Unloading Stress Paths Research Articles

Simulation of downhole and crosshole seismic tests on sand using the hydraulic gradient similitude method

A method of simulating downhole and crosshole seismic shear-wave tests in a model under controlled stress conditionsis described. The downhole and shear wave in horizontal plane (SH) crosshole shear waves are generated and received along the principal stress axes using piezoceramic bender elements. The K0in situ stress conditions, including loading and unloading stress paths, are simulated by the hydraulic gradient similitude method, which allows high stresses simulating field conditions to be obtained. The horizontal stress during the tests is directly measured by a lateral total-stress transducer. The test data are used to evaluate various published empirical equations that relate shear-wave velocity and soil stress state. It is found that although the various empirical equations can predict the in situ shear-wave velocity profile reasonably well, only the equation that relates the shear-wave velocity to the individual principal stresses in the directions of wave propagation and particle motion can predict the variation of the velocity ratio between the downhole and SH crosshole tests. It was also found that the stress ratio has some effects on the downhole (or shear wave in vertical plane (SV) crosshole) shear-wave velocity, but not on the SH crosshole shear-wave velocity. This indicates that it is only the stress ratio in the plane of wave propagation that is important to the shear-wave velocity. Comparison between the downhole and SH crosshole shows that structure anisotropy is in the order of 10%. In addjtion, K0 values are predicted from shear-wave measurement and compared with measured ones. The difficulties in obtaining K0 values from shear-wave measurement are also discussed. Key words: hydraulic gradient, model tests, downhole and crosshole shear-wave tests, sand.

A continuous plasticity version of the critical state model

AbstractA modification to a conventional critical state model is described which introduces some plasticity in the stress region which is normally fully elastic. The effect is to blur the otherwise sharp distinction between elastic and yielding states. The modification is conceptually simple and does not require any additional material parameters. The model was developed for the dual purpose of providing a more robust constitutive law for finite element applications and to provide a more realistic representation of the soil. Its potential is demonstrated by simulating the response of a plane strain sample tested over a range of loading and unloading stress paths. The results are in accordance with expectations from physical tests. Further, they show that the model has some potential for at least the first few cycles of cyclic loading. A finite element boundary value analysis of an excavation is also described. This shows how the model copes with the severe test of stress paths which unload while yielding. Finally, the model is used to match stress‐strain curves obtained from large‐scale laboratory tests on a compacted clay. Close agreement with three of the five tests was obtained using a single set of material parameters.