A well foundation is a deep foundation which is used to support bridges and other large structures. The well foundation is generally provided below the water level, in order to provide extra support for the structure. The well foundation consists of a series of holes or shafts that are dug into the ground, and then reinforced with concrete. The concrete helps to transfer the load of the structure down to the soil or rock below, which helps to keep the structure stable.
Cassion or wells have been in use for centuries as a foundation for bridges and other structures. The term cassion is derived from the French word caisse which means box or chest. This term refers to a type of foundation that is composed of a series of arches, vaults, or tunnels that are constructed underground. The use of a cassion also helps to prevent erosion of the soil around the foundation. They are an ideal way to provide a solid foundation in unstable or wet ground, as the soil can be packed around the cassion to create a stable base. The use of a cassion also allows for the foundation to be constructed in difficult-to-access locations.
Types of Cassion
There are three types of caisson:
Open Caisson: Caissons of this type are used for the deepest foundations for bridge piers, abutments, and other structures. In this case, the caisson is rectangular or square in plan, and is further divided into smaller sections from the inside, forming open walls. Both the external and internal walls of a divider are usually made of reinforced concrete.
Box Caisson: In contrast to open caissons, this type is closed at the bottom. When necessary, the caisson is launched in water and towed to the site for sinking, after being cast and cured on land. A caisson is sunk by filling its interior with sand, gravel, or concrete. Caisson bases must rest on a level surface, and as such box caissons are used near the ground where the strata have a sufficient bearing capacity.
Pneumatic Caisson: A caisson of this type is closed at the top and open (during construction) at the bottom. During construction, compressed air is used to keep water out of the caisson.
Advantages of well foundations
A well foundation can be used in almost any soil condition. This is because the weight of the building is carried by the walls of the well, rather than by the soil itself. This also means that a well foundation can be used in areas where there is a lot of clay or other soil that is unstable.
A well foundation is a foundation that is deep enough so that the soil below it can support the weight of the structure above it. A well foundation can withstand large lateral loads, or loads that are applied to the side of the foundation.
There is no danger of damage to adjacent structures because the foundation piles will be driven deep into the ground and will be spaced far apart. This will ensure that the weight of the building is distributed evenly and will not cause any damage to the surrounding area.
Shapes of Wells
(i) Single Circular: A circular well foundation is ideal for the piers of single-line railway bridges and for two-lane road bridges. For excessively long piers, the diameter of circular wells cannot be increased beyond 9m because it is not economically viable.
(ii) Twin circular: In this case, two circular, identical wells are drilled very close to one another, so that they are held together by a common cap. Each well is drilled at the same time, adjacent to the other. It is preferable to use these wells when the length of the pier cannot be accommodated by a double-D or double-octagonal well. Typically, these wells are located where the sinking depth is smaller and the soil strata are capable of bearing more weight.
(iii) Double-D: Usually, these wells are used on the piers and abutments of bridges that are too long to fit into a circular 9m diameter well. A partition well has corners that are squared to prevent sinking.
(iv) Twin octagonal: In many aspects, these wells are superior to Double-D wells. A square corner is preferably eliminated so that bending stresses are significantly reduced. The larger area of these wells also provides a higher resistance against sinking than double-D wells.
(v) Rectangular: Most rectangular wells with depths up to 7m-8m are used for bridge foundations. The use of double-rectangular wells is appropriate in cases of large foundations. In rectangular wells, the loading stresses at the steining are high.
Twin hexagonal and Dumb Well are also common shapes of well foundations.
Components of Well Foundation
Steining: The walls of the wells are called steining. These are constructed of brick, stone or reinforced concrete. Skin friction and the unit weight of the well are used to design steining reinforcement. The thickness of steining is designed so that all platforms of wells sink under their own weight.
Curb: A well’s curb transfers all superimposed loads to the soil as it sinks through the cutting edge. The curb may be made of wood or RCC.
Top plug: After the filling is complete, the top plug is provided. The top plug helps transfer the load of the pier and superstructure to the steining.
Bottom plug: The bottom of the well is plugged with concrete after the well has been sunk. By enclosing the bottom plug, the well curb acts as a raft against soil pressure from below.
Sand Filling: After curing, the bottom plug concrete is removed and the well is filled with saturated sand. With sand filling, the bottom of the well will be stable and the tensile forces will not be generated at the bottom.
Well Cap: It is designed to transfer the weight of a pier to the well. It consists of a slab resisting the well. The well cap forms an interim layer between the well pier and cap because their shapes are different. There is a minimum all-round offset provided by the well cap at the base of the pier.
Forces Acting On a Well Foundation
Dead loads: These are the weight of the superstructure as well as the weight of the well.
Live loads: The weight of vehicles on roads and bridges, the weight of humans, furniture and floors, and other items.
Impact loads: These are the result of live loads, and they should only be taken into consideration in the design of pier caps and bridge seats on abutments. Other components of the well should not be affected by this effect.
Wind loads: Wind loads affect only the exposed areas of the bridge and thus act laterally.
Water pressure: Water currents exert pressure on portions of the substructure between the water level and scour level.
Longitudinal forces: Tractional and braking forces result in longitudinal forces. Longitudinal forces are determined by the type of vehicle and bearing.