The primary way that drainage prevents liquefaction is by reducing the amount of water in the soil. When the soil isn’t saturated, liquefaction cannot occur (see Liquefaction).
However, some drainage techniques use other mechanisms to mitigate liquefaction damage by:
- delaying the development of excess pore water pressure during earthquake shaking
- preventing high pore water pressure in liquefied areas from spreading to adjacent areas and causing secondary liquefaction.
Drainage remediation methods are most suitable for use in sands with less than 5% fines. One of the greatest advantages of drainage is that it induces relatively small horizontal earth pressures and can be installed with relatively low vibration.
Nevertheless, it is difficult to verify how effective drainage will be prior to an actual earthquake.
Permanent dewatering involves permanently altering the level of the groundwater table. The reduced water content increases the amount of non-liquefiable soil on the surface, and if the water table is lowered below the liquefiable soil layer, the liquefaction hazard can be entirely eliminated.
In most cases, permanent dewatering is a major engineering undertaking. For sites with abundant sources of water or those that require a substantial change to the groundwater level, the cost of groundwork or permanent pumping solutions makes this technique prohibitively expensive. However, it may be a viable option for some small, high-investment sites, such as urban areas.
Vertical draining involves installing column drains either in a grid pattern across the site or in a wall-like perimeter pattern around the edge of remediated areas to isolate them from liquefiable soils.
A vertical column drain is constructed by drilling a hollow casing into the soil to the target depth, filling the hole with gravel and withdrawing the casing, leaving the vertical column in place. The uncompacted fill remains loose and acts as a natural drain.
Artificial vertical drains can be constructed from plastic composites or by using specialised piles with drainage functions. These are often easier to install than gravel drains, but they typically have a lower drainage capacity so require closer spacing and greater numbers to protect the same site area.
Vertical drains inhibit liquefaction by rapidly reducing the pore water pressure in the first few critical seconds of an earthquake. If the drainage system cannot relieve sufficient pressure or relieves it too slowly, liquefaction will still occur, and the drains will do little to improve the performance of the soil.
For this reason, vertical drains are usually combined with a densification technique.