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Horizontal Drains - State of Practice The Past Seven Decades in the US

Lee, Thomas S.

Next only to gravity, groundwater is the most important factor triggering slope instability. Subsurface water reduces the stability of cut/fill slopes and embankment/levee foundations through reduction of the shearing resistance of the soil, and increases the weight of the ground mass and seepage forces which add to the driving forces. Over the past seven decades, millions of linear feet of horizontal drains have been used in the US to stabilize marginally stable or failing slopes composed of a wide variety of soils/rocks and geologic, topographic, climatic, and groundwater conditions. Some horizontal drains were installed effectively, a few had been installed that were not effective, and the remainder were effective in various degrees. Horizontal drains were first installed in 1939 by the then California Division of Highways (CDH) to stabilize landslides in California, and subsequently adopted by many Departments of Transportation (DOTs) in the US for landslide treatment. Despite its successful use for many years, there has been a dearth of publications written about design, installation, and long term maintenance aspects of horizontal drains in the US. This paper discusses the state of practice concerning design, installation, field instrumentation, and long term maintenance of horizontal drains starting in late 1939 and ending with the present (2013). In addition, the paper discusses how to evaluate the effectiveness of horizontal drains and practical aspects of an effective horizontal drainage program as gleaned from maintenance data of existing horizontal drains throughout the US.