Stiffness characterization of mechanically-compressed cohesive soils using wave propagation
Author(s)
Marjanovic, Jana
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Massachusetts Institute of Technology. Department of Civil and Environmental Engineering.
Advisor
John T. Germaine.
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Wave propagation through soils is a non-destructive method used to characterize soil stiffness properties and is the basis for geophysical interpretations. Due to the difficulty of measuring shear waves through soft cohesive soils with ultrasonic transducers, bender elements are typically used, which couple to the soft clay. Unfortunately, the stress limitation of bender elements precludes it from testing above 1.5 MPa. A novel setup using ultrasonic transducers, with electronic conditioning, is developed in order to overcome the difficulties of transmitting a shear wave through a soft material. The successful fabrication of triaxial cell endcaps fitted with ultrasonic piezoceramic elements has enabled the measurement of compressional and shear wave velocity of clays over a wide stress range (0.5 - 10 MPa). This research compares the shear modulus measurements using three different technologies that encompass a stress range of 0.1 to 70 MPa. A variety of resedimented materials and plasticities are tested in order to characterize the Vp and Vs as a function of stress during K₀-consolidation. Resedimentation creates controlled, uniform specimens whose results can develop a backbone for velocity behavior independent of field heterogeneities and disturbance. The Vp and Vs results show different trends as a function of stress for the different plasticity materials. However, the Vp/Vs ratio, which is a common indicator of unloading and lithology, has a strong dependence on the liquid limit (wL). A model is developed to predict the Vp/Vs as a function of stress for a given wL. In addition to K₀ loading, the effects of unloading are investigated. Unloading introduces secondary compression, which significantly alters the stiffness results. A method is developed to integrate the secondary compression data with normally consolidated data based on the concept of apparent preconsolidation pressure. Finally, the dynamically-obtained stiffness parameters are compared to statically-obtained constrained modulus values from static compression. By using Gassmann's fluid substitution and a corrective X factor, a new technique is developed that can predict the dynamic bulk modulus based on the static CRS measurements.
Description
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2016. Cataloged from PDF version of thesis. Includes bibliographical references (pages 291-302).
Date issued
2016Department
Massachusetts Institute of Technology. Department of Civil and Environmental EngineeringPublisher
Massachusetts Institute of Technology
Keywords
Civil and Environmental Engineering.