The removal of groundwater from the subsurface is one of the most common remediation practices in slope stabilization. The use of horizontal drains has often proved to be an efficient and economical dewatering option for slope stability. Despite their frequent use, a comprehensive review of the state of the art that includes modern research and contributions from related fields has not been performed for nearly 30 years. The objective of this paper is to provide a summary of the current state of practice, including application of recent research. In addition, this paper provides some suggestions for possible improvements to areas of current practice that have been identified as lacking complete answers. Recent research that may be applied to the design of horizontal drains includes (1) Zhou and Maerz's (2002) method for optimizing drilling directions to intersect as many discontinuities as possible in a rock mass, (2) Crenshaw and Santi's (2004) method for calculating an average drain spacing for designs implementing nonuniform drain spacing, and (3) Crenshaw and Santi's (2004) method for calculating an average two-dimensional groundwater profile representative of a corrugated three-dimensional groundwater table, which is low at drain locations and high between drains. Examples of issues that are not adequately addressed by current practice include (1) calculation of drain spacing values required to lower the groundwater level in a slope by a specific amount, (2) prediction of groundwater changes at various distances away from a drain, and (3) proper approaches to modeling complex landslide and groundwater geometries in two dimensions.

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