Mechanical behavior and deformation texture of fine-grained quartz sand in drained and undrained shear experiments at a large displacement
Abstract
Recent studies of fault rocks documented examples of fluidized textures of fault gouge possibly produced during ancient seismic fault motion (Otsuki et al., 2003, Rowe et al., 2005). Knowledge of the process involved in the fluidization of fault materials is therefore important to understand the mechanisms of dynamic weakening of faults during earthquakes. In this study, shear experiments were performed on water-saturated fine-grained quartz sand (silt) to simulate fluidized (liquefied) fault deformation at a shallow depth, and mechanical behavior and the deformation texture of the tested materials was examined. In the experiments, a ring-shear apparatus developed and improved by K. Sassa and his colleagues at Kyoto University for the study of landslides (Sassa, 1997; 2000) was used.. An artificial sand made by grinding silica sandstone was used in this study, same as that used in the previous study of the same type experiment with this machine (Wafid Agung, 2004). Cylindrical space of the sample box (inner and outer diameter is 120mm and 180mm, respectively) was filled with oven-dried sand specimen firstly. Sand specimen was then saturated with the help of CO2 gas and de- aired water. After infiltration of water, sample was consolidated at normal stress of 500kPa for an hour before to start shear loading. During shear loading, shear stress was increased at a constant rate of 0.1kPa/sec. Pore-water pressure was monitored 2mm above the boundary of the upper and the lower shear box, i.e., close to the shear zone. In the undrained test, shear resistance of the sample increased monotonically until it reached to the failure line. Pore pressure also increased during this period gradually and effective stress decreased from 500kPa to about 320kPa. Once it reached to the failure line, rotation side of the sample box begun to rotate. With the onset of the rotation, pore pressure increased rapidly and shear resistance of the sample decreased along the failure line to a steady-state value of about 80kPa. For the drained test, no excess pore-water pressure built up during shear loading and effective pressure was constant at 500kPa during the whole period of the run. When shear resistance of the sample reached to the failure line at about 400kPa, sample box begun to rotate and the shear resistant decreased to a steady state value at around 350kPa. Preliminary observation of the texture of the shear zone on the surface of the sample after the tests showed that texture was different for the drained and undrained test. Deformation zone of the sample from drained test showed foliation (planar surfaces) inside the zone. On the contrary, shear zone of the sample from undrained test, in which the sample experienced liquefaction, seemed to have no clear foliation within the zone, although the sample has experienced large displacement comparable to that of the undrained test.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.T21D0453T
- Keywords:
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- 8004 Dynamics and mechanics of faulting (8118);
- 8045 Role of fluids;
- 8118 Dynamics and mechanics of faulting (8004)