Abstract
In order to evaluate numerically the seismic-induced liquefaction potential of gravelly soils, three-dimensional discrete-element method cyclic undrained triaxial tests were performed in a periodic cell. The undrained tests were conducted by deforming the samples under constant volume conditions. The paper presents a detailed investigation into the mechanical phenomena occurring at the grain scale leading to the liquefaction of gravel when subjected to cyclic straining. The influence of variables characterising the granular assembly, such as inertial number, geometrical coordination number, mechanical coordination number, redundancy index and Reynolds stresses, on liquefaction is investigated. Results of the simulations show that the onset of liquefaction occurs when the granular medium becomes on average isostatic – that is the redundancy index is equal to 1, the geometrical coordination number is equal to 2 and the mechanical coordination number is equal to 3. To elucidate further the transition from solid-like to liquid-like behaviour, an analysis of how particles cluster together is presented. It is shown that, as the mean effective stress decreases, contacts are lost, with the maximum cluster size gradually reducing at an increasing rate. The loss of contacts is uniformly distributed throughout the sample and there is no break-up of the largest cluster into smaller clusters.
Highlights
Liquefaction is a very common natural phenomenon that occurs in saturated loose sands during an earthquake, causing very significant destruction to the built environment
Thornton (2000) introduced the so-called mechanical coordination number Zm to exclude particles that do not contribute to load bearing – that is floaters and rattlers, as
After this point the effective pressure increases due to the cyclic nature of the loading, until after a few more cycles p′/p0 remains stable. It emerges that at the identified onset of liquefaction the geometric coordination number is very close to 2, indicating the sample to be isostatic on average. This is a rather remarkable finding since it indicates that the onset of liquefaction from a stress state point of view – that is, the occurrence of p′/p0 = 0 – is associated with the geometrical coordination number approaching the value of the threshold geometrical coordination number expressing the isostatic condition for the granular system which has not previously been shown for assemblies of granular particles with rolling resistance
Summary
Liquefaction is a very common natural phenomenon that occurs in saturated loose sands during an earthquake, causing very significant destruction to the built environment.
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