Design and construction of well foundations
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Well foundations are commonly used for transferring heavy loads to deep soil strata for bridges. They have a large cross-sectional area and can take large vertical and horizontal loads. Designing well foundations involves determining the depth, shape, size, and type based on factors like minimum grip length and permissible base pressures. Common well foundation types include open, box, and pneumatic caissons. Precautions during construction like uniform dredging are important to avoid tilting and shifts. Well foundations are a low-cost and trusted option for bridge construction due to their high success rates and long life spans, though sinking can be time consuming.
Design and construction of well foundations
- 1. Presented By Hilal Ahmad Dar Civil-C Roll Number: 120107094
- 2. Structural foundations are the substructure elements which transmit the structural load to the earth in such a way that the supporting soil is not over stressed and not undergo deformations that would cause excessive settlement of the structure. The various types of structural foundations can be broadly grouped into 2 categories, namely, Shallow foundations Deep foundations
- 3. Well foundations are deep foundations. They provide a solid and massive foundation for heavy loads and are commonly used for transferring heavy loads to deep strata in river or sea bed. Wells provide a large section modulus with the minimum cross-sectional area and hence effectively in taking large vertical and horizontal loads even when the unsupported length is large. Concreting of well steining is done under dry and controlled conditions and hence quality of work is assured. Wells have a large cross sectional area.
- 4. Design of well foundation involves the following: Determining depth of well foundation Determining shape of well foundation Determining size of well foundation Determining type of well foundation
- 5. The two important requirements that influence the depth of a well foundation are: 1. Minimum grip length below the scour depth 2. Base pressures to be within permissible limits The grip length is taken as one third the maximum scour depth. Thus as per IS:3955 (1967), the depth of the foundation should not be less than 1.33 times the deepest scour below HFL. The maximum base pressure should be less than the allowable bearing pressure. This is important from the consideration of safety of well.
- 6. The choice of a particular shape of well is dependent mainly on the base dimensions of pier or abutment, the ease and cost of construction, tilt and shift during sinking and the magnitude of forces to be resisted. Shapes of well foundation are: 1. Circular well 2. Double D well 3. Twin circular well 4. Double octagonal well 5. Rectangular well
- 8. Wells are of three types, namely, open wells, box wells and pneumatic wells Open caisson or well: The top and bottom of the caisson is open during construction. It may have any shape in plan. Box caisson: It is open at the top but closed at the bottom. Pneumatic caisson: It has a working chamber at the bottom of the caisson which is kept dry by forcing out water under pressure, thus permitting excavation under dry conditions.
- 9. Well foundations are designed in such a way that the loads are directly transmitted by bearing to the base of the well ignoring the skin fraction developed by grip length of the well and passive pressure offered by soil. The size of the well is determined to a large extent taking into consideration a safe bearing capacity of the soil at the founding level. The size of the well has to be such that it can accommodate the pier without much need of cantilevering out the cap.
- 10. The commonly adopted method for laying out a bridge is by laying out the station point line at right angles to the center line of the bridge on the high bank on one side of the proposed bridge or anywhere between the abutments where level ground may be available. Masonry pillars are constructed on this line to serve as station points for checking location of piers
- 11. The curb is usually pitched at about 15 cm above the low water level. Excavation should be carried out upto the level at which the well curb is proposed to be pitched and the center of the well curb carefully marked. The well curb should then be assembled on wooden blocks or sand bags placed at suitable intervals so that while assembling the curb, it does not sink. Concreting is done after placing the reinforcement.
- 12. Well steining could be of masonry or R.C.C. Initially, when the well has a very small grip length, the chances of tilting are more. The chances of tilting increase considerably, if the well is made top heavy by raising the steining too high in the first instance. A better approach is to first sink the well curb alone without raising any steining above it. The steining should then be raised in heights of above 1.5 m at a time, after allowing at least 24 hours for setting. Once the well has attained a grip length of about 6 m or more, the steining can be raised in installments of about 3 m.
- 13. The well is sunk by excavating the soil from within the dredge hole. A large size spade jham is used for excavation under water. The jham tied to a rope moving over a pulley is lowered into the well. The blade is pushed into the soil by the driver. The jham full of soil is then pulled up by men and emptied outside. The process is continued. When a clay stratum is to be pierced, rail chisel is more effective.
- 14. The following precautions are useful in avoiding tilts and shifts in wells: The outer surface of the well curb and well steining must be as regular and as smooth as possible. The radius of the well curb should be kept about 20 to 40 mm larger than the outer radius of the well steining. Cutting edge should be of uniform thickness and sharpness. Dredging should be done uniformly and on all sides of the well. For a twin well, dredging should be done in both dredge holes uniformly.
- 15. Well foundations are very common and trusted for bridge construction. They have a huge success rate and tend to have large life span. Well foundations are very time consuming. It may take years together for well sinking (in worst cases). It still remains a favourite and a good choice because of low cost, life span and success rates. Well foundations have varied shapes and types to accommodate to various conditions. Since the cost is also low, it is the ideal foundation to be used.