Well Foundations

A well (caisson) foundation is a hollow, box-like masonry or concrete structure, circular, rectangular or D-shaped in plan, sunk through the soil by dredging from inside until it reaches the founding level. It is then plugged and filled, and carries the bridge pier above. It is widely used for bridges over rivers where scour is deep.

        Pier
     ____|____
    |  Well   |  <- Well cap
    |  cap    |
  --|---------|-- GL / LWL
    |  Top    |
    |  plug   |
    | Sand/   |  <- Steining (walls)
    | filling |
    |         |
    |Interm.  |
    |plug     |
    |  Bottom |
    |  plug   |
    |_Curb____|  <- Cutting edge
     \       /

Components and functions

1. Cutting edge: steel shoe at the bottom that cuts the soil as the well sinks; protects the curb.
2. Well curb: reinforced concrete wedge-shaped ring above the cutting edge (with an inner slope of 30° to 37°); it facilitates sinking and transfers the load to soil.
3. Steining: the walls of the well, of masonry or reinforced concrete; carry load to the base and provide weight for sinking; usually 0.5 to 1.5 m thick.
4. Bottom plug: concrete plug placed under water (tremie) in the bottom of the well to seal the base and transfer the load to the soil; thickness at least 1/2 the internal diameter or by design.
5. Sand filling: clean sand placed above the bottom plug to give weight and stability and to transfer the load.
6. Top plug: a layer of concrete (about 300 mm) at the top of the sand filling, to support the well cap.
7. Well cap: a reinforced concrete slab on top of the steining that distributes the pier load uniformly to the well walls.
8. Intermediate plug (if required) and dredge hole: the dredge hole is the central opening for removing soil by grabs.

Description

Well foundations resist large vertical and horizontal loads (by their weight and by passive resistance of soil on the sides, with a grip length below the maximum scour level). They are suitable for sinking through soft soils, boulders, or sand, and are common in Nepal for large river bridges.

Tilt is the inclination of the axis of the well from the vertical, and shift is the horizontal displacement of the well from its intended position. The permissible tilt is 1 in 80 and shift about 150 mm at the founding level (IRC 78). They are corrected during sinking.

Causes

Uneven dredging, obstructions (boulders, logs), non-uniform soil, uneven kentledge, sudden slipping of the well and eccentric load.

Remedial measures for tilt

1. Dredging unevenly: remove more soil from the high side and none on the low side so the well sinks on the high side.
2. Eccentric kentledge: place extra load (kentledge) on the high side.
3. Water jetting: jet water at the high side to reduce skin friction (or pumping).
4. Pulling with wire ropes (winches): pull the top of the well against the tilt using ropes anchored to piles or to deadmen, with a jack.
5. Strutting the low side against a firm abutment (props or anchor) to prevent more tilt.
6. Blasting or removing the obstruction (boulders, wood) at the low side.
7. Pushing with jacks against the sides, in sand, if only a small tilt remains.

Remedial measures for shift

1. Pulling the well sideways with wire ropes/winches anchored to a fixed anchor.
2. Excavation on the side opposite to the direction of required movement to allow the well to move into the dredged space.
3. Intentional tilt: tilt the well in the opposite direction and then sink it vertically so it returns to position.
4. Eccentric sinking and the use of jacks against the sides.

 Tilted well       Correction
   \   \         kentledge   -> 
    \   \        dredge high side
     \___\       pull rope  <---

Measures during sinking: keep the well central with guides, sink uniformly with dredging symmetrical about the centre, check levels and verticality regularly, and use the soil type information.

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. Normal scour depth by Lacey's formula $d = 1.34\left(\dfrac{q^2}{f}\right)^{1/3}$ ($q$ = 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 which 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. Also, the depth to avoid the effect of liquefaction or soft layers and for cutoff.

Sinking of a well (in 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: 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 it sinks, keeping about 1 m above water.
4. Aids to sinking: kentledge (extra weights), jetting water to reduce skin friction, use of bentonite slurry (lubrication), or pulling.
5. Verticality control: correct tilt and shift as sinking continues.
6. Founding: at the founding level the base is cleaned, inspected and the bottom plug is placed by tremie under water.
7. Finishing: the well is dewatered, sand-filled, topped with a top plug and capped by the well cap.

A well foundation is more suitable than other foundations in the following situations.

1. Deep scour: in rivers with deep scour and where the founding level must be well below the bed (bridge piers and abutments).
2. Large vertical and horizontal loads: a large cross-section gives high bearing capacity and large lateral resistance.
3. Soil with boulders or hard layers where piles cannot be driven easily.
4. Pier shape: the well can be sunk through a variable depth of soil to a firm stratum.
5. Difficult dewatering: it can be sunk in water without sheeting and open excavation.

Advantages

• It has a large rigidity and gives a large moment of resistance.
• It is constructed with simple equipment and local labour.
• Very high load capacity: the structure is solid, and settlement is small.
• It can be inspected at the base, and the quality of founding stratum is verified.
• Economical for deep foundations of bridges and river training works.

The forces acting on a well foundation are:

1. Vertical loads: dead load of the superstructure and the pier, live load on the bridge, self-weight of the well, and buoyancy (uplift).
2. Horizontal forces: braking/tractive force of vehicles, centrifugal force on curves, wind load, water current force on the pier and well, and earthquake (seismic) forces.
3. Earth pressure: active and passive earth pressure on the well and surcharge on the soil, for abutments.
4. Temperature, shrinkage and friction at bearings (secondary forces).
5. Forces during sinking: skin friction of the soil and the stresses from the weight and kentledge.
6. Moments due to the above forces at the scour level (the horizontal force times its lever arm), and buoyancy and seepage (water pressure).

