EVALUATION OF ANTI-SEEPAGE QUALITY FOR THE PAC MA HYDROPOWER DAM FOUNDATION USING GROUTING METHOD

Abstract

The Pac Ma Hydropower Project is constructed on a dam foundation characterized by fractured porous rock and complex geological and hydrogeological conditions, resulting in a high risk of seepage within the foundation zone. This paper presents an assessment of anti-seepage performance through staged curtain grouting using cement slurry and bentonite mortar incorporating sodium silicate. Based on engineering geological investigations and Vietnamese standards, six reinforcing rows and three curtain rows were installed with a treatment depth of 10.5 m and borehole spacing of 0.75–1.0 m. Field permeability was evaluated using Lugeon water pressure tests before and after grouting. Before large-scale construction, experimental grouting was conducted in four stages with different grout materials and borehole layouts to determine technical parameters. The results showed that only the dense borehole configuration combined with cement–bentonite–sodium silicate mortar achieved the required permeability reduction. The initial permeability ranged from 26 to 50 Lu (approximately 2.6 × 10⁻⁶ to 5.0 × 10⁻⁶ m/s). After treatment, Lugeon values in 25 boreholes decreased to 1.14–4.88 Lu (approximately 1.1 × 10⁻⁷ to 4.9 × 10⁻⁷ m/s), all below the 5 Lu design requirement. These results confirm that fractures and voids were effectively sealed, forming a continuous grout curtain beneath the dam. The near one-order-of-magnitude reduction in hydraulic conductivity demonstrates the efficiency of the optimized multi-row system under fracture-controlled conditions. The findings also provide quantitative documentation of the tectonic structure, U-shaped valley morphology, and fracture network of the Da River basin, highlighting its geological significance alongside engineering implications.