To disperse load over a large area, tiers of beams are installed inside the concrete layer of foundations, such a foundation set-up is known as a grillage foundation. In grillage foundation, tiers of beams are placed at a right angle to each other at the foundation of the columns piers and scaffolds.
While constructing heavy structures to avoid deep excavation, a grillage foundation is used which restricts foundation depth to 1 to 1.5m only. Usual foundation transfers load from structure to ground but grillage foundation disperses the heavy load over a large area providing immense balance to heavy structures.
Types of grillage foundation:
There are mainly two types of grillage foundation:
1. Timber grillage foundation
2. Steel grillage foundation
1. Timber grillage foundation: Heavy construction in waterlogged areas where the bearing capacity of soil is less, requires timber grillage as a strong withstanding foundation. Massively loaded timber columns or load-bearing brick walls are provided with timber grillage foundation for the load to be dispersed over a large area and are economical too.
Wooden planks are of thickness 8-10 cm one longitudinally and the other across the wall exceeding 45 to 60 cm on each side of the foundation stacked in 2 layers. The bearing capacity of soil is limited to 50 to 60 kN\m thus timber platforms are provided at the foundation. Timber joints are exempted from the use of any concrete as concrete fillings don’t play any role in the transfer of load to the ground.
2. Steel grillage foundation: While casting foundation for heavy structures, the foundation is provided with a steel framework to get the load of the structure distributed over the ground known as a steel grillage foundation. The heavy steel beams induced are in two or more tiers and are also known as rolled steel joist(RSJ).
Over a concrete bed of a minimum depth of 15cm, a steel framework with a minimum separation of 100mm positioned using 20mm space bars and 25mm pipe separators are placed. Then concrete is poured with a minimum cover of 10cm on the outer beam and upper flanges. Subsequently, the next tier of the beam is placed at a right angle and concrete is poured and both tiers are connected to act as a monolithic structure.
It is to be noted that concrete in this setup does not play any role in the transfer of load but rather gets load dispersing beams connected and provides protection against corrosion.
Construction of grillage foundation
The steps to construct a grillage foundation are:
1. Excavation of around 1 to 1.5m is only needed.
2. A concrete bed of a minimum thickness of 15cm of specified grade is spread over the foundation area for steel grillage and timber planks of 8 to 10cm are placed for timber grillage.
3. In the case of timber grillage, the 2nd tier of timber is placed and compacted together with minimum spacing and the steel framework of equally spaced grill positioned together is placed with optimum space for pouring concrete in case of steel grillage.
4. The 2nd tier of a steel beam or wooden plank is placed at right angles to the prior one with proper spacing using separation batons.
5. In timber grillage, wooden planks are extended longitudinally along the masonry wall and in steel grillage with another layer of concrete a fully compact monolithic foundation is obtained.
6. Properly extended timber flange is provided and specified cover to steel is provided.
Pros of Grillage foundation
There are many advantages of installing the grillage foundation and some of them are:
1. Can be executed within less time and with less material.
2. Can successfully disperse heavy load over a large area.
3. Minimizes the vibratory frequency of structures.
4. Easily adaptable in areas with the soil of very low bearing capacity.
5. Limits deep excavations to 1-1.5m only.
6. It’s an environment-friendly procedure as the steel used can be recycled and reused.
7. Grillage foundations allow control over the evocation of heat.
8. Can be accomplished with less manpower.
Cons of Grillage foundation
The disadvantages of grillage foundation are:
1. The cost of excavation can be expensive.
2. Steel joints if exposed to spillage can get rusted and corroded to breakdown.
3. Space beneath requires to be filled and warmed.
4. Skilled workers are required.
5. Not suitable for high rise multi-storey buildings as they demand deep excavated foundations.
6. Maintaining an alignment for huge columns is difficult.
7. Mobility of heavy materials is expensive.