These produce a resultant horizontal force $H$ and moment $M$ at the scour level, which the lateral resistance of the soil must resist.

The lateral stability of a well foundation (resistance to horizontal force and moment) is analysed by:

1. Terzaghi's analysis (simplified), for light and heavy wells.
2. IRC:45 method (the Indian Roads Congress code, based on the modified Terzaghi method).
3. Banerjee and Gupta's method (the elastic / subgrade-reaction approach).
4. Subgrade reaction (Winkler) method.
5. Finite element methods for complex cases.

To prevent tilt and shift of a well during sinking, the following precautions are taken.

1. Proper foundation for the sinking: the sand island or the ground is made level and firm before the cutting edge is laid.
2. Accurate construction of curb and steining: the well curb and steining are built truly vertical and symmetrical, and in uniform thickness with equal lifts.
3. Uniform dredging: dredging must be done evenly around the dredge hole, taking out the soil from the centre first and not from the sides.
4. Even kentledge and loading: load must be placed symmetrically.
5. Regular checks: check the tilt and shift at least once per shift with a plumb and levels; keep records of the levels.
6. Guide frames / dolphin, to keep the well in position during the initial sinking.
7. Obstructions: examine the soil first and remove boulders and tree trunks in advance, using divers and blasting.
8. Avoid sudden sinking by controlling dredging and by not dredging much below the cutting edge.
9. Care during the final stage when the well reaches the founding level.

Grip length is the depth of the well foundation below the maximum scour level (up to the bottom of the well). IRC practice takes it as at least $\tfrac{1}{3}$ of the maximum scour depth below HFL.

Importance

• It gives the lateral stability: the passive resistance of the soil on the sides of the grip length resists the horizontal force and the overturning moment.
• It gives safety against scour that is greater than expected, and protects the base of the well from exposure.
• It provides fixity and reduces the tilt and settlement of the well.
• It makes sure the base rests in firm undisturbed soil.

• Grip length: the depth of a well foundation below the maximum scour level; at least one-third of the maximum scour depth. It gives the lateral stability of the well.
• Tilt: the inclination of the axis of the well from the vertical, found during sinking from uneven dredging or obstructions; the permissible tilt is 1 in 80.
• Shift: the lateral displacement of the centre of the well from its designed position, at the base or at the top; the permissible shift is about 150 mm.

Terzaghi's simplified approach for a light well (self-weight small, so the base does not resist horizontal force) treats the well as a rigid body in cohesionless soil, rotating about a point O at a depth $z_0$ below the scour level.

Assumptions:

• The soil reaction at depth $z$ is the net passive resistance $\sigma = \dfrac{\gamma'z(K_p - K_a)}{FS} = kz$, with $k = \dfrac{\gamma'(K_p - K_a)}{FS}$, acting on the width $B$ of the well.
• Above O the reaction opposes the horizontal force; below O it acts in the same direction as the force.
• The horizontal force $H$ acts at a height $h = M/H$ above the scour level, and $D$ is the depth of embedment.

   H -->  (height h)
   ----|----- scour level
       |  <--- sigma = kz   (above O)
      -O----  rotation point z0
       |  ---> sigma = kz   (below O)
       |_____ D

Derivation

Horizontal equilibrium:

Moments about the scour level:

Eliminating $H$ gives the equation for the depth of the point of rotation:

Solve for $z_0$, then the allowable equivalent resisting force is $H = Bk\left(z_0^2 - \tfrac{D^2}{2}\right)$. The well is stable if this value is greater than the applied horizontal force.

Given: circular well, external diameter $B = 4.5$ m, steining 0.75 m, embedment $D = 12$ m in sand, $\phi = 30^\circ$, $\gamma' = 1\ \text{t/m}^3$; $H = 50$ t and $M = 400$ t·m at the scour level; light well; $FS = 2$ for soil resistance.

Method: Terzaghi's simplified approach for a light well (net passive resistance $kz$, rotation about a point O at depth $z_0$) as derived for a light well. The width taken is the external diameter.

Soil constants

Height of the force

Depth of rotation point

Allowable equivalent resisting force

(Check by moment: $\dfrac{Bk}{3h}(D^3 - 2z_0^3) = 60.3$ t.)

Answer: allowable equivalent resisting force due to earth pressure $\approx 60.3$ t, which is greater than the applied horizontal force of 50 t, so the light well is stable.

Considerations governing the depth of a well foundation

• Maximum scour depth (Lacey's formula) and a grip length of at least one-third of it below the maximum scour level.
• Bearing capacity and settlement: the founding stratum must carry the load safely.
• Lateral stability: enough embedment so that passive resistance resists the horizontal force and moment.
• Depth of firm stratum or rock, soil layers prone to scour or liquefaction, and ease of sinking.

Numerical

Given: internal diameter 4 m, steining 0.75 m, so external diameter $B = 4 + 2(0.75) = 5.5$ m; $D = 12$ m; $H = 500$ kN, $M = 4000$ kN·m at scour level; $\gamma_{sat} = 20$ kN/m³, $\phi = 30^\circ$, $FS = 2$ for passive resistance. $\gamma_w = 9.81$ kN/m³.

Method: Terzaghi's simplified analysis. The net passive resistance at depth $z$ is $\sigma = kz$.

(a) Light well

The well rotates about a point O at depth $z_0$ below the scour level (the base offers no resistance).

(b) Heavy well

A heavy well has enough weight and base friction to resist sliding, so it rotates about the centre of its base. The soil pressure $\sigma = kz$ acts on the front over the full depth, and moments are taken about the base:

Answer: allowable total equivalent resisting force due to earth pressure $\approx 752$ kN (light well) and $\approx 1076$ kN (heavy well), both greater than the applied 500 kN.