Water saturated compressible soils can experience significant settlement when they are underneath constructed embankments. Often a preload will be put in place before embankment construction to preconsolidate the soil limiting the settlement that will occur after construction. The problem is that preloads may need to remain in place a long time. Often the soils most suceptible to settlement (such as marine silts and clays) are the ones that are the most difficult to drain.
Prefabricated Vertical Drains (also called PVDs or wick drains) are inserted into the saturated soil under a preload to create a drainage matrix within the soil. This allows the water of consolidation to escape easily speeding the settlement of the preload. Typically a preload that would take years can be reduced to months with PVDs. The PVDs allow the engineer to specify how long the preload will need to be in place greatly simplifying (and advancing) project scheduling.
The most common use for PVDs is underneath embankments such as bridge abutments and highway overpasses. Wherever significant settlement would occur the PVDs can speed the settlement so that construction can progress quickly.