T Doanh

Undrained instability of very loose Hostun sand in triaxial compression and extension. Part 1: experimental observations

The undrained behavior of very loose Hostun RF sand in triaxial compression and extension tests is described. The samples are consolidated isotropically or anisotropically along constant effective stress ratio paths. Very loose sand exhibits partial liquefaction, deviator peak stress at relatively low to very low axial strain, gradual post-peak stress reduction to a small residual value at large strains, sharp loss of effective mean pressure due to generation of large pore pressure and overall volume reduction. The instability line of Lade is examined in the case of extension tests and extended for anisotropic samples. It is shown that monotonic and anisotropic consolidation strongly influences the instability concept. A higher positive anisotropic consolidation level produces a greater slope of the instability line in compression and a reverse trend can occur in extension. Complex stress history can develop a fossilized instability line depending on the amount of axial strain attained. Effective strain ratio increment at peak has an asymptotic stabilization effect.

Deformation characteristics of geomaterials : recent investigations and prospects

Testing and apparatus Recent developments in deformation and strength testing of geomaterials 3 Characterisation On the mechanics of reconstituted and natural sands Time effects on the behaviour of geomaterials From very small strains to failure Integrated ground behaviour Particle-level phenomena and macroscale soil behavior Integrated ground behavior: in-situ and labs tests Modelling Diffuse modes of failure in geomaterials Case history and field or physical model measurements Lessons learned from full scale observations and the practical application of advanced testing and modelling

Effects of Previous Deviatoric Strain Histories on the Undrained Behaviour of Hostun RF Loose Sand

The effects of previous deviatoric strain histories on the undrained behaviour of very loose and saturated Hostun RF clean sand are investigated in this paper. From an initial isotropic stress state in the triaxial plane, strain histories are generated by isotropic consolidation followed by standard drained triaxial preshear or presheared cycle, either in compression or in extension, up to a desired value of axial strain or mobilized stress ratio. Deviatoric strain histories are achieved by having nearly the same void ratio at the beginning of the undrained shearing for all tested samples. Subsequent undrained behaviour in triaxial compression and extension is analyzed in detail. Previous deviatoric strain histories can progressively transform the compressive and unstable behaviour of loose sand into a dilative and stable behaviour of dense-like sand, while being loose. Experiments show a common response induced by recent strain histories in terms of effective stress paths, independently of the axial strain attained during the drained presheared cycle, a unique initial gradient of the effective stress paths, a progressive appearance of dilatancy, an evolution the undrained behaviour and a systematic partial static liquefaction associated with softer behaviour when sheared in the opposite direction of the initial presheared direction. This paper offers a comprehensive understanding of the mechanisms of a specialized induced anisotropy created by simple linear stress paths in the classical triaxial plane.

Liquefaction of immersed granular media under isotropic compression

We report an observation of the spontaneous liquefaction of glass beads immersed in water and compacted by external isotropic stress. We show that during compression, loose granular samples exhibit a series of sudden rearrangements accompanied by a transient overpressure of interstitial fluid. Ultimately, spontaneous liquefaction with large deformation of the sample is observed. By contrast, denser samples do not show a liquefaction by maintaining its shape integrity. We then discuss the potential mechanisms which could explain this unexpected liquefaction.

Ambient Noise Site Investigation of A Representative MSW Landfill In Bishkek, Kyrgyzstan

This paper explores the possibilities offered by the ambient noise vibration technique to evaluate some dynamic characteristics of the waste and the underlying loess deposit of a representative municipal solid waste (MSW) landfill site in Bishkek, Kyrgyzstan. The paper demonstrates that the ambient noise technique provides the source for a simple and low-cost in-situ dynamic characterization of landfill site. The result can be of great benefit for many developing countries.