United States Department of Transportation
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Reliability of FEQDrain for Modeling Performance of Sand Treated With Large Diameter Prefabricated Vertical Drains for Liquefaction Mitigation
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2018-08-01
[PDF-8.78 MB]
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Edition:Final Report May 2013 to August 2018
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Abstract:The objective of the modeling in this study was to evaluate the reliability of the axisymmetric finite element computer program FEQDrain for computing excess pore pressure ratios (Ru) and settlement at sand sites treated with prefabricated vertical drains (PVDs). FEQDrain was found to be capable of successfully modeling measured excess pore pressure ratio time histories from the laminar shear box experiment, as long as an appropriate combination of ‘number of equivalent cycles’ and ‘shaking duration’ was chosen, and sensitive soil parameters were in the range of measured values. Hydraulic conductivity, soil compressibility, and cycles to liquefaction are sensitive parameters and govern the computed Ru values. Computed Ru values decreased as hydraulic conductivity increased and compressibility decreased. Computed settlement was somewhat overestimated relative to measured values. Modeling shows that the loading rate in the laminar shear box (15 cycles at 2 Hz) likely induced higher Ru values than would be expected in a typical earthquake event with a longer duration. Results from models in this study compare favorably with those from computer modeling performed by Howell et al. (2014) in connection with centrifuge tests. In both cases, the drains were more effective at decreasing pore pressures at greater depths than at shallow depths presumably owing to upward fluid flow. Similar Ru values can be modeled with different combinations of hydraulic conductivity and compressibility. Based on computer analyses, wick drains and 2” diameter PVDs were found to be relatively ineffective for preventing liquefaction. However, 3” diameter PVDs are fairly effective but can be overwhelmed during intense shaking. In contrast, 4” diameter and larger PVDs are significantly more effective.
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