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Everything you need to know about raft foundations

raft foundations
admin / December 10, 2018

Raft foundations, also known as Mat Foundations, are a large concrete slab that provides support for structures. They cover a large area under a building, imposed by many columns and walls to lower the contact pressure.

The raft foundation is a type of shallow foundation. Buildings use it where loads imposed by a structure are low relative to the bearing capacity of the surface soils. They are also cheaper and simpler to install as they don’t need much excavation.

When to use raft foundation?

People use raft foundations for unstable strata or when a normal strip foundation covers more than half of the ground area of the building. An example of an area with movement in the strata includes former mining areas.

Other situations when raft foundations are a choice are when building commercial buildings, constructing on low-bearing capacity soils, when the walls of the buildings are close, and when the ratio of individual footings to total floor space is high.

Types of raft foundation

Different structural systems require different types of raft foundations. Here are a few types of rafts:

  • Flat raft mat

Construction of small dwellings usually uses this type of foundation. It covers the whole bearing area and reinforced by two steel meshes.

  • Slab beam raft

When there are unequally-distributed column loads, slab beam rafts are used. To add stiffness to the slab, concrete or steel columns are positioned on intersections of reinforced concrete grand beams.

  • Cellular raft foundation

raft foundations

A cellular raft foundation consists of two slabs with two-way interlocking ground beams. They are rigid and suitable for heavy loads or loose soil that can be subject to uneven settlement.

Advantages and disadvantages

Raft foundations have their pros and cons when constructing structures. First of all, compared to traditional footings, they are cheaper and quicker to use. Using raft foundations also saves time and materials, as less excavation is required.

Another advantage is they are perfect poor ground conditions where they cannot spread loads effectively. They also reduce other problems like cracking.

However, the disadvantage of using raft foundations is they are prone to edge erosion. They are not suitable when the building load is focused on a single point.

Conclusion

Raft foundations can be an excellent choice when constructing commercial buildings for their engineering benefits.

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