The siphon drain® process is an innovative and proven solution to slope stability problems developed by TP Geo in France.
Wells are inserted in a vertical or inclined borehole in or above the unstable zone of a slope. The wells are then pumped using a siphon drain® driven by the fall in elevation of the slope. The wells reduce the pore pressure in the slope, increasing the effective stress and stabilising the slope.
The priming of the siphon is controlled by an automatic flushing system located at the downstream outlet of each siphon pipe. This system enables the siphon to operate intermittently at low flows, thus maintaining the prime by preventing the build up of air in the siphon. At high flows the flushing system automatically switches to continuous running.
The siphon drain® system is a very effective solution to slope stability problems in terms of adaptability and durability. Maintaining a low water table level prevents the build up of localised pore water pressures in the unstable zone of a slope. The water table can be lowered to 8.5 m vertically below the surface when the suction inlet is placed at 10 m below the crown of the siphon. Depending on the gradient of the slope, it is possible to achieve greater effective lowering of the water table if the length and the slope of the wells are modified. Alternatively greater drawdown can be achieved with pneumatic wells. Both the diameter and the number of siphon pipes depend on the drainage flow. Diameters range from 10 mm for 150 l/hour per well, to 25 mm for 1 cubic metre/hour per well.
The first siphon drain® system was installed by TP Geo in 1986 and remains in use today. Since then about 200 siphon drain® systems have been installed in Europe.
Examples are:
- 200 m landslide treated with 40 siphon drains®, Rhone-Alpes, France
- Five landslides treated on public roads, Normandy, France
- 80 m landslide treated with 15 siphon drains®, Champagne-Ardenne, France
- Three sites treated in Sestriere, Valtournanche, Varazze, Italy
- One site treated with 116 siphon drains® and 35 pneumatic wells at Castlehaven, Isle of Wight, United Kingdom