Pore Pressure Anomaly In Dam Cores

In many earth dams, the core zone is constructed of earth materials. Because of the sealing role expected from the core, the used materials are chosen among the most impermeable available soils. Depending on the geological characteristics of the region and thus the origin of the impermeable material, the hydraulic behavior of core may vary from case to case. However, the condition of material during the placement and compaction step is the other parameter, which do affect the performance of the core.In this paper, the data resulted from monitoring of a series of earth dams are studied and analyzed. The cores are constructed of a wide grain size material ranging from cobble to silt and few percent of colloidal material. The used materials are nonplastic moraine tills mostly resulted from glacial activities.In many of these cores, a common anomaly was observed. The piezometric readings were considerably different with expected values obtained by theoretical seepage analysis. In the upstream side of cores, during the reservoir filling and in the early years of service, the reading values were higher than expected values,while they were lower in the downstream side. By the time, the differences in pore pressure were reduced, and after some years, the piezometric readings matched the expected pore pressure at respected positions.The results of analysis show that the magnitude of the observed anomaly,is a function of the following parameters: the geometry of the core, the water content of material at the time of compaction, the height of the dam and the rate of reservoir filling.

Keywords:

Earth dams , piezometer , water content , non-plastic material , compaction