The sinking of a well foundation is carried out through the following steps.
Laying of Curbs – In dry ground excavate up to 50 cm in river bed and place the cutting edge at the required position. If the curb is to be laid under water and depth of water is greater than 5 m, prepare Sand Island and lay the curb. If depth of water exceeds 5 m built curb in dry ground and float it to the site. A typical sand island is shown in Fig.1 below.
Construction of Well Steining – The steining should be built in short height of 1.5 m initially and 3 m after a 6 m grip length is achieved. The verticality should be maintained. The aim of the well sinking is to sink the well vertically and at the correct position. The following precautions are to be taken as far as possible.
Precautions – The following precautions should be taken during well sinking.
- Outer surface should be regular and smooth.
- Radius of the curb should be 2 to 4 cm larger than the radius of the steining.
- Cutting edge should be of uniform thickness and sharpness.
- Dredging should be done uniformly on all sides.
Sinking Operation – The following operation is carried out while sinking the caisson.
- Excavate material under the inside of well curb mechanically or manually
- Allow the well to remain vertical.
- Up to a depth of 1 m, excavation underwater can be made manually. When the depth of water exceeds 1 m excavate by Jhams or grabs.
- When well goes on sinking skin friction increases and weight of well decreased due to buoyancy.
- When the well does not sink, sunk by applying kentledge. If this operation is not sufficient jet outside the well or grease the outside. A typical loading on steining by kentledge is shown in Fig 2.
- Go on adding sections of steining (2 to 5 m in length) up to the required founding strata.
Problem of Sinking – Generally the following problems are encountered while sinking a well.
Tilt and Shift- Adopt the following measures if tilt exceeds 1/60. Dredge at the higher side. Not effective at greater depths. This is shown in Fig.3 and 4 below.