Prefabricated vertical drains

In the framework of a land reclamation project, electrokinetics was applied in combination with preloading and prefabricated vertical drains to improve the mechanical strength of marine sediments intermittent current and noninsulated electrodes were adopted (Lo et al, 2000). The results showed that the mechanical properties of sediment were signiflcantly improved in the vicinity of the electrodes after 22-34 days, by applying voltage gradients ranging between 9.5 and 28.2V/m. 

Installation of prefabricated vertical drains is not only carried out on land (see Figure 7.3), but also over water using floating pontoons or barges to accommodate the drain stitchers (see Figure 7.4). The installation process over water is, however, slower than drains installed on land. 

The European Standard EN 15237 2007, Execution of Special Geotechnical Works, Vertical drainage describes the installation and testing procedures of prefabricated vertical drains. 

The most evident technique of soil improvement of soft material is the use of a surcharge in combination with prefabricated vertical drains (PVDs) in order to improve the rate of consolidation. 

The consolidation rate can be accelerated by installing prefabricated vertical drains. The design of the vertical drain system is usually the responsibility of the Contractor, since it is related to the adopted construction program. Sometimes the drain spacing (triangular or rectangular) and depth are specified in Contract documents. Since the performance of a vertical drain system is not accurately predictable, field trials are recommended. Sufficient time for such trials should then be foreseen in the project schedule or executed before tender stage. 

In soil layers where Prefabricated Vertical Drains (PVDs) are installed, part of the consolidation can be attributed to the horizontal water flow towards the drains. The formulas to calculate the consolidation as a result of horizontal drainage, are based on Barron s theory. Reference is made to the literature about this subject (Holtz et al., 1991). In Figure D.5 the basic formulas are given for the design of PVDs. However, alternative formulas exist in which smear effects caused by penetration of the drain in cohesive soil is taken into account as well. 

Imanishi. Design of prefabricated vertical drain method on reclaimed marine clay, 2000. 

Holtz, R.D., Jamiolkowski, M., Lancellatta, M., Pedroni, S., 1991. Prefabricated Vertical Drains Design and Performance. Butterworths Publ., London, England, 131 p. 

Prefabricated vertical drains providing dewatering until water evacuation and associated soil consolidation occur... 

Geotextiles in conjunction with a strip drainage core of continuous drainage channels, typically forming prefabricated vertical drains (PVDs)... 

Table 13.1 Physical properties of prefabricated vertical drains...

Sampath Kumar, J.P., Venkatappa Rao, G., Baneijee, P.K., 2003. A review on physical properties of prefabricated vertical drains. Journal of the Textile Association 64 (2), 79-82. 

---