What Is Well Foundation?
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.
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 section 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.
The advantages of well foundations over pile foundations are:
(i) A well foundation, because of its large cross-sectional area and rigidity, can withstand the effect of scouring better.
(ii) 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.
Thus, it is possible to ensure that it rests upon a suitable bearing stratum of uniform nature and bearing power.
(iii) A well foundation can withstand large lateral loads and moments that occur in the case of bridge piers, tall chimneys, and towers.
(iv) There is no danger of damage to adjacent structures since the sinking of a well does not cause any vibrations.
Type of Wells
• Single Circular well
Single Circular well
• Twin Circular well
Twin Circular well
• Dumb Well
• Double – D Well
Double – D Well
• Twin – Hexanol Well
Twin – Hexanol Well
• Twin – Octagonal Well
Twin – Octagonal Well
• Rectangular Well
Component of Well Foundation
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.
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.
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
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
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)|
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.
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.
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:
t = thickness of the concrete or steel plug
W = total bearing pressure on the base of the well
ƒc = flexural strength of concrete seal
μ = Poisson’s ratio = 0.15 for concrete
R = Radius of well base
q = Unit bearing pressure against base of well
b = width or short side of well
∝ = width / length or, Short side / long side of well.