Thursday, February 16, 2012

Electro-Osmosis | Building Construction

What is Electro-Osmosis? | Civil Engineering

Electro Osmosis is a method of drainage of cohesive soils in which a direct current is used. When a direct current Is passed through a saturated soil between a positive electrode (anode) and a negative electrode (cathode), pore water migrates to the cathode.

The cathode is a well point which collects the water drained from the soil. The water collected is discharged, as in a conventional well point system.

Before you proceed with the post on "Electro-osmosis", check out the following articles on Well Points and Well Systems.
  • Single Stage Well Points
  • Multi-stage Well Points
  • Vacuum Well Points
  • Shallow Well System
  • Deep Well System
  • Horizontal Wells

The phenomenon of electro-osmosis can be explained with the help of the electrical double layer. Cations are formed in pore water when the dissolved minerals go into solution. These cations move towards the negatively charged surface of clay minerals to satisfy the electrical charge. As the water molecules acts as dipoles, the cations also attract the negative end of dipoles. When the cations move to the cathode, they take with them the attracted water molecules.

In fact, the entire outer part of the diffuse double layer which is loosely adsorbed to the soil particles gets sheared along a plane.

Anodes are in the form of steel rods located near the toe of the slope of the excavation. Cathodes are in the form of perforated pipes, resembling well points, installed in the soil mass about 4 to 5m away from the slope of the cut.

The electrodes are so arranged that the natural direction of flow of water is reversed and is directed away from excavation. This arrangement is required to prevent sloughing of the slopes. In many cases, mere reversing of the direction of flow helps in increasing the stability of the slope even if there is no significant decrease in the water content of the soil.

The system requires about 20 to 30 amperes of electricity per well at a voltage of 40 to 180. The consumption of energy is between 0.5 to 10 kWh/m3 of soil drained. Because of specialised equipment and high electricity consumption, drainage by electro-osmosis is expensive compared with other methods. The drain water in a cohesive soil of low permeability (k=1x10-5 to 1x10-8m/sec)

Electro-osmosis also helps in increasing the shear strength of the cohesive soil.