Construction of well foundation consists of the following stages: 1. Laying the Cutting Edge 2. Alignment Control 3. Construction of Well Curb 4. Construction of Well Steining 5. Well Sinking.
Stage # 1. Laying the Cutting Edge:
Well foundations are constructed in stages by sinking under self-weight as well as dredging the soil inside the dredge hole and on the sides outside. The first step in the construction of a well foundation is to lay the cutting edge and well curb. If the river bed is dry, the cutting edge is placed in position after removing the top loose layer of sand.
Otherwise, a temporary structure, known as a sand island, is constructed using cofferdams for the purpose of excluding water, and soil is sufficiently constructed using continuous sheet piles all around the well foundation and filling the space with sand, which serves as a working platform for the work force and equipment. If the water is deeper than about 5 m, the cutting edge and the well curb are fabricated on the shore and towed to the sand island for installation.
Stage # 2. Alignment Control:
The centerline of the wells should coincide with the centerline of the abutments and piers and of the bridge. Masonry pillars are constructed on the centerline of the bridge to serve as station points for checking the alignment of the abutments and piers.
Stage # 3. Construction of Well Curb:
The well curb is assembled on wooden blocks or sand bags placed at suitable intervals so that it does not sink, while assembling the curb. Concreting is done after placing the reinforcement. M15 or richer grade concrete is used for the well curb. The well curb is then allowed to set for a week before sinking is started. The well curb is allowed to sink alone before raising the steining above it.
Stage # 4. Construction of Well Steining:
The grip length of the well is very small at the beginning of the sinking operation and the chances for tilting are more. The steining should not be raised too high in the initial stages of well sinking, which would otherwise lead to increasing the tilting further. Hence, the well curb is first allowed to sink alone and the steining is then raised in small heights of 1.5 m at a time, allowing minimum 24 h before adding the next height of steining.
The steining can be raised in installments of about 3 m, once the well has sunk to a sufficient depth to get a minimum grip length of 6 m. The steining masonry should be constructed perfectly vertical to ensure vertical sinking of the Well. Straight edges of about 2 m length should be used for this purpose along the outer periphery of the steining at suitable intervals. When the steining is raised by this height, the straight edges are removed and fixed at a higher level and the entire height of the steining should be raised using this procedure.
Stage # 5. Well Sinking:
The soil in the dredge hole is excavated to facilitate sinking of the well. A large-size spade, known as jham, is used for excavation under water. Jham consists of a sector-shaped steel pan with edges connected by a rope and wooden bullies. When Jham is used, excavation is done manually and hence well sinking is slow. Alternatively, automatic grabs or dredgers can be used for excavation operated by a winch and crane. Bell’s dredger is commonly used for sandy soils.
As the sinking proceeds, more depth of the well foundation will be below the scour level, increasing the frictional resistance between the steining surface and the surrounding soil. The steining is loaded with kentledge through a suitable platform and with sand bags piled on it to aid well sinking, overcoming this frictional resistance. The platform for the kentledge is constructed in such a way that it does not obstruct the dredging process. Air and water jets are also used in addition to the kentledge to further overcome the frictional resistance. These jets consist of G.I. pipes of 2.5-5 cm diameter with a nozzle at one end.
Well sinking may stop and tilting may also occur when the cutting edge encounters an obstruction below it. The obstruction can be removed by dewatering the well using pulsometer pumps and blowing of sand into it. The dewatering process should be continuously watched and should be suspended if there is a tendency for tilting of the well. Dewatering also should not be used unless the well has sunk to a sufficient length of a minimum of 9 m.
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