Here are some materials used for the erection of Raft foundation. Materials Used for Raft Foundation Construction This type of raft is very rigid and is economical when the required slab thickness is very high.
Here two concrete slabs are placed, one on top of another and connected with foundation walls in both directions and thus forms a cellular raft foundation. Rigid frame mat is referred when columns carry extremely heavy loads and the connecting beams exceeds 90cm depth. In this type of raft, the foundation walls act as a deep beam. Piles under raft help in reducing settlement and provides resistance against buoyancy. A piled raft is used when the soil at a shallow depth is highly compressible and the water table is high. This type of raft foundation is supported on piles. Purpose of this type of foundation is same as flat plate thickened under columns. In this type of mat, a pedestal is provided at the base of the columns. This type of raft is suitable when the columns are placed at a larger distance and the loads on the columns are variable. In this type of raft, beams are cast monolithically with the raft slab connecting the columns and walls. When the columns and load bearing walls are subjected to heavier loads, the slab is thickened under the columns and walls and extra reinforcement is provided to resist against diagonal shear and negative reinforcement. The thickness of this types of raft foundation is generally restricted within 300mm for economic reason. Reinforcement is placed in both directions and more reinforcement is required at the column locations and load-bearing walls. This type of mat is used when the columns and walls are uniformly spaced at small intervals and the subjected loads are relatively small. This is the simplest form of raft foundation. Rigid Frame Mat or Cellular Raft Foundationĭifferent types of mat foundation are briefly discussed below.Several types of Raft foundation may be used depending on the condition of soil and the load imposed on the foundation.įollowings are the different types of raft foundation used in construction: Any other type of footing cannot be used advantageously.When soil strata are unpredictable and contain pockets of compressible soil.There is a possibility of differential settlement in case individual footing is used.The columns or walls are placed so closely that the individual footings would overlap.Individual or any other foundation area would approximately cover 50% of the total ground area beneath the structure.Load of the structure has to be distributed over a large area.When to Choose Raft Foundationįor foundation design, one of the most important aspects is choosing the right type of foundation. Total weight of the structure and self-weight of the mat is calculated and is divided by the total area of the foundation it is covering to calculate the stress on the soil.Īs in case of raft foundation the contact area of the foundation with soil is much more than any other type of foundation, so the load is distributed over a larger area and thus the stress on soil is lesser and the possibility of shear failure of soil is also reduced. Stress distribution mechanism of raft foundation is very simple. Raft foundation transmits the total load from the building to the entire ground floor area. Let’s get a quick review of its working principle. To get a better idea about when to use raft foundation, it is important to understand how raft foundation works. Raft foundation is generally used to support structures like residential or commercial buildings where soil condition is poor, storage tanks, silos, foundations for heavy industrial equipment etc. Usually, mat foundation is spread over the entire area of the structure it is supporting. Raft foundation is actually a thick concrete slab resting on a large area of soil reinforced with steel, supporting columns or walls and transfer loads from the structure to the soil. Definition of raft foundation along with working principle, when to choose raft, types of raft foundation, materials of raft foundation, raft construction steps etc are discussed below. Raft foundation is also known as Mat foundation. The raft foundation is a very commonly used type of foundation system.