Influence of core thickness on stability of downstream slope of earth and rockfill dams under end-of-construction and steady-state-seepage: a comparison

The steepness of the slopes of earth and rockfill dam is governed by stability considerations which in turn are influenced by the strength of the core and shell materials as well as their geometry. An analytical study has been undertaken to evaluate and compare the variation in the factor of safety of the downstream slope of a dam on account of variation in the thickness of vertical core for the two conditions, namely, steady-state seepage condition and end-of-construction condition. A 180-m high earth and rockfill darn section, founded on strong base, having section and slopes similar to a high dam in the northern India was used as a base section for analysis. The study shows that as core thickness increases beyond a critical value, it causes a progressive decrease in the stability of downstream slope. This critical core thickness is observed to lie in the range of 50-150% of darn height for both conditions and a core within this zone does not influence stability of downstream slope. The rate of decrease in factor of safety is also observed to be similar in both conditions. The influence of relative strength of shell to core, height of dam and pore water parameter on critical core thickness is highlighted.