Well Foundation or Caissons Foundation have been in usage for foundations of bridges and different structures since the Roman and Mughal periods.
Well Foundations are utilized in India for centuries because of giving deep foundations below water for aqueducts, bridges, and monuments. For instance, the popular Taj Mahal of Agra exists on good foundations.
Well Foundations are comparable to open caissons and are mainly utilized to support bridge piers and abutments since they give various advantages over different types of deep foundations for such large jobs.
What Is Well Foundation?
Well Foundation is the type of deep foundation which is mainly equipped below the water level for bridges. A Well foundation is the same as an open caisson and it can be established on the dry bed or after making a sand island.
At locations where the velocity of water is high and the depth of water is higher than 5 to 6 m, wells can be created on the river bank and then floated to the last position and grounded.
Much care is needed while grounding a well of assuring that the position of the well is exact. To prevent scour, sandbags are protected around the well. Wells or caissons are large-diameter foundations obtained in underwater conditions such as bridge foundations in rivers.
Caissons are large-width foundations that transmit the load of the superstructure through the layer of weak soil to a firm stratum at significant depth. The well may sink into the river bed by 500 mm to 600 mm under its self-weight. Additional sinking operation is equal to the sinking of wells on the dry bed.
Shapes Of Well Foundation
Wells have various shapes and accordingly, they are named as,
- Circular Well
- Double D Well
- Twin Circular Well
- Double octagonal well
- Square / Rectangular Well
1. Circular Well Foundation
Circular wells are simple in construction, easy to sink, and need minimum staining thickness due to lessening flexural stresses than in different types. A Circular Well has the minimum surface area for a provided weight, leading to considerable sinking effort available.
Circular wells acquire high strength and are subject to lower bending stresses due to the lack of sharp corners. They need just one dredger for sinking.
As every point on the cutting edge is at a constant distance from the center of the dredge hole, the possibilities of tilting the wells during sinking are less. Circular wells can be utilized for piers of single-line railway or road bridges.
If the length of the pier is lengthy, the usage of the circular wells will become uneconomical and similarly cause excessive obstruction to the flow.
Circular wells give limited resistance to lateral loads. The maximum diameter of circular wells is commonly limited to 9 m.
2. Double-D Well
Double-D wells are mainly utilized for large piers because it is more economical than compared to the single circular wells.
The wells of this shape can be sunk easily, but, significant bending moments cause in.
The steining due to the pressure difference between the inner and outside of the well. Resistance to sinking is given by the square corners at the partition wall.
3. Twin Circular Well
Twin circular wells are beneficial when the depth of the foundation is little and the foundation soil contains a high bearing capacity.
But, the major drawback is that there will be a differential settlement and tilting between the two wells with the loosening of sand between them or due to uneven sinking, even though the two wells are rigidly attached by a heavy top cap unless the soil or rock is uniform.
4. Double Octagonal Well
Double Octagonal Wells are satisfactorily in any respect than the Double-D wells. In this shape, square corners are excluded, and being stresses are significantly decreased.
Nonetheless, they give greater resistance against a sinking than double-D wells. The construction of this shape is similarly more complicated.
5. Square / Rectangular Well
A square or rectangular section is utilized for Well Foundations of a lesser depth of up to 8 m. For large-size abutments or piers, double rectangular wells with two dredge holes are utilized.
The bending stresses are, but, extremely high in the steining due to sharp corners. A rectangular section is more reasonable than a square section.
Components Of Well Foundation
The various well foundation components are as follows,
- Well Curb
- Cutting Edge
- Bottom Plug
- Sand Filling
- Top Plug
- Well Cap
1. Well Curb
The well curb is composed to support the weight of the well. Other loads that require to be evaluated in the design of a well curb are sand blows and blasting, which may be resorted to, for the sinking of the well.
The well curb should be reinforced with the minimum M-25 grade of concrete, with a minimum reinforcement of 72 kgf/m3.
In case blasting is expected during the sinking, the internal faces of the well curb should be conserved with steel plates of minimum thickness of 10 mm up to the top of the well curb.
2. Cutting Edge
The lowest part of the well foundation is cutting edge and it enables penetration of the bottom of the well during the sinking.
The thickness of the cutting edge and the slope angle of the inside edge should be such that it should be sharp sufficiently for easy sinking, and at a similar time should not break when penetrating through the hard rock or soil.
The outward edge of the cutting edge is kept vertical, while the inside edge has a slope of 1H:2V.
The walls of the well foundation are known as steining. Steining earlier utilized to be created with brick or stone, but today it is mainly prepared of concrete.
The steining is similarly reinforced suitably to resist the stresses created during the sinking of the well as well as the design stresses.
The thickness of steining is normally about one-fourth (1/4) of the diameter for railway bridges, and one-tenth (1/10) of the diameter for road bridges.
The thickness of steining is determined based on the following considerations:
- It should not be extremely thick so that unnecessary resistance is avoided during the sinking of the well.
- It should not be extremely thin so that it gets destroyed either during the sinking of the well or under design loads. The stresses generated in the steining should be within permissible limits under design loads.
- The steining should resist the more stresses generated when corrective measures are taken for tilting and shifting of the well.
