A pier foundation is a collection of large diameter cylindrical columns to support the superstructure and transfer large super-imposed loads to the firm strata below. It stood several feet above the ground. It is also known as “post foundation”.
Types of Pier Foundation
Usually two types of pier foundation are used. These are:
Masonry or Concrete Piers
Masonry or concrete piers depend on the level of the stratum. If a good bearing stratum exists up to 5m, masonry piers are used. The size and the shape of the piers depend on the nature of the soil, depth of the bed etc.
Drilled caissons usually refer to the cylindrical foundation. A drilled caisson is largely a compressed member subjected to an axial load at the top and reaction at the bottom. There are three types of drilled caissons:
- Concrete caisson with enlarged bottom
- Caisson of steel pipe with concrete filled in the pipe
- Caissons with concrete and steel core in the steel pipe
Advantages of Pier Foundation
Pier Foundation is found in the coastal areas. There are many advantages of it:
- This method is easy and requires less amount of materials and labor. The materials required here is easily available
- It has a wide range of variety when comes to design. There are varied materials we can use here to increase the aesthetic view and also it remains in our budget
- Pier foundation save money and time as it doesn’t need extensive excavation and lot of concrete
- It causes minimal disruption to the soil environment. a shovel can be used for excavation and existing roots and soil organisms remain mostly undisturbed. At the end of the building's useful life, the site will be easier to restore to a natural state than a site with a full basement.
- As it lifts the house above the ground, floods cannot do any damage to the structures.
- The space between house and ground is enough to install utilities like plumbing and electrical wires among them.
- Workers can easily get under the space between house and ground to solve issues related to plumbing and electrical as there is enough space to crawl
- It’s comfortable to walk on the floor which doesn’t rest on a firm surface and it is good for the people who have arthritis and back pain
- Inspection is possible as the diameter of the shafts are larger
- Engineers can change the design whenever they want if necessary
- The ground vibration that is normally associated with driven piles is absent in case of drilled pier construction.
- Bearing capacity can be increased by under-reaming the bottom (in non-caving materials).
Pier Foundation Construction Details
Spacing of Piers
Pier foundations usually are built 1-1.5 feet above the ground. This gap (how far apart are foundation piers) is necessary to prevent the moisture as the moisture damages the wooden structures.
Shape and Size of piers
The shape of the piers is:
The diameter of piers is usually 6 in, 8 in, 10in, 16in. The depth of pier foundation is below the freezing depth. The depth is around 5-6ft.
The piers can be made from varied materials
- Solid concrete
Ways of Performing
There are many ways to do pier foundation. Masonry is the most convenient way among them. But the process is not ideal. The stack of bricks is not directly put together in the hole. Fold pillar is completely on the ground, and then lower it into the wall and also does not look fast and enjoyable.
Wood, Brick, concrete etc. are used in pier foundation. But the most used material is reinforced concrete. It provides most compressive strength and has a high tolerance tensile strain. Furthermore, reinforced monolithic pillars can withstand any type of frosting and will not crack under these forces. And it is quite easy to dilute the concrete mixture and pouring it into the holes.
Types of cross section
There are many types of foundation pillars’ cross-sections. It can be cylindrical, helical or box-shaped or more complex form with the broadening of the bottom of the posts. The broadening can increase the area of the base and thus increase the load-bearing capacity of the foundation. The weight of the house will be distributed over a larger area.
Technologies of Pier foundation
The most used way consists of digging a square or rectangular hole. The size of which is 4-8 inches longer than the required diameter of the post. Then formwork is set in the pit which settles the shape of the future foundation; then the reinforcement cage is placed, and concrete is poured. After that, the formwork is removed and covered with a pillar. This technology has made it possible for us to make a reinforced concrete pillar of different shapes but requires a relatively substantial number of earthworks and the use of removable shuttering. Installation of every new post for the foundation is the following - in the dug well we set formwork which propped up the sides of the spacer. Inside the formwork, there is the reinforcement cage. The reinforcement cage makes sure that the reinforcements remain in place.
The top of the pier foundation which connects the individual posts into a single structure is called grillage. When heavy structural loads from columns, piers or stanchions are required to be transferred to a soil of low bearing capacity, grillage foundation is used. Grillage is often found to be lighter and economical.
Suitable Condition for Pier Foundation
Pier foundation is used in the below conditions:
- When decomposed rocks are present in the top strata, and there are underlying strata of sound rock below them, in such condition pier foundations, are used.
- As stiff clays offer a lot of resistance when driving a bearing pile, pier foundations can be conveniently used in such situation.
- It is used if the house is built from log, timber, frame as the pillars are small relative to other foundations
- If structure needed to be built on a slope, pier foundation is used
- The soil must have a low bearing capacity of water unless the pillars will sink under the weight of the house