WHAT IS RAFT FOUNDATION IN CONSTRUCTION
Raft foundation is the type of foundation with thick concrete slab spread across the entire area of the building reinforced with steel to support heavy structural loads from columns and walls and transfer loads from the structure to stop the soil.
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.
Raft Foundation is also known as Mat Foundation.
The use of mat foundation is for columns and walls foundations where the loads from structure on columns and walls are very high. This is used to prevent differential settlement of individual footings, thus designed as a single mat (or combined footing) of all the load bearing elements of the structure.
It is suitable for expansive soils whose bearing capacity is less for suitability of spread footings and wall footings. Raft foundation is economical when one-half area of the structure is covered with individual footings and wall footings are provided.
These foundations should not be used where the groundwater table is above the bearing surface of the soil. Use of foundation in such conditions may lead to scour and liquefaction.
When to use Raft Foundation:
In construction, raft foundation is usually suitable and recommended when –
- The soil has a low bearing capacity.
- Load of the structure has to be distributed over a large area.
- Individual or any other foundation area would approximately cover 50% of the total ground area beneath the structure.
- The columns or walls are placed so closely that the individual footings would overlap.
- Stress on soil needs to be reduced.
- There is a possibility of differential settlement in case individual footing is used.
- When soil strata are unpredictable and contain pockets of compressible soil.
- Basement is to be constructed.
Materials Used for Raft Foundation Construction:
The following are some of the materials used for the construction of Raft foundation-
Steps to Construct Raft foundation
The following are the Key steps involved in the construction process of raft foundation-
- Identify the desired depth at which foundation is to be provided.
- Excavate soil up to the required depth.
- Compact the soil.
- Provide a waterproofing membrane.
- Pour 3” of plane cement-sand paste.
- Lay reinforcement maintaining the required spacing using spacers.
- Pour concrete to the desired depth.