Effect of backfilling surface settlement trough on waste cover leakage

The effect of backfilling of a surface differential settlement trough to reduce leakage is explored both experimentally and numerically. The field experiment examined two lined sections each with an 11 mm-diameter hole in the liner on a nominally 4 horizontal:1 vertical slope. A 2 m by 3 m, 0.3 m deep depression was filled with a 50-50 sand-snow mixture in winter to give a continuous 4H:1V slope prior to covering with the liner and 0.3 m of cover soil. Spring thaw induced a differential settlement trough up to 0.14 m deep. A second section with a similar trough was backfilled with cover soil to reinstate the 4H:1V surface while the settlement depression in the liner remained. Over the 15 months of monitoring, the backfilling reduced leakage by 57% from a annual total of 565 L to 244 L (i.e., a 60% reduction in colder seasons, from 351.3 L to 137.8 L together with a 45% reduction in warmer seasons, from 141.8 L to 77.6 L). A 3D numerical model showed encouraging agreement with the experimental results. The model indicated an inverse relationship between leakage and slope gradient, and a direct relationship between leakage and depression depth and upgradient distance to the depression. The effect of cover hydraulic conductivity was complex.