Recent stress history effects in clays and associated improvements to the BRICK model

It is known that the stiffness of many soils is higher at small strains than at the large
strains used in typical triaxial testing. Understanding this initial high stiffness and
the factors affecting it is key to accurate displacement predictions.
The recent stress history (RSH) effect describes how the stiffness degradation curve
at small strains is affected by the immediately preceding stress path. The effect is
usually described in terms of stress path rotation angle between an approach path
and the shear probe upon which stiffness degradation is measured.
The principle aim of the work was to investigate the relationship between the RSH
effect, approach path length, and time dependent effects in more detail. A combination
of physical testing and simulations with the BRICK constitutive model were
used to achieve this. This has shown that in multistage testing the residual RSH
effect from the first test increased the stiffness exhibited in the second, thus masking
the natural behaviour. In addition the stress path rotation from consolidation to
testing caused a stiffness reduction in the first test. This effect, newly termed recent
consolidation history (RCH), was found to be degraded by the first test and so did
not impact on the second. When allowance was made for these effects trends seen
in the literature were supported.
Due to the complex stress paths used in the physical testing, a number of improvements
were required to the strain rate dependent (SRD) BRICK model to allow accurate
simulations to be run. These included implementation of a bisecting iteration
within the strain calculations, and a coordinate decent routine to allow creep and
creep rupture to be modelled. The model was then used to simulate the physical
tests to allow more insight into the interplay between the effects identified in the
physical testing, ultimately supporting the conclusions drawn.