What is Pier foundation?
The foundation which is constructed to bear the heavy load from the superstructure with the help of a pile below the soil and cylindrical columns called a pier above the ground is known as a pier foundation.
Pier foundation is also known as post foundation. There is a flat structure between the pile and cylindrical column (pier) called a pile cap which helps the superstructure withstand with column and be stable.
In the pier foundation, the hollow vertical shaft is sunk up to the hard bed and hollow portions are then filled is up with an inert material such as sand or lean concrete.
Where can we construct pier foundation?
A pier foundation is especially suitable for heavy structures such as flyovers in Sandy soil or soft soil overlying hard beds at a reasonable depth. This foundation transfers the load by bearing and load is taken only by a bearing. This foundation is open at the top as well as bottom is called open Caissons.
Pier foundation is suitable where the bearing stratum is available at shallow and no chances of erosion below the foundation. This is close at the top and opens at the bottom is called closed Caissons. This is also called a floating Caissons.
A pier foundation consists of a cylindrical column of large diameter to support and transfer a large super-imposed load to the firm of strata below. A difference between pile foundation and pier foundation lies in the method of construction.
Through pile foundation transfer through friction and or a bearing, pier foundation transfers the load only through the bearing. Generally, a pier foundation is shallower in a depth than a pile foundation.
Pier foundation is preferred in a location where the top strata consist of decomposed rock overlying strata of sound rock. On this such a condition, it becomes difficult to drive the bearing piles through the decomposed rock.
In this case of stiff clays, which offer large resistance to the driving of a bearing pile, pier foundation can be conveniently constructed.
Caissons:- The term “caissons” Is derived from the French word, Caisse means a chest or box. Caissons have the mean of box-like structure, round or rectangular, which is sunk from the surface of either land or water to some of the desired depth. Caissons are the three types.
Types of Caissons
a) Box Caissons
b) Open Caissons
c) Pneumatic Caissons
a) Box Caissons:-
The box Caissons is open at the top and closed at the bottom and is a mode of timber, reinforcement concrete or steel. A Caissons is built on land, then launched and floated to a pier site where this is sunk in a position. Such a type of a Caisson is a user where bearing stratum is available at a shallow depth, and where loads are not very heavy. Closed box Caissons are used to breakwater and sea walls.
Before placing a precast launched Caissons, the level bearing surface is prepared by dredging or by the divers. The sand filling is usually done to achieve this. A launched Caissons is then sunk, by filling it with suitable material, usually sand or gravel.
b) Open caisson:-
An open Caisson reinforced concrete masonry which is open both at the top and at the bottom and is used for building and bridge foundation. Open Caissons are called Wells. A good foundation from the most common type of a deep foundation for the bridge in India.
Whenever consideration for a scours or bearing capacity requires foundation being taken to a depth of more than five to seven-meter, open excavation becomes costly on uneconomical as happy timbering has to provide. Also, because of the greater earthwork involved due to side slopes, the progress of work it open excavation will be very slow.
Another disadvantage in adopting the ordinary type of footing is that excavated material refilled around the structure is loose and hence easily scourable as compared to natural ground. The above disadvantage is avoided in a good foundation which is a shell sunk by digging inside of it and which finally becomes part of a permanent structure.
c) Pneumatic Caissons
In pneumatic Caissons, the structure is the same as the open Caissons but the difference is that the lower parts of these Caissons are open and the upper part is closed. These types of foundations are constructed generally underwater. The structure may be a bridge in a river, culvert, multistorey building, etc.
Methods of pier foundation.
1) Masonry or concrete pier:-
When a good bearing stratum exists up to 5m below ground level, brick, masonry, or concrete foundation piers in an excavated pits may be used. The size and space of the pier depending on the depth of the hard bed, nature of overlying soil, and super-imposed load.
2) Drilled caissons:-
The term drilled caissons, foundation pier, or sub-pier are interchangeably used by an engineer to denote cylindrical foundation. A drilled caisson is largely a compressed member subjected to an axial load at the top and a reaction at the bottom.
Drilled caissons are generally drilled with mechanical means. Drilled caissons may be of three types. They are given below.
a) Concrete caisson with the enlarged bottom.
b) Caissons of steel pipe with concrete filled in the pipe.
c) Caissons with concrete and steel core in steel pipe.
C) Pier foundation with arches:-
A wall is carrying heavy loads piers are dug at regular intervals and filled with cement concrete. Piers may rest on good-bearing strata.
These piers are connected by a concrete or masonry arch, over which the wall may be constructed. If required, a concrete beam may be provided over the arch if an arch is constructed of masonry. Arches are constructed with a gap above ground level.
This gap would permit the free vertical movement of soil during swelling and shrinkage operation.
Advantage of the Pier foundation
- It is easy to construct.
- It is required less money for material and labor.
- It can be pre-casted.
- It saves time.
- It is not any cause of damage to the structure.
- The designer can change the design if necessary.
Disadvantage of pier foundation
- If the space is low then it is difficult to construct.
- Some time pier spoilt due to entering of moister inside it.
- Heavy equipment are needed for handling.
Different Shapes and sizes of the pier foundation are;
The choice of a particular shape depending upon the dimension of a base of the pier or abutments, the care and cost of sinking, the consideration of tilt and shift during the sinking, and the vertical and horizontal forces to which the well is subjected.
A circular well has a minimum per meter for a given dredge area and hence the ratio of sinking afford to skin friction is maximum. Also, since the Perimeter is equidistant at all points from the center of the dredge hole, the sinking is more uniform than for another shape.
However, the disadvantage of the circular pier is that in the direction parallel to the span of the bridge, the diameter of the pier is much more than the minimum size required to accommodate the bridge pier and hence the circular well causes more obstructing the waterway than the bridge pier does.
This disadvantage is avoided in the case of a double- D shape which conforms to the shape of a bridge pier in the plan. The dredge area is similar to a double-D. Hence for the large pier, a double-D is more economical than a single circular well.
A twin circular well is at combining the advantage of a circular well and of a double D, but the only snag is that two well sunk close to each other have a tendency to close in or move apart.
However, in abutment and wing walls where the tilt and shift in position are not important, a battery of small diameter Wells is provided.
So, friends, I hope this article on “What is a pier foundation” remains helpful for you.