India Vertical Band Drains and Soil Stabilisation Methods

 

A network of vertical band drains is used to control ground water levels. The draining system uses wicking material, which is supplied in 280-metre rolls. The material is then threaded through a mandrel on a specially-adapted crawler rig. The mandrel then threads the lance and is connected to a slotted anchor plate at the foot of the lance. The lance is then pushed into the silt, which overlies the sandstone aquifer.

Vertical band drains are particularly effective for soft clay because they increase the shear strength of the soil. In one study, an embankment raised on 1.5 m spacing and stabilized with vertical band drains showed 90 percent consolidation after 430 days. The embankment was also reinforced with granular piles, which increased the slope stability safety factor by as much as 25 percent.

The design of vertical band drains is dependent on a number of parameters. These parameters include the width-to-thickness ratio, the surface area per unit length, and the spacing between vertical band drains. We will discuss these parameters in more detail in the following section. In general, the width-to-thickness ratio should be between 30-35 mm, and the spacing should be between three and eight feet.

The use of a composite type of vertical band drain has been proven to reduce pore pressure and prevent settlement. In addition to these benefits, this drain also provides excellent shear reinforcement for liquefied soil. A case study in India found that the use of vertical band drains in India reduced overall settlement and reduced the amount of pore water pressure. The use of vertical band drains on these embankments resulted in significant savings.

Indraratna, B., and Mesri, G., presented their results at the 2009 EH Davis Memorial Lecture. Their studies on the ground improvement of vertical band drains used vacuum preloading and mandrel action. The results of these studies have helped geotechnical engineers to come up with innovative solutions and improve their designs.

The design of vertical band drains in India has evolved since their initial application in the US in the 1920s. Various techniques such as vacuum preloading and electro osmosis have been developed since then. Some of these techniques have reduced the required surcharge. Nevertheless, the practice of installing vertical band drains is a widely-used practice in soft clay regions.

The effect of smear on vertical band drains on soft clay foundations has also been studied. With the use of a large radial drainage consolidometer, the smear zone is estimated to be between two and three times the diameter of the band-shaped drain. It shows a significant reduction in horizontal permeability toward the central band drain, but otherwise the vertical permeability remains unchanged.

The construction of vertical band drains is very simple and quick. The plastic strip is pushed into the ground using a crawler-mounted rig. The polyester strip acts as a wick that allows pore water to rise, while the polyester core allows water to flow more easily. These drains are installed in triangular patterns between eight to 12 metres deep.