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 threedimensional situations.

A new method has been developed to stabilize slopes and landslides using driven, geotextile wick drains rather than drilled polyvinyl chloride or steel drains. Compared to drilled horizontal drains, wick drains can be significantly deformed without rupturing. They resist clogging, and they can be installed quickly and economically by workers with no previous training. Since 1998, more than 170 drains, totaling more than 2,600 m (8,600 ft) in length, have been installed at eight sites in Missouri, Colorado, and Indiana. At three of the sites, drains were installed at a density and layout considered to be appropriate for full-scale stabilization: 27 to 44 drains were driven at each site, averaging 15 to 20 m (50-70 ft) in length. A method was developed to accurately estimate the groundwater-table profile after drain installation, and computer slope-stability analysis using this method showed significant increases (10-40 percent) in factors of safety after drain installation. The limitations of the methodology should be recognized: it may be applied in specific situations requiring relatively short (<30-45 m, or <100-150 ft) drains in materials ranging from soft to very stiff (up to 30 blows/ft in standard penetration tests). The potential effectiveness of drainage as a mitigation option should be verified through analysis of site geology and hydrogeology before drain installation.