Pore water pressure is critical to slide stability, so drainage is usually very effective, and is the only economical method to stabilize large landslides within natural slopes.
Surface drains: concrete diversion ditches intercept surface flows; drains on slides reduce infiltration.
Shallow drains: stone drains in trenches 1–2 m deep lined with geotextile; have limited effect by reducing soil water; granular fill in deepe counterfort drains also provides shear resistance.
Deep drains: most effective; mined adits with leaky walls and sealed floor, or boreholes with perforated casings, inclined to drain out from slide toe.
Relief wells: drain up or down through aquiclude; need pumping unless draining into a lower aquifer. Some London Clay slopes have been drained into lower sand, through 100 mm boreholes filled with sand in polypropylene tubing, on 2–5 m centres.
Interception tunnels: cut behind the slide to reduce groundwater inflow; used in 1800 to stabilize failing slopes of limestone and clay above the city of Bath.
Impermeable clays respond poorly to normal drainage.
Electro-osmosis, or heating with ducted hot air, can improve clay stability, but both are expensive.