The modern development of vertical drain methods is geosynthetic drains, though sand drains perform well, geosynthetic drains offer some extra advantages. At first we will discuss about basics of geosynthetic drains.
Band or strip shaped drains are most popular though plastic drainage conduits wrapped with geotextile can be used as vertical drains.
The Swedish institute first developed strip drain in 1948 where cardboard was used having internal ducts. The type of drain was later replaced by thin fluted drains where cardboard is supported by PVC materials.Now, more than 50 varieties of drains are available in the market which are mostly manufactured with composite materials;say a studded or corrugated construction, inner core are wrapped with filter fabric.
Normally nonwoven geotextile is chosen as filter fabric. In general, strip drains have width 100 mm and usual thickness is (2~6) mm. Some strip drains available in the market are listed below with details of their dimension, filter materials and core materials:
Advantages of synthetic drains:
• Installation is rapid and easy, saves time and offer greater economy.
• They are made of uniform material.
• Can be stored and transported easily.
• Rigs and equipment required are relatively lighter than that required in traditional sand drains.
• The continuity of strips is established by its own tensile strength.
• Cost involvement is low and treatment at site can be done at one-fourth cost of conventional sand drains.
• Placement rate is quite high, a typical installation for 15 m long synthetic drains having spacing (1~2.5), required a placement rate of maximum of 375 meter/hour.While calculating radial consolidation, the theory assumes a circular cross-section of vertical drains, which requires conversion of equipment diameter for strip drains. In this case two approaches are proposed:
1. Equal void area
2. Equal circumference
t= strip thickness
ns= porosity of traditional sand drain
Equal void area was established by per koerner in 1986; Equal circumference approach is proposed by Hansbo in 1979 which is found more conservative than equal void area approach i.e. equal void area approach produces smaller equaivalent diameter.