A pier foundation is a collection of large diameter cylindrical columns to support the superstructure for transfer large super-imposed loads to the firm strata below, it stood several feet above the ground also known as “post foundation”.
It is preferred in a location where the top level consists of the decomposed rock overlying strata of sound rocks, the difference between a cast-in-situ pile and a pier is arbitrary, a cast-in-situ pile exceeding 0.6 m in diameter
Pile foundation transfer loads through friction and bearing and pier foundation transfer loads only through bearing.
Introduction to pier foundation:
A pile is a vertical column of a relatively large cross-section than the pile, which is installed in a dry area by digging a large diameter cylindrical hole to the desired depth then backfilling from the concrete.
In some situation, it becomes difficult to move the bearing pile through the decomposed rock.
In the case of rigid clays, which offer large resistance to the driving of a bearing pile, the pier foundation can be easily built.
Types of pier foundation:
1.Masonry or concrete pier:
When a good-bearing stratum exits up to 5 m below ground level, brick masonry or concrete foundation piers may be used in excavated pits.
The size and spacing of the pier are dependent on the depth of the hard bed and the nature of the soil depends on the super-imposed load.
The concrete pier is made up of concrete, it is precast as well as cast in situ.
Precast concrete pier are manufactured in a factor then driven into the ground at the place required.
The precast concrete pier generally reinforces with steel wires generally 30 cm to 50 cm in cross-section and up to 20 m in length and a cast steel pier is provided at the bottom of the pier.
Drilled caissons are foundation holes or sub pier interchangeably used to represent a cylindrical foundation, while axial load at the top and a drilled caisson is a large compressed member subject to reaction at the bottom.
Drilled caissons are usually drilled with mechanical means, it can be of three types as follows:
- Concrete caisson with increased bottom.
- Steel pipe with concrete filled in pipe.
- Cassion with concrete and steel core in steel pipe.
Advantages of Pier Foundation:
- This method is easy which requires a small amount of material and labor.
- The necessary material is easily available and has a wide range of design variations.
- The pier foundation saves money and time because it does not require extensive excavation and much concrete which causes minimal disruption to the soil environment.
- The space between the house and the ground is sufficient to install utilities such as plumbing and electrical wiring between them.
- It is comfortable for walking on the floor, which does not rest on a solid surface and it is good for those who have stiffness and back pain.
- Inspection is possible because the diameter of the shaft is large.
- Workers can easily find the space between the house and the ground to solve issues related to plumbing and electrical as there is enough space to crawl.
- Ground vibration that is normally associated with driven piles is absent in the case of drilled pier construction.
- Bearing capacity can be increased by under-reaming the bottom (in non-caching material).
Disadvantages Pier Foundations:
- The installation of drilled piers requires careful supervision and quality control of all materials used in construction.
- The method is bulky so adequate storage space is required for all materials used in construction.
- The construction of piers in a heavy flow of groundwater due to the pressure of the artesian is very difficult.
Also read: Caisson Foundation, Pier and Beam Foundation & Floating Foundation
The pier foundation is preferred at a location where the top layer consists of the decomposed rock leveling.