Development of a Geophyscis-Based Multiphase Constitutive Model

Grants and Contracts Details


A supplemental research phase is proposed to, in part, address the concerns of the reviewers. More importantly, a supplemental research phase is proposed to strengthen the fundamental basis of the theories and verify the hypotheses put forth in the original proposal. This supplemental effort will provide the foundational framework that will increase the chance of success of the original proposed research. The tasks associated with the proposed supplemental research align with the four basic thematic questions concerning stress history, stress state, loading conditions, and parameter definitions. Task #1 - Answer Question about Stress History An automated triaxial testing system will be used for this supplemental research. The automated system can apply a time-dependent load while measuring porewater pressures. Thus, a full consolidation curve will be developed during the consolidation phase by and setting the rate of consolidation such that the target consolidation pressure is reached within the ASTM standard of 24 hours. An unload/reload cycle can be achieved by using a stress reversal once the target consolidation pressure and the target unload pressure are reached. At least one unload/reload cycle will be included in the loading schedule. Elastic wave measurements will be obtained throughout the consolidation phase. The proposed loading schedule will test the veracity of the original geophysical-based consolidation function proposed by the PI under elastic unloading and reloading conditions, and under plastic loading conditions. Task #2 - Answer Question about Stress State The reference soils will be tested in shear starting at three different consolidation stress states. Specifically, triaxial tests will be performed on a normally consolidated (i.e. overconsolidation ratio (OCR) = 1.0) sample, a lightly overconsolidated (i.e. OCR . 1.5) sample, and a heavily overconsolidated (i.e. OCR . 2.0) of the reference soils. The initial saturated research performed by the PI developed a geophysics-based stress function that predicted the variation of normalized shear wave velocities with variation of mean normal stress from the beginning of shear to the yield point. For normally consolidated samples, the yield point and the critical point are the same. Therefore, any difference in geophysical measurements under peak conditions versus measurements under critical state conditions were not established. The intent of triaxial shear program for this supplemental research is to test the samples to approximately 20 percent to 25 percent strain in order to reach critical state of the material. At critical state, the peak and critical state shear strength parameters can be obtained from the three consolidation stress states. Also, the yield surface contracts during shearing of a heavily overconsolidated sample due to dilation. The variation of the normalized shear wave velocities with respect to dilative behavior was not investigated during the original saturated research. Consequently, the variation of the normalized shear wave velocities under strain softening/hardening conditions (i.e. post-yield) could not be evaluated. The supplemental research will evaluate the relationships between the post-yield plastic shear and volumetric strains and the geophysical behavior Task #3 - Answer Question about Loading Conditions The original model presented by the PI was based on isotropically consolidated undrained (CIU) triaxial tests performed on saturated samples. The original research presented by the PI was unique in that it showed the shear wave velocity is related to the effective stresses acting in the direction of wave propagation and in the direction of soil particle motion. Thus, it can be concluded that variations in shear wave velocity reflect changes in effective stress and in state properties that affect particle motion, such as void ratio and water content. For undrained loading, variation in void ratio (i.e. volumetric strain) during shear is assumed to be negligible. Thus, variation in shear wave velocity during shear is cause by a decrease in the particle-to-particle contact stress that results from an increase in effective stress. CID tests will be performed for this supplemental research to quantify the changes in elastic wave velocities as functions of changes in volumetric and shear strain during drained shear. The geophysical functions will be integrated with the volume change functions developed as part of the consolidation phase. The intent of this effort is to develop continuous geophysical functions that go from the consolidation process through the shearing process. Task #4 - Answer Question about Parameter Definitions The original research presented by the PI defined the consolidation phase fitting parameters and the shear phase fitting parameters in terms of physical behavior. However, these definitions were not evaluated over the range of behaviors and loading conditions proposed under this supplemental research. Also, empirical relations were presented to obtain some of the parameters from index properties of the soil samples. The additional data that will be produced during the supplemental research will allow the veracity of the physical definitions to be tested. Also, the additional data will allow for the applicability of the empirical relations to be evaluated for a broad range of soils. With the additional data, it may also be possible to reduce the empiricism by further establishing physical definitions.
Effective start/end date1/21/217/20/22


  • Army Research Office: $90,883.00


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