One of the most common and effective means of slope stabilization is lowering the water level within a soil mass. Frequently, horizontal drains are installed for this purpose. Computer-aided slope stability analyses are then used to evaluate the increase in factor of safety produced by drain installation. Critical to these analyses is the location and shape of the water table surface above the drain field. However, evaluation of the water table surface is complicated by its complex corrugated shape, with troughs corresponding to drain locations and ridges at the midpoints between drains. The objective of this research was to accurately describe the water table surface within a drain field using easily measured field and laboratory parameters. To accomplish this, physical and computer modeling of the water table along and between drains was conducted. The results of these analyses were compared to an analytical solution of the water table profile between drains that was derived by modifying groundwater equations developed for agricultural engineering applications. Based on these comparisons, a method was developed to describe the water table surface using the analytical solution and an experimentally derived correction factor. The method was confirmed by comparisons to field data. As a result of this research, water table surface heights can be approximated along and between drains. Additionally, an average water table surface height may be calculated and used in stability analyses, allowing accurate substitution of two-dimensional analyses for more complex three-dimensional situations.

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