Horizontal drains are drilled, slightly upward-inclined relief wells installed into the body of a slope, embankment, or cut face to lower the regional groundwater table and reduce the pore pressure that drives landslide movement. The classic detail, refined by the California Division of Highways through the 1930s and 1940s and codified in TRB Special Report 247 and Cornforth's Landslides in Practice, is a 2 to 4 inch slotted PVC pipe drilled 50 to 500 ft into the slope at 2 to 10 degrees upward inclination, with passive gravity flow to a daylighted outlet at the slope face.
The mechanism is direct, lower the water table inside the slope mass, raise the effective stress between soil grains, and the factor of safety against sliding goes up. On active landslides and saturated cut slope sites, drilled drains are usually the first stabilization measure deployed because they address the root cause of failure (water) before structural reinforcement like soil nails, tieback anchors, or micropiles is committed. They pair routinely with weep drains at any new wall facing, the horizontal drain handles the regional water table inside the soil mass and the weep clears residual seepage at the face for a fully dewatered system.
For installation, our drilling services include horizontal drains for slope and landslide work. At Lava Hot Springs, we installed horizontal drains as part of a tiered soil nail wall project. At Terwilliger Plaza, drainage accompanied soil nails, mesh, and shotcrete on a constrained urban slope.
To discuss a drain installation project, send the site location, available geotechnical and groundwater information, drainage plans, access constraints, discharge arrangements, and target schedule. If drainage is part of a larger repair, include the landslide remediation or stabilization scope so we can discuss the construction sequence with your project team.
What Is a Horizontal Drain?
A horizontal drain is a long, slightly upward-inclined relief well drilled into the body of a slope, embankment, or cut face to lower the groundwater table and relieve the pore pressure that drives slope failure. The standard detail is a 2 to 4 inch slotted or perforated PVC (or HDPE) pipe installed 50 to 500 plus ft into the slope at a 2 to 10 degree upward inclination, with passive gravity flow to a daylighted outlet at the slope face. Terms used interchangeably across the literature include horizontal drain, subhorizontal drain, drilled drain, and (in the older landslide literature) Hercules drain after the early Caltrans installations on the Pacific Coast Highway.
The technique was developed by the California Division of Highways through the 1930s and 1940s for landslide remediation on coast highways and is documented as the canonical slope drainage tool in TRB Special Report 247 (Landslides: Investigation and Mitigation, Turner and Schuster eds. 1996) and Derek Cornforth's Landslides in Practice (Wiley 2005). FHWA-NHI-08-097 (Highway Subdrainage Design Reference Manual) covers design and construction details for transportation applications, and USACE EM 1110-2-1901 covers drainage galleries with radial horizontal drains for dam abutments. The mechanism is the same across all of these references, lower the water table inside the slope mass, raise the effective stress, and the factor of safety against sliding improves.
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Erosion Control 
