Variable Direction Dynamic Cyclic Simple Shear

Centrifuge modelling is an advanced physical modelling technique for testing reduced scale geotechnical engineering models in the enhanced gravity field of a centrifuge. The main principle of centrifuge testing is the equivalence between the small scale model and the full-scale prototype via well-established scaling laws. In order to scale the stresses correctly, the centrifuge test is performed at increased gravity forces equal to the number of times the model is scaled to. The outcome of the modelling is stress and soil behaviour similarity between the scale model and the full size field structure (Schofield, 1980).

Centrifuge modelling has been applied to many disciplines, such as earthquake engineering, where soil models are prepared and tested under extreme seismic events, which can provoke large displacements failures and liquefaction. Conventional centrifuge testing proposes the use of identical soil in the model and prototype. However, in some cases, such as centrifuge testing of stone columns as a countermeasure against liquefaction, the stone columns should be modelled using reduced scale particles, because prototype aggregate is too large in diameter for the model columns. For consistency, the surrounding soil must be scaled by the same rule. Moreover, the fine material should be capable of liquefaction, as it represents a liquefiable soil. Here, the proposed fine soil is a liquefiable coarse silt. However, before using large amounts of soil in a centrifuge model and testing it under earthquake motions, it is necessary to test the material under shearing to check its behaviour. (Apostolou et al., 2016)

 

BEHAVIOUR OF SILT UNDER SIMPLE SHEARING ON VDDCSS The chosen material used for further investigation as a scaled reference was coarse silt A50 Silica. According to geological reference and case studies silt is a material that can liquefy (Carr et al., 2004), (Sartain et al., 2014). Thus, it was investigated extensively in the lab to understand its behaviour under cycling loading. A series of drained tests were carried out in order to assess the degree of contraction under cyclic simple shear, which may be interpreted as indicative of its potential to undergo liquefaction. The equipment used for the tests was the GDS Variable Direction Dynamic Cyclic Simple Shear System (VDDCSS) (Fig. 1). Simple Shear was preferred over a conventional direct shear device, as soil behaviour under simple shearing mimics better the soil stress response under seismic events in real field conditions. More precisely, a conventional shear box splits horizontally in halves during the direct shear test and as a result, it allows a specific plane of failure for the specimen. 


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