| Summary: | In zoned dams, it is expected that pore pressure is uniformly dissipated from the upstream to the downstream side of the core. Based on this hypothesis, the potential profile across the core in a steady state seepage flow is expected to be more or less linear and the equipotential lines should be distributed quasi-uniformly. This analysis is based on a homogeneous saturated permeability throughout the core. For some large cores made of glacial moraine (till), it has been reported that the pore pressures were significantly in excess of expected values during and after reservoir filling. The materials used in the cores are well graded silty sand soils including a small percentage of colloidal particles and cobble. This investigation is aimed at understanding this problem, its evolution with time and the influencing factors. The research study includes a series of experimental tests and numerical simulations. Deformations of till material after construction and during impoundment were studied. The mechanism of water infiltration and the evolution of water pressure through the unsaturated till were investigated. The influences of the initial degree of saturation, the volume of water circulating, the effective stress and the back pressure on the degree of saturation and hydraulic conductivity were investigated. An attempt was also made to study the influence of compaction water content on the hydraulic and mechanical behavior of till. Numerical modeling of the LG-4 dam was carried out to illustrate the influence of the degree of saturation on the pore pressure dissipation and flow discharge. The results show that pore pressures in excess in cores made of till are due to heterogeneity in the degree of saturation, and hence in the hydraulic conductivity (coefficient of permeability) across the core. By impounding, reservoir water pressure creates a relatively high hydraulic conductivity zone in the upstream and central portions of the core, resulting in a high hydraulic gradient in the downstream side. After a certain time, the phenomenon of progressive saturation causes the coefficient of permeability to increase in the downstream side, which results in the reduction of the heterogeneity. As a consequence, pore pressures decrease toward theoretical expected values.
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