| Abnormal pore water pressure distributions have been encountered in some fine-grained earth dam cores, characterized by higher than anticipated pressures in the central portion with a rapid decrease near the downstream side of the core. This situation results in higher than expected pore water pressures near the downstream side of the core and can be detrimental to the stability of a dam. The theory that is being explored in this study, put forward by Guy St-Arnaud, is that unsaturated (90-95% saturation) wet-of-optimum placement of the core material results in the entrapment of occluded air bubbles. These bubbles dissolve in the high-pressure upstream portion of the core and reform as the seepage water flows to the lower pressure downstream portion. The hydraulic conductivity of the core is decreased where the bubble is reformed, which causes an increase in hydraulic gradient and upstream water pressure. This in turn causes the air bubbles to dissolve again and migrate further downstream before reforming. As the air bubbles advance toward the downstream side of the core by this dissolution-reformation process, a high gradient pore water pressure front moves through the core over time. (Abstract shortened by UMI.)... |
