Real-Time Measurement of Shear Zone Development in Soil Models Using Time Domain Reflectometry

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***9908342PierceThe objective of this research is to develop a time domain reflectometry(TDR)-based method of detecting and measuring, in real-time, the development ofinternal shear zones within centrifuge models of soil structures. TDR isanalogous to radar in that voltage is pulsed along a coaxial cable to detectelectromagnetic faults. When a cable is embedded in a deforming geologicmaterial, the cable shears and reflects a small percentage of voltage. Theability to measure internal shear zone development during testing will providea powerful tool to investigate the behavior of soil subjected to a wide varietyof loading conditions. It will also allow more comprehensive comparisons ofphysical model behavior to numerical predictions than currently possible. Theadvantages of real-time measurement of internal shear zone development will bedemonstrated through the modeling of overconsolidated clay slopes. Priorcentrifuge studies of overconsolidated clay slopes have shown that suddenfailures involve block-type motion along a distinct failure surface. Inaddition to the benefits provided by the instrumentation aspects of thisproject, the investigators will evaluate the effects of grain size, soil shearstrength and stiffness, and the use of grout on TDR measurements in soil. Finally, the results of this work will aid in the development of field use ofTDR in soils.The project is divided into four main tasks: 1) TDR instrumentationdevelopment; 2) laboratory feasibility testing; 3) centrifuge developmenttesting; and 4) centrifuge modeling of overconsolidated clay slopes. In thefirst task, the investigators will use a modified direct shear device todetermine the effects of soil grain size, soil strength and grout encapsulationon shear displacement measurements using different TDR cables. In the secondtask, the investigators will conduct small- and large-scale bearing capacitymodel tests to evaluate the ability of TDR to detect multiple shear zones, thequality and reproducibility of the data, and the robustness of selected cablesunder these conditions. In the third task, bearing capacity tests will beconducted at the University of Florida using a geotechnical centrifuge with aradius of 1.85m, a maximum acceleration of 100g and a maximum payload ofapproximately 70kg to demonstrate the ability of the TDR system to work in acentrifuge environment. The accuracy and reliability of TDR measurements willbe evaluated and centrifuge results will be compared to the large-scalelaboratory tests. The final task utilizes the new instrumentation to evaluateinternal shear development in overconsolidated slopes. Long-term andshort-term tests on overconsolidated clay slopes instrumented with TDR cablesand LVDTs will be conducted. The objectives are to determine if internal sheardisplacements occur prior to failure; if shear displacements can be detectedprior to surface movement; and how the location of these displacements relatesto the internal stress-state of the slopes.***