4. Bottom Plug
After the sinking of the well is finalized to the essential depth, the hollow dredge hole at the underside is concreted up to some depth, which is known as the Bottom Plug.
The bottom plug will increase the bearing resistance of the well evaluated and gives long-term stability to the well foundation. The surface of the bottom plug is created bowl-shaped to have an inverted arch action.
The bottom plug is mainly designed as a thick plate subjected to a uniform bearing pressure under the maximum design loads.
It has to expand for a minimum height of 300 mm above the top of the well curb. Concreting should be finished at one stretch for the bottom plug.
5. Sand Filling
The Dredge Hole between the top and the bottom plug is 5injected with sand after sinking is finished, to enhance the self-weight of the well, to enhance its stability, and to assure that no tensile bending stresses are created at the base of the Well foundation.
Sand filling, but is not assumed to take any design loads and the whole design load is assumed to be taken by the steining.
6. Top Plug
The Top Plug is of M-15 cement concrete grade of 300 mm thickness above the sand filling.
7. Well Cap
The Reinforced Cement Concrete (RCC) slab contributed at the top of the well is identified as the well cap.
The well cap contributes the required surface for positioning the pier or abutment and transfers its loads safely to the well foundation.
The bottom of the Well cap is mainly kept at the LWL. The least reinforcement in the Well cap is about 80 kg/m3.
The longitudinal bars from Well steining, as several in number as feasible, should be anchored into the Well cap.
Types Of Well Foundation
There are three types of caissons or well are commonly utilized, which are as follows,
- Open caissons
- Box caissons
- Pneumatic caissons
1. Open Caissons
Open caissons are similarly called the Well foundations, which are caissons in which the top and bottom of the caisson are open during the construction. An open caisson may be rectangular, oblong, or circular in the plan.
It includes a cutting edge at the bottom, which is composed at the site along with the first portion of the shaft.
When the Well sinks by self-weight, the soil inside the shaft is dredged by desirable means, which aids further sinking into the soil.
Another segment of the shaft is then added to it. The procedure of sinking by self-weight, as well as by dredging, is continued till it achieves the essential depth.
The bottom of the well is then closed up with concrete, which similarly forms the base of the Well foundation.
The Hollow Shaft is injected with sand and a concrete seal is given at the top, known as a top plug. Open caissons can be constructed up to any depth and the expense of construction is considerably low.
The depth up to which the caisson is to be settled relies on the loads on the caisson, the bearing capacity of the soil, skin friction resistance of the sides, and the minimum grip length to be utilized below the scour level.
2. Box Caissons
The Box Caissons are open at the top, but closed at the bottom, as indicated in the figure. It is the early cast on the soil and then towed to the location, where it is sunk onto a previously leveled foundation base.
The sinking of the caisson is enabled by filling with gravel, sand, or concrete blocks inside the caisson. Box caissons are similarly called Floating Caissons and are utilized where loads are not extremely heavy and a bearing stratum is available at shallow depth.
3. Pneumatic Caissons
In Pneumatic Caissons, the inside air pressure of the sealed chamber is kept high to avoid water from entering the chamber.
The working chamber is consequently kept dry to enable skilled persons to work in the chamber. Airlocks are given at the top. The Caisson is Sunk under complete controlled situations by trained persons and supervisory staff in the working chamber.
The working chamber is filled with concrete after the final depth is obtained and the sinking of the caisson is finished.
Advantages Of Well Foundation
The advantages of the well foundation are as follow,
- It can withstand large lateral loads and moments which occur in the case of bridge piers, abutments, towers, and tall chimneys.
- It can withstand the reaction of scouring due to its large cross-sectional area.
- It decreases vibrations and has limited noise because the foundation is based on piers there are limited vibrations that will disturb the structure.
- Its depth can be determined due to the sinking procedure.
- No requirement for a pile cap, since the piers are filled, with concrete, there is no requirement for a pile cap.
- It is inexpensive, the cost of drilling and establishing a well or caisson is less as compared to a conventional foundation.
- They are easily adaptable in different site situations. It is simple to put well or caisson at any place. The extensively tough thing to place them is drilling the hole.
- Comparable to piles they are shorter.
- If the rock (soil) with a high bearing capacity is between 3 m to 7 m, it may be extremely effective.
- If the load is not relatively large, however, the upper 3 m to 7 m soil is not desirable and it may be applied effectively.
- The cost of the construction is comparatively low on the bed level or lower side.
The Advantages Of Well Foundation Over Pile Foundation
- A Well Foundation because of its large cross-sectional area and rigidity can resist the effect of scouring nicely.
- The depth can be determined as sinking progresses, since the nature of the layers can be examined and tested, if essential, at any desired stage.
- Thus, it is possible to assure that it lays upon a desirable bearing stratum of uniform nature and bearing power.
- A Well Foundation can resist large lateral loads and moments that happen in the case of bridge piers, towers, and tall chimneys.
- There is no danger of destruction to adjacent structures since the sinking of a well does not induce any vibrations.
Disadvantages Of Well Foundation
The disadvantages of the well foundation are as follow,
- It can not be established in contaminated areas due to the high amount of drilling needed to locate the caisson. There is a risk of additional contamination throughout the area.
- Professionals are essential for the construction of this type of foundation.
- Lack of skilled workers aware of this type of foundation.
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