What Is Well Foundation | Component of Well Foundation

What Is Well Foundation?

Well foundation is a type of deep foundation which is generally provided below the water level for bridges. Cassions or well have been in use for foundations of bridges and other structures since the Roman and Mughal periods. The term ‘ cassions ’ is derived from the French word Caisse which means box or chest.

Well foundations are used in India for centuries because of providing deep foundations below water for monuments, bridges, and aqueducts. For example, the famous Taj Mahal of Agra stands on well foundations.

Well foundations are similar to open caissons and are generally used to support bridge piers and abutments since they offer a number of advantages over other types of deep foundations for such large jobs.

The construction of a well foundation is, in principle, similar to the conventional wells sunk for obtaining underground water; in fact, it derives its name owing to this construction procedure.

well foundations

It’s a monolithic and massive foundation and is relatively rigid in its engineering behavior. The plan shape of a well foundation is similar to that of a caisson.

A single circular well becomes uneconomical to support a bridge pier since it must encircle the pier. In these cases, rectangular, twin-circular, twin-octagonal, or double-D sections might be used to advantage.

Dumb-bell and rectangular wells with multiple dredge holes arc two other types used for heavy bridge piers and abutments.

Box title

Well Foundation

Well foundation is a type of deep foundation which is generally provided below the water level for bridges. Cassions or well have been in use for foundations of bridges and other structures since Roman and Mughal periods. The term ‘cassion’ is derived from the French word caisse which means box or chest.

Also, read: What Is Inverted Beam | Advantages of Inverted Beam | Purpose of Inverted Beam

Advantages of Well Foundations

The advantages of well foundations over pile foundations are:

  1. A well foundation, because of its large cross-sectional area and rigidity, can withstand the effect of scouring better.
  2. The depth can be decided as sinking progresses, since the nature of the strata can be inspected and tested, if necessary, at any desired stage.
  3. Thus, it is possible to ensure that it rests upon a suitable bearing stratum of uniform nature and bearing power.
  4. A well foundation can withstand large lateral loads and moments that occur in the case of bridge piers, tall chimneys, and towers.
  5. There is no danger of damage to adjacent structures since the sinking of a well does not cause any vibrations.

Shapes Of Well Foundation

7 Different types of shapes of well foundation.

1. Single Circular well

single Circular well

Single Circular well

They require only one dredger for sinking. As every point on the cutting edge is at a constant distance from the center of the dredge hole, the chances of tilting of wells during sinking are less. Circular wells can be used for piers of single-line railway or road bridges

2. Twin Circular well

Twin Circular well

Twin Circular well

In this case, two independent circular wells placed very close to each other and having a common well cap. The wells are sunk simultaneously. Twin-circular wells are commonly used where the length of the pier is considerable, which can not be accommodated on a double-D or double-octagonal well.

3. Dumb Well

Dumb Well

Dumb Well

4. Double-D Well

Double - D Well

Double – D Well

Double-D shape wells are an improvement over twin rectangular and twin circular wells and have two dredge holes each in the shape of the letter D, as shown above fig.

The main advantage of double-D wells is their high lateral stability. When the size of the pier or abutment is large and cannot be accommodated on a single circular well economically, double-D wells will become the most economical and commonly used alternative shape.

5. Twin-Hexanol Well

Twin - Hexanol Well

Twin – Hexanol Well

6. Twin-Octagonal Well

Twin - Octagonal Well

Twin – Octagonal Well

These wells are considered to be better than Double-D wells in numerous aspects. Most preferably the square corners are eliminated such that bending stresses are reduced considerably.

Additionally these wells provide higher resistance against sinking than double-D wells because of increased area.

7. Rectangular Well

Rectangular Well 

Rectangular Well 

Rectangular wells are principally employed on bridge foundations with depths up to 7m-8m. In case of larger foundations, double-rectangular wells can be used. The loading stresses at the steining are very high in rectangular wells.

Component of Well Foundation

Well

1. Well Curb

The well curb is designed for supporting the weight of the well with partial support at the bottom of the cutting edge, i.e. when only part of the cutting edge is in contact with soil and the remaining portion is only held by skin friction.

Three-point support of the cutting edge resting on a log may be assumed for design purposes. The load coming on the cutting edge is uncertain as a considerable part of it is borne by skin friction.

Another factor of uncertainty is in regard to the effective depth of the well curb since the entire well acts as a deep girder to resist torsion and bending.

Since the load is occasional, working stress up to 99% of yield stress may be permitted. The well curb has also to withstand stress due to sand blows, as well as due to light blasting required when boulder obstructs the sinking of the well.

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2. Cutting Edge

The cutting edge should have as sharp an angle as practicable for knifing into the soil without making it too weak to resist the various stresses induced by boulders, blows, blasting, etc.

An angle to the vertical of 30° or a slope of I horizontal to 2 vertical has been found satisfactory in practice. In concrete caissons, the lower portion of the cutting edge is wrapped with 12 mm steel plates which are anchored to the concrete by means of steel straps.

A sharp vertical edge is generally provided along the outside face of the caisson. Such an edge facilitates the rate of sinking and prevents air leakage in the case of pneumatic caissons.

3. Steining Thickness

The thickness of steining is designed in such a way that at all stages the well can be sunk under its own weight, as the need for weighting with kentledge takes time and retards progress considerably.

For a circular well with outer diameter D and thickness I of the steining, we have

Self-weight per unit height = π ( D – t ) t ρ

Skin friction forces per unit weight  = π D r ƒ

Where

ρ = unit weight of concrete or masonry of the steining

r ƒ = Unit Skin Friction

Equating the two, we get  π ( D – t ) t ρ = π D r ƒ

From Which 

Formula

It will be seen from this equation that for a given value of skin friction, the steining thickness comes out to be less with increasing value of the diameter of the well.

This is, however, contrary to the usual practice of providing a greater thickness of steining with increasing diameter of the well as given in the following table:

D (Outside Dia of Wall) t (steining thickness)
3m 0.75m
5m 1.20m
7m 2.00m

This is so because of large diameter well is taken deeper and the skin friction increase with depth. Moreover, for deeper wells, water is invariably met with and the effective self-weight is reduced by buoyancy in the well below the water level, and hence larger steining thickness is required.

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4. Skin Friction

The unit skin friction increases with depth, and at a given depth, the skin friction is equal to the coefficient of friction it times the lateral earth pressure.

However, it is not possible to evaluate the skin friction from laboratory tests as the lateral earth pressure depends upon a state of stress.

It is also not possible to accurately determine the value of i.t. For the purpose of design, the values of skin friction given in the following table (Terazaghi and Peck, 1948) may be used:

Type of Soil Skin Friction (t/m2)
Silt and Soft Clay 0.73 – 2.93
Very Stiff Clay 4.9 – 19.5
Loose Sand 1.22 – 3.42
Dense Sand 3.42 – 6.84
Dense Gravel 4.9 – 9.4

Greater skin friction requires greater sinking efforts, and hence retards the sinking of the well. Hence, methods should be used to reduce skin friction while sinking the well.

Since the frictional resistance depends on the roughness of the surface of contact, a smoothly plastered well steining surface which is in a true plane without kinks or warps will considerably reduce skin friction.

Skin friction is also reduced by flaring the well. In order to reduce skin friction on the San Francisco Oakland Bay Bridge, a coating which gave a smooth oily surface and which was tough enough not to be rubbed off during the sinking process was used on the walls of the caissons and it was estimated that this reduced the friction between the concrete and fairly stiff clay by roughly 40%.

It has also been reported that bentonite solution injected on the external surface considerably reduces skin friction.

5. Bottom Plug

The bottom plug of concrete to be designed for an upward load equal to the soil pressure (including the pore pressure) minus self-weight of bottom plug and filling.

The bottom plug is made bowl-shaped so as to have inverted arch action. As generally under-water concreting has to be done for the bottom plug, no reinforcement can be provided.

The bottom plug is generally designed as a thick plate subjected to a unit bearing pressure under the maximum vertical load which is transmitted from the vertical walls of the well.

Based on the theory of elasticity, the thickness of the bottom plug is as follows:

Formula

and

Formula

Where,

t = Thickness of the concrete or steel plug

W = Total bearing pressure on the base of the well

ƒc = Flexural strength of a concrete seal

μ = Poisson’s ratio = 0.15 for concrete

R = Radius of well base

q = Unit bearing pressure against the base of well

b = Width or short side of well

= Width / length or, Short side / long side of well.


FAQ

Well Foundation

Well foundation is a type of deep foundation which is generally provided below the water level for bridges. Cassions or well have been in use for foundations of bridges and other structures since Roman and Mughal periods. The term ‘cassion’ is derived from the French word caisse which means box or chest.

What Is Well Foundation?

Well foundation is a box of timber, metal, reinforced concrete or masonry which open both at the top and bottom, and is used for building for building and bridge foundations.

Types of Well Foundation

  • Circular: These are more commonly used shape is circular, as it has high structural strength and is convenient in sinking. the chance of tilting is also minimum in this type.
  • Twin circular: These are more commonly used shape is circular, as it has high structural strength and is convenient in sinking. the chance of tilting is also minimum in this type.
  • Square/Rectangular: They are generally used for bridge foundations having depth up to 7-8 m. for large foundations, double-rectangular wells are used. For piers and abutment of very large size, rectangular wells with multiple dredge holes are used.
  • Double D: They are generally used for the piers and abutments of bridges which are too long to be accommodated on a circular well of 9 m diameter.
  • Hexagonal/Octagonal: These are better than the double-D wells in many respects. The square corners are eliminated and bending stresses are considerably reduced. However, they offer greater resistance than double-D well against sinking on account of increased surface area. Moreover, construction is more difficult.

Well Foundation Construction Procedure

  1. Laying the Cutting Edge:
  2. Alignment Control:
  3. Construction of Well Curb:
  4. Construction of Well Steining:
  5. Well Sinking:

Components of Well Foundation

  • Well-Cap: It is an RCC slab laid at the top of the well steining to transmit the loads and moments from the pier to the well or wells below.
  • Steining: It is the main body of the well which transfers load to the base of the foundation. Steining is normally of reinforced concrete.
  • Well Curb: It is the wedge-shaped RCC ring beam located at the lower portion of the well steining provided to facilitate sinking.
  • Bottom Plug: After the well is sunk to the required depth, the base of the well is plugged with concrete. This is called the bottom plug. It acts like an inverted dome supported by the steining on all the sides and transmits the load to the subsoil and acts as a raft against soil pressure from below.
  • Top Plug: The top plug is an unreinforced concrete plug, generally provided with a thickness of about 600 mm beneath the well cap to transmit the loads from the pier to the steining. The minimum grade of concrete used in top plug is M15.
  • Intermediate Plug: As discussed above, for wells resting on clayey strata, it is not preferable to fill the space inside the well completely with sand. In such cases, a sand filling is not done or sand is filled up to the scour level.

Why Well Foundation is Used?

Well foundation is a type of deep foundation which is generally provided below the water level for bridges. Cassions or well have been in use for foundations of bridges and other structures since Roman and Mughal periods.

What Is Cutting Edge in Well Foundation?

Sharp edge which is provided at the lower end of the well or open and pneumatic caisson for accelerating sinking operation is called cutting edge. It is made up of steel or it is made in R.C.C. Its angle to vertical is 30 and normally slope of 1 horizontal to 2 vertical given a better result.

Difference Between Well Foundation and Pile Foundation

The foundation has been used in India for hundreds of years, i.e. the well-known Taj Mahal in Agra stands on well foundation. The main difference between a well and a pile foundation: The pile is versatile like a beam under horizontal load. The well undergoes rigorous body movement below such load.

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