Subsurface water may act in many ways to reduce the stability of cuts and embankments. Among these are decrease in cohesion, subsurface erosion, lateral pressure in fractures and joints, and excess pore-water pressure. One way of removing subsurface water is to use horizontal drains, which are holes drilled into an embankment or cut slope and cased with a perforated-metal or slottedplastic liner. The equipment, materials, and procedures used in the drilling and installation of horizontal drains have been improved and refined considerably since the California Division of Highways first introduced the Hydrauger in 1939. The development of polyvinyl chloride pipe, improvements in drill bits and drill stem, and the development of drilling machines capable of producing high thrust and torque have made subsurface drainage a significant and economical alternative in the repair and prevention of some types of landslides.

Water in all its forms (rain, snow, fog, ice, etc.) and in all its occurrences (streams, lakes, oceans, the subsurface, etc.) is the single most troublesome and perplexing substance that must be dealt with by transportation engineers. Of all these occurrences, subsurface water is probably the most perplexing because it is the least predictable, especially as it relates to the stability of cuts and embankments in geologically complex areas. Subsurface water may act in many ways to reduce the stability of cuts and embankments. These include subsurface erosion, lateral pressure in fractures and joints, decrease in cohesion, reduction in moisture tension, viscous drag due to seepage flow, and excess pore-water pressure (l). By far the most common and significant of these is excess pore-water pressure, which is also referred to as neutral stress and is defined as" the stress transmitted through the fluid that fills the voids between particles of a soil or rock mass"(2)-. Pore-water pressure increases in a cut or embankm; nt when what may be termed the normal balance among infiltration, migration, and discharge is upset. This can happen rather suddenly during periods of heavy rain when there is high infiltration, or it may develop over longer periods of time due to blockage that results, for example, from consolidation along the contact or along a zone between an embankment and its underlying foundation. A reduction in stability, often to the point of failure, frequently accompanies excess increases in pore-water pressure. One way of reducing excess pore-water pressure and high seepage forces created by perched water tables or of lowering the normal water table is through the use of horizontal drains. Horizontal drains are holes drilled into an embankment or cut slope and cased with a perforated-metal or slotted-plastic liner.