Abstract
Principal stress rotation (PSR) has been known to act as ‘loading’ on soils since the seminal work of Arthur et al. some 30 years ago, using the directional shear cell. However, the key insight – reflected in the titles of the original papers – that soils yield under constant stress invariants if the principal stresses rotate, has been consistently neglected in virtually all constitutive models of soil as well as in all procedures/methods of geotechnical engineering practice. This neglect is a pity, as a rather simple softening (annealing) of yield surface size, proportional to the rotation of principal stress, captures much of soil behaviour that is measured in the cyclic simple shear test – an idealisation demonstrated in this paper using a minimal extension of the generalised critical state model NorSand (although the idealised effect of PSR could be implemented in any comparable model as the ideas are general). The extended model captures much of the behaviour encountered in the large body of test data on Fraser River sand. Equally, the implementation of PSR annealing makes clear that there is a further loading process for soil: it is suggested that the rotation of the strain increment direction is the missing additional factor. The model is provided as a downloadable, open-source code Excel/VBA program as a supplement to this paper.
Highlights
Principal stress rotation (PSR) is pervasive in engineering works involving soil, arising in the simplest static loading of a spread footing through to very complicated cyclic rotations during wave loading of offshore structures
Fitting methodology for CSS The basic premise of critical state models, including NorSand, is that soil properties do not vary with void ratio or confining stress: the calibration reported earlier and giving the properties shown in Table 2 must be used ‘as is’
The proposition explored in the present work is that PSR softens the yield surface of a soil
Summary
Principal stress rotation (PSR) is pervasive in engineering works involving soil, arising in the simplest static loading of a spread footing through to very complicated cyclic rotations during wave loading of offshore structures. The NorSand yield surface has the familiar bullet-like shape of the original Cam-Clay model but with one important difference – there is an internal cap so that the soil cannot unload to very low mean stress without yielding This internal cap is taken as a flat plane, and its location depends on the soil’s current state parameter.
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