USSD will insert footer text here 1 UNCERTAINTY OF SOIL COMPRESSIBILITY IN TRANSIENT AND UNSATURATED SEEPAGE ANALYSES Timothy D. Stark, Ph.D., P.E., D.GE 1 Navid H. Jafari, NSF Fellow 2 ABSTRACT Most levee underseepage and uplift analyses are based on steady-state seepage and can yield conservative results. Transient and unsaturated seepage analyses are more representative of levee seepage conditions because boundary conditions acting on the levee or floodwall and saturation changes with time, which induce pore-water pressure and seepage changes with time in the embankment and foundation strata. In addition, these boundary conditions, e.g., flood surge or storm event, are rapid such that steady- state conditions may not have time to develop in the embankment and some foundation materials. Transient seepage analyses using a case study floodwall indicate that as soil compressibility of the underseepage layer decreases, rapid landside pore-water pressures increase and can approach steady-state values. The uncertainty analysis shows that the Factor of Safety Most Likely Value (FSMLV) is 1.09 but coefficient of variation is 18%, which indicates high uncertainty. The hydraulic conductivity controls Factor of Safety (FS) standard deviation while soil compressibility does not contribute to overall uncertainty. INTRODUCTION Over 100,000 miles of flood protection infrastructure are currently operating in the United States (National Committee on Levee Safety 2009), e.g., along Mississippi, Sacramento, Trinity, Missouri, and American Rivers. The increase in development behind levees and floodwalls poses increased risk to public health and safety. The societal, economic, and environmental risks will consequently play a greater role in assessing the required performance of flood protection. Current performance of urban levees and floodwalls to hurricane and flood events are primarily based on steady-state seepage analyses. Hurricane and flood conditions typically only act for a period of hours to weeks, which may not allow sufficient time to develop steady-state conditions (Peter 1982). As a result, a transient seepage analysis, e.g., wetting front or other movement of water in unsaturated soil, provides a more realistic approach to evaluating levee seepage and slope stability. Lambe and Whitman (1969) define transient flow as the condition during water flow where pore-water pressure, and thus total head, changes with time. During transient conditions, changes in hydraulic boundary conditions and boundary total stresses cause: (1) saturated seepage through relatively pervious foundation strata (Casagrande 1937; 1 Professor, Department of Civil Environmental Engineering, University of Illinois, 205 N. Mathews Ave., Urbana, IL 61801, tstark@illinois.edu 2 Doctoral Candidate, Department of Civil Environmental Engineering, University of Illinois, 205 N. Mathews Ave., Urbana, IL 61801, njafari2@illinois.edu