Sunk wells are constructed in soft formations. The subsoil formation may be sandy or clayey. This type is called sunk well because the lining is first constructed above the ground level and then it is sunk in the sub-soil formation by putting a load on the lining. This procedure of constructing the lining and loading is done in small lifts successively.
The actual procedure may be outlined as mentioned below:
A hole, about 1.25 metres bigger in diameter than the finished well is dug in the ground. The depth of digging is such that the natural sides stand vertical without support. At this depth a curb is placed on the bottom. The curb is of the same thickness as that of a proposed lining. The inner diameter of the curb ring is same as that of a finished well.
Then the masonry is constructed on the curb in the form of a cylinder till it emerges about 1.5 metres above the ground surface. A temporary circular ring platform is now constructed on the top of the lining cylinder and the platform is loaded with locally available material for example, rock blocks or sand filled in gunny bags.
An opening is necessary for removing the earth from the bottom of the well while sinking is in progress. The opening of 2 metres diameter is left in the platform. The platform extends on the outer side of the steening of lining cylinder to provide more area for loading. Under the load the cutting edge penetrates into the earth.
The material below the curb and at the bottom of the well can now be removed by some suitable device. Most common method is by a bamboo basket running over a pulley. Depending upon the size of the well number of pulleys may be fixed at various points along the circumference of the well.
Thus, sinking of the well cylinder proceeds. (Fig. 8.7) When sinking takes place to such an extent that the platform touches the ground surface it is removed and masonry cylinder is built again on the top of old masonry. It is taken to a sufficient height above the ground surface. The platform is again mounted on it and load is applied.
When the well reaches water table water starts coming in which can be, removed together with excavated material from the bottom. To see that the well is sunk vertically four plumb bobs may be suspended from four sides of the well. The space between the lining and the natural face is filled with clayey earth and compacted.
