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Development of Hollow Cylinder Apparatus for Testing of Unsaturated Sandy Soils

Hosseinpour, Saeed | 2017

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 49394 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Jafarzadeh, Fardin
  7. Abstract:
  8. The main purpose of this study is to develop the Hollow Cylinder Apparatus (HCA) for unsaturated testing on granular materials and to investigate the effect of stress anisotropy on dynamic parameters of soils like shear modulus and damping ratio. Direction of loading and the magnitude of intermediate principal stress have significant effect on the anisotropic response of soils. In many of in-situ loading the direction of major principal stress does not coincide with deposition direction of soil or because of the three dimensional loading condition of soil, the effect of the intermediate principal stress cannot be ignored. On the other hand, in many areas, soils are in unsaturated condition. The necessity to consider the actual condition of a soil as an unsaturated media in geotechnical applications and also the increasing need to study the anisotropic behavior of unsaturated soil lead to promotion of the HCA of the advanced geotechnical laboratory for unsaturated testing of soils. To reach the final goal of this research, suction controlling system was added to HCA as a large strain device. For this purpose, the HCA was promoted by replacing the bottom platen with a new platen with a High Air Entry Value (HAVE) ceramic disk and using the combination of negative water column method (hanging column) and axis translation (pressure panel) to apply wide range of suctions. Using the tensiometer method and by modifying the top pedestal, suction uniformity along the height of the soil specimen was controlled. To prevent any unexpected error, the inner cell pressure, the outer cell pressure, vertical and horizontal load of the apparatus was calibrated. Considering the importance of controlling the volume change during the unsaturated tests, the volume change of apparatus was calibrated and one of them was placed in outer cell pressure path. Therefore, by measuring the changes in outer and inner cell volume, the sample volume change can be measured. Some changes applying pressure path system of HCA has done. This means that, the pressure apply route on top of the sample went away and replaced by air pressure and suction route. This enables applying suction by axis translation. To study the effect of matric suction and degree of saturation on dynamic properties of granular soil, the cyclic hollow cylinder tests with approach to soil water characteristic curve on suctions equal to zero (fully saturated condition), suction equals to 3 kPa (a point in transition zone), suctions equal to 8.4 and 30 kPa (2 points in residual zone) were done on Babolsar sand. Then the soil water characteristic curve which has measured by tests is compared with Fredlund-Xing model. The main parameters of tests such as relative density equal to 15%, shear stress to initial confining pressure ratio of 0.2, stress controlled condition with Sinusoidal shape, loading frequency of 0.1 Hz, confining pressure of 50 kPa and drying path were remained constant in all of tests. Inconsiderable increase in shear modulus were observed from saturated condition to residual zone. In residual zone, increase in suction brings up significant increase in shear modulus. Damping ratio is climbed in transition zone with the increase of suction and dropped in residual zone. Generally, suction increase leads to increase in shear modulus and decrease in damping ratio of the soil. Although the changing procedure was not completely regular. Also the change in shear modulus with the major principal stress direction showed that by increasing α from 10 to 30 degrees, the shear modulus increased and after that by rising up to α=45° another slight increase was observed. In the unloading path, shear modulus decrement was seen in results. The changing procedure of shear modulus with the intermediate principal stress parameter (b) has showed a reduction of soil stiffness. Trends for damping ratio is almost in reverse order although it is not totally regular
  9. Keywords:
  10. Direction Of Major Principal Stress Axis ; Matric Suction ; Saturation Degree ; Hollow Cylinder Apparatus (HCA) ; Babolsar Sand ; Intermediate Principal Stress ; Unsaturated Soil ; Granular Soil

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