and residual shear stress in-situ, extending on the current prescribed testing
procedure (CUR 236, 2011).
The only constitutive model available that describes the critical state soil behaviour for sand
is the hypoplastic sand model. However, the combination of the tensile loading, softening
implementation into the hypoplastic sand model in Plaxis and the deformations, induce
considerable inaccuracies and physically impossible behaviour. This rejects further
elaboration with this approach. The direct shear tests are performed on only sand, based on
the results of Uesugi & Kishida (1987), assuming a rough surface. In summary, a clear
dependency on relative density and average confining pressure is observed. Varying from a
reduction of approximately 30% for samples with the maximum possible density to 0% for a
sand with a medium dense packing. Furthermore, the displacements necessary to mobilise
from peak to residual shear stress show a uniform trend, independent of relative density or
pressure. Quantitatively, the displacement ranges between 2 and 3 mm. In the large scale
tests, due to unexpected, structural failure, the majority of the test results, unfortunately,
were unusable. The pile test that did produce workable results, behaved as expected. It
could be deduced that the used methodology works.
The use of the hypoplastic constitutive model in Plaxis, does not give the expected results. It
can thus be concluded that a fully coupled stress – strain analysis of the problem is
unfortunately impossible. Concluding on the direct shear tests performed, the implementation
in the CUR 236 addendum is too simplistic. In terms of ratio between peak and residual
shear stress, a dependency based on relative density and average confining pressure should
be included. In terms of displacement, the found displacement is larger. It should however be
noted that the direct shear tests are only a simplification, approximating the actual situation.
The performed large scale tests do show results in line with expectations and could be used
in the future to assess the softening behaviour in-situ.