Depth of well and sinking process

Depth of a well foundation

The depth is fixed by:

1. Scour depth. The founding level is the maximum scour depth plus a grip length. The normal scour depth by Lacey's formula is $d = 1.34\left(\dfrac{q^2}{f}\right)^{1/3}$ ($q$ = discharge per metre width, $f$ = silt factor). The maximum scour depth is taken as $2d$ at piers (or $1.27d$ to $1.5d$ for abutments), measured below HFL.
2. Grip length: at least $\tfrac{1}{3}$ of the maximum scour depth below the maximum scour level, for lateral stability.
3. Bearing capacity: a stratum that can carry the load safely (settlement within limits).
4. Stability against overturning and sliding (passive resistance of soil), and the depth at which a firm stratum or rock is found.
5. The depth must also avoid liquefiable or soft layers.

Sinking of a well (in a site with water above ground level)

1. Construction site: a sand island (a temporary sand-filled island enclosed by sandbags/bamboo) is built above the water level in shallow water, or a floating caisson (a steel shell) is used in deep water.
2. Cutting edge and curb: these are placed on the island and a first lift of steining (about 2 to 3 m) is cast over them.
3. Dredging: soil is removed from the dredge hole by a grab, an air-lift or divers, so the well sinks under its own weight. Steining is built in lifts as the well sinks, keeping its top about 1 m above water.
4. Aids to sinking: kentledge (extra weights), water jetting to reduce skin friction, bentonite slurry (lubrication), or pulling.
5. Verticality control: tilt and shift are corrected as sinking continues.
6. Founding: at the founding level the base is cleaned and inspected, and the bottom plug is placed by tremie under water.
7. Finishing: the well is dewatered, filled with sand, topped with a top plug and capped by the well cap.