Prefabricated Vertical Drains, also known as Wick Drains or band drains are prefabricated geotextile filter-wrapped plastic strips with moulded channels. These act as drainage paths to take pore water out of soft compressible soils that consolidate faster under a constant surcharge load.
Typically used in soft, saturated fine-grain soils, such as silts, clays, peat, sludges, mine tailings, and dredge fills, with big pore capacity and normally filled with water (fully saturated)
Wick drains or PVDs consist of a prefabricated strip which is ideal for water transportation. The flexible core is typically manufactured of polypropylene and both sides have grooves allowing water to flow unimpeded. The core is wrapped in a strong and durable geotextile filter fabric with excellent filtration properties, allowing free access of pore water into the drain. This also prevents the piping of fines from adjacent soils without clogging.
The drain is fed down through a hollow mandrel mounted on an excavator or crane mast, connected at the bottom to an expendable anchor plate. A vibratory hammer or static method is used to insert the mandrel to design depth. It’s then removed, leaving the wick drain in place. The wick drain is then cut at the ground surface, a new anchor plate connected to it, and the mandrel moved to the next location. A pattern of installed vertical wick drains provides short drainage paths for pore water, which accelerates the consolidation process and the construction schedule.
Advantages are High-speed installation, Accelerated drainage, preventing potential soil settlement because of load and in turn construction schedules. This process can be done by using the following two methods.
The preloading method is used to remove pore water over time. Heating is used to form a crystalline or glass product by electric current. Ground freezing converts pore water to ice to increase their combined strength and make them impervious. Vibro-replacement stone columns improve the bearing capacity of soil whereas the Vibrio displacement method displaces the soil. Electro osmosis makes water flow through fine-grained soils.
Vacuum Consolidation is an effective means for the improvement of saturated soft soils. The soil site is covered with an airtight membrane and a vacuum is created underneath it by using dual venture and vacuum pump. The technology can provide an equivalent pre-loading of about 4.5m high conventional surcharge fill. Vacuum-assisted consolidation preloads the soil by reducing the pore pressure while maintaining constant total stress. It would take time for the removal of water by using the pumps instead of a conventional surcharge just after applying as par the processes of the preloading method instantly.
As being a supportive way to obtain an easier and effective project schedule full of agility, these techniques have been contributing to the projects in Bangkok. According to the needs of the projects, both of the ordinary method and vacuum method has been applied then the following ways were used for a counter measuring and testing the ground stability.
a) Surface Settlement Plate
b) Extensometer (Magnetic Type)
c) Piezometer Vibrating Wire Type
e) Surface Displacement Stake
For the needs form the perspective of Architect, the backfilling process would proceed after the adjustment and modification of the ground to create the innovated and elevated garden which included the King’s Statue in the very center of the lake that would represent the Rama 9 with its massive shape as the artworks. The company is going to cooperate for these creations for the lake by using prestressed concrete piles and other additional civil works just after the soil adjustment progress which can be remarked as one of the soil engineering challenges.