Well Foundation Types and Construction

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1.1 Introduction, type and shape of well foundation

  • 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 = 1.76m1/2

d = Scour depth

m = mean particle size

  • Regime width of water way (W)

W = CQ1/2

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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