Effect of levee foundation conditions on the saturated zone during flooding for rapid drawdown analysis

Abstract

Analysis of the rapid drawdown loading condition for dams and levees typically assumes full saturation prior to drawdown. This assumption is likely incorrect for levees due to the relatively short duration of floods prior to drawdown; and hence requires that methods be developed to estimate the extent of the saturation zone within levees after floods for undrained rapid drawdown analysis. Levees can be founded on soils with a range of permeability, and therefore it is important to investigate how levee foundations influence levee through-seepage under transient conditions. A very important parameter which dictates response and extent of saturation in levees is the coefficient of consolidation, cv. This parameter is a function of soil hydraulic conductivity and volumetric compressibility, and dictates how quickly or otherwise water flows through levees and their foundations during floods. Numerical transient seepage models were analyzed with cv values for the foundation soils (cvf) ranging from 50 cm2/s (sand-like soil) to 1×10-3 cm2/s (clay-like soil). The levee models considered a silt-like levee on (i) sand foundation, (ii) silt foundation, (iii) clay foundation, and (iv) impermeable foundation. Similar to the theory of consolidation settlement, for a given levee soil, a rapid build-up of pore pressures and subsequent establishment of steady-state conditions in foundation soils for increasing values of cvf under changing boundary conditions forces upward seepage through levees due to the high total heads developed in the foundation soils. Decreasing ratios of levee soil and foundation soils cv values resulted in increasing saturation zone area within levees.