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Development a Double Cell Triaxial Permeameter for Measuring Saturated Hydraulic Conductivity of Highly Deformable Soils

Ali Panahi, Pouya | 2020

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 52636 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Sadeghi, Hamed
  7. Abstract:
  8. The measurement of saturated hydraulic conductivity in conventional methods is based on the assumption of non-deformable porous media. However, this assumption is violated in most cases, especially when dealing with highly deformable soils such as collapsible and expansive soils. An alternative is to use flexible wall triaxial devices, yet the deformation characteristics during wetting and sample saturation are missed. This issue is of great importance in soils with considerable amount of volume changes (e.g., problematic soils). It should be noted that errors included in measuring sample volume changes, water inflow/outflow rates, and the creation of sidewall leakage are essential factors leading to unreliable estimation of hydraulic conductivity. In order to cope with all the limitations mentioned, a new saturated permeameter has been developed. The most important advantage of this device is the accurate and independent measurement of the specimen volume change from the volume of water passed through the specimen during all testing stages. This feature makes the new device superior to all available devices. It also allows simulation of in-situ stress conditions and determination of permeability coefficient as a function of stress level or void ratio for a single sample. To apply the confining pressure and pressure gradient along with the specimen height, a pressure and volume control panel has been developed for monitoring and measuring the inlet/outlet volume of the sample as well as changes in the volume and void ratio of the sample during testing. All routes needed for saturation, consolidation, and steady-state flow of water within the sample are considered in the design. This panel is equipped with three thripple sets of: 1) Pressure regulator, 2) Pressure transducer, 3) Double wall burette with inner glass burette providing a measuring resolution 35 times more than that of conventional panels. Results confirm that the inclusion of an inner cell and inner burettes to the design result in the complete elimination of the compliance error in volume measurements. The merits of newly developed apparatus are shown and highlighted through comparing the results with those obtained from other techniques on two types of problematic soils, i.e. collapsible and expansive soils. The conventional tests considered are (1) Triaxial single cell and burette, (2) Standard method for flexible wall permeameter (ASTM D 5084), (3) Oedometer apparatus using consolidation theory and (4) Rigid wall cell in conjunction with a falling head method (ASTM D 5856). Results revealed that it is not possible to accurately measure the volume changes of highly deformable samples in triaxial devices by conventional methods. A significant error in measuring permeability, up to 24 times, was observed in this method. In a rigid wall and oedometer device, sample saturation is not entirely possible, and therefore the permeability coefficient is lower than the actual value. The permeability measurement error in the oedometer apparatus of the collapsible and expansive soil was measured 6.5 and 2.2 times, respectively. The permeability measurement error of the rigid wall method was also up to 15 times, lower than that presented in this study. The permeability coefficients obtained from the new apparatus were also compared with prediction models. It was concluded that these models underestimate the permeability due to the fact that the database for these models was obtained from consolidation test results which could be far from the fully saturated condition
  9. Keywords:
  10. Collapsible Soils ; Pressure Control and Volume Measurement Panel ; Expansive Soil ; Saturated Permeability ; Triaxial Double Cell

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