Study on the Effects of Compound Seepage Control Measures by Combining Seepage Prevention with Drainage Measure in Hannan Yangtze River Dike

The piping burst danger control project of Hannan Yangtze River dike is affected by the West-east Gas Transmission pipeline through the dike, making the enclosed cutoff wall separated and discontinuous. This discontinuity will cause by-pass seepage at the end of the cutoff wall. To solve the potential seepage problem at the unsealed section of the cutoff wall, this paper proposes a compound seepage control and drainage measure to reduce the excessive groundwater pressure. The proposed compound measure applies the fully enclosed cutoff wall in the dike as anti-seepage measure and installs relief wells in the section of West-east Gas Transmission pipeline to control the groundwater head. To assess the effects of compound seepage control measures, a three-dimensional seepage numerical model of the Hannan Yangtze River dike was constructed based on MODFLOW. The numerical simulation method was used to compare the seepage stability of the levee without any seepage control measures, using only fully enclosed cutoff wall and using the compound seepage control measures. Effects of the wellhead elevation of relief wells on the seepage control effect in Hannan Yangtze River dike were also investigated. Results show that in the natural state, and when only enclosed cutoff wall is constructed, there is seepage risk in the weak parts of Hannan Yangtze River dike. Moreover, the seepage around the cutoff wall gap is obvious when only enclosed cutoff wall is constructed. However, when relief wells are installed in the unenclosed section of the cutoff wall, and the combined measures of fully enclosed cut-off wall and pressure relief well are adopted, the seepage control effects are significantly affected by the wellhead elevation. When the elevation of the wellhead gradually decreases, the pressure head difference in the levee increases, the confluence capacity of relief wells is enhanced, and the drainage and decompression capacity of relief wells are also obviously improved. It is suggested that the wellhead elevation of relief wells should be carefully considered in the design stage.