• Deep foundation provided below water level.
• Also known as caissons foundation.
• Used for bridge construction.

Types of well foundation

1. Open well

• Both top and bottom are open during construction.
• Cost is cheap.
• Rate of progress is slow.

2. Pneumatic caissons

• Open at bottom and closed at top.
• Sunk vertically.
• Construction cost is quite high.

3. Box caissons

• Open at top and closed at bottom.
• Cost of construction is low.

Shape of well foundation

1. Circular well

• Commonly used shape.
• Maximum diameter is 9 m.

2. Doub D-well

• Sunk easily.
• Used for pier.

3. Double octagonal well

• Shape of well is better than double D-well.

4. Twin circular well

• Two indepent wall.
• Small depth of sinking.

5. Rectangular wall

• Used for bridge foundation.
• Large foundation, double rectangular well.

1.2 Component of well foundation

• Well cap: Transmit load of super structure to steining.
• Steining: Transmit load to subsoil.
• Well curb: Facilitates process of sinking.
• Cutting edge: Cuts soil during sinking.
• Bottom plug: Transmit load to sub soil.
• Dredge hole: Hole formed during excavation.
• Top plug: Concrete plug constructed at top.

1.3 Depth of well foundation

• Depth is dependent on:

1. Minimum grip length below scour depth.

2. Base pressure to be within permissible load.

• Normal depth of scour is calculated by lacy’s formula.

d = 0.473 (Q/f)^1/2

Where,

Q = Design discharge in cumecs

f = Lacy’s factor = 1.76m^1/2

d = Scour depth

m = mean particle size

• Regime width of water way (W)

W = CQ^1/2

Where,

W = Regime width

C = Constant

• Actual water way length (L) is less than regime width.
• The actual depth(d^|) = d(W/L)0.67
• Grip length: Depth of bottom of well below maximum scour level.

1.4 Force acting on well foundation

• Live load: Load is not constant and change with time.
• Impact load: Sudden load.
• Wind load: Horizontal load.
• Force due to water.
• Seismic force.
• Earth pressure.

1.5 Construction and sinking of well

1. Sinking of a well

Steps:

a. Laying of curb

• If river bed is dry the cutting edge is placed.
• If water table is upt 5 m sand is land is created.
• If water table is more than 5 m more economical curb is built.

b. Construction of well steining

• Steining is constructed with a height of 1.5 m at time of sinking.

c. Sinking operation

• Material is excavated mechanically or manually. Manual work can be done upto 1 m height.
• Well os allowed to remain vertical.
• Sinking in well increase skin friction.

2. Tilt and shift of well

• Objective is to well sunk should sunk straight and vertical.

Cause of tilt and shift:

• No – uniform bearing capacity.
• Obstraction on one side.
• Unequal removal of soil.

Precausion:

• Uniform thickness cutting edge should be provided.
• Tilt and shift should be carefully noted.

Remedial measure of tilt and shift:

1. Control dredging

• Done more on higher side.

2. Eccentric loading

• Provide greater sinking effort on higher side of wall.

3. Pushing the wall

• Applied on lower side of wall.

4. Pulling the wall

• Applied on higher side of wall.

5. Water jetting

• Used for outer face of higher side.

References:

• Terzaghi, Karl, Peck, R.B & John, Wiley (1969) Soil mechanics in engineering practice, New York.
• Arora , K.R (2008), Soil mechanics and foundation engineering, Delhi: Standard Publisher Distribution.

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