WELL FOUNDATION

Well foundations are deep foundations below water for monuments, bridges and aqueducts.

Example: Taj Mahal @ Agra

Construction of well foundation is in principle, similar to the conventional wells sunk for obtaining underground water

It is a monolithic and massive foundation and is relatively rigid in its engineering behavior.

Major applications: Bridge piers and abutments

Advantages of well foundations:

1. The effect of scour ca be better withstood by a well foundation because of its large cross sectional area and rigidity.

2. The depth can be decided as the sinking progresses, since the nature of the strata can be inspected and tested, if necessary at any desired stage.

3. It can withstand large lateral loads and moments that occur in the case of bridge piers, abutments, tall chimneys and towers

4. There is no danger of damage to adjacent structures since sinking of well does not cause any vibrations.

Elements of a well foundation:

1. Cutting edge

2. Curb

3. Concrete seal bottom plug

4. Steining

5. Top plug

6. Well cap

Different shapes of Well foundations:

In a plan view the following shapes can be construct:

1. Circular 2. Square 3. Rectangular 4. Octogonal

5. Twin – circular 6. Twin rectangular 7. Twin – hexagonal

8. Double – D

Point of view – Sinking & Skin friction – Circular is ideal

Lateral stability – square or rectangular section

Different shapes of Well foundation:

Grip length:

Definition: A well foundation should be sunk below the max. scour depth

such that there is adequate lateral stability. The depth of the bottom of

the well below the max. scour level is known as the grip length.

How do we calculate the grip length:

Step . 1 - Calculate the max. scour depth

a. With soundings method b. Lacey’s formula

d = normal scour depth, Q = design discharge, f = laceys silt factor,

Step .2 , Regime width of the waterway,

3/1)/(473.0 fQd

mdf 76.1

2/1.75.4 QW

The grip length for wells of railway bridges - 50% max. scour depth

Road bridges – 30% max. scour depth

The base of the well is taken to a depth of - 2.67 d’ below the HFL

As per IS 3955-1967, depth should not be less than 1.33 times the max. scour depth

Depth of the base level below the max.scour level - 2m (piers, abutments)

Forces acting on well foundations:

1. Dead loads 2. Live loads

3. Impact loads 4. Wing loads

5. Water pressure 6. Longitudinal forces

7. Earth pressure 8. Centrifugal force

9. Buoyancy force 10. Temperature stresses

11. Seismic forces 12. Resultant forces

Terzaghi’s Analysis:

• Compared the free bulkhead

• When a rigid bulkheads embedded in sand moves parallel to its originalposition.

• Assume the both passive and active pressures are fully mobilized, the net pressure at any depth

Considering unit length, and applying H = 0, rotation above the base,

q’max = Area ABC – Area FEC

)(. ap KKzp

1'2' ).)(2(

2

1)(

2

1DKKDKKD aPap

Finally, calculating the depth of zero shear point.

Substituting this value in first equation, q’max can be calculated. Kp and Ka rankines values can be used

Case 1. Heavy wells: rotation at the base

Case 2. Effect of surcharge:

Allowable Transverse load:

FS = factor of safety is not less than 2.

FS

LqQa

.max'

Design of the individual components of a well foundation:

1. Cutting edge: • it should have a sharp angle for cutting

through the soil• It should be strong enough so that it does

bend when penetrating through a soil containing boulders.• It should be properly anchored to the well curb

2. Well curb:

Q = u.P

Resolve the forces vertically & Horizontal

)cossin.(

)cos.(sin

N

H

Calculating the hoop tension: T = H x d/2,

3. Well steining: The thickness of the steining should be adequate for the stresses developed during sinking and after installation.

B = external diameter, P = weight added for sinking, h = height at which it has got suspended, H = depth at which the well has progressed

dNT ).cossin.

cos.sin(5.0

PHttBfhHB c ..).()..(.

Reinforcement provided – 5 to 6kg/m3 of concrete steining

Empirically thickness of the steining:

For railway bridges- 1/4th of the outside diameter

road bridges - 1/8th of the outside diameter

Thumb rule commonly used:

K = constant, 1 for sandy soils, 1.1 for soft clay, 1.25 for hard clay and boulders

4. Bottom plug:

a. Vertical load transfer to the soil

b. It shape of an electric bulb to produce an arch action, to reduce the hoop tension.

c. It is constructed in 1:2:4 cement concrete, which is trimie concrete

d. If rock bed is need to anchored 25cm to 30cm deep.

)100/8/( HBKt

5. Well cap:

a. The bottom of the well cap is kept at low water level.

b. It is designed as a slab resting on the well.

c. Min. reinforcement is provided 80 kg/m3.

6. Top plug:

a. Function is transmit the loads from pier to the well steining

b. Its provided for extra safety precautions

7. Sand filling:

a. Main purpose of the sand filling is to provide the stability to well by increasing its weight and reduce the tensile stresses cuased at the base by bending moment

b. Its not contributing towards structural strength of the well.

Sinking process of the well:

Process:

1. Construction of the well curb

2. Construction of well steining

3. Sinking process

4. Kentledge

Shifts and Tilts:

• Well should be sunk straight and vertical at the correct position.• The well tilts onto its side or it shifts away from the desired position.

The following precautions may be taken to avoid tilts and Shifts:

1. The outer surface of the well curb and steining should be smooth2. The curb diameter should be kept 40 to 80mm larger than the outer

diameter of the steining, and the well should be symmetrically placed.3. The cutting edge should be uniformly thick and sharp4. Dredgeing should be done uniformly on all sides and in all the pockets.

Tilts and shifts must be carefully noted and recorde.Limits as per IS 3955: A. Tilt - 1 in 60.

Shift - 1% of the depth sunk.

If its exceed these limits, suitable remedial measures are to be taken for rectification.

Remedial measures for correction of tilt jacks

Problem. A circular well of 6m external diameter and 4m internal diameter is embedded to a depth of 15m below the max. scour level in a sandy soil deposit. The well is subjected to a horizontal force of 800kN acting at a height of 8m above the scour level. Determine the allowable total equivalent resisting force due to earth pressure, assuming a. The rotation is about a point above the base, b. the rotation is at the base. Take sat = 20 kN/m3,= 30, factor of safety for passive resistance = 2.0, use terzaghi’s analysis.

Sol:

Step1: Calculate the Kp, Ka from value

Step 2: a. Rotation above the base – H = 8+15 = 23m

Calculate D1 from the terzaghi’s formula, this is the point where well rotates above the base

Step3 : calculate qmax

4.Qallowable = qmax. L/ FS.

Case b. Rotation at the base: heavy well case, qmax need to calculate,

Qallowable = qmax.L / FS