Comparison of Flownets for Varying Thickness of Vertical Core Earth and Rockfill Dam Under Rapid Draw-Down and Steady-State Seepage and Its Effect on Stability Using Numerical Modeling

Abstract

AbstractStability of Earth and Rockfill Dams under steady-state seepage and rapid draw-down conditions largely depends upon the flownet generated in the core of dam. Direction of flow lines in flow direction or in reverse direction of flow in core may destabilize the factor of safety of dam. An analytical study has been undertaken to evaluate and compare the variation in the flownet under Steady-State Seepage and Rapid Draw-Down in the core of a dam on account of variation in the thickness of core. A 180 m high earth and rockfill dam section, founded on strong base, was used as a base section for analysis. The flow lines were obtained for Steady-State Seepage under full reservoir condition and Rapid Draw-Down (Hdd = H/2) using software (which employs FEM technique). It is observed that for thin and thick core, the direction of flow lines in core are always from upstream slope to downstream slope under Steady-State Seepage, while for all cases of Rapid Draw-Down for Hdd = H/2 condition, when the vertical core is very thin, bulk of the flow lines are towards downstream slope and some are towards upstream slope. As the thickness of core increases, more flow lines are observed towards upstream face including below draw-down level. Stability analysis shows that the factor of safety is constant for core thickness in the range of 1.0 V: 0.25 H to 1.0 V: 0.50 H for steady-state seepage and from 1.0 V: 0.25 H to 1.0 V: 0.75 H under rapid draw-down condition. However, after this range, there is a decrease in factor of safety. The rate of decrease in the factor of safety is also observed to be similar in both conditions.KeywordsEarth and rockfill damVertical coreFlownet