Shear Creep in Sensitive Clays
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Stability of natural clay slopes subjected to environmental changes is today under close investigation in Norway. This is stimulated by the NIFS project (Natural Hazards, Infrastructure, Flow and Slides), a cooperation research project between Norwegian Public Road Administration, Norwegian Energy Directorate and Norwegian National Rail Administration, involving most research institution and consultancy companies in Norway.This project is a continuation of a MSc thesis from 2014 by Guro Torpe. The challenge in the testing of soft sensitive quick clays has been the consolidation phase, where the samples failed at K0 -levels far below expected failure. The pore pressure condition inside the sample is suspected to be the reason for this, and therefore a project including instrumentation of the central pore pressure is now started. The intention is to run the consolidation procedure under full control of the effective stress level, apply shear and finally under constant shear measure the behaviour in creep.In standard triaxial test, pore water pressure in a sample is measured at the surface with external pore pressure sensor. The pore water pressure inside the clay sample is different from the pore pressure at the surface. This variation in the pore pressure can cause high mobilisation of the shear strength during consolidation phase of K0 triaxial test. This can cause failure of the sample far below the failure line during the undrained shear test. The pore water pressure in the sample can also cause the failure due to accumulation of pore pressure even if the axial load is held constant. Therefore, to confirm the role of the pore water pressure in the failure of the clay sample during K0 triaxial test, undrained K0 shear creep test was done for Tiller clay and Goteborg clay in the modified triaxial apparatus. The apparatus contained the pore pressure sensor installed in it that measures the pore pressure in the middle of the sample of height 50mm and diameter 54mm. The filter plates were not used during the test to reduce the end restraint effect, and drainage of the pore water was only in lateral direction. The consolidation of the samples were done with different loading steps. After that, they were sheared at different rates to different degree of shear mobilisation and then creep phase was started. Pore pressure measurement in the middle and surface of the sample was done for consolidation, shearing and creep phases. The effective stress plot at consolidation phase showed that the sample was apparently mobilised to higher degree of mobilisation or consolidated to higher K0 value, although it was consolidated to their actual K0 value. It showed that the consolidation along K0 line should be done with the small loading steps and adequate time should be provided for each loading steps to release all the pore water pressure inside the sample otherwise pore water pressure will be accumulated in the consecutive loading steps. The pore water pressure variation in the middle and surface of the sample was about 4 to 6KPa. This variation was different for the different rate of strain. The variation was higher for the high strain rate than low strain rate.During the creep phase, the strain rate decreased initially and remained constant and then increased leading to the failure of the sample. The sample that was sheared to higher degree of shear mobilization failed earlier than that of sheared to lower degree of shear mobilization. The sample could not reach the allowed mobilization degree when sheared at low strain rate.