Title page for ETD etd-08182004-161435


Type of Document Dissertation
Author Porras Ortiz, Oscar F
Author's Email Address oportiz@lsu.edu
URN etd-08182004-161435
Title Small and Large Strain Monitoring of Unsaturated Soil Behavior by Means of Multiaxial Testing and Shear Wave Propagation
Degree Doctor of Philosophy (Ph.D.)
Department Civil & Environmental Engineering
Advisory Committee
Advisor Name Title
Dante Fratta Committee Chair
Emir Macari Committee Member
John Sansalone Committee Member
Khalid Alshibli Committee Member
Jim Wang Dean's Representative
Keywords
  • unsaturated soil
  • shear wave
  • small strain
Date of Defense 2004-08-10
Availability unrestricted
Abstract
The deformation and strength behavior of dry and saturated soils is controlled by the effective stresses as defined by Terzaghi. However, Terzaghi’s definition of the effective stresses fails for unsaturated soils, as capillarity force influence is also important. The effects of capillarity forces in soil are evaluated by the concept of matrix suction. Several techniques are used to evaluate soil suction however their applications involve difficult calibrations and tedious methodology. Furthermore, suction is a microscopic property and it is influenced by interparticle soil attraction, which can change by sampling disturbance.

This research program evaluates the effect of suction on stiffness and strength of soils at small strain (at constant fabric) and large strain (with fabric changes) levels. The phenomena are studied using a modified oedometer cell and a multi-axial device with matric suction control that have been equipped with bender elements for shear-wave velocity measurements. The test program consists on testing dry and unsaturated specimens under different boundary conditions: Ko-loading and multi-axial loading. To test the Ko-loading condition, the soil is loaded in the oedometer cell while the bender-elements monitor the changes in state of stresses by evaluating the changes in the velocity of wave propagation. Similarly, triaxial compression and conventional triaxial compression tests, along with monitoring of shear wave velocities, are conducted on 10-cm side cubical specimens of reconstituted soil specimens to study the stress-strain behavior of an unsaturated soil over a range of degrees of suctions and stress paths and the effect they have on the propagation velocity of shear waves.

Results show the adequacy of methods and equipment used in this investigation to monitor the behavior of unsaturated soils under the application of a range of suctions and several stress paths. Experimental results are analyzed using simple, yet robust wave propagation models and geo-material behavior. Their interpretation bring a better understanding to low and large strain-stress behavior of near sub-surface soils. Results provide a stronger base for the development of models for the imaging of near-surface geo-materials using elastic wave-based imaging techniques and for better interpretation of geotechnical models of design.

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