What are vertical drains?
Vertical drainage is a technique for letting water flow out of soft, poorly permeable ground such as clay and peat more quickly, so that the settlement under a fill or load happens much faster. To do this, vertical drains are installed in the ground at regular spacing. They are also called wick drains. In Dutch the technique is called verticale drainage.
How does it work?
If you place a sand fill on clay or peat, the water in the pores of the ground is put under pressure: excess pore water pressure develops. The ground can only consolidate once that water has drained away. In clay this happens very slowly, because the water has to travel a long way vertically through a poorly permeable layer. Without measures, the consolidation process can take many years.
Vertical drains shorten that path. The water now flows mainly horizontally to the nearest drain and escapes upwards along it, to a draining sand layer or horizontal drains. As a result, settlement takes months instead of years.
Construction
- Band drains: usually a flat plastic core with grooves, wrapped in filter fabric. They are pushed through the soft layer into the ground with a steel mandrel. There are also drains made of natural fibres such as jute.
- Sand drains: an older variant, in which columns of sand act as drains.
- Grid and depth: the spacing and depth are determined by a geotechnical consultant based on ground investigation and the desired rate of settlement.
- Vacuum consolidation: in some systems, a vacuum is applied to the drains so that atmospheric pressure acts as an additional load.
Application
Vertical drainage is used when preparing land for new housing estates, and for roads, dykes and railways on soft ground. The aim is for most of the settlement to take place before building or construction, so that less residual settlement occurs later.
Points of attention
- Monitoring: settlement plates and piezometers are used to check that settlement proceeds as calculated before building starts.
- Effect on surroundings: settlement and groundwater flow can also affect neighbouring buildings and pipes.
- Residual settlement from creep: especially in peat, settlement due to creep continues even after consolidation. Vertical drainage mainly speeds up consolidation, not creep.
Related terms
- Settlement
- Consolidation
- Excess pore water pressure
- Ground improvement
- Permeability